A highly &#946;-stereoselective catalytic epoxidation of &#916;<SUP>5</SUP>-unsaturated steroids with a novel ruthenium (II) complex under aerobic conditions

Catalytic &#946;-stereoselective epoxidation of &#916;<SUP>5</SUP>-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 5&#946;,6&#946;-epoxides with high degree of stereoselectivity (88-96%) in very good yields, while oxidation of steroid derivatives with peracids leads to 5&#945;,6&#945;-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 5&#946;,6&#946;-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 &#946;-face of the steroidal skeleton