Concise Synthesis of the Multiply Oxygenated ABC-Ring System of the Dihydro-β-agarofurans

The multiply oxygenated ABC-ring system of the dihydro-β-agarofurans was synthesized by employing two highly stereoselective reactions. The quinidine-catalyzed Diels–Alder reaction between a chiral dienophile and 3-hydroxy-4-methyl-2-pyrone simultaneously installed the C2-stereogenic center and two contiguous tetrasubstituted carbon centers (C5 and C10) of the A-ring. After 12 additional transformations, the aldol reaction of the resulting spiral AC-ring cyclized the B-ring with stereoselective introduction of the C7- and C8-centers.