Biosynthesis of the diterpene verrucosan-2β-ol in the phototrophic Eubacterium Chloroflexus aurantiacus -: A retrobiosynthetic NMR study

The biosynthesis of verrucosan-2 beta-ol in the green phototrophic eubacterium Chloroflexus aurantiacus was investigated by in vivo incorporation of singly or doubly C-13-labeled acetate, The C-13 labeling of the isolated diterpene was analyzed by one- and two-dimensional NMR spectroscopy. The C-13-labeling patterns of verrucosan-2 beta-ol were compared with the labeling patterns of intermediary metabolites (acetyl-CoA, pyruvate, and glyceraldehyde 3-phosphate) which were deduced from amino acids and nucleosides by retrobiosynthetic analysis. The results show that verrucosan-2 beta-ol is synthesized via mevalonate and not via the deoxyxylulose pathway, which was discovered recently in some eubacteria, algae, and plants. A scheme for the formation of the unusual tetracyclic ring system is offered. The cyclization process is initiated by the solvolysis of pyrophosphate from geranyllinaloyl pyrophosphate and the mechanism involves a Wagner-Meerwein rearrangement, a 1,5-hydride shift, and a cyclopropylcarbinyl to cyclopropylcarbinyl rearrangement.