ENZYME-CATALYZED IRREVERSIBLE TRANSESTERIFICATIONS WITH VINYL-ACETATE- ARE THEY REALLY IRREVERSIBLE

When racemic beta-methyl-(2-thiophene)propanol (1) was resolved via tr ansesterification catalysed by lipase from Pseudomonas fluorescens (PF L) using an excess vinyl acetate in chloroform the enantioselectivity was high E approximate to 200 (conversion, c = 39%) when calculated fr om the ee of the (S)-ester S-1Ac. However, based on the ee of the rema ining (R)-alcohol R-1, assuming irreversibility and when measured at c = 57%, E = 37 was obtained. Similarly, when the 5-(1-ethoxyethoxy)-3- pentyn-2-ol (2) was resolved using immobilised lipase B from Candida a ntarctica (Novozym SP435) under similar conditions, the (R)-ester prod uct R-2Ac at c = 22% gave E approximate to 140 compared with that of t he remaining alcohol S-2 at c = 65% which gave E = 15. These results a re interpreted as a consequence of a reversible process occurring in t ransesterifications of this type, which are commonly referred to as ir reversible.