Total Synthesis of (+)-Ambruticin S: Probing the Pharmacophoric Subunit

An enantioselective synthesis of the antifungal natural product (+)-ambruticin S has been accomplished starting with the readily available methyl α-d-glucopyranoside, (R)-Roche ester, and (S)-glycidol as chirons, which encompassed seven of the 10 stereogenic centers of the target molecule. The remaining three centers were set by a highly diastereoselective, asymmetric cyclopropanation employing a chiral, nonracemic phosphonamide reagent. Our strategy for the construction of the dihydropyran subunit involved a highly syn-selective Lewis acid catalyzed 6-endo-trig cyclization. Other key steps in the synthesis featured an epoxide opening with a dithiane anion, two efficient phosphonamide-anion based olefinations, and a late-stage C-glycosylation.