Introduction

The considerable interest in employing environmentally acceptable processes has motivated the use of biocatalysts for the production of different chemical structures. Thus, many researchers are utilizing these natural catalysts for the smart insertion of chirality and selectivity with regard to regio- and stereo-chemistry for producing optically active, higher value molecules under milder reaction conditions (Santacoloma et al. 2011; Hudlicky and Reed 2009; Pollard and Woodley 2007; Schmid et al. 2001; Clouthier and Pelletier 2012; Patel 2008; Van Rantwijk and Sheldon 2007).The Biginelli reaction (Biginelli 1893) involving condensation of aldehyde, β-ketoester and urea ranks as one of the most recognized multicomponent reaction (Dömling 2006; Zhu and Bienaymé 2005; Wipf et al. 2003; Biggs-Houck et al. 2010) and widely employed reaction for the preparation of 3,4-dihydropyrimidin-2-(1H)-ones (DHPMs). DHPM derivatives exhibit a wide range of biological and pharmacological properties such as antiviral, antimitotic, anticarcinogenic, antihypertensive and most importantly, as calcium channel modulators (Kappe 1998, 2000; Yarim et al. 2003; Kidwai et al. 2005; Kumar et al. 2009; Jain et al. 2008; Mayer et al. 1999). Additionally, the biological activity of potent HIV gp-120-CD4 inhibitor Batzelladine A and B has also been attributed to DHPM moiety (Hojati et al. 2010).

In the past decade, a series of procedures has been developed (Gore et al. 2011; Ramalingan et al. 2010; Shen et al. 2010; Tamaddon et al. 2010; Chitra and Pandiarajan 2009; Lannou et al. 2008; Chari et al. 2009; Suzuki et al. 2008; Polshettiwar and Varma 2007; Yu et al. 2007; Debache et al. 2006, 2008; Bhosale et al. 2004; Rafiee and Jafari 2006; Banik et al. 2007; Bose et al. 2004; Li et al. 2003; Saha and Moorthy 2011) to overcome the relatively harsh acidic conditions of the original Biginelli reaction. However, in spite of their potential utility, most of these methods involve expensive reagents, stoichiometric amount of catalyst, longer reaction times, unsatisfactory yields and generation of waste materials besides cumbersome product isolation procedures and incompatibility with certain functional groups, thus are not closer to the principles of green chemistry. We envisioned that an improvement addressing these issues might be achieved by the use of a biocatalyst particularly for the asymmetric version.

Exploiting the promiscuous activity of a biocatalyst to achieve multicomponent reaction has always remained challenging (Znabet et al. 2010a, b; Wang et al. 2011) and in this case too, only few reports are there where enzymatic systems have been used for the resolution of DHPM esters (Prasad et al. 2009; Schnell et al. 2000) or towards the synthesis of racemic DHPMs (Borse et al. 2012; Kumar and Maurya 2007). Following our recent report on role of amino acid based ionic liquid (Sharma et al. 2012a) in DHPMs synthesis and exploitation of biocatalyst for various organic transformations (Sharma et al. 2009, 2011a, b, 2012b; Kasana et al. 2007), we targeted the waste-free synthesis of DHPMs involving a biocatalytic procedure where water ought to be the only by-product.

Initially, lipases (Table 1; entries 1–6) were screened for obtaining enantiomeric pure DHPM (1b) by cyclocondensation of 0.25 mmol of benzaldehyde (1a), ethyl acetoacetate (2) (1 equiv.) and urea (3) (1 equiv.) in EtOH at 28 °C for 6 days. Among all, Candida antarctica lipase-B (CAL-B) and porcine pancreas lipase (PPL) provided 1b in 28 % yields (entries 1 and 3) with inferior enantioselectivity (5 % ee) at room temperature. Even the use of different solvents with CAL-B as catalyst, did not improve the yield and % ee of chiral 1b (Table 1; entries 7–13). Thereafter, the effect of temperature was probed to stir the reaction in desired direction. It was observed that though increase in temperature from 28 °C to 60 °C brought about a linear change on the yield (62 %), yet it could not exert any positive effect on % ee (Table 1; entry 1 vs 14, 15). In order to exclude background activity, we decided to perform some control experiments where denatured CAL-B or bovine serum albumin (BSA) was used to mediate the desired transformation (Table 1; entry 16, 17).

Table 1 Screening of different enzymes/proteins and solvents for asymmetric Biginelli reaction
Full size table

Surprisingly, 52 % conversion was reached with denatured CAL-B (entry 16) and a very high conversion value (80 %) using BSA (entry 17). With these results in hand (lack of enantioselectivity together with considerable yield observed in case of both denatured lipase and BSA), we hypothesized the role of protein catalysis instead of specific promiscuous catalysis by enzymes which is in contrast to some recent reports (Borse et al. 2012; Kumar and Maurya 2007) where the possibility of protein catalysis has not been considered. Moreover, in comparison to enzyme CAL-B (Walker 2005), BSA have a rich diversity of surface amino acids which might account for the enhanced yield of 1b. Replacing the BSA with porcine albumin and egg albumin also provided the desired DHPM (1b) in 77 % and 74 % yield (entries 18, 19), respectively whereas denatured BSA (Norberto et al. 2012) provided 1b in 28 % yield (entry 20).To validate the above hypothesis, further optimization of reaction conditions using BSA as catalyst was carried out. The effect of solvent, temperature, reaction time and the amount of catalyst on the yield of 1b was examined (Table 2). Best results were obtained when the reaction of 1a was carried out with 50 mg BSA in EtOH at 60 °C with shorter reaction time of 8 h in 82 % yield (Table 2, entry13).

Table 2 Optimization of BSA catalyzed Biginelli reaction
Full size table

Afterwards, to extend the substrate scope of BSA triggered Biginelli reaction, various benzaldehydes 1a–15a were treated with urea and 1,3 dicarbonyl compounds for the synthesis of dihydropyrimidones 1b–15b (Table 3, entries 1–15). All the obtained products were characterized by NMR and HRMS data (see supporting information). Thiourea was also used with similar success to provide the corresponding dihydropyrimidine-(2H)-thiones (Table 3, entries 16, 17), which are also of much interest with regard to their biological activity (e.g. monastrol Table 3, entry 17). As shown in Table 3 (entries 1–17), yields of this one-pot protocol following recrystallization from ethanol were of the order 69–83 %, which is quite favourable. In particular, the catalyst exhibited remarkable activity for Cl, Br, OH, NO2 and OCH3 functional group containing compounds. In order to check the standard deviation (SD), three independent experiments were conducted for the substrate with electron neutral, withdrawing and releasing groups (Table 3, entries 1, 10 and 13) where deviation in the yield of the products were 83 ± 3, 78 ± 5 and 78 ± 2 percent, respectively.

Table 3 Substrate scope of BSA catalysed Biginelli reaction
Full size table

A plausible mechanism (Scheme 1) suggested the probable role of amino acid side chain in BSA as the catalytic base for the reaction where basic character of the amino group might be responsible for the catalysis as per prior reports (Taylor et al. 1975; Riva et al. 1998; Hollfelder et al. 1996; Boucher et al. 2005; Klein and Reymond 1998; Reetz et al. 2007; Strohmeier et al. 2009). Firstly, the urea attacks on the aldehyde group of substrate leading to the formation of an iminium ion (Kappe 1997; De Souza et al. 2009) followed by the removal of proton from active methylene group of ethylacetoacetate (Sharma et al. 2011b) by the free basic amino group of BSA which subsequently condensed with iminium ion to give desired product as shown in scheme 1. As expected, use of acetylated BSA drastically reduced the formation of 1b in 7 % yield only (Table 1, entry 21) which clearly indicates the importance of free amino group present in BSA indispensable for synthesis of DHPM.

Scheme 1
scheme 1

Plausible reaction mechanism for BSA catalysed Biginelli reaction

Full size image

Subsequently, we studied the recyclability of the BSA for the above reaction. Complete conversion was observed up to third reaction cycle with benzaldehyde (1a), ethyl acetoacetate (2) and urea (3) as reaction substrates. However, slight loss of activity (74 %) was observed after fourth catalytic cycle onwards. Further, to demonstrate the practical applicability of the developed method, preparative scale reaction (1 g batch) of 3-hydroxybenzaldehyde (17a) with ethyl acetoacetate and thiourea was effectively accomplished using BSA as catalyst leading to the formation of a potent mitotic kinesin Eg5 inhibitor monastrol (Mayer et al. 1999) (17b) in good yield (Scheme 2).

Scheme 2
scheme 2

BSA catalysed gram scale synthesis of bioactive monastrol (17b)

Full size image

Conclusion

In conclusion, we have successfully developed a simple, inexpensive and waste-free methodology for the synthesis of bioactive 3,4-dihydropyrimidin-2(1H)-ones using “off-the shelf” protein- bovine serum albumin (BSA) while exploring the promiscuous lipase catalysed system for the asymmetric version of Biginelli reaction. Moreover, catalyst recyclability and gram scale synthesis have also been demonstrated to enhance the practical utility of process.

Experimental Section

General Procedure for the synthesis of 3,4-dihydropyrimidin-2(1H)-ones (1b–17b) from substituted benzaldehydes (Table 3, entries 1–17 and Scheme 2)

Substituted benzaldehyde (1a–17a, 0.25 mmol), ethyl acetoacetate (1 equiv.), urea or thiourea (1 equiv.) and BSA (50 mg) was taken in 3 mL ethanol in a round bottom flask and the reaction mixture was incubated in a orbital shaker at 60 °C for 8–12 h. Afterwards, the reaction mixture was worked up and analyzed with the HPLC. Further, the crude product was purified by recrystallization from water–ethanol mixture giving an isolated yield of 1b–17b in the range of 69–83 %. 1H and 13C NMR spectra were recorded and matched with reported values and further confirmed by HRMS/MS.