Lipase-catalyzed regioselective synthesis of monoester of pyridoxine (vitamin B6) in acetonitrile

Candida antarctica lipase B (Novozyme 435)-catalyzed esterification of pyridoxine (PN) was studied. In order to improve the solubility of PN and to elevate the production of 5-O-acetylpyridoxine (5-AcPN), two media, acetonitrile and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6), were used in the reaction. The site-selectivity in [bmim]PF6 was different from that of acetonitrile, i.e., in [bmim]PF6, the reactions provided 4-O-acetylpyridoxine (4-AcPN) as the major product by employing acetic anhydride as acyl donor; however, the main product was 5-AcPN in acetonitrile. By employing different acyl donor in the transesterification reaction, it was found that the acyl donor affected not only the degree of conversion but also the regioselectivity. In addition, the influence of several parameters such as water activity, reaction temperature, substrate mole ratio and enzyme loading on the esterification of PN were analyzed systematically. As a result, C. antarctica lipase B (Novozyme 435)-catalyzed esterification of PN gave a maximum conversion of 99% and the best regioselectivity of 93% in acetonitrile when vinyl acetate was used as the acyl donor.