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.