Enzymes in organic synthesis, 4. Investigation of the pancreatin‐catalyzed acylation of cis‐cyclopent‐2‐ene‐1,4‐diol with various trichloroethyl and vinyl Alkanoates
During the pancreatin-catalyzed acetylation of the meso-diol 1 with 2,2,2-trichloroethyl acetate (2a) in tetrahydrofuran/tri-ethylamine, the enantiomeric monoacetates 3a and ent-3a are formed at nearly equal rates. ent-3a is rapidly acetylated in a second enzyme-catalyzed step, forming 4a, whereas 3a resists further enzymatic acetylation. Thus, the monoacetate 3a can be obtained in 48% yield with an enantiomeric excess (e.e.) of more than 99%. 2,2,2-Trichloroethyl propanoate and butanoate give the corresponding monoacylation products even in slightly better yield, whereas the octanoate affords the mono-acylation product with a lower enantiomeric excess. 2,2,2-Trichloroethyl monochloroacetate provides the monoacylation product in a 40% yield with an e.e. of 90%. The dichloroacetate, however, affords the diacylation product exclusively in an enzyme-independent chemical reaction. With the 2,2,2-trichloroethyl esters of isobutyric, phenylacetic, and benzoic acid no transesterification could be achieved within 24 hours. The application of vinyl acetate, however, represents a significant improvement in the synthesis of enantiomerically pure monoacetate 3a from meso-diol 1.