Enantioselective synthesis of (-)-menthyl acetate in a batch stirred tank reactor: effects of fed batch feeding strategy

Abstract

Investigation involving enantioselective synthesis of chiral compounds catalysed by immobilised enzyme lipase is a viable option. In this study, acetic anhydride as an acylating agent was employed with (±)-menthol, a secondary alcohol in enantioselective synthesis of (-)-menthyl acetate in a fed batch reactor. (-)-menthyl acetate is an optically active flavour and fragrance compound. Molar ratio concentration of acetic anhydride to (±)-menthol is 1:4. The ratio of alcohol to acyl donor, acid anhydride to (±)-menthol was prepared at 15 mmole/L : 60 mmole/L in 25 ml n-heptane. The reaction was catalysed by commercially available immobilised enzyme lipase, chirazymeL-3, c. -f, C2 lyo from candida rugosa. The incorporation of a fed batch feeding system to the batch reactor configuration was investigated. The system was evaluated in terms of molar percentage yield and productivity. The reactions were carried out at 30°C in n-heptane and molar yield as high as 8.72% -29.57% were attained after 48 hours of reaction. This fed batch feeding system was thought able to minimise the hydrolysis of acetic anhydride. A high concentration of acetic anhydride in a batch reaction system is not a viable option since water present in the reaction mixture would promote the hydrolysis of acetic anhydride catalysed by the same enzyme. A fed batch feeding strategy of 12uL, 10uL, 8uL and 6uL produced the highest molar percentage yield with 29.57% of (-)-menthyl acetate formation and productivity of 0.00154mM/min. The enzyme loading used was only 40% from the actual equivalent amount.