Enantioselective Total Syntheses of Kuwanon X, Kuwanon Y, and Kuwanol A.

The first enantioselective total syntheses of (-)-kuwanon X, (+)-kuwanon Y, and (+)-kuwanol A have been accomplished by using asymmetric Diels-Alder cycloaddition promoted by chiral VANOL or VAPOL/boron Lewis acid. The biosynthesis-inspired asymmetric Diels-Alder cycloaddition shows high exo selectivity (exo/endo = 13/1), which was unprecedented in the previous total syntheses of related prenylflavonoid Diels-Alder natural products. An acid catalyzed intramolecular ketalization process enabled a biomimetic transformation to construct the polycyclic skeleton of kuwanol A efficiently.