New approaches to the structural modification of olean-type pentacylic triterpenes via microbial oxidation and glycosylation

Microbial transformation of 4 olean-type pentacyclic triterpenes (OPTs), 3-oxo oleanolic acid (1), 3-acetyl oleanolic acid (2), oleanolic acid (3), and esculentoside A (4) was studied. After the screening of 12 strains of microbes, preparative biotransformation by two strains of Streptomyces griseus ATCC 13273 and Aspergillus ochraceus CICC 40330 resulted in the isolation of 10 metabolites. The microbial catalyzed high efficient regio-selective methyl oxidation and glycosylation were discovered, which could be provided as an alternative method to expand the structural diversity of OPTs. All the structures of the metabolites were elucidated unambiguously by ESI-MS, ¹H NMR, ¹³C NMR, and 2D-NMR spectroscopy.