Stereoselective synthesis of optically active 1-benzyl-4,5,6,7-tetrahydro-6,6-dimethyl-4-oxo-1H-indol-7-yl acetate and 1-benzyl-6,7-dihydro-7-hydroxy-6,6-dimethyl-1H-indol-4(5H)-one through lipase-catalyzed esterification and transesterification processes

The enantioselective synthesis of 1-benzyl-4,5,6,7-tetrahydro-6,6-dimethyl-4-oxo-1H-indol-7-yl acetate (4) and 1-benzyl-6,7-dihydro-7-hydroxy-6,6-dimethyl-1H-indol-4(5H)-one (5), which are important intermediates in pharmaceutical industry, was carried out for the first time, both by enzyme-mediated hydrolysis and transesterification reactions with high enantiomeric excesses in the presence of various lipases. In either case S enantiomer of (5) was obtained with high enantiomeric excesses at low rate of conversion and E value. However, R enantiomer of (5) was also obtained by transesterification reaction with high optical purity. In the transesterification reaction of (rac-5a) with several lipases in different solvent systems in the peresence of DMAP as an additive and vinyl acetate, E value of the reaction were raised for some enzyme and solvent combination (THF-MJL with >99% ee and E value: 41; for acetonitrile–MJL with 91% ee and E value: 51; for acetonitrile-Amano with 99% ee, E value: 68) showed R-(5) selectivity. Furthermore the conversion value was also increased. The best conversion of the transesterification reaction was 39% with DMSO-HPL showed 73% ee and E value: 15 for R-(5) selectivity and 47% for S-(4) selectivity. The two procedures can therefore be considered as complementary with respect to the final composition.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slide