Synthesis of (S)-N-(2-Ethyl-6-methylphenyl) alanine by Enzyme-catalyzed Resolution and Chemical Racemization

A practical synthesis of (S)-N-(2-ethyl-6-methylphenyl)alanine, a key intermediate for (S)-metolachlor, was completed by means of lipase-catalyzed hydrolytic kinetic resolution and chemical racemization of the remaining ester. The effects of operating temperature and enzyme concentration on the activity and enantioselectivity of enzyme were initially studied, and it was found that the enantioselectivity of CAL-B towards the resolution was not high enough to obtain enantiomerically pure compound(E=12.1). When diethyl ether(15%, volume fraction) was added in the reaction medium, the lipase gave an excellent enantioselectivity(E=117.8), which is about 9.7-fold that in pure buffered aqueous solution. For overcoming the limitation of a maximum theoretical yield of 50%, the acid product was separated from the remaining ester by a simple extraction procedure and the remaining ester was racemized with aldehyde and acetic acid under microwave irradiation or conventional heating condition. The results show the microwave irradiation was more effective than the conventional heating method and gave the desired (R,S)-N-(2-ethyl-6-methylphenyl)alanine methyl ester a high yield(92%) with R/S=50/50 in 1 h.