Biocatalytic enantiomeric resolution of l-menthol from an eight isomeric menthol mixture

Abstract

The four diastereomers of menthol and their enantiomers, namely dl-menthol, dl-neomenthol, dl-neoisomenthol and dl isomenthol, were synthesised by the hydrogenation of thymol to yield an eight isomer liquid menthol. A suitably selective lipase was sought to preferentially esterify l-menthol in hexane, hence simplifying separation from this diasteromeric mix through distillation. From an initial screen of over 70 enzyme preparations, a commercial Pseudomonas fluorescens lipase (Amano AK) was selected, and vinyl acetate was chosen as a suitable irreversible acyl donor for transesterification. The enzyme was recycled a total of 150 times in 5 ml batch reactions using liquid menthol and achieving an overall yield of 184.31 g dl-menthol/g enzyme. An enantiomeric excess of l-menthol of greater than 95% was reproducibly achievable at a conversion of 30% dl-menthol (0.68 M) at =50°C. On the basis of the composition of liquid menthol the reaction had a diastereomeric ratio of 81%. The resolution reaction was scaled up 400 fold to 2 L and the ezyme recycled 38 times with an average conversion of the available l-menthol of 59%.