Microbial Transformation of Tetracyclic Diterpenes: Conversion of Ent-Beyerenes by Rhizopus nigricans Cultures

Abstract

Microbial transformation of ent-7a, 18-diacetoxy- 14P-hydroxybeyer- 15- ene (2) was carried out with Rhizopus nigricans. The fungus produced an mt- 15a, lba-epoxida- tion and/or mt-3P-hydroxylation. The structures of resulting products were established by chemical and spectroscopic methods. Occasionally, microbial transformations constitute an alternative for introducing chemical functions into inaccessible sites of molecules. Thus, a great number of micro- bial transformations of steroids have been performed for this purpose (1). Microbial transformations have been also applied to nonsteroid compounds (2, 3). On certain occasions, the products formed by this procedure can be easily obtained by chemical reactions. When transformations are made on natural substrates, the intro- duction of functions into biogenetically favorable sites is a frequent phenomenon (4). As far as we know, only one paper on the microbial transformation of ent-beyerenes by hydroxylating fungi has been published (5): Aspergilh ochraceous catalyzes a reduc- tion at C-3 on ent-3-ketobeyer-15-en-17-oic acid to give an axial alcohol at this posi- tion, and Calonectria decora catalyzes hydroxylation at C-6, giving an equatorial hydro- xyl group. No transformation were observed using Rhizopus nigricans cultures. Incuba- tion of a 19-norderivative of this substrate with C. decora cultures produced an equato- rial hydroxylation at C-6 of this substrate. Incubation of this latter substrate with A. ochraceous and R. nigricans yielded no metabolites. As indicated above, reports on microbial transformations of ent-beyerenic systems are scarce, and no information is available on the transformation of 14-hydroxy- beyerenes. As a continuation of our studies on microbial transformations of tetracyclic diter- penes (6, 7), we now report the action of hydroxylating fungi (R. nigricans and A. oc- hraceous) on ent-beyer- 15-enic substrates with an ent- 14p hydroxylation.