Diastereoselective Total Syntheses of (±)-Caseabalansin A and (±)-18-Epicaseabalansin A via Intramolecular Robinson-type Annulation

Highly diastereoselective total syntheses of (+/-)-caseabalansin A (1) and (+/-)-18-epicaseabalansin A (2) are described in this paper. We revealed that the intramolecular Robinson-type annulation of an alkynone was effective in the stereocontrolled construction of the bicyclic skeleton of 1 and 2. Further transformation of the resulting enone, including diastereoselective reduction by LiAlH(OtBu)(3), hydroxy-group-directed hydrogenation, cyclization to form the cyclic acetal moiety, and introduction of a side chain by a C(sp(3))-C(sp(3)) Stille coupling reaction, resulted in the total syntheses of (+/-)-1 and (+/-)-2.