Total Synthesis of Polygalolide A by Intramolecular C-Glycosylation

Total synyhesis of polygalolide A, a secondary metabolite isolated from a Chinese medicinal plant, was achieved. In order to construct an oxabicyclo [3.2.1] core skeleton, we have developed an intramolecular Ferrier-type C-glycosylation of a glucal modified with siloxyfuran as an internal nucleophile. Although the conditions found in the model experiments did not undergo the C-glycosylation, further examination led us to find that combination of TMSOTf and 2,4,6-collidine was the best condition, giving oxabicyclo [3.2.1] octene with correct quaternary stereogenic centers in high yield. The successful production of the single diastereomer indicated that the siloxy group at the C-3 position played a crucial role for the stereocontrol. After transformation into tetracyclic intermediate, a phenolic moiety was finally introduced by Mukaiyama aldol reaction to furnish polygalolide A.