Enantioselectivity of enzymatic acylation of some structurally various racemic alcohols in anhydrous aprotic media

Partial acylation of (R,S)-3,7-dimethyloctan-1-ol (1) and (R,S)-7-methoxy-3,7-dimethyl-octan-1-ol (2) with vinyl acetate catalyzed by the lipase from Candida cylindracea affords in good yields the corresponding S-configured acetates with 92-98% enantiomeric excess (ee). Under similar conditions, racemic alpha-cyclogeraniol (3), drim-7-en-11-ol, methyl 4-(3-hydroxy-2-methylpropyl)benzoate, and its eta(6)-chromium(tricarbonyl) complex (6) are acylated with rather poor (and, for the two latter, opposite) enantioselectivity, whereas (R,S)-2,4:3,5-di-O-benzylidenexylitol remains unaffected. Racemic isoborneol (8) and 2-nitro-1-phenylethanol also remain almost or completely unconverted. Attempts to perform enantioselective acylation of alcohols 3 and 8 with Ac2O in the presence of porcine pancreatic lipase (PPL) proved equally unsuccessful. By contrast, the PPL-catalyzed acylation of alcohol 6 with vinyl acetate at 17% conversion affords the levorotatory acetate (S)-6a with ca. 100% ee. PPL-Mediated partial acylation of (R,S)-pantolactone with Ac2O, followed by mild deacylation of the resulting R acetate, gives (R)-(-)-pantolactone of 97% enantiomeric purity in 60% overall yield.