Preparation of isoxazolidinyl nucleoside enantiomers by lipase-catalysed kinetic resolution

An efficient biocatalytic procedure to obtain chiral N,O-nucleoside derivatives consisting of a lipase-catalysed resolution of the corresponding racemates in organic solvent has been developed. Enantioselective esterification of thymine and cytosine derivatives, (+/-)-9a and (+/-)-9b, has been Investigated by comparing the efficiency of different lipases and acyl donors. Since esterification of (+/-)-9a and (+/-)-9b, Occurred with low enantioselectivity (E <= 14) in the presence of the lipases considered, for preparative purposes we resorted, with Success, to a double sequential kinetic resolution, using Lipozyme IM as the best catalyst. This approach enables LIS to obtain all the enantiomers with ee >= 95%. The absolute configuration of the new chiral N-acetyl cytosine derivative 9b was established by circular dichroism spectroscopy. (c) 2009 Elsevier Ltd. All rights reserved.