O-glycoside Formation

Abstract

When a monosaccharide (or a sugar fragment of any size) is condensed with either an aliphatic or aromatic alcohol, or another sugar moiety through oxygen, a glycoside bond is formed. General examples of -glycosides are shown in Scheme 2.1.

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2.1 General Methods

When a monosaccharide (or a sugar fragment of any size) is condensed with either an aliphatic or aromatic alcohol, or another sugar moiety through oxygen, a glycoside bond is formed. General examples of O-glycosides are shown in Scheme 2.1.

Scheme 2.1

Examples of O-glycosides

The most common coupling reaction methodologies used for preparing the vast majority of O-glycosides known thus far are as follows: [1]

• Michael reaction

• Fischer reaction

• Koenigs–Knorr reaction

• Helferich reaction

• Fusion method

• Imidate reaction

• Glycal reaction

• Sulfur reaction

• Armed–disarmed approach

• Unprotected anomeric carbon

• Unprotected glycosylations

• Miscellaneous leaving groups

• Solid phase approach

2.1.1 Michael Reaction

Promoter	Conditions
NaH	THF
K2CO3, NaOH	Acetone

This pioneering methodology for O-glycosylation consists of the condensation reaction between 2,3,4,6-tetraacetyl-α-d-glucopyranosyl chloride and potassium phenoxide to generate the acetylated derivate that undergoes basic hydrolysis to give phenyl-β-d-glucopyranoside (Scheme 2.2). Since its original methodology, some modifications have been introduced especially for aromatic glycosides.

Scheme 2.2

Synthesis of paranitrophenyl-β-d-glucopyranosyl tetraacetate

Some of the main features associated with this methodology are:

• Preserves the pyranose or furanose ring

• Drives the addition of the aromatic aglycon to the anomeric position

• Uses protecting groups which are easily removed in basic medium

• Produces exclusively the β-O-glycoside as a result of neighboring group participation

This reaction has been employed for the preparation of O-glycosides that are used as substrates for detection and measurement of enzymatic activity of most of the known glycosidases.

Using this methodology, several chromophores have been attached to most of the common monosaccharides. After O-glycoside cleavage by the enzyme, the release of the chromophore will indicate the sites and eventually will quantify the enzymatic activity. Some of the chromophores currently used for these purposes are represented in Scheme 2.3.

Scheme 2.3

O-glycoside chromophores used for enzymatic detection

The highly fluorescent O-glycoside substrate 7-hydroxy-4-methylcoumarin-β-d-glucopyranose is prepared by condensation between acetobromoglucose and 4-methylumbelliferone in the presence of potassium carbonate in acetone. The intermediate is deacetylated under basic conditions to form umbelliferyl β-d-glucopyranoside (Scheme 2.4).

Scheme 2.4

Michael approach for preparation umbelliferyl-O-glycoside

Anderson and Leaback [2] were able to prepare 5-bromo indoxyl-β-d-N-acetylglucopyranoside, a histochemical substrate for enzymatic detection of chitinase by condensing 3,4,6-triacetyl-β-d-N-acetylglucopyranoside chloride with 5-bromo-hydroxy-N acetyl indole at 0 °C under nitrogen atmosphere (Scheme 2.5).

Scheme 2.5

Synthesis of indole O-glycoside derivative

An alternative method for preparing the indoxyl glycosides was described more recently consisting in the coupling reaction between fucosyl bromide donor and indoxylic acid allyl ester under basic medium providing the O-glycosides in 84 % yield as β-anomer. This protocol was extended in the synthesis of sialic acid indoxyl glycosides (Scheme 2.6) [3].

Scheme 2.6

Alternative method for preparing the indoxyl glycosides

2.1.2 Fischer Reaction

Promoter	Conditions
HCl gas	CH2Cl2, r.t.
pTsOH	CH2Cl2, r.t.

This straightforward strategy is used specially for the preparation of simple O-glycosides and the advantage of this methodology is that it does not require the use of protecting groups and simply by combining the free sugar with an alcohol under acidic condition we furnish the corresponding O-glycoside. However, contrary to the previous method, this procedure is not stereo selective and therefore it provides a mixture of anomers. Also it has been found satisfactory only for small aliphatic alcohols (Scheme 2.7).

Scheme 2.7

Fischer O-glycoside reaction

The addition of a controlled stream of dry HCl during a period of around 10 min at room temperature generally is the condition of choice. However, the use of Lewis acid, ion exchange resin and more recently triflic acid have been also reported providing good yields [4].

It is worth mentioning that besides the main product, a mixture of isomers has been detected, suggesting that a rather complex mechanism is involved. It is also seen that the amount of these isomers depends importantly on the condition reactions employed (Scheme 2.8).

Scheme 2.8

Fischer O-glycoside isomers

The Fischer methodology has been applied successfully for the synthesis of benzyl O-glycosides. l-Fucose was converted into benzyl fucopyranoside [5] by treatment with benzyl alcohol under saturation with HCl at 0 °C, to furnish the α and β anomers (ratio 5:1) in 80 % yield (Scheme 2.9).

Scheme 2.9

Fischer conditions for preparation of benzyl l-fucose

2.1.3 Koenigs–Knorr Reaction

Promoter	Conditions
Ag2CO3	PhH, drierite (drying agent), I2
Ag2O	s-collidine (acid scavenger).
	CH2Cl2, 23 °C, borinic ester, ref. [6]
AgNO3	HgO (acid scavenger)
AgClO4	Ag2ClO3 (acid scavenger), THF or toluene, r.t.
AgOTf	CH2Cl2, r.t.
Silver silicate	CH2Cl2, 4 Å MS, −60 °C ref. [7]

This reaction reported in 1901 is still one of the most useful reactions for preparing a wide variety of O-glycosides [8]. It is useful for coupling reactions with either alkyl or aromatic alcohols as well as for coupling between sugars. The methodology requires silver salts as catalyst and among them the oxide, carbonate, nitrate, and triflate silver salts are the most commonly employed (Scheme 2.10). Also a drying agent such as calcium sulfate (drierite), calcium chloride, or molecular sieves is recommended. Improved yields are obtained with iodide, vigorous stirring, and protection against light during the course of the reaction.

Scheme 2.10

Koenigs–Knorr reaction

The stereochemistry observed is 1,2 trans type in most of the cases reported, as a consequence of neighboring group participation. When the protecting group is acetate at C (2), there is an intra molecular nucleophilic displacement of the leaving group, generating an orthoester [9]. This intermediate is responsible for the incorporation of the alcohol on the β-position (Scheme 2.11). Only until recently a method for preparing 1,2-cis glycosides has been developed involving the use of (1S)-phenyl-2-(phenylsulfanyl)ethyl moiety at C-2 of a glycosyl donor to give a quasi-stable anomeric sulfonium ion. The sulfonium ion is formed as a trans-decalin ring system. Displacement of the sulfonium ion by a hydroxyl leads to the stereoselective formation of α-glycosides [10].

Scheme 2.11

Proposed mechanism for Koenigs–Knorr glycosidic reaction

This versatile methodology can be applied for preparation of alky, aryl, and oligosaccharide O-glycosides. A steroidal glycoside cholesterol absorption inhibitor was prepared by condensation between acetobromocellobiose and (3β,5α, 25R)-3-hydroxyspirostan-11-one with anhydrous ZnF₂ as catalyst in acetonitrile to provide the steroidal glycoside in 93 % yield (Scheme 2.12) [11].

Scheme 2.12

Synthesis of steroidal glycoside

The steroidal glycoside estrone-β-d-glucuronide was prepared by condensation between methyl tri-O-glucopyranosylbromide uronate and estrone, employing cadmium instead of silver carbonate (Scheme 2.13) [12]. For recent developments for the synthesis of O-glucuronides [13].

Scheme 2.13

Synthesis of a steroidal O-glycoside

The syntheses of various disaccharides have been reported under Koenigs-Knorr conditions. Gentobiose octaacetate was prepared through condensation of acetobromoglucose with 1,2,3,4-tetra-O-acetyl-O-trityl-β-d-glucopyranose in nitromethane using silver perchlorate as catalyst (Scheme 2.14) [14].

Scheme 2.14

Synthesis of gentobiose

Bächli and Percival [15] reported the synthesis of laminaribiose by reacting 1,2,5,6-diisopropylidenglucose with acetobromoglucose in the presence of silver carbonate, iodine, and drierite to produce an acetonide intermediate which upon treatment with oxalic acid and sodium methoxide furnished the 1,3-disaccharide (Scheme 2.15).

Scheme 2.15

Synthesis of laminaribiose

The synthesis of various disaccharides containing N-acetylneuraminic acid (Neu5Ac) was achieved by using acetochloro and acetobromo neuraminic acids as glycosyl donors with active glycosyl acceptors under Ag₂CO₃-promoted reactions conditions (Scheme 2.16) [16, 17].

Scheme 2.16

Silver carbonate promoted synthesis of Neu5Ac(2 → 6) disaccharides

These conditions are also suitable for preparing short oligosaccharides such as the one presented in Scheme 2.17. The donor sugar acetobromogentobiose is coupled to the acceptor intermediate using silver triflate as glycosidation catalyst [18].

Scheme 2.17

Synthesis of tetrasaccharide

Total synthesis of bleomycin group antibiotic has been achieved by Katano and Hecht [19]. Thus, glycoside coupling reaction of protected disaccharide glycosyl donor with histidine derivative using silver triflate as glycoside promoter provided bleomycin key intermediate in 21 % (Scheme 2.18).

Scheme 2.18

Glycosylation reaction for preparation of bleomycin precursor

O-glycosidation reactions promoted via silver N-heterocyclic carbene complexes formed in situ in ionic liquids have been implemented. Good to excellent yields were obtained using Ag–NHC complexes derived from imidazolium halide salts to promote the glycosidation reaction (Scheme 2.19) [20].

Scheme 2.19

O-glycosidation reactions promoted via silver N-heterocyclic carbene complexes

On the other hand it has been found that 1,2-cis glycosides can be synthesized from α-glycosyl bromide with aliphatic alcohols in the presence of tetraethylammonium bromide, under mild conditions reporting high yields. The α-stereoselectivity can be explained by an equilibrium between the glycosyl bromide promoted by the tetraethylammonium bromide and the nucleophilic attack on the oxonium ion generated during the interconversion (Scheme 2.20) [21].

Scheme 2.20

Preparation of α-glycosyl bromide with aliphatic alcohols in the presence of tetraethylammonium bromide

Deoxy aceto chloro glucose has been also used as glycosyl donors under silver oxide conditions providing disaccharides in high yields. Moreover, the use of borinic acid derived catalyst enhance the regioselective and β-selective reactions with acceptors having unprotected cis-1,2- and 1,3-diol groups (Scheme 2.21) [22].

Scheme 2.21

Glycosylation reaction in the presence of silver oxide and borinic acid derived catalyst

2.1.4 Helferich Reaction

Promoter	Conditions
Hg(CN)2	CH3CN
HgBr2	CH3CN
HgI2	CH3CN
ZnI2	MS, CH2Cl2

This methodology is considered a modification of the previous one, and the main change being the use of mercury and zinc salts instead of silver. Also more polar solvents are used such as acetonitrile or nitromethane (Scheme 2.22). The yields reported for this reaction are up to 70 %, or higher; however, a mixture of anomers is often observed.

Scheme 2.22

Helferich general reaction

By following this strategy, Umezawa et al. [23, 159, 160] prepared kanamycin A by condensing 6-O-[2-O-benzyl-3-(benzyloxycarbonylamino)-3-deoxy-4,6-O-isopropylidene-α-d-glucopyranosyl]-N,N′-di(benzoyloxycarbonyl)-2-deoxyestreptamine, as glycosyl acceptor with 2,3,4-tri-O-benzyl-6-(N-benzylacetamido)-6-deoxy-α-d-glycopyranosyl chloride, as glycosyl donor. The catalyst employed was mercury (II) cyanide (Scheme 2.23).

Scheme 2.23

Synthesis of a kanamycin A derivative

The antitumoral O-glycoside epirubicine was prepared under Helferich conditions [24] using the acetonide form of adriamycinone and 2,3,6-trideoxy-3-trifluoroacetamido-4-O-trifluoroacetyl-α-l-arabinohexopyranosyl chloride, and a mixture of mercury (II) oxide and bromide as shown in Scheme 2.24.

Scheme 2.24

Synthesis of epirubicin

Other coupling reactions between sugars under Helferich conditions have been as well described [25]. For example the case of trisaccharide raffinose prepared by condensation between tetra-O-benzyl-α-d-galactopyranosyl chloride as donor and 2,3,4,1′,3′,4′,6′-hepta-O-acetyl sucrose as acceptor (Scheme 2.25).

Scheme 2.25

Synthesis of raffinose derivative

Helferich conditions have been used for preparing disaccharides containing Neu5Ac(2 → 6)Gal and Glc in good yields, although with low stereocontrol (α:β 3:4) (Scheme 2.26).

Scheme 2.26

Helferich conditions for the preparation of sialic disaccharide

2.1.5 Acetate Donors

Promoter	Conditions
ZnCl2	Heat, or MW
ZnCl2	120 °C
SnCl4	5–10 °C
TsOH	120 °C

This method has been used for preparing long chain and aromatic glycosides under different acid promoters such as ZnCl₂, SnCl₄, FeCl₃, TsOH, or zeolite. Particularly the use of ZnCl₂ as promoter has been successfully utilized to attach long chain alcohol to peracetate saccharides with moderate heating or microwave conditions to produce amphipathic glycosides in moderate to good yields as mainly the 1,2-trans-glycosides or as a mixture of anomers (Scheme 2.27) [26, 27].

Scheme 2.27

Preparation of long chain and aromatic glycosides under different acid promoters

A one-step procedure for the preparation of α-O-glycosamine pentaacetylated glycosides with yields up to 70 % and high α-stereoselectivity was achieved by condensation between commercially available d-glycosamine pentaacetates and fluorogenic coumarins, substituted phenols, and protected serine acceptors under ferric chloride conditions (Scheme 2.28) [28].

Scheme 2.28

Preparation of α-O-glycosamine pentaacetylated glycosides

Another simple method for O-glycosidation under SnCl₄ or silver triflate an SnCl₄ is described reporting high yields as a mixture of anomers depending on the as bulkiness, presence of electron-withdrawing groups or polyethoxy motifs (Scheme 2.29) [29].

Scheme 2.29

O-glycosidation protocol under SnCl₄ or silver triflate an SnCl₄ conditions

The application of zeolites as heterogeneous catalysts for the preparations of alkyl glycosides is an alternative method due to the acid strength and larger pore openings and channel intersections. Thus, the Fe-β zeolite gave the maximum yield of 63 % of cetyl galactopyranoside as a mixture of anomers (Scheme 2.30) [30].

Scheme 2.30

Heterogeneous catalysts for the preparations of alkyl glycosides

This methodology has been also useful to synthesize 1-naphthyl 2,3,4,6-tetra-O-acetyl-α,β-l-idopyranoside by mixing 1,2,3,4,6-penta-O-acetyl-α-l-idopyranose, 1-naphthol, zinc chloride and heating up to 120 °C during 1 h (Scheme 2.31) [31].

Scheme 2.31

Preparation of naphthyl O-glycosides with peracetylated sugars with naphthols under ZnCl₂ catalyst

2.1.6 Imidate Reaction

Imidate	Promoter	Conditions
OC(NH)CCl3	AgOTf	CH2Cl2, 0 °C → r.t.
OC(NH)CCl3	TMSOTf	CH2Cl2 or MeCN, 0 °C
OC(NH)CCl3	BF3-OEt2	CH2Cl2 or MeCN, −20 °C
OC(NH)CCl3	NaH	CH2Cl2
OC(NH)CCl3	PhBF2	CH2Cl2, −78 °C, ref. [32]
OC(NH)CCl3	Chiral Brønsted acid catalyst	toluene ref. [33]
OC(NH)CCl3	2 mol% Pd(PhCN)2Cl2, 4 mol% AgOTf	CH2Cl2, −78 °C, ref. [34]
OC(NPh)CF3	TBSOTf	4 Å MS, toluene, −40 °C

This protocol is attributed to Schmidt and coworkers [35, 161] who introduced trichloroacetimidate as a good leaving group for preparation of O-glycosides. A significant number of simple and complex O-glycosides involving the imidate coupling reaction have been described. This strategy involves the use of trichloroacetonitrile that in the presence of a base is incorporated on the anomeric hydroxyl group to generate trichloroacetimidate (Scheme 2.32). It should be noted that the resulting imidate derivative is air sensitive and should be used in coupling reactions immediately following preparation. Imidate formation might be spectroscopically detected by ¹H NMR through a signal appearing down field at 6.2 ppm [36].

Scheme 2.32

Preparation of glycosyl imidate and ¹H NMR of imidate rhamnosyl derivative

Once the imidate if formed, it can be subjected to nucleophilic attack to provide the corresponding S-, N-, C-, or O-glycoside, depending on the chosen nucleophile. The use of a catalyst such as BF₃.OEt₂, TMSOTf, or AgOTf is necessary to carry out the reaction to completion (Scheme 2.33). Although the unquestionable applicability of this approach, an undesirable side reaction has been encountered with glycosyl trichloroacetimidates in the presence of Lewis acid catalysis via the Chapman rearrangement [35, 161].

Scheme 2.33

Nucleophilic displacement of imidate leaving group

Hasegawa et al. [37] has prepared the ganglioside shown in Scheme 2.34 using 2,3,4,6-tetrabenzylglucopyranosyl-α-acetimidate with the lipophilic alcohol, to generate a ganglioside .

Scheme 2.34

Coupling reaction for the preparation of ganglioside

The total synthesis of calicheamicin α and dynemicin A has been described by Danishefsky’s group [38], and involves glycosylation of calicheamicinone congener with the complex glycosyl imidate using BF₃.OEt₂ as Lewis acid catalyst (Scheme 2.35).

Scheme 2.35

Glycosylation of calicheamicinone congener

Naturally occurring herbicides known as tricolorin A, F and G were isolated from the plant Ipomoea tricolor and since then synthesized involving glycoside coupling reactions. The first total synthesis of tricolorin A was performed by Larson and Heathcock [39], involving three coupling reactions steps with imidate intermediates used as glycosyl donors (Scheme 2.36). The lactonization key step for the preparation of the synthesized tricolorins has been achieved either under macrolactonization conditions reported by Yamaguchi [40, 41] and also under ring closure methathesis conditions [36].

Scheme 2.36

Synthesis of tricolorin A precursor

Another hetero-trisaccharide resin glycoside of jalapinolic acid known as tricolorin F has been synthesized involving coupling reactions with imidates as glycosyl donors. In this way disaccharide and trisaccharide were prepared sequentially. The resulting tricoloric acid C derivative was deprotected and subjected to lactonization under Yamaguchi conditions to produce protected macrolactone. Final removal of acetonide and benzyl protecting groups provided Tricolorin F (Scheme 2.37) [41].

Scheme 2.37

Synthesis of tricolorin F

A convergent approach for obtaining a tumoral antigen fragment of Lewis^X has been developed by Boons et al. [42, 162] Condensation of the imidate glycosyl donor and the trisaccharide glycosyl acceptor provided the hexasaccharide, which was further allowed to react with trichloroacetimidate to generate a hexasaccharide glycosyl donor. The final coupling reaction with the disaccharide using BF₃.OEt₂, furnished the tumoral fragment Lewis^X (Scheme 2.38).

Scheme 2.38

Convergent synthesis of Lewis^X fragment

Selectins (E, P, and L) are mammalian C-type lectins involved in the recognition process between blood cells or cancer cells and vascular endothelium. L-selectins plays a key role in the initial cell-adhesive phenomena during the inflammatory process, whereas E-selectins binds strongly to sialyl Lewis^A and Lewis^X [43, 44, 163–165]. It has been found that the tetrasaccharide sialyl Lewis^X is the recognition molecule and the preparation of sialyl Lewis^X confirmed the hypothesis that sulfation increase the affinity for L-selectins [45]. The chemical synthesis of 3e- and 6e-monosulfated and 3e,6e-disulfated Lewis^X pentasaccharides has been prepared according to the Scheme 2.39.

Scheme 2.39

Coupling reaction for the preparation of Lewis^X pentasaccharide intermediate

Likewise, thioaryl donors can also be suitably converted to acetimidates for performing glycoside coupling reactions. This is the case of arabinosyl thio derivative which is deprotected under NBS-pyridine conditions forming the lactol in 80 % yield as a mixture of anomers (2:1). Treatment with NaH, followed by addition of Cl₃CCN provided the desired trichloroacetimidate intermediate. This strategy has been successfully applied in the syntheses of cytotoxic marine natural products eleutherobin (Scheme 2.40) [46].

Scheme 2.40

Cytotoxic marine glycoside eleutherobin

Fluorogenic aglycones such as 4-methylumbelliferyl have been attached to peracetylated imidates providing the alpha anomer only when TMSOTf was used as promoter at −20 °C (Scheme 2.41). The resulting glycoside was further used for preparing a 4-MU α-T-anitgen [47].

Scheme 2.41

Synthesis of α-4-methylumbelliferyl glycosides

In order to understand the α-stereoselectivity the authors proposed that the imidates in the presence of TMSOTf generate an oxocarbenium triflate ion pair which in turn will accept the nucleophilic attack, favoring an alpha glycoside formation due to the extra stability arising from through-space electrostatic interaction between the axially disposed C-4 acetyl function and ring oxygen atom of the corresponding α-glycosyl oxonium ion (Scheme 2.42).

Scheme 2.42

Proposed oxocarbenium triflate ion intermediates leading to α-stereoselectivity

Another approach leading to the preparation of amino acid glycosides with enhanced α-stereoselectivity was described involving trichloroacetimidate donors with non-participating protecting groups with protected amino acids using the heterogeneous catalyst, HClO₄–SiO₂, reporting high yields (Scheme 2.43) [48].

Scheme 2.43

Preparation of α-amino acid glycosides from imidates

An additional utility of trichloroacetimidates as leaving group is its ability to be transformed to ureas with α-stereoselectivity via nickel-catalyzed [1,3]-rearrangement and subsequent treatment with secondary amines under the conditions described in Scheme 2.44 [49].

Scheme 2.44

Preparation of glycosyl ureas from imidates

Another approach involving imidates was assayed with trifluoroacetimidate as leaving group and a disaccharide acceptor, using CH₂Cl₂ as solvent and TBSOTf as the promoter. Under these conditions different α:β ratios were observed, however by lowering the temperature from −20 °C to −40 °C and improved α:β ratio was obtained while keeping the good yields (Scheme 2.45) [50].

Scheme 2.45

Synthesis of tetrasaccharides from phenyl trifluoroacetimidate as glycosyl donor

Likewise, fructofuranosides having N-phenyl trifluoroacetimidate as leaving group formed α-O-glycosides for different aglycons such as admantanol, protected sugars, phenols, and flavonoids, when TMSOTf is used as promoter at low temperature (Scheme 2.46) [51].

Scheme 2.46

Preparation of fructofuranosyl glycosides from N-phenyl trifluoroacetimidate as leaving group

2.1.7 Sulfur Reaction

Promoter	Conditions
NIS-TfOH	0 °C → r.t.
HgCl2	CH2Cl2 or MeCN, 0 °C
CuBr2-Bu4NBr-AgOTf	CH2Cl2 or MeCN, −20 °C
MeOTf	Et2O, r.t.
MeSOTf	Et2O, r.t.
AgOTf-Br2	CH2Cl2
DMTST	MeCN, −15 °C
NBS-TfOH	EtCN, −78 °C

Promoter	Conditions
NIS/TfOH	MeCN
NBS	CH2Cl2, r.t.
BSP	CH2Cl2, MS, r.t
DMTST	CH2Cl2
MeOTf	CH2Cl2
MeSOTf	CH2Cl2
(a) Ph2SO, Tf2O (b) TBAI	CH2Cl2, MS, −78 °C, ref. [52]
NIS, AgOTf	CH2Cl2, MS, −45 °C ref. [53]

Thioglycosides are useful glycosyl donors widely used in the preparation of O-glycosides. An example of their applicability for the preparation of saccharide synthesis is represented in Scheme 2.47. Thus, the synthesis of trisaccharide intermediate was obtained by combining the thioglycoside donor with a monosaccharide acceptor in the presence of methyltriflate, to provide the target trisaccharide in 72 % yield [54].

Scheme 2.47

Thioglycoside coupling reaction for preparation of a trisaccharide intermediate

A convergent synthesis of the trisaccharide unit belonging to an antigen polysaccharide from streptococcus has been performed by Ley and Priepke [55]. In this approach rhamnosylalkylsulfur was used as the glycosyl donor, and cyclohexane-1,2-diacetal as the protecting group (Scheme 2.48).

Scheme 2.48

Synthesis of an antigen polysaccharide fragment

Thioalkyl donors are also useful derivatives for the preparation of biologically important natural sugars known as sialic acids [23, 159, 160]. An efficient procedure for introducing thioalkyl groups as leaving groups involves the conversion of acetate into thiomethyl by treatment with methylthiotrimethylsilane in the presence of TMS-triflate. O-glycosylation reaction proceeds between the thioglycosylsialic donor and a glycosyl acceptor (bearing an -OH group available), using a catalyst such as N-iodosuccinimide-TfOH as promoter (Scheme 2.49) [56].

Scheme 2.49

Thioalkyl donor for the preparation of sialic acids

The synthesis of aryl 2-deoxy-d-glycopyranosides from 2-deoxy-1-thioglycosides and differently substituted phenols and naphthols under N-iodosuccinimide/triflic acid conditions is reported. The analysis of the reaction mixtures was followed by HPLC technique showing that the α-anomers are the major product (Scheme 2.50) [57].

Scheme 2.50

Synthesis of aryl 2-deoxy-d-glycopyranosides from 2-deoxy-1-thioglycosides

2-thiophenyl glycosides were used as glycosyl donor for preparing complex oligosaccharides containing sialyl moieties. A remarkable convergent approach was described for preparing a sialyl octasaccharide consisting in the initial glycosidic reaction between 2-thiophenyl Neu5Ac donor and trisaccharide intermediate to produce the expected tetrasaccharide in 45 % having an α(2 → 6)-linkage. The resulting tetrasaccharide was coupled with dimeric sialyl donor to yield hexasaccharide in 42 %. Acetal hydrolysis was followed by coupling reaction with Neu5Acα(2 → 3)GalSMe donor to give the octasaccharide in 85 % yield (Scheme 2.51) [58].

Scheme 2.51

Convergent synthesis of sialyl oligosaccharide

Crich and Li [59] introduced the use of 1-(Benezenesulfinyl)piperidine/triflic anhydride as promoter conditions for preparing O-glycosides from thioglycoside donors. These conditions were applied for preparing Salmonella type E1 core trisaccharide (Scheme 2.52). This method has been adopted as an alternative approach known as “iterative or preactivation” glycosylation which consist in treatment of the thioglycoside with 1-benzenesulfinyl piperidine (BSP) or morpholine analog (BSM) and triflic anhydride at low temperature, and the resulting “glycosyl triflate” intermediate treated with a thioglycosides acceptor having a free alcohol suitable for attachment [60].

Scheme 2.52

Preparation of Salmonella type E₁ core trisaccharide under BSP-Tf₂O conditions

This method has been extended as an alternative approach known as “iterative or preactivation” glycosylation which consist in the treatment of the thioglycoside with 1-benzenesulfinyl piperidine (BSP) or morpholine analog (BSM) and triflic anhydride at low temperature, and the resulting “glycosyl triflate” intermediate treated with a thioglycosides acceptor having a free alcohol suitable for coupling reaction (Scheme 2.53) [60, 61].

Scheme 2.53

Iterative or preactivation protocol

Highly fluorinated thiols have been developed and used as donors in the preparation of disaccharides. The reactivity of these novel fluorinated thiols were examined using different acceptors. Thus, disaccharide formation under glycosidic conditions provided the disaccharides in high yields (Scheme 2.54) [62].

Scheme 2.54

Highly fluorinated thiols glycosyl donor for glycosidation

Thioglycosides have been used as donor models for glycosylations with imidazolium-based ionic liquids promoters under N-iodosuccinimide conditions. Thus it was observed that tetra-O-benzyl-1-thio-β-d-glucopyranoside as donor and 1,2:3,4-di-O-isopropylidene-α-D-galactopyranose as glycoside acceptor gave the disaccharide in almost 1:1 α/β ratio in 84 % yield. This methodology claims to have the ability of recycling the ionic liquid promoter which make it attractive as a cost effective protocol (Scheme 2.55) [63].

Scheme 2.55

glycosylations with imidazolium-based ionic liquids promoters under N-iodosuccinimide

An study using protected thio gluco and galactoside bearing and acetate group at 6-position was conducted to determine the influence of solvent in the stereoselectivity of the glycosylation reaction with small and reactive acceptors has been carried out, observing a high α-stereoselectivity when using NIS/TfOH as activator and ethyl ether as the solvent at −60 °C. Other solvents did not improve the α/β ratio, although yields were high (Scheme 2.56) [64].

Scheme 2.56

α-stereoselectivity under NIS/TfOH activation

Fully substituted and deoxy thioglycoside donors were converted to cholesterol and disaccharide O-glycosides by reaction with an air- and water-stable iodonium salt phenyl(trifluoroethyl)-iodonium triflimide as an activator for glycosylation reporting 68–97 % yield as a mixture of isomers (Scheme 2.57) [65].

Scheme 2.57

O-glycoside formation with an air- and water-stable iodonium salt

Thioperoxide in combination with trimethylsilyl trifluoromethanesulfonate (TMSOTf) was designed as thioglycosides activators as it can be seen in the O-glycoside synthesis of disaccharides reporting high yields and β stereoselectivity or as a mixture of anomers (Scheme 2.58) [66].

Scheme 2.58

O-glycosylation under thioperoxide-TMSOTf conditions

Another report for preparing 1,2-cis-R-glycosides from thioglycosyl donors without directing groups involved activating conditions of Ph₂SO/Tf₂O at low temperature. It was observed that the use of tetrabutylammonium iodide (TBAI) and N-methylmaleimide leds to a increase of yield accompanied by high 1,2-cis stereoselectivity (Scheme 2.59) [67].

Scheme 2.59

1,2-Cis glycosylation under Ph₂SO/Tf₂O conditions

Toluylglycoside was chosen as a glycosyl donor for preparing glycosyl sulfonium ions, via electrochemically generated glycosyl triflate, which in turn served for preparing β-disaccharides from moderate to good yields depending on the temperature at which glycosylation was performed (Scheme 2.60) [68].

Scheme 2.60

O-glycosylation method via electrochemically generated glycosyl triflate

2.1.8 Unprotected Glycosylations

Attempts for preparing straight glycosylations using unprotected sugars with a variety of aglycons such as aliphatic, aromatic and other sugars have been implemented in the presence of different promoters. For instance simple benzyl glycosides and disaccharides of glucose, mannose and N-acetylgalactosamine were obtained in 1-ethyl-3-methylimidazolium benzoate with Amberlite IR-120 (H+) resin or p-toluenesulfonic acid as promoters in modest yields (Scheme 2.61) [69].

Scheme 2.61

Unprotected glycosylation in the presence of acidified liquid ion solvents

Brønsted acid ionic liquids (BAILs) have been designed as promoters for glycosylations of unprotected sugars due to their ability to adjust solubility properties by different cation–anion combinations. Under these conditions the yields reported range from 19 to 67 depending on the alcohol assayed, providing mainly the α-anomer. It has been observed that the reaction between different aldose monosaccharides and octanol produces a mixture of pyranosides and furanosides as a mixture of anomers (Scheme 2.62) [70].

Scheme 2.62

Unprotected glycosylation in the presence of Brønsted acid ionic liquids (BAILs)

Glycosylation of unprotected ribose with a variety of alcohols, have been carried out by following a variation of the Apple reaction which substitute a hydroxyl group by a bromine in situ, under triphenylphosphine and tetrabromomethane conditions. An improvement in the reaction was observed when lithium perchlorate was used in arabinose, xylose, and lyxose providing good yields although the glycosides were obtained in the pyranoid form with different α/β ratios (Scheme 2.63) [71].

Scheme 2.63

Unprotected glycosylation via the Apple reaction

Previously this group was able to prepare isopropyl glycosides by direct glycosylation reaction of unprotected riboside with isopropanol in the presence of mandelic acid and titanium tert-butoxide [72]. On the other hand, Meng et al. [73] reported the 1,2-cis-alkyl glycosidation protocol with unprotected phenyl 1-thioglycosyl donors with a variety of alcohol acceptors under the activation of N-iodosuccinimide–trimethylsilyl triflate (although other Lewis acids such as TfOH or BF₃.OEt₂ provide good yields). The desired product was obtained in 75–76 % yields and with high α stereoselectivity (Scheme 2.64).

Scheme 2.64

Unprotected glycosylation with unprotected phenyl 1-thioglycosyl donors

Another protecting group free glycosidations was proposed by using p-toluenesulfonylhydrazide as leaving group followed by coupling reaction with alcohols in the presence of NBS in DMF at room temperature, providing the O-glycoside in good yields 70–87 % mainly as a β-isomer (Scheme 2.65) [74].

Scheme 2.65

Unprotected glycosylation by using p-toluenesulfonylhydrazide as donor

Gold (III) activation of unprotected glycosyl donors bearing 2-butynyl as leaving group has been used in combination with primary alcohols and protected saccharides as acceptors, providing the corresponding O-glycosides as a mixture of anomers in moderate yields (Scheme 2.66) [75].

Scheme 2.66

Unprotected glycosylation from 2-butynyl glycosyl donors in the presence of gold (III) activation

2.1.9 Armed–Disarmed Method

This versatile approach has been attributed to Mootoo and Fraiser-Reid [76], and considers the use of a glycosyl donor in the classical sense coined with the term “armed saccharide” (because the reducing end is armed for further coupling reaction), and an acceptor in this case “disarmed saccharide” which contains both a free alcohol and a leaving group sufficiently resistant for the ongoing coupling reaction. The resulting disaccharide now becomes and armed disaccharide which in turn is reacted with another glycosyl acceptor or disarmed sugar to produce the oligosaccharide chain elongation (Scheme 2.67).

Scheme 2.67

General scheme for the armed–disarmed approach

This method was first implemented in the preparation of 1–6 linked trisaccharide shown in Scheme 2.68. As it can be observed the disarmed sugar intermediates function as glycosyl acceptor bearing the hydroxyl group at position 6 available for establishing a glycosidic linkage with the armed unit.

Scheme 2.68

Armed–disarmed approach

Despite the usefulness of pentenyl as protecting group, clear preference in the use of thioglycoside donors as armed and disarmed donors is often observed (Scheme 2.69) [77].

Scheme 2.69

General scheme of the armed–disarmed approach with thioglycosyl sugars

This concept was applied successfully in the stereocontrolled synthesis of Le^x oligosaccharide derivatives by using two glycosylation steps as described by Yoshida et al. [78]. The first coupling between “armed” thiophenyl fucopyranosyl derivative and “disarmed” thiophenyl lactose derivative under NIS-TfOH conditions provided trisacccharide which was subjected without purification to second condensation with different acceptors, one of which is indicated in Scheme 2.70.

Scheme 2.70

Preparation of Lewis^X tetrasaccharide using armed–disarmed coupling method

The construction of α-linked mannoside disaccharide was achieved under the armed–disarmed approach by using armed thiogalactoside donor activated by BSP/Tf₂O and condensed with disarmed thiomannoazide intermediate bearing a free hydroxyl group. Addition of triethyl phosphate prior to the aqueous work up led to the generation of the expected α-linked disaccharide in 74 % (Scheme 2.71) [77].

Scheme 2.71

Synthesis of α-linked mannosyl disaccharide following an armed–disarmed strategy

Recently S-benzoxazol thio glycoside (SBox) was synthesized and introduced as alternative glycosyl donor for preparing disaccharides under the armed–disarmed approach. Thus, the SBox glycosyl donor was used as armed donor and condensed with disarmed thioglycoside to provide the target disaccharide (Scheme 2.72) [79].

Scheme 2.72

Armed–disarmed synthesis using S-benzoxazol (SBox) as disarmed glycosyl donor

2.1.10 Glycal Reaction

Promoter	Conditions
ZnCl2	THF

The glycals are unsaturated sugars with a double bond located between C1 and C2. These useful intermediates were discovered by Fischer and Zach in 1913 [80] and their utility in the preparation of building blocks for oligosaccharide synthesis is increasingly important. Different routes for the preparation of triacetyl glucals have been examined by Fraser-Reid et al. [81], involving the Ferrier rearrangement. Moreover, a suitable one-pot preparation of glucals has been more recently described, starting from reducing sugars by Shull et al. [82] The general procedure for preparing these valuable intermediates is based on the reductive removal of a halogen and neighboring acetate group through the use of zinc in acetic acid (Scheme 2.73). The completion of this reaction can be followed by ¹H NMR, where the presence of a signal around 6.3 ppm as double of double with J _1,2 = 6.2 Hz, J _1,3 = 0.3 Hz is expected for H-1, and a multiple shifted upfield for H-2.

Scheme 2.73

Fischer–Sachs glucal and ¹H NMR of benzylfucopyranosyl glycal

More recently the use of alternative catalysts such as titanium complex, Li/NH₃, Sodium, Cr (II) and vitamin B-12 as catalysts has been described as improved method, for preparing especially acid sensitive glycals.

As for any double bond, these unsaturated sugars may undergo electrophilic addition, which takes place at the C2 position leaving a positive charge at C1, which instantly reacts with the conjugate base. This reaction is particularly useful for the preparation of 2-deoxypyranosides (Scheme 2.74).

Scheme 2.74

Electrophilic addition

A more extended application for glycoside bond formation has been developed recently. Such strategies consist of the conversion of glycals into Brigl’s epoxide, and then further treatment with nucleophiles to effect ring opening. The oxidation of the double bond has been successfully achieved with dimethyl dioxirane (DMDO) in acetone (Scheme 2.75).

Scheme 2.75

Brigl epoxide formation

The standard procedure for generation of DMDO was developed by Murray and Jeyaraman [83], and optimized by Adam et al. [84]. Such procedure involves the use of potassium monoperoxysulfate as oxidizing agent, and the reaction conditions require temperatures below 15 °C and efficient stirring. The DMDO–acetone solution generated must be immediately distilled under moderate vacuum. The concentrations of DMDO are in the order of 0.09–0.11 M (5 %), and it is used as acetone solution. The transformation of the glycal to the epoxide can be verified by ¹H NMR, where it is observed the disappearance of the signal at 6.3 ppm for H-1 double bond, and it is expected the presence of a signal at 5.0, as double for H-1 and at 3.1 as double of double for H-2 (Scheme 2.76).

Scheme 2.76

¹H NMR spectra of 1,2-anhydro-3,4-di-O-benzyl-α-d-fucopyranose (and traces of acetone)

The stereo selectivity of epoxide formation is protecting group dependent, observing in the case of acetate protecting group a mixture of epoxide anomers, and preferentially the α-anomers if the protecting groups are benzyl, or methyl groups (α:β ratio 20:1). As expected, the epoxide ring opening by nucleophiles occurs with inversion of configuration, providing β-glycosides exclusively (Scheme 2.77).

Scheme 2.77

Ring opening for β-glycoside formation

Likewise, alternative epoxide conditions from glycals have been assayed besides DMDO treatment. Among them, cyclization of a bromohydrin [85], m-chloroperoxybenzoic acid-potassium fluoride complex oxidation of the glycal [86], and potassium tertbutoxide oxidation of fluoride glycosyl donor [87] has been described (Scheme 2.78).

Scheme 2.78

Alternative glycal-epoxidations

The potential of 1,2-anhydro sugars as glycosyl donor for the preparation of β-linked saccharides was established by Halcomb and Danishefsky [88] and such strategy consist in the treatment of the glucal having available a hydroxyl group at position 6, with the sugar epoxide under Lewis acid conditions (ZnCl₂) at low temperature. The resulting glucal disaccharide generated as a single coupling product was further converted to the epoxide which eventually lead to the next coupling reaction with another glucal acceptor (Scheme 2.79).

Scheme 2.79

Epoxide glycal as glycosyl donors

The tetrasaccharide Cap Domain of the antigenic lipophosphoglycan of Leishmania donovani has been prepared under the glycal approach by Upreti and Vishwakarma [89]. Thus, the preparation of the hexa-O-benzyl-lactal under standard procedures was followed by oxirane formation with dimethyl dioxirane to generate the corresponding oxirane. Methanolysis ring opening and gluco → manno conversion generated the disaccharide intermediate. This was coupled to the mannobiose donor to produce the tetrasaccharide, which after deprotection lead to the tetrasaccharide Cap domain (Scheme 2.80).

Scheme 2.80

Synthesis of a tetrasaccharide using an epoxide disaccharide as glycosyl donor

Brigl’s epoxide has been exploited successfully for the preparation of glycosylated peptides such as collagen type II derived glycosides carrying β-Gal and αGlc-1,2-βGal side chains [90, 166]. Galactosyl glycal is reacted with DMDO–acetone solution and the resulting epoxide reacted with hydroxylysine and ZnCl₂ as promoter (Scheme 2.81). General procedures for preparation of glycosidic bond of glycopeptides can be reviewed in the comprehensive study reported by Kunz [91].

Scheme 2.81

Amino acids glycosidation

A Gold (I)-catalyzed glycosidation approach was developed by reaction of anhydro glycals with protected sugar acceptors or cholesterol, using as promoter Ph₃PAuNTf₂ producing the glycosylation product as a mixture of anomers in moderate to good yields (Scheme 2.82) [92].

Scheme 2.82

O-glycosylation from anhydro glycals promoted by gold complex

Glycals can lead to 2-deoxy-O-glycosides by treatment of protected d-glucal and d-galactal with the alcohol in the presence of trimethylsilyl iodide and triphenylphosphine to produce the O-glycoside favoring the α-selectivity (Scheme 2.83) [93].

Scheme 2.83

Preparation of 2-deoxy-O-glycosides from glycals promoted by TMSI-PPh₃

Likewise the preparation of unsaturated O- and S-glycosides can be accomplished properly by glycosidic reaction of glycal triacetate with alcohol or thiol under erbium triflate-catalysis, observing that in dry CH₃NO₂ during 2 h the higher yields of the Ferrier product (90 %) mainly as the α-isomer (Scheme 2.84) [94].

Scheme 2.84

Preparation of unsaturated O- and S-glycosides under erbium triflate-catalysis

This methodology has been extended for the preparation of E-selectin ligand tetrasaccharide sialyl Lewis^X (SLe^x), which is located at the terminus of glycolipids present on the surface of neutrophils. The chemoenzymatic sequence consisted in the reaction of the 6-acetylated glucal with β-galactosidase transferase to produce disaccharide which was subjected to further transformations according to the pathway presented in Scheme 2.55 (Scheme 2.85) [95].

Scheme 2.85

Chemoenzymatic synthesis of tetrasaccharide sialyl Le^a

2.1.11 Fluorine Reaction

Promoter	Conditions
SnCl2-AgClO4	Et2O, −15 → r.t.
Cp2HfCl2-AgOTf	CH2Cl2, −25 °C
SnCl2-AgOTf	CH2Cl2, 0 °C

Fluorine is considered a poor leaving group, and its use for glycoside bond formation has been more restricted than chlorine and bromine, although display higher thermal and chemical stability. Nonetheless several O-glycoside synthesis involving glycosyl donors with fluorine as leaving group has been described, specially for the preparation of α-O-glycosides with high stereoselectivity [96].

Based in the use of fluorine glycosyl donors, the synthesis of the marine algae α-agelaspines was carried out through the condensation of perbenzylated galactopyranosyl fluorine as anomeric mixture with the long chain alcohol in the presence of a mixture of SnCl₂ and AgClO₄ as catalyst (Scheme 2.86) [97].

Scheme 2.86

Fluorine monosaccharide as glycosyl donor

A general procedure for the preparation of ribofuranosyl fluorides and their use as glycosyl donors for O-glycosylation with α-stereocontrol was developed by Mukaiyama et al. [98], and consist in the conversion of 2,3,5-tri-O-benzyl-d-ribofuranoside that react under mild conditions with 2-fluoro-1-methylpyridinium tosylate at room temperature to give an anomeric mixture (α:β 58:42) in 84 % yield. These two fluorines could be either separate or interconverted by treating the α-anomer with boron trifluoride etherate in ether at room temperature (Scheme 2.87).

Scheme 2.87

The Mukaiyama protocol for preparation of ribofuranosyl fluoride

It has been observed that the glycosylation reaction between the glycosyl fluorine and different alcohols under Lewis acid conditions provides mainly α-riboglucosides in high yield as it is shown in Scheme 2.88

Scheme 2.88

N-glycosylation reaction using ribofuranosyl fluorine

Sulfated Le^x and Le^a-type oligosaccharide selectin ligands were synthetically prepared as described below. Thus, glycosyl donor and acceptor were condensed under Mukaiyama conditions (AgClO₄-SnCl₂) to form the β-glycoside in 90 % yield. The sulfated tetrasaccharide was formed by reaction of tetrasaccharide acceptor with SO₃.NM₃ complex in anhydrous pyridine (Scheme 2.89) [99].

Scheme 2.89

Total synthesis of sulfated Le^x

2.1.12 Iodine Reaction

Promoter	Conditions
NBS (1.2), TMSOTf (0.4), TMU (0.2)	5 °C to rt, 4 h
ZnCl2 (1.4)	rt, 12 h
NBS (1.2), Cu(OTf)2 (0.12)	5 °C to rt, 28 h
Bu4NI	DIPEA, PhH, 4 Å MS
NIS, I2, TMSOTf	3 Å MS, DCE

Glycosyl iodides have been increasingly adopted as glycosyl donors for the synthesis of O-, S, and C glycosides, on one side because of the introduction of suitable reagents for iodination such as iodotrimethylsilane (Me₃SiI), and hexamethyldisilane (HMDS) with molecular iodine, and on the other because of the feasibility for generating either α and β glycosides (Scheme 2.90) [100].

Scheme 2.90

O-glycosylation from protected glycosyl iodides under NBS-ZnI₂ conditions

In general the stereocontrol on glycosylations depends on a combination of factors mainly the protecting group at C-2 position, the nature of the leaving group and the promoter conditions. It is well accepted that there are two possible mechanism S_N1-like and S_N2-like which define the final α/β ratio or the major anomer produced. Usually the intermediate oxacarbenium ion has poor stereochemical control, because it can be attacked from both the α- and β-side while in the S_N2-type the protected glycosyl donor is activated by an electrophile and the leaving group is displaced by the nucleophile being in this case the sugar acceptor or any other aglycone (Scheme 2.91) [101–103].

Scheme 2.91

Schematic representation of α-glycosylation stereocontrol involving glycosyl iodides

The nature of the aglycones linked to glycosyl iodide donors are diverse and among them morphine, uridine diphosphate, and steroidal alcohols have been glycosylated with promoters such as and Bu₄NF, NBS-I₂-TMSOTf (Scheme 2.92) [104–108].

Scheme 2.92

Example of O-glycosylations from glycosyl iodides in the presence of different promoters

2.1.13 Silyl Reaction

R	Promoter	Conditions
Me3Si	TMSOTf or BF3.Et2O	CH2Cl2, −5 °C
tBuMe2Si	TMSOTf	CH2Cl2–acetone, −35 °C

Silyl groups are best known as versatile protecting groups, and their use as leaving groups for glycoside bond formation has been more limited. An example of glycoside formation involving a silyl group as leaving group is reported for the preparation of luganol O-glycoside [109]. In this work, the glycosyl donor is combined with luganine in the presence of trimethylsilyltriflate at low temperature (Scheme 2.93). It is worth mentioning that stereoselectivity is dependent on C-2 neighboring group participation. When acetate is the C-2 protecting group, the β-anomer is obtained, while if the protecting group is benzyl, the α-anomer is preferred.

Scheme 2.93

Sialyl derivatives as glycosyl donors

2.1.14 Phosphate Reaction

R	Promoter	Conditions
P(=O)(OPh)2	TMSOTf	CH2Cl2, −5 °C
P(=S)(Me)2	TrClO4
P(=O)(NMe2)2	TMSOTf	CH3CN, −40 °C
P(=NTs)(NMe2)2	BF3-Et2O	CH2Cl2

Phosphorous glycosyl donors are another option for preparing oligosaccharides. These donors have been used for the preparation of sialyl oligosaccharides however the yield reported were moderate. This is the case of the preparation of sialyl tetrasaccharide derivative which was carried out by condensation between sialyl phosphate and trisaccharide acceptor under TMSOTf as catalyst (Scheme 2.94) [110, 111].

Scheme 2.94

Phosphorous glycosyl donors for oligosaccharide synthesis

2.1.15 Pool Strategy

This term applies to define a one-step reaction used to build up two β-linkages simultaneously from three sugar intermediates [112]. This approach has been described for the preparation of the glycosyl ceramide Globo H hexasaccharide identified as an antigen on prostate and breast cancer cells. The synthesis consisted in the initial synthesis of the trisaccharide building block from the one-pot reaction of the three suitable sugar intermediates under N-iodosuccinimide and triflic acid conditions in 67 % yield (Scheme 2.95).

Scheme 2.95

One-pot reaction for two β-linkages formation

2.1.16 Enzymatic Approach

Enzymes in organic chemistry has become an essential tool for the synthesis of important target molecules and in many cases they are considered the first choice specially for those key steps involving stereospecifically controlled reaction conditions. In general enzymes are considered efficient catalysts which perform the desired transformation under mild conditions with high selectivity and specificity, usually avoiding epimerization, racemization and rearrangements processes. Besides there is a current need of developing economical and environment friendly processes for synthesis. However still some aspects needs close attention in order to fulfill thoroughly the requirements specially for high scale production. Thus, many enzymes are unstable, high cost, difficult to handle, and requires expensive cofactors.

Glycosyltransferases are important enzymes involved in essential processes related to oligosaccharide biosynthesis and they have found also very useful as biocatalyst for the chemoenzymatic synthesis of interesting oligosaccharides and nucleotides [113, 114]. They have been classified as Leloir if they are involved in the biosynthesis of most of N- and O-linked glycoproteins in mammalians, and require monophosphates and diphosphates as glycosyl donors, and non-Leloir enzymes which utilize sugar phosphates as substrates.

Glycosylations with galactosyltransferases can be performed through the use of glucose-1-phosphate as donor. A general sequence consists in the conversion by using UDP-Glc pyrophosphorylase to give UDP-glucose. Epimerization with UDP-glucose epimerase forms UDP-galactose which is used for glycosylation with galactosyltransferase (Scheme 2.96) [115].

Scheme 2.96

Glycosylation with galactosyltransferases

The use of phosphorylase enzymes emerge as a potentially useful enzymatic tool for glycosylation, and an array of these enzymes such as glucan, sucrose, glucosyl glycerol, laminaribiose, nigerose, and maltose phosphorylases, have been isolated and identified from different microorganisms and considered for synthesis even at industrial scale synthesis [116].

Several chemoenzymatic synthesis of α(2 → 6) and α(2 → 3)-oligosaccharides have been reported through the use of sialyltransferases for glycosidic coupling reactions. One described approach involves the in situ regeneration of CMP-Neu5Ac, requiring catalytic amount of CMP-Neu5Ac (Scheme 2.97) [117].

Scheme 2.97

Synthesis of sialyl trisaccharide mediated by sialyl glycosyltransferase

Sialyltransferases also proved to be efficient biocatalysts in the preparation of gangliosides, being involved in (2 → 6) linkage formation between the tetrasaccharide ceramide and CMP-Neu5Ac (Scheme 2.98) [118].

Scheme 2.98

Enzymatic synthesis of ganglioside

Glucosamine may be enzymatically transformed to glucosamine 6-phosphate by treatment with hexokinase from yeast, and ultimately to glucosamine 1-phosphate by the action of phosphoglucomutase (Scheme 2.99) [119].

Scheme 2.99

Enzymatic preparation of glucosamine 6- and 1-phosphate

UDP-glucuronic acid was prepared from UDP glucose by the action of UDP-Glc dehydrogenase along with NAD. This cofactor was regenerated with lactate dehydrogenase in the presence of pyruvate (Scheme 2.100) [120].

Scheme 2.100

Enzymatic preparation of UDP-glucuronide

CMP-N-acetylneuraminic acid has been prepared form CTP and NeuAc under catalysis by CMP-NeuAc synthetase. In a cascade representation, it is observed that CTP is synthesized from CMP with adenylate kinase and pyruvate kinase (Scheme 2.101) [121].

Scheme 2.101

Synthesis of CMP-N-acetylneuraminic acid

2.1.16.1 Enzymatic Synthesis of Oligosaccharides

Mutated glycosidase also known as glycosynthase AbgGlu358Ala in combination with activated glycosyl donors and suitable acceptors can generate synthetic oligosaccharides. Thus, for this transformation the conditions selected were α-glycosyl fluoride as glycosyl donor and p-nitrophenyl as glycosyl acceptor in the presence of ammonium bicarbonate buffer. The proposed mechanism of glycosynthase-catalyzed reaction is illustrated in Scheme 2.102 [122].

Scheme 2.102

Glycosynthase-catalyzed oligosaccharide synthesis

The Regioselective preparation of α-1,3 and α-1,6 disaccharides by using α-glycosidase as biocatalyst has been described. Thus, by combining p-nitrophenyl-α-galactose functioning as glycosyl donor, with the glycosyl acceptor methoxygalactose, the expected 1,3- and 1,6-disaccharide were obtained in the form of α- and β-anomers (Scheme 2.103) [123].

Scheme 2.103

Example of microbial catalyzed coupling reaction

A transglycosylation reaction mediated by α-l-fucosidase from Alcaligenes sp. was performed by combination of p-nitrophenylglycosides donors, with different acceptors such as N-acetylglucosamine, lactose, d-GlcNAc, and d-Glc, providing the corresponding p-nitrophenyl glycosides of disaccharides and trisaccharides containing a (1 → 2)-, (1 → 3)-, (1 → 4)-, or (1 → 6)-linked to the α-l-fucosyl group. In the general procedure illustrated in Scheme 2.76 the p-nitrophenyl fucoside donor was combined with p-nitrophenyl lactosamine acceptor, being incubated with α-l-fucosidase at 50 °C to produce the 2- and 3-linked trisaccharides (Scheme 2.104) [124].

Scheme 2.104

Transglycosylation reaction for the preparation of 2- and 3-linked trisaccharides

Sulfotransferases provides a versatile method for the preparation of glycoside sulfates. A recent report describes the use of 3′-phosphoadenosine-5′-phosphosulfate (PAPS), and GlcNAc-6-sulfotransferase as catalyst (Scheme 2.105) [125].

Scheme 2.105

Transfer of the sulfuryl group from PAPS to the glycoside

A chemoenzymatic synthesis of rhodiooctanoside isolated from Chinese medicines was described. The synthesis was carried out by direct β-glucosidation between 1,8-octanediol and d-glucose using immobilized β-glucosidase from almonds with the synthetic propolymer ENTP-4000 to generate the glycoside in 58 % yield (Scheme 2.106) [126].

Scheme 2.106

Chemoenzymatic synthesis of rhodiooctanoside

Lactosamine was prepared using and enzymatic approach consisting in the preparation of UDP glucose and condensation with N-acetyl glucosamine (GlcNAc) in the presence of galactosyl transferase (Scheme 2.107) [127].

Scheme 2.107

Enzymatic synthesis of lactosamine

Unprotected glycosyl fluorides also have been used as donors for the enzymatic synthesis of disaccharides. For instance, glycosynthase and glycosidase mutants obtained from Thermotoga maritima and Thermus thermophilus have been used effectively for the regioselective synthesis of disaccharides (1 → 3) in higher of 80 % yield (Scheme 2.108) [128].

Scheme 2.108

Enzymatic glycosylation from unprotected glycosyl fluorides

Another example of enzymatic glycosylation using unprotected fluorides donors was achieved by using α-d-glucuronyl fluoride with engineered Escherichia coli glucuronylsynthase, providing β-glucuronides in moderated to good yield depending on the alcohol acceptor employed (Scheme 2.109) [129].

Scheme 2.109

Enzymatic glycosylation from unprotected glycosyl fluorides

2.1.17 Solid Phase Methodology

Perhaps what remains as the most challenging task for sugar chemistry is the synthesis of complex oligosaccharides such as that found in bacterial membranes or wall cells, and that are usually in the form of glycopeptides. Different types of monosaccharides can be present as constitutive parts such as glucose, galactose, mannose, N-acetylglucosamine, sialic acid and l-fucose. Also, the order of linkage and stereoselectivity between them is rarely conserved.

The different nature, stereoselectivity and linkage sequence have been a formidable obstacle for the development of general procedures of the type used for peptides and oligonucleotides which can be prepared on machine synthesizers with high efficiency.

The main advantage of the solid phase methodology is the coupling of sugar units to the resin, which allows easy washing away of the non reacted reagents, avoiding tedious purifications steps.

Nonetheless despite the difficulties, interesting progress has been made for preparing oligosaccharides [130, 167, 168], and glycopeptides [131], suggesting that in the solid phase technology for complex sugars will be affordable.

The solid phase approach involves three elements namely the glycosyl donor, glycosyl acceptor and the resin which is properly activated with a group susceptible for attachment either with the glycosyl donor or acceptor depending on the strategy of choice. Although it appears obvious, it is important to remain that the linkage between the resin and the sugar should be easily cleaved under compatible conditions for the glycoside bond.

According to a comprehensive review [132], the synthetic strategies are classified into: (a) donor-bound, (b) acceptor-bound, and (c) bidirectional Strategies.

One general approach involves the initial attachment of a glycosyl donor (halides, trichloroacetimidate, sulfoxides, phosphate (one is repeated), thio, allyl and glycals) to the resin (polystyrene-base). The attached sugar is selectively deprotected depending on the required position (1,2- 1,3- 1,4- 1,6-), transforming the resin–sugar complex in a sugar acceptor which will be coupled to the next glycosyl donor to produce a second linkage. By repeating this sequence an elongated chain is obtained. The final release and full deprotection will produce the free oligosaccharide (Scheme 2.110) [133].

Scheme 2.110

General scheme for solid-phase oligosaccharide synthesis 1,4-linkage case

An example of the donor bound strategy is the bounding of sulfur glycoside to polystyrene resin to form a sulfur linkage between the donor and the resin (Scheme 2.111). Suitable hydroxyl group from the donor will serve as linkage site with de next sugar unit for chain elongation.

Scheme 2.111

Example of donor bound strategy for solid-phase glycosylation reactions

It should be noted that the glycosyl donor also contains a position available for the linkage with the next sugar. In other words the glycosyl donor once attached to the resin becomes a glycosyl acceptor, as can be seen for the next coupling sequence (Scheme 2.112) [132].

Scheme 2.112

Sulfur mediated solid-phase coupling reaction

The synthesis of β-(1 → 6) gentotetraose was accomplished by using a benzoyl propionate as resin linker. The glycosyl donor chosen was acetobromoglucose functionalized with trichloroacetate group as a temporary protecting group at position 5. Glycosylation reactions were effected under Helferich conditions and cleavage from resin was performed with hydrazinium acetate (Scheme 2.113).

Scheme 2.113

Solid-phase coupling promoted by Helferich conditions

Polymer solid phase has been also exploited successfully by Crich et al. [134], for the synthesis of sensitive β-mannosides, using a variation of sulfhoxide method, consisting in the transformation of sulfoxide to triflic group as leaving group. The subsequent addition of alcohol acceptor to the donor attached to the Wang resin will result in the glycoside β-mannoside formation (Scheme 2.114).

Scheme 2.114

Solid-phase synthesis of β-mannoside glycoside

The N-phenyl trifluoroacetimidate donor was incorporated as a building block for solid-phase assembly as described in Scheme 2.115, starting from the coupling with a resin under TfOH conditions, and subsequent condensation with S-phenylglucuronic acid, to furnish dimer which was transformed into imidate donor until reaching a building block at multigram scale (Scheme 2.115) [135].

Scheme 2.115

Solid-phase assembly by using N-phenyl trifluoroacetimidate donors

The enzymatic solid-phase oligosaccharide synthesis relies mainly by the use of glycosyltransferases, glycosidases, and glycosynthases. By taking advantage on their high stereoselectivity and regioselectivity, various oligosaccharides and glycopeptides have been prepared usually under mild conditions without the need of using protecting groups. Unfortunately the enzymatic approach is still in some cases unaffordable due to its high cost for large scale processes, lower yields provided and their limited capability for recognizing a broad range of sugars specially those not common. Two general approaches have been proposed for the preparation of oligosaccharides through the solid-phase approach (Scheme 2.116) [136].

Scheme 2.116

Two general approaches for immobilized solid-phase oligosaccharide synthesis

A solid-phase enzymatic approach for extending the oligosaccharide chain was described by Gijsen et al. [136] in which a disaccharide-linker fragment attached to a resin was coupled with the glycosyltransferases UDP-galactose and CMP-NeuAc in the presence of galactosyltransferases and sialyltransferase as enzymatic catalyst. Final treatment with hydrazine was used to release the tetrasaccharide from the solid support (Scheme 2.117).

Scheme 2.117

Enzymatic-solid phase glycosylation reaction

2.1.18 Miscellaneous Glycosylations

2.1.18.1 Selenosyl Donors

The use of selenoglycosides as glycosyl donors and acceptor in glycosylation reactions has also been described by Metha and Pinto [137]. A typical glycosidation procedure with phenylselenoglycoside donors involves the glycosyl acceptor, 4-Å molecular sieves, silver triflate, and potassium carbonate in dichloromethane (Scheme 2.118).

Scheme 2.118

Phenylselenosugars as glycosyl donors

2.1.18.2 Tetrazol as Leaving Group

Tetrazol has also been tested as a leaving group for the preparation of an antibiotic fragment [138]. A coupling reaction with the methoxyphenyl glycosyl acceptor was catalyzed with (Me₃)₃OBF₄ as shown in Scheme 2.119.

Scheme 2.119

The use of tetrazol as a leaving group

2.1.18.3 Sigmatropic Glyosylations

2-aminodisaccharides were obtained by an elegant [3,3] sigmatropic rearrangement, by Takeda et al. [139] The addition of thiophenol to an unsaturated C-1 in the presence of Lewis acid, was followed by a sigmatropic rearrangement with an imidate group which migrates from C-4 to C-2. Disaccharide formation was catalyzed with Pd(CH₃CN)₂-AgOTf complex in dichloromethane (Scheme 2.120).

Scheme 2.120

Sigmatropic rearrangement

2.1.18.4 Zinc Promoted Glycosylation

The total synthesis of the cyclic glycolipid arthrobacilin A , a cell growth inhibitor was achieved by Garcia and Nizhikawa [140], under zinc p-tert-butylbenzoate salt as glycoside catalyst, obtaining the β-galactoside glycoside in 73 % along with α-isomer in 27 % (Scheme 2.121).

Scheme 2.121

Glycosylation reaction for preparation of arthrobacilin A

2.1.18.5 Heterogenous Catalysis

Stereocontrolled α- and β-glycosylations by using environmentally benign heterogenous catalyst has been developed as a novel approach for stereoselective formation of β-O-glycosidic linkages. Polymeric materials such as montmorillonite K-10 [141], heteropoly acid (H₄SiW₁₂O₄₀) [142], sulfated zirconia (SO₄/ZrO₂) [143], and perfluorinated solid-supported sulfonic acids (Nafion resins) [144] have been assayed successfully providing series of stereocontrolled O-glycosides in high yield (Scheme 2.122).

Scheme 2.122

Stereocontrolled O-glycosidations using heterogeneous polymeric materials

Glycosyl N-trichloroacetylcarbamate obtained from reaction of tetrabenzyl glucopyranoside hemiacetals with trichloroacetyl isocyanate was used as glycosyl donors. Various Lewis acids were tested for α-selective glycosylation observing that the promoters TMSOTf and TMSClO₄ yield the best results (Scheme 2.123) [145].

Scheme 2.123

O-glycosylation via N-trichloroacetylcarbamate

N-Sulfonyl imidazole has been used as activating agent for preparing 2-deoxy monosaccharides through deprotonation of the anomeric hydroxyl group with KHMDS at low temperature. Further reaction with N-sulfonyl imidazole resulted in the glycosyl sulfonates intermediate generated in situ which was finally reacted with the desired nucleophile to produce the β-glycoside in moderate to good yields (Scheme 2.124) [146, 147].

Scheme 2.124

Preparation of β-glycosides via glycosyl sulfonate formation

On the other hand 1,2-cyclopropaneacetylated sugar has been proposed as glycosyl donors for O-glycosylations, allowing stereoselective control depending on the catalyst employed. Thus, β-anomeric products were obtained with BF₃.OEt₂ as catalyst, whereas TMSOTf-catalyzed glycosylation prefers the α-anomeric products (Scheme 2.125).

Scheme 2.125

Stereocontrolled glycosylations from 1,2-cyclopropaneacetylated sugar as glycosyl donors

Gem-dimethyl 4-n-pentenyl glycosides were proposed as glycosyl donors for glycosylation and hydrolysis of the anomeric carbon when using NBS as the sole stoichiometric activator with yield reported around 80 % mainly with β selectivity (Scheme 2.126) [148].

Scheme 2.126

Preparation of protected β-1,6 disaccharide form Gem-dimethyl 4-n-pentenyl glycosides

2.1.19 Cyclic Oligosaccharides

The synthesis of cyclic oligosaccharides involves the preparation of linear saccharides which ultimately are joined together to form a cyclic macromolecule. There are two main approaches proposed based on the cycloglycosylation step. The first involves the preparation of a long chain having and each end the donor and acceptor functionalities that will be interconnected through a glycosidic bond at a final step, and the second involving the polycondensation of smallest repeating unit called “saccharide monomers.” It has been observed that the latter strategy is considered less laborious; however, it produces cyclic oligomers of different size since under these conditions the ring formation step is not controllable.

The chemical synthesis of cyclic oligosaccharides has been mainly driven to obtain cyclic (1 → 4)-linked oligopyranosides, however (1 → 3), and (1 → 6) linked cycloforms are also described. In the case of (1 → 2)-linked oligosaccharides, the ring closure require about 17 or more glucopyranoside residues because (1 → 2)-linkage composed of pyranoside connected by one equatorial and one axial bond assumes rigid conformations and cannot cyclize [149].

The pioneering total synthesis of cyclic oligosaccharide α-Cyclodextrin was carried out by Ogawa’s group in 1985 [150] and since then alternative chemical or enzymatic methodologies appeared for preparing cyclic oligosaccharides. Nowadays the industrial production of cyclodextrins relies on the enzymatic conversion of prehydrolyzed starch into a mixture of cyclic and acyclic oligomers.

A full report about cyclic oligosaccharides [150] proposes four approaches to the synthesis of cyclic oligosaccharides developed during the last 10 years. (1) the stepwise preparation of a linear precursor that is subjected to cycloglycosylation; (2) the one-pot polycondensation/cycloglycosylation of a small “oligosaccharide monomer” typically, a disaccharide or trisaccharide that can yield a range of macrocycles of different sizes; (3) the enzyme-assisted synthesis of natural or unnatural cyclic oligosaccharides; (4) the ring opening of cyclodextrins followed by oligosaccharide chain elongation and cycloglycosylation (Scheme 2.127).

Scheme 2.127

The four suggested approaches to the synthesis of cyclic oligosaccharides

Despite the significant advances observed in cyclic oligosaccharide synthesis, their preparation is time consuming, producing the target compounds with low regioselective and stereoselective in low yields. The total synthesis of α-CD and γ-CD was described according to Scheme 2.128 [151, 152].

Scheme 2.128

Chemical synthesis of cyclic α(1 → 4)-oligosaccharide γ-CD

In 1990, the chemical synthesis of β-(1 → 3) linked hexasaccharide was reported. The chemical approach involved the glycosidic reaction between benzylidene acceptor and protected glycosyl bromide as glycosyl donor, under silver triflate-promoter conditions. As it can be seen in Scheme 2.89, the construction of the linear oligosaccharide and its final cycloglycosylation was performed by using glycosyl bromides which were prepared by photolytic brominolysis of 1,2-O-benzylidene glucose with BrCCl₃ (Scheme 2.129) [153].

Scheme 2.129

Synthesis of cyclic β-(1 → 3)-linked oligosaccharide

The formation of (1 → 6)-glycopyranosidic linkages might produce cyclic disaccharides, trisaccharides, and tetrasaccharides. An early synthesis of β-(1 → 6)-glucopyranan under Helferich conditions, generated along with the linear oligomer, a cyclic disaccharide and tetrasaccharide in 12 % and 6 % respectively (Scheme 2.130) [154].

Scheme 2.130

Preparation of linear, and cyclic β(1 → 6) disaccharides and tetrasaccharides

An improved synthesis of cyclotetraoside was described by the same group 10 years later, consisting in the preparation from the peracetylated tetrasaccharide into the tetrasaccharide derivative having both the acceptor and the donor components. The final cyclization was performed under Helferich conditions providing a mixture of trisaccharide and tetrasaccharide in 22 % and 25 % yield respectively (Scheme 2.131) [118, 155].

Scheme 2.131

Improved synthesis of cyclic β(1 → 6) trisaccharides and tetrasaccharides

2.1.19.1 Chemoenzymatic and Enzymatic Synthesis

The use of enzyme is as mentioned for many O- or N-glycosides the parallel possibility for preparing cyclic oligosaccharides. The limitation continue to be the availability and affordability; however, some enzymes such as glycosidases and cycloglycosyltransferases (CGTases) which are involved in the preparation of cyclodextrins from starch and other α-(1 → 4)-glucans are accessible and more versatile [155].

The feasibility of the chemoenzymatic approach was established in the preparation of cyclic β(1 → 4) hexasaccharides, heptasaccharides, and octasaccharides, from 6-O-methylmaltosyl fluoride when incubated with CGTase. Thus, a mixture of 6^I, 6^III, 6^V-tri-O-methyl-α-CD (42 %), 6^I, 6^III, 6^V-tetra-O-methyl-γ-CD (16 %) and in less proportion 6^I, 6^III, 6^V-tri-O-methyl-β-CD were obtained (Scheme 2.132) [136, 156].

Scheme 2.132

Synthesis of 6^I, 6^III, 6^V-tri-O-methyl-α-CD, 6^I, 6^III, 6^V-tetra-O-methyl-γ-CD and 6^I, 6^III, 6^V-tri-O-methyl-β-CD

Furthermore, under the same conditions it was possible to prepare from the maltotriosyl fluoride the cyclic α(1 → 4) hexasaccharide (6^I, 6^II-dideoxy-6^I,6^II-diiodo-α-CD) in 38 % (Scheme 2.133) [118, 157].

Scheme 2.133

Enzymatic synthesis of 6^I, 6^II-dideoxy-6^I,6^II-diiodo-α-CD

An alternative option for the enzymatic preparation of cyclic oligosaccharides besides CGTases is glycosidases which exerts its action on polysaccharides. This possibility is exploited in the preparation of cyclic fructins by conversion of β-(1 → 2)-fructofuranan by bacterial fructotransferases isolated from Bacillus circulans (Scheme 2.134) [158].

Scheme 2.134

Enzymatic synthesis of cycloinulooligosaccharides

2.1.19.2 Summary for Preparing Conventional Glycosyl Donors