Regio- and Stereochemistry of the Cycloadditions of Dichloroketene to 2-Methyl- and 3-Methyl-2-cholestene

Electronic and stereoelectronic factors governing the cycloaddition of 2-methyl- and 3-methyl-2-cholestene (7 and 8) with dichloroketene were examined. In each case, the reaction was regio- and stereospecific: 2-methyl-2-cholestene (7) afforded 2a, 2a-dichloro-2a, 3a-ethano-2/3-methylcholestan-3a-one (9), while 3-methyl-2-cholestene (8) afforded 3a, 3a-dichloro-2a, 3a-ethano-3d-methylcholestan-2a-one (13). The results indicate that the cycloaddition proceeds exclusively via attack of the ketene from the a side of the steroid and that the regiochemistry is guided largely by electronic factors. The structures of the cycloadducts were elucidated by chemical means (reduction, Baeyer-Villiger oxidation) and NMR, while circular dichroism proved useful in the conformational analysis of the fused steroidal cyclobutanones and lactones.