Stereo- and region-specific biotransformation of physapubescin by four fungal strains.

Biotransformations of physapubescin (1) were performed by four fungal strains-Mucor subtilissimus AS 3.2454, Mucor polymorphosporus AS 3.3443, Aspergillus niger AS 3.795, and Syncephalastrum racemosum AS 3.264. Four metabolites were prepared in the biotransformation process of 1, and their structures were elucidated as 15α-acetoxy-5,6β:22,26:24,25-triepoxy-26α-hydroxy-3β-methoxy 4β-hydroxyergost-1-one (2), 15α-acetoxy-5,6β:22,26-diepoxy-4β,24β,25α,26(α, β)-tetrahydroxyergost-3β-methoxy-1-one (3a/3b), 15α-acetoxy-5,6β:22,26-diepoxy-4β,24β,25α,26(α, β)-tetrahydroxyergost-2-en-1-one (4a/4b), and physapubescin D (5), by spectroscopic data analysis. Among them, metabolites 2 and 3 are new. All of these fungal strains showed the ability to be highly stereo- and region-specific for the bioconversion of substrate (1). Our research provides a reference for the structural derivatization of withanolides or possibly even other natural products.