A new approach to steroid D-ring annelation: stereoselective, intramolecular alkyne–carbonium ion collapse
Abstract
A short, efficient method for generating the D-ring of 20-keto-steroids is described, wherein a t-cyclohexyl cation, corresponding to C-13, attacks an adjacent, equatorial hex-3-ynyl side chain; the predominant result is five-membered trans-fused ring formation.