(+)-Thalianatriene and (-)-Retigeranin B Catalyzed by Sesterterpene Synthases from Arabidopsis thaliana

Two GFPPS linked (+)-thalianatriene (1) and (−)-retigeranin B (2) sesterterpene synthase genes were identified from the genome of Arabidopsis thaliana. 1 possesses an unprecedented 11–6–5 tricyclic ring system, while 2 contains a characteristic 5–5–5–6–5 pentacyclic ring system. Their structures were determined by extensive NMR spectroscopy, chemical derivatization, and X-ray crystallography. The variable-temp NMR measurement of 3, a diepoxy-bearing derivative of 1, enables us to completely assign the NMR signals of the two conformers as 3a (67%, UUU) and 3b (33%, UUD). A plausible biosynthesis mechanism of 1 was proposed.