Identification of (8S,9S,10S)-8,10-dimethyl-1-octalin, a key intermediate in the biosynthesis of geosmin in bacteria.

Abstract

(−)-Geosmin (1, blue) is an important odor component produced by many bacteria, including actinomycetes, myxobacteria, and cyanobacteria, but has also been reported for eukaryotic organisms such as fungi, liverworts, insects, and plants. Recent research has shown that the biosynthesis of 1 starts with the cyclization of farnesyl pyrophosphate to (1(10)E,5E)-germacradien-11-ol (2, yellow), the first key intermediate en route. In a retro-Prins-reaction acetone is lost, and the second key intermediate, (8S,9S,10S)-8,10-dimethyl-1-octalin (3, red), is formed. After reprotonation, a 1,2-H-shift and attack of water, geosmin is finally released. Octalin 3 occurs in the bouquets of volatiles released by myxobacteria as well as in enzyme extracts of incubation experiments of the purified geosmin synthase from Streptomyces coelicolor. Here the identification of 3, a new natural compound, the elucidation of its stereochemistry, and its synthesis are reported.