Alkyne Cyclizations through Vinyl Cations and Stereoselective Allene Formation by Terpene Synthases

contained an allene function embedded in a macrocyclic ring. A heating experiment revealed racemization of this compound at 120 °C with a half-life of about 24 h. Also, elongation of the FPP analog with one more isoprene unit by a geranylgeranyl pyrophosphate synthase and subsequent cyclization using a diterpene synthase proved to be possible. The absolute configurations of the isolated compounds and the cyclization mechanisms toward them were investigated through isotopic labeling experiments and through computational methods.