Lipase-catalyzed stereoselective esterification of DL-menthol in organic solvents using acid anhydrides as acylating agents

Stereoselective esterification of racemic menthol in n-hexane using acid anhydrides as acylating agents was successfully achieved with lipase AY-30 from Candida cylindracea. Molar yields of menthyl esters as high as 23-30% were observed in the control without the presence of enzyme after 48 h incubation at 30 degrees C in n-hexane. Molar percentage yield, enantiomeric ratio (E), and percentage enantiomeric excess (ee%) of product menthyl esters were estimated using capillary gas chromatography and a chiral column. The (-) menthol was preferentially esterified by lipase AY-30 with a yield as high as 64%. The highest E (14) and ee% (86%) were achieved in n-hexane solution containing a 1:1 molar ratio (+/-)menthol:butyric anhydride. The addition of a weak base, NaHCO3, did not enhance enantioselectivity of lipase AY-30; however; it did improve the overall yields in both the control and the systems containing lipase AY-30.