Microbial transformations of natural antitumour agents. Part 15. Metabolism of bruceantin by Streptomyces griseus

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Abstract
Microbial transformations were conducted with the quassinoid bruceantin (1). Screening studies with 193 micro-organisms provided several streptomycetes capable of accumulating bruceantin metabolites. Streptomyces griseus(ATCC 10137) totally metabolized bruceantin within 19 h, and a preparative-scale incubation using bruceantin provided bruceine C (4), bruceantin 4′,5′-epoxide (3), and bruceantin-5′-ol (5). Structures of metabolites were assigned based on their 1H n.m.r. and mass spectral characteristics. Metabolites (3) and (5) are novel analogues of bruceantin, and are formed by an unusual pathway involving oxidation of bruceantin, probably through the unisolated 4′,5′-bruceantin olefin (2) to the epoxide (3), which serves as an obligatory precursor for bruceine C (4) and bruceantin-5′-ol (5).