Synthesis of Unnatural Steroids Using the Bistro Strategy
Résumé
Lithium-mediated reductive dimerization of buta-1,3-diene in the presence of chloro(trimethyl)silane readily provides 1,8-bis(trimethylsilyl)octa-2,6-diene, commonly known as Bistro'. This bisallylsilane can react with various electrophilic reagents to give 1,1-disubstituted 2,5-divinylcyclopentanes, which are precursors of the D rings of steroids. After slight alterations, these precursors can be coupled with benzocyclobutene derivatives. Heat induces ring opening of the benzocyclobutene to form an o-xylylene that subsequently undergoes intramolecular cycloaddition with a vinyl substituent to form the skeleton of a new unnatural steroid, according to the A + D AD ABCD strategy. In this way, more than 250 steroids have been prepared, including 3-, 11-, 12-heterosteroids bearing a 17-vinyl group that can be readily modified to form a 17-acetyl or 17-(2-oxoethyl) group, as well as some steroid building blocks. The steroids were obtained in few steps from buta-1,3-diene and benzocyclobutene derivatives in overall yields in excess of 25%. This powerful strategy, which has not yet been exhausted, paves the way towards various related synthetic pathways. 1 Introduction 2 Series A 3 Series B 4 Series C 5 Series D 6 Series E 7 X-Ray Crystallographic Data 8 Conclusion