BIOSYNTHESIS OF PENTALENENE AND PENTALENOLACTONE

Pentalenene synthase catalyzes the cyclization of farnesyl pyrophosphate to the sesquiterpene hydrocarbon pentalenene (3). Incubation of both (9R)- and (9S)-[9-13H,4,8-14C2] FPP (2c and 2d) with pentalenene synthase isolated from Streptomyces UC5319 and analysis of the derived labeled pentalenene by a combination of chemical and microbial methods established that H-9 re of 2 becomes H-8 of pentalenene, while H-9 si undergoes net intramolecular transfer to H-l of 3. The stereochemistry of the proton transfer was determined by incubation of [10-2H,11-13C]FPP (2e) with pentalenene synthase and analysis of the product by a combination of2H and13C NMR spectroscopy, which established that the deuterium atom occupied exclusively the predicted H-l si (H-l β) position in pentalenene. Deuterium NMR analysis of pentalenene derived from both (1S)- and (1R)-[1-2H]FPP (2h and 2i) established that the cyclization takes place with inversion of configuration at C-l of FPP. These results are fully consistent with a stereochemical model of the cyclization reaction in which FPP is cyclized to humulene (11), folded in the RSR-CT conformation. The humulene is in turn protonated on the 10 re face of the C-9,10 double bond, leading, after further cyclization and rearrangement, to the formation of pentalenene (3). The stereochemical course of the oxidative metabolism of pentalenene was also examined. The hydroxylation at C-l that leads to the formation of pentalenic acid was shown to take place with retention of configuration. The fact that tritium label from either (1R)-[1,8-3H2]- or (1R)-[1-3H]pentalenene (3b or 3d) was lost upon formation of pentalenic acid but was retained in the eventually formed pentalenolactone ruled out 10 as an intermediate in the formation of 1. Administration of both (3S)- and (3R)-[3-3H, 14,15-14C2]pentalenene (3j and 3k) to cultures of Streptomyces UC5319 and analysis of the3H/14C ratio in the derived pentalenolactone established that the A-ring rearrangement takes place with stereospecific loss of H-3 re of 3, anti to the migrating methyl group. © 1990, American Chemical Society. All rights reserved.