Stereoselective epoxidation of 2,2-dimethyl-2H-1- benzopyran-6-carbonitrile

The chiral intermediate (3 S ,4 R )- trans -3,4-dihydro-3,4-dihydroxy-2,2-dimethyl-2 H -1-benzopyran-6-carbonitrile [(+)- trans diol 3 ] was made by the stereoselective microbial epoxidation of 2,2-dimethyl-2 H -1-benzopyran-6-carbonitrile 1 . This compound is a potential intermediate for the total synthesis of potassium-channel openers. Several microbial cultures were found which catalyzed the transformation of 1 to the corresponding (3 S ,4 S )-epoxide 2 and (+)- trans diol 3 . The two best cultures, Corynebacterium sp. SC 13876 and Mortierella ramanniana SC 13840 gave reaction yields of 32 M% and 67.5 M% and optical purities of 88 and 96 %, respectively, for (+)- trans diol 3 . A single-stage process (fermentation-epoxidation) for the biotransformation of 1 was developed using Corynebacterium sp. SC 13876 and M. ramanniana SC 13840. In a 25-L fermentor, the (+)- trans diol 3 was obtained in 38.6 M% yield with an optical purity of 90 % using Corynebacterium SC 13876. The reaction yield of 60.7 M% and optical purity of 92.5 % were obtained for (+)- trans diol 23 using M. ramanniana SC 13840. A two-stage process for the preparation of (+)- trans diol 3 was also developed using a 3 L cell-suspension (10 % w/v, wet cells) of M. ramanniana SC 13840. The reaction was carried out in a 5-L Bioflo fermentor. The concentration of substrate 1 was 2 g L −1 with glucose present at 10 g L −1 . After 48 h, (+)- trans diol 3 was obtained in 76 M% yield with an optical purity of 96 %. From the reaction mixture, (+)- trans diol 3 was isolated in 65 M% (4.6 g) overall yield. An optical purity of 97 % and a chemical purity of 98 % were obtained for the isolated (+)- trans diol 3 .