Microbial transformation of the anti-diabetic agent corosolic acid.

Biotransformation of corosolic acid (1) by Cochliobolus lunatus and Streptomyces asparaginoviolaceus afforded four metabolites, which were identified by using (1)H NMR, (13)C NMR, DEPT, HSQC, HMBC and NOESY spectral data. Biotransformation of corosolic acid by C. lunatus R.R. Nelson & Haasis CGMCC 3.4381 produced three metabolites: 2α,3β,21β-trihydroxyurs-12-en-28-oic acid (2), 2α,3β,7β,21β-tetrahydroxy-urs-12-en-28-oic acid (3) and 2α,3β-dihydroxy-21-oxours-12-en-28-oic acid (4). Incubation of corosolic acid with growing cultures of S. asparaginoviolaceus CGMCC 4.0175 afforded metabolite 2α,3β,30-trihydroxyurs-12-en-28-oic acid (5). All the metabolites were reported for the first time. The substrate and four metabolites, along with four products obtained previously, were evaluated for their inhibitory effects on α-glucosidase; all the triterpenes tested showed potent inhibitory effects.