A Diene Transmissive Diels−Alder Strategy for Oxygenated Nor-Steroid and Triterpenoid Skeletons†

A diene transmissive cycloaddition strategy for the synthesis of tetracyclic skeletons is described. Initially both l-gulonolactone and l-arabinose were converted independently to related acetal aldehydes 13 and 14. A pentadienyl indium reagent supplied the triene unit for 16. The cis-isopropylidene acetal controlled the initial intramolecular [4 + 2] cycloaddition to the decalin 21, and a second (tandem) intermolecular cyclization afforded highly oxygenated nor-steroid and triterpenoid skeletons as chiral nonracemic compounds.