title: A Highly beta-Stereoselective Catalytic Epoxidation of Delta(5)-Unsaturated Steroids with a Novel Ruthenium(II) Complex under Aerobic Conditions.
abstract: Catalytic beta-stereoselective epoxidation of Delta(5)-unsaturated steroid derivatives has been effected by a novel ruthenium(II) bioxazoline complex under aerobic conditions. The reactions are regio- and stereoselective. The reaction conditions provide the corresponding 5beta,6beta-epoxides with high degree of stereoselectivity (88-96%) in very good yields, while oxidation of steroid derivatives with peracids leads to 5alpha,6alpha-epoxides as the major products. The overall conformation of the steroid nucleus is nearly planar in the cholesteryl ester, while it is bent at the junction between the rings A and B in the 5beta,6beta-epoxide. This change from pseudo-trans- to cis-stereochemistry of the A-B ring junction provides more room for the catalyst to approach from the beta-face of the steroidal skeleton.
authors.affiliation: Department of Organic Chemistry, Indian Institute of Science, Bangalore-560 012, India.
authors.forename: Venkitasamy
authors.initials: V
authors.lastname: Kesavan
authors.forename: Srinivasan
authors.initials: S
authors.lastname: Chandrasekaran