SYNTHESIS OF PENTACYCLIC STEROIDS VIA TANDEM STILLE COUPLING AND DIELS-ALDER REACTIONS

Tandem Stille coupling and Diels-Alder reaction of steroidal iodoalken yl substrates [17-iodoandrosta-16-ene (2), 17-iodo-3-keto-4-methyl-4-a zaandrosta-16-ene (10), and 17-iodo-3-keto-4-azaandrosta-16-ene (11)] were examined in the presence of vinyltributyltin, a dienophile (dieth yl maleate, methyl acrylate, dimethyl acetylenedicarboxylate, diethyl fumarate, (-)-di[(LR)-menthyl] fumarate, maleic anhydride), and a pall adium catalyst. The yields of novel pentacyclic steroids and the stere ochemical outcome of the cycloaddition reaction were investigated unde r different reaction conditions. In some cases mixtures of isomers wer e formed, but with dimethyl acetylenedicarboxylate, diethyl fumarate, (-)-di[(1R)-menthyl] fumarate, and maleic anhydride the products could be produced in good yields with high stereoselectivity. The unequivoc al assignment of stereoisomers was carried out by various NMR techniqu es including H-1-H-1 COSY and NOE experiments.