SYNTHETIC STUDIES ON FURANOHELIANGOLIDES - STEREOCONTROLLED CONSTRUCTION OF THE OXYGEN-BRIDGED TRICYCLIC FRAMEWORK

The heavily functionalized 6,9-epoxycyclodeca[b]furan-11-ones 34 and 37 have been prepared in 13 steps from 4-methyl-2-[(phenylmethoxy)methyl]furan. The key elements of the scheme include a high-pressure Diels-Alder cycloaddition to 1-cyanovinyl acetate, highly regioselective hydroboration, controlled stepwise oxidation to give keto aldehyde 20, and thermal oxy-Cope rearrangement of both 32 and 36b. The prior introduction of a phenylthio substituent provides for the accommodation of different levels of unsaturation at a more advanced stage of furanoheliangolide construction. While the present strategy is developed around a racemic model, the potential for adoption of enantioselective features is immediate. The overall stereocontrolled sequence provides a general and flexible entry into oxygen-bridged frameworks closely related to substructures occurring in many furan-type germacranolides.