Cycloaddition of Nitrosoaromatics with Steroidal Dienes:  Unexpected Dependence of the Chemoselectivity on the Aryl Ring Substituent

Tandem palladium-catalyzed Stille coupling of steroidal alkenyl iodides (17-iodo-androst-16-ene (1), 17-iodo-4-methyl-4-aza-androst-16-en-3-one (4), and 17-iodo-4-aza-androst-16-en-3-one (5)) with vinyltributylstannane and hetero Diels−Alder reaction of the products using nitrosobenzene or some para-substituted nitrosoaromatics (p-NO2, p-Br, p-Me, p-OMe) as dienophiles were investigated. Cycloaddition was regioselective in each case, but two stereoisomers were obtained in a 2:1 ratio with unsubstituted pregnadienes as reaction partners. Stereoselectivity was improved by the use of Lewis acid catalysts. The similar reaction of substituted nitrosoaromatics with highly negative Hammett substituent-constants (p-OH, p-NMe2) resulted in the spontaneous dehydrogenation of the dihydro-oxazine, which was followed by the formation of rearrangement products. The assignment of stereoisomers was carried out by various NMR techniques including 1H−1H COSY and NOESY experiments.