An Efficient Approach to the Unnatural Chirality of Steroid Side Chains at C-20

Abstract

Although considerable efforts have gone into the stereocontrolled synthesis of side chains of important steroids with the “natural” chirality at C-20, little has been done on the stereocontrolled introduction of the “unnatural” chirality present in unusual marine steroids.¹ Recently, we reported for the first time that steroidal α-face (2,3)-sigmatropic rearrangements of alkoxy-organolithium reagents provide an efficient method for the stereospecific introduction of the “natural” chirality at C-20.^2a We now describe how this method can be applied to the stereospecific construction of the “unnatural” chirality at C-20 through the more hindered β-face.