Enzymatic Process for the Synthesis of cis/ trans-(1R,5R)-Bicyclo[3.2.0]hept- 6-ylidene-acetate:  Solvent Effect and NMR Study

An efficient enzymatic process has been developed to resolve a diastereomeric mixture of racemic ethyl bicyclo[3.2.0]hept-6-ylidene-acetate (1). Using 40% acetone, not only was the enantioselectivity of Candida antarctica lipase B (CAL-B) significantly improved to E > 200 from E = 2.7, remarkably the enzyme is able to maintain low diastereoselectivity for the Z (cis)- and E (trans)-isomers leading to an overall high isolated yield (40−45%, vs maximum theoretical yield 50%) and excellent enantiomeric excess (>98.5% ee). Preliminary studies using 2D TROSY shows that there is probably a global conformational change in the N15-labeled enzyme CAL-B when the content of organic cosolvents increases from 0% to 30%. The route was successfully scaled to 63 kg for the synthesis of GABA analogues.