A diene transmissive Diels-Alder strategy for oxygenated nor-steroid and triterpenoid skeletons

[GRAPHICS] A diene transmissive cycloaddition strategy for the synthesis of tetracycli c skeletons is described, Initially both L-gulonolactone and L-arabinose we re converted independently to related acetal aldehydes 13 and 14, A pentadi enyl indium reagent supplied the triene unit for 16. The cisisopropylidene acetal controlled the initial intramolecular [4 + 2] cycloaddition to the d ecalin 21, and a second (tandem) intermolecular cyclization afforded highly oxygenated nor-steroid and triterpenoid skeletons as chiral nonracemic com pounds.