Syntheses of Bufospirostenin A and Ophiopogonol A by a Conformation-Controlled Transannular Prins Cyclization

Controlling the conformation of medium-sized rings is challenging because of their flexibility and ring strain effects. Herein, we report non-Curtin–Hammett conditions for the precise control of the conformation of cyclodecenones to effect the first cis-selective transannular Prins cyclization, which enabled concise syntheses of the 5(10→1)abeo-steroids bufospirostenin A and ophiopogonol A in only seven steps from inexpensive starting materials. Computational results indicated that the key cyclization was kinetically controlled and proceeded via either a Prins pathway or a carbonyl–ene pathway, depending on the reaction conditions. Moreover, conformational isomerization played a critical role in determining the stereochemistry of the products.