Introduction

The search for new methods for the stereoselective synthesis of nonproteinogenic amino acids is a topical task because these compounds are widely used in biochemistry, pharmacology, and synthetic chemistry (Barrett 1985; Kuchar’ and Soloshonok 1994; Goodman and Ro 1995; Haufe and Kroger 1996; Kroger and Haufe 1997; Williams 1989; Cativiela and Díaz de Villegas 1998, 2000; Wirth 1997; North 1996; Duthaler 1994; Ager and Laneman 2004; Nájera et al. 2000; Nájera and Sansano 2007; O’Donnell 2004; Hughes et al. 2004; Lygo and Andrews 2004; Maruoka and Ooi 2003; Ma 2003; Kotha 2003).

In particular, the fluorinated analogs of proteinogenic amino acids find wide application in biochemistry, pharmacology, and microbiology (Goodman and Ro 1995). Stereoselective fluorination may drastically modify the characteristics of those amino acids, significantly influencing their biological and chemical activities. The comparable sizes of fluorine and hydrogen atoms diminish steric inhibition of binding of such a fluorinated analog at the active site of the receptor or enzyme. On the other hand, the fluorinated amino acids often cannot be metabolized in the same way as the usual amino acids. As a result, the fluorinated amino acids can be used as potent agonists and antagonists of natural amino acids, sometimes functioning as antiproliferation agents, inhibiting the synthesis of proteins (Goodman and Ro 1995). In addition, β-fluorinated amino acids sometimes serve as irreversible inhibitors of pyridoxal enzymes (Barrett 1985).

Naturally, the properties of the amino acids crucially depend on their absolute configuration. Thus, the elaboration of the novel synthesis of fluorinated amino acids should involve enantioselective stages in the protocol (Barrett 1985; Kuchar’ and Soloshonok 1994; Goodman and Ro 1995; Williams 1989; Cativiela and Díaz de Villegas 1998, 2000; Wirth 1997; North 1996; Duthaler 1994; Ager and Laneman 2004; Nájera et al. 2000; Nájera and Sansano 2007; O’Donnell 2004; Hughes et al. 2004; Lygo and Andrews 2004; Maruoka and Ooi 2003; Ma 2003; Kotha 2003).

Glu is a mammal central nervous system neurotransmitter and activate several types of synaptic sites subdivided on subspecies (Dodd et al. 2000; Girault et al. 1997). Additionally, Glu is a human vital function important agent and glutamine-precursor (Newsholme et al. 2003). Elaboration of new medicines for Parkinsonism and Alzheimer’s dementia, myocardial and cerebral ischemia and several types of epilepsy are based, at present, on Glu derivatives (Dodd et al. 2000).

The impact of fluorine atom introduction at the C-4 position of Glu has been tested in the screening of modulators for folate poly-γ-glutamate biosynthesis and to study the role of analogous derivatives of antifolates such as methotrexate in the cytotoxic action of these drugs. In one of the crucial steps, the γ-carboxyl group of the C-terminal glutamate is activated by the enzyme prior to peptide coupling. Therefore, introducing one or more fluorine atoms in the γ-position of the side chain could interfere with the biological processes (Hart et al. 1996; Tsukamoto and Coward 1996). As a second illustration, enantiomerically pure FGlu were employed for investigating the mechanism of the vitamin K-dependent carboxylation of Glu residues present in several proteins (Dubois et al. 1983).

Another highly important off-shoot of enantiomerically pure FGlu might be its [18F]-substituted derivative. [18F]-Labeled compounds and [18F]-labeled amino acids, in particular, are perspective tools believed to become perspective diagnostic molecular imaging agents in PET (Anderson and Shokeen 2009; Krasikova et al. 2004, 2008 and references cited therein). Because Glu plays a crucial role in ammonia exchange in organism, its [18F]-derivative might be expected to concentrate in fast-growing tissues such as tumors and, thus, become a novel radiopharmaceutical for PET cancer imagining, supplementing [18F]-labeled FDG, the only PET radiotracer practically used, at present, for routine clinical evaluation.

The existing procedures for the synthesis of enantiomerically pure forms of FGlu utilize tedious resolution of racemic mixtures (Dubois et al. 1983; Tolman 1993 and Tolman and Simek 2000) or involve use of enantiomerically pure precursors in a multistage procedure (Hudlický and Merola 1990, 1991, Hudlický 1993; Konas and Coward 2001). For this purpose both (S)- or (R)-hydroxyproline (Hudlický and Merola 1990, 1991 and (S)-Glu (Konas and Coward 2001) were employed, furnishing the target compound in a low total yield (15–18%).

Earlier, we developed a simple synthetic protocol for the synthesis of enantiomerically and diasteroisomerically pure nonproteinogenic amino acids, including alkyl substituted Glu via Michael addition to a Ni-BPB-Gly to substituted acrylic acid (Belokon et al. 1985, 1986, 1998; Belokon 1992). The method was later further elaborated by Soloshonok et al. for the synthesis of Glu derivatives and its alkylated analogs (Soloshonok et al. 1997, 1999).

In this work, we put ourselves a task of developing a simple procedure for the synthesis of enantiomerically pure FGlu. In addition, we planned to develop a protocol for the asymmetric synthesis of 4-Br substituted Glu which could become a useful precursor for many 4-substituted Glu (Krasnov et al. 2003) (including 18F-substituted enantiomerically pure Glu) or aminocyclopropanecarboxylic acids.

Herein, we report a novel simple synthetic protocol for the preparation of both (2S,4R)- and (2S,4S)-FGlu, applying Michael addition of methyl α-fluoroacrylate (2) to Ni-BPB-Gly (1) (Schemes 1, 2). In addition, the diastereoselective addition of 1 to ethyl α-bromoacrylate was shown to lead to the NiII complex of the Schiff base of BPB with (2S,4R)-4-bromo-glutamic acid monoester. Further transformations of the complex into the corresponding complexes of 1-aminocyclopropane-1,2-dicarboxylic acid was elaborated.

Scheme 1
scheme 1

Key step in the asymmetric synthesis of FGlu

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Scheme 2
scheme 2

Reagents and conditions: i) HCl, MeOH, H2O, reflux 20 min; ii) Dowex 50 W × 8 in H+ form, eluent – 5%-NH3; iii) 6N HCl, reflux 1 h

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Results and discussion

As expected, methyl α-fluoroacrylate (2) added to 1 in MeOH in the presence of Pr i2 NH at 20°C within a 10-day-period (see Scheme 1) to give a mixture of two diastereoisomeric complexes, differing at their γ-carbon atom configuration of the side chain and having the same (S)-configuration of the α-carbon atom. This type of stereoselectivity in Ni-BPB-AA complexes is well documented (Belokon and Soloshonok).

The resulting mixture of the diastereomeric complexes, incorporating (2S,4R)- and (2S,4S)-moieties of FGlu (3a and 3b correspondingly), was isolated in a 70% isolated yield after flash chromatography on silica. The ratio of 3a/3b was 3/2, according to 1H NMR data. The separation of the diastereomeric (2S,4R)- and (2S,4S)-complexes (3a and 3b correspondingly) was carried out on a Toyopearl HW-55F column.

The individual diastereomeric complexes 3a and 3b were characterized with 1H, 19F and 13C NMR spectroscopy, elemental analysis, and specific rotations. The configurations of the FGlu moiety of 3a (with greater mobility on Toyopearl HW-55F (benzene/THF, 7:2) were assigned as (2S,4R)- according to the X-ray structure analysis data (see Fig. 1).

Fig. 1
figure 1

Structure of NiII complex of (2S,4R)-FGlu Schiff base with BPB as revealed by X-ray analysis data

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The configuration of the AA moiety of another diastereoisomeric complex 3b was assigned as (2S,4S)- accordingly. Additionally, 1H and 19F NMR spectroscopy data corroborate this assignment. For example, the resonance of 19F in the 3a spectrum was shifted 5 ppm upfield relative to that of 3b as a result of the aromatic ring anisotropy effect, as presented on Fig. 2. The 0.12 ppm upfield shift of γ-hydrogen resonances of 3b relative to that of 3a can also rationalized in the same way.

Fig. 2
figure 2

Magnetic anisotropy effects influencing relative 19F chemical shifts in 3a and 3b

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Enantiomerically enriched (2S,4R)- and (2S,4S)-FGlu (4a and 4b correspondingly) were isolated (see Scheme 2) after the decomposition of complexes 3a and 3b with aqueous 6N HCl, accompanied by simultaneous ester group hydrolysis.

After recovery of the initial BPB (BPB × HCl in 90–95% yield) the amino acids were purified by the ion-exchange technique. The resulting amino acid solutions consisted of both open and closed forms of the FGlu, according to their 1H NMR spectra. The additional acidic hydrolysis of the mixture with 6N HCl furnished the hydrochlorides of target amino acids (2S,4R)- or (2S,4S)-FGlu (4a and 4b) correspondingly. D.r. of amino acids, as determined with chiral GLC, exceeded 20/1 for 4a × HCl and 26/1 for 4b × HCl, and for each stereoisomer e.e. >96%. Once crystallization of samples 4a × HCl and 4b × HCl allowed raising e.e. and d.e. of the products up to 100%.

It seemed likely that a straightforward Michael condensation of 1 with unsaturated ester 5 in EtOH would furnish the desired product. The use of 2,6-diethylpyridine as a promoter did not lead to the addition of compound 1 to 2 (see Scheme 3) even on heating in EtOH. However, DBU-promoted reaction of complex 1 with unsaturated ester 5 in EtOH afforded individual diastereomers of NiII complexes of Schiff bases of BPB with stereoisomers of 1-aminocyclo-propane-1,2-dicarboxylic acid monoesters 6a, 6b, and 6c (Scheme 3) easily separated by chromatography (SiO2).

Scheme 3
scheme 3

Reaction product manifold formed in addition of 5 to 1

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The configuration of the proline residue is omitted for simplicity, because the configuration of this residue in the starting complex 1 (R N ,S) is retained in the reaction. The absolute configuration of the minor isomer 6a was not determined. X-ray diffraction study demonstrated that compounds 6b and 6c (Fig. 3) represented NiII complexes of the Schiff bases of (1S,2S)- and (1S,2R)-1-aminocyclopropane-1,2-dicarboxylic acid monoesters, respectively (Fig. 4). The ratio of isomers 6b and 6c determined by 1H NMR spectroscopy was 1:1, and their total yield was 70%.

Fig. 3
figure 3

Molecular structure of the NiII complex of the Schiff base of BPB with (1S,2R)-1-aminocyclopropane-1,2-dicarboxylic acid monoethyl ester, 6c, with displacement ellipsoids drawn at the 50% probability level

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Fig. 4
figure 4

Molecular structure of the NiII complex of the Schiff base of BPB with (1S,2S)-1-aminocyclopropane-1,2-dicarboxylic acid monoethyl ester, 6b, with displacement ellipsoids drawn at the 50% probability level

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Most likely, the formation of cyclic complexes 6a–c instead of the expected linear Michael adduct, viz., the Schiff base of 4-bromo-Glu monoester, results from the secondary reaction of this derivative. Presumably, the NiII complex of the Schiff base of BPB with 4-bromo-Glu that formed in the first step undergoes cyclization under the reaction conditions (Scheme 4).

Scheme 4
scheme 4

A possible mechanism of aminocyclopropanedicarboxylic acid formation, according to Scheme 3

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An attempt to recover the corresponding 1-aminocyclopropane-1,2-dicarboxylic acids from the parent complexes by simple hydrolysis, followed by ion exchange technique, failed. The result was not a surprise as the decomposition of the racemic amino acids in basic media was earlier described (Krasnov et al. 2003). A special procedure should be developed to avoid the decomposition and it is now under elaboration.

Synthesis of the Michael adduct requires that cyclization was retarded, which could be achieved if sterically hindered amine Pr i2 NH was used instead of DBU. The reaction in the presence of this base did produce diastereomeric complexes 7a–c containing a fragment of 4-bromo-Glu monoester (see Scheme 3) with a total yield of 80%. Complexes 6b and 6c were also obtained as by-products (<5%). The diastereomer ratio 7a:7b:7c = 1:11:22 was determined by 1H NMR spectroscopy. Complex 7c was obtained as the major product, which was isolated in 52% yield. Studies by elemental analysis, 1H and 13C NMR spectroscopy, and X-ray diffraction analysis (Fig. 5) showed that this compound is the NiII complex of the Schiff base of BPB with (2S,4R)-4-bromo-Glu monoester.

Fig. 5
figure 5

Molecular structure of the NiII complex of the Schiff base of BPB with (2S,4R)-4-bromoglutamic acid monoethyl ester, 7c, with displacement ellipsoids drawn at the 50% probability level

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The recovery of pure 4-bromo-Glu from the parent complex by the routine simple hydrolysis of the parent complex, followed by ion exchange technique, led to an unidentified mixture of amino acids.

Treatment of complex 7c with DBU in EtOH resulted in its selective transformation into 6c in full accord with Schemes 3 and 4. The formation of compounds 6a and 6b was not observed. Presumably, the cyclization proceeds by either the E2 mechanism, which involves simultaneous α-proton abstraction from the amino acid fragment under the action of DBU accompanied by intramolecular replacement of the bromide ion and the three-membered ring closure, or the E1cB mechanism involving the successive α-proton abstraction and generation of the carbanion followed by cyclization and elimination of the Br− ion (see Scheme 4).

The stereochemistry of the transformation can be adequately described by both the E2 and E1cB mechanisms. Apparently, the DBU-catalyzed synthesis of 6b involves intramolecular cyclization of the initially formed 7b. Hence, the (S,S)-configuration can be tentatively assigned to the amino acid fragment of 7b.

To summarize, the NiII complex of the Schiff base of BPB with (2S,4R)-4-bromoglutamic acid monoester (7c) was synthesized and characterized. This complex is a convenient precursor of various derivatives that can be prepared by nucleophilic substitution reactions in analogy with literature data (Krasnov et al. 1989, 2004). Preliminary experiments indicated that substitution of Br by CN or RO-groups could be easily brought about.