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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(sp3 )-C(sp3 ) Stille coupling reaction, resulted in the total syntheses of (±)-1 and (±)-2.