(1 S,5R,6R,7R)-( -)-7-Hydroxy-6-hydroxymethyl-2-oxabicyclo[3.3.0]octan-3-one (1) as well as the corresponding racemate rac-1 are versatile intermediates in the synthesis of natural prostaglandins2) and racemic prostaglandin analogs, such as cl~prostenol'*~) and fluprostenol'). The classical way to obtain enantiomerically pure 1 is the resolution Recently, we *) and other g r o u p~~~'~) have already advantageously used enzymatic methods for obtaining enantiomerically pure compounds of this type. The aim of this investigation has been to apply the lipase-catalyzed transesterification in organic solvents ''J') to the resolution of rac-1 into its enantiomers. As standard conditions for this investigation we have used lipases of different origin in tetrahydrofuran/triethylamine as solvent and vinyl acetate as acylating agent. We have already applied this solvent system successfully to other enantio-L''-'~) and regioselective 17) lipase-catalyzed transformations. To find out a suitable enzyme for the optical resolution of rac-1 we have tested five lipases of different origin using 1 mmol of substrate rac-1 ( Table 1).The lipase of choice for an effective separation of the enantiomers is the lipase Amano PS. After 50% conversion of the starting material rac-1 the unreacted enantiomer ent-1 can be obtained enantiomerically pure, whereas the monoacetate 2 is isolated with an e.e. of 86% (entry 3). After deacetylation of 2 the enantiomeric purity of the resulting 1 can be easily enhanced up to more than 99% by recrystallization from ethyl acetate/acetone'). The negative optical rotation of 1 proves the (lS,5R,6R,7R) configuration" of the monoacetate 2. Unreacted ent-1 shows a positive optical rotation indicating the (lR,5S,6S,7S) configuration for this enantiomer. An earlier termination of the reaction with lipase Amano PS (entries 4 and 5) yields 2 with an e.e. higher than 86%, however with a loss of chemical yield. Pancreatin shows a significantly lower enantioselectivity in this reaction (entry l), but allows the preparation of racemic monoacetate