PETER YATES and FRANFOISE M. WINNIK. Can. J. Chem. 59, 1641 (1981).Bridged steroids with a bicyclo[2.2.l]heptane ring A system are formed on thermolysis of 5-vinyl 3-keto steroids. 5-Vinyl-Sa-cholestan-3-one (26) on being heated at 350°C in decalin gave 2a,5-(syn-methylmethano)-Sa-cholestan-3-one (27). Similar treatment of 17~-hydroxy-S-vinyl-S~-estran-3-one (21) gave 17P-hydroxy-2P,5-(syn-methylmethano)-SP-estran-3-one (M), which was also obtained by reduction of 2P,S-(syn-methylmethano)-SP-estrane-3,17-dione (31) with sodium borohydride on alumina. Compound 31 was formed as the major product on thermolysis of 5-vinyl-5P-estrane-3,17-dione (24) at 350°C, together with 5-(trans-propeny1)-A-nor-SP-estrane-3,17-dione (32). Compounds 27, 30, and 31 are considered to arise via intramolecular ene reactions of the A*-enols of compounds 26, 21, and 24, respectively. Compound 32 is postulated to be formed via an analogous reaction of the A3-enol of 24, followed by thermolysis of the resulting 3'-methyl-4~,5-dihydrocyclopropa[4,5]-5~-estrane-3,17-dione (36). Photolysis of 30 in methanol results in a-cleavage of the C2-C3 bond and the formation of a ketene-derived ester and two enal-derived oxetanes.PETER YATES et FRAN~OISE M. WINNIK. Can. J. Chem. 59, 1641 (1981). La thermolyse des vinyl-5 ceto-3 stero~des conduit a la formation de stero~des pontes portant au niveau du cycle A un bicyclo[2.2.1] heptane. Si on chauffe la vinyl-5 5a-cholestanone-3 (26) dans de la decaline a 350"C, on obtient la (methylmCthano syn)-2a,5 5a-cholestanone-3 (27). L'hydroxy-17P vinyl-5 SP-estranone-3 (211, traitee de la mime f a~o n , fournit I'hydroxy-17P (methylmethano syn)-2P,5 SP-estranone-3 (30) que I'on obtient egalement par reduction de la (methylmethano syn)-2P,S SP-estraned~one-3,17 (31) par le borohydrure de sodium sur I'alumine. Le composk 31 se forme comme le produ~t principal de la thermolyse de la Bridged steroids are of interest both because of their potential biological activity and because they can serve as substrates for the study of the effects of the constraints imposed on bridged bicyclic systems by the steroid skeleton in the course of the I reactions of such systems.We and others have previously investigated the synthesis of steroids incorporating a bicyclo-[2.2.2]octane (2, and references therein quoted) or bicyclo[3.2. ]]octane system (3). Here we are concerned with the incorporation of a bicyclo-[2.2.l]heptane system. Solo et al. have made extensive investigations of bridged ring D steroids of this type, which they synthesized by Diels-Alder addition of dienophiles to steroid 14,16-dienes (4, and earlier papers in this series). However, the only previous work to our knowledge that led to the incorporation of a bicyclo[2.2. llheptane system into a steroid ring other than ring D is that of Nazer ( 9 , who obtained compounds la and l b by treatment of 2a and 2b, respectively, with lithium (3), C2 and C5 (4), or C3 and C10 (5) (the last being possible only in the 19-nor series). Our initial approach was to a system of typ...