Synthetic studies on a marine polyether toxin, gambierol: Stereoselective synthesis of the EFGH ring system via B-alkyl Suzuki coupling

A synthetic route to the EFGH ring system (3) of gambierol (1), a marine polyether toxin isolated from the dinoflagellate Gambierdiscus toxicus, has been developed. The present synthesis features convergent coupling of the F and H rings followed by ring-closure of the G ring based on the B-alkyl Suzuki reaction of lactone-derived enol phosphates. An angular methyl group at C23 was stereoselectively introduced by treatment of sulfone 32 with trimethylaluminum. Installation of a tertiary alcohol at C21 was accomplished through stereoselcetive dihydroxylation of exo-methylene 36 followed by selective formation of the primary p-toluensulfonate and treatment of the resultant monotosylate 40 with lithium aluminum hydride. Finally, formation of the E ring as a lactone form completed the synthesis of 3.