Stille-heck coupling sequences applied in a versatile new access to steroid skeletons

Abstract

A variety of enantiomerically pure steroidal compounds was synthesized utilizing a sequence of Stille and Heck cross-coupling reactions and subsequent thermal 6π-electrocyclizations. Highly chemoselective Stille couplings on the triflate moiety of several 2-bromocyclohex-1-enyl triflates with cis- and rraws-fused bicyclo[4.3.0]-nonenylstannanes furnished the corresponding tricyclic bromobutadienes in good to excellent yields (70-97%). These were subjected to Heck reactions with ten-butyl acrylate to provide pentasubstituted tricyclic 1,3,5-hexatrienes. A significant increase in efficiency could be achieved by applying a novel protocol with a precatalyst on the basis of the palladacycle prepared from Pd(OAc)2 and P(o-Tol)3 with added triarylphosphines as co-ligands (73-90% yield). Upon heating to 205-215°C in decalin or to 140°C in toluene (for certain cases), these hexatrienes yielded (78-90%) various unsaturated steroid analogues as single diastereomers. A particular oxohexatriene, obtained after deprotection of an adjacent carbonyl group, underwent 6π- electrocyclization at the unusually low temperature of 140°C to yield (75%) an interesting 7-carboxyl- substituted steroidal dienone. Attempts to remove the remaining protecting groups from some of the other new steroidal compounds under acidic conditions furnished a novel 3-oxo-7-carboxyl steroid analogue and a 3-hydroxy-substituted steroidal diene. A novel estradiol derivative could be obtained in 69% yield from the synthesized steroidal dienone. Deprotection furnished the corresponding unprotected 7-carboxylestradiol in 81 % yield. © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.