Preparation Of L-menthol By Biocatalyzed Resolution

Due to its unique flavour,cooling and refreshing effects,L-menthol is of much more importance from industrial point of view than DL-menthol or the mixture of eight menthols.In this work,L-menthol was prepared from DL-menthol and from the mixture of eight menthols by biocatalyzed resolutions including microbial-catalyzed asymmetric hydrolysis and lipase-catalyzed asymmetric transesterification.GC method was established to analyze conversion rate,yield and optical purity of L-menthol in these two reaction systems employing two columns such as CP-CYCLODEXTRINβ-2,3,6-M-19 and DB 210.About one hundred and twenty strains were isolated from soil samples to hydrolyze DL-menthyl acetate,among which,a strain designated as D2 exhibited high enantioselectivity to L-menthyl acetate with e.e.p%value of over 98%.The optimum carbon resource and nitrogen resource were tween-80 and yeast extract,and the optimum cultivating temperature was 30℃.The production of hydrolase was increased from 40U/L to 200U/L,nearly 400%enhancement.Then the hydrolase was applied to the hydrolyzation of the mixture of eight menthyl acetates.It showed very high enantioselectivity with the d.e.%value of over 98%,which meant that the percentage of L-menthol in the products was over 99%.The conversion rate of L-menthyl acetate could reach nearly 70%.Lipase-catalyzed resolution of menthols was also investigated.More than twenty kinds of lipases were screened and lipase 25# was found to be the optimum enzyme. n-hexan was chosen as the optimum solvent.The e.e._p%value of the lipase-catalyzed asymmetric transesterification of DL-menthol was about 98%with the conversion rate of 41%.When the lipase was used to catalyze the resolution of the mixture of eight menthols,the d.e.%value of L-menthyl acetate was about 85%.