Direct organocatalytic stereoselective transfer hydrogenation of conjugated olefins of steroids

Kinetically controlled and organocatalytic syn-selective transfer hydrogenation has been successfully demonstrated for the reduction of the enone functional group of various steroids. Herein, diastereoselective synthesis of many 5β-steroids have been reported through organocatalysis, which have broad medicinal and pharmaceutical applications. The mechanistic studies and the selectivity of the products clearly indicated that the catalyst 1b·D-CSA is mild enough to activate the various chiral cyclic enones through iminium ion formation during the organocatalytic transfer hydrogenations with Hantzsch ester 2a as a hydrogen source. Further, clear evidence for the selective formation of intermediate iminium species [I]+ have been characterized through on-line monitoring of controlled experiments by NMR and ESI-HRMS analyses.