Enantioselective Total Syntheses of (−)-Silicine and (−)-20-Episilicine

The enantioselective total syntheses of (-)-silicine and (-)-20-episilicine, which contain a chiral piperidine with three contiguous chiral centers (D-ring) and a strained seven-membered ring (C-ring) attached to an indole, were achieved. The key steps of these syntheses included a chiral secondary amine-catalyzed formal aza-[3 + 3] cycloaddition reaction and Lewis acid-mediated irreversible ring-closing reaction. In addition, the stereochemistry at C20 was controlled at a later stage in the syntheses.