High-performance continuous operation for enanthioselective esterification of menthol by use of acid anhydride and free lipase in organic solvent

By using propionic anydride as a highly reactive and non-water-producing acyl donor, and free lipase OF 360 suspended in cyclohexane as a biocatalyst under the optimized conditions, efficient and stable enantioselective esterification of (+-)-menthol is possible and has been carried out very stably for 2 months in a continuous stirred-tank reactor at 30°C, retaining a 47%–35% conversion of menthol and high optical purity of the (–)-menthyl ester produced (95%–98% e.e.). However, when the corresponding propionic acid was used as an acyl donor, the conversion ratio of menthol decreased rapidly after the start of continuous operation, indicating a higher productivity with the anhydride than with the free acid. In the case of the acid anhydride, the water content of the reaction medium, which has a great effect on the reactivity and stability of lipase, was easily controlled in long-term operation by the occasional micro-adjustment of the flow rate or concentration of the feed solution of the acid anhydride, and could essentially be kept constant (2–4 mM), avoiding the inactivation of the enzyme.