A concise synthesis of the steroidal core of clathsterol

The protected steroidal core of clathsterol, a marine natural product with remarkable inhibitory activity against HIV-1 reverse transcriptase, was synthesized starting from readily available tigogenin. The synthetic strategy involved three key reactions: abnormal Baeyer–Villiger rearrangement of the spiroketal of tigogenin, xanthation of steroidal 16-hydroxyl-22-carboxylate dianion, and regioselective epoxide-opening lactonization.