THE TANDEM CYCLOADDITION CHEMISTRY OF NITROALKENES - A NOVEL SYNTHESIS OF (-)-HASTANECINE

A short and efficient asymmetric synthesis of the pyrrolizidine necine base (-)-hastanecine is described. The key reaction in the synthesis is a sequential inter [4+2]/inter[3+2] cycloaddition. The Lewis acid promoted [4+2] cycloaddition between 2-acyloxy nitroalkene 16 and chiral vinyl ether 17 afforded a nitronate which underwent facile [3+2] cycloaddition with dimethyl maleate. The resulting nitroso acetal 19 had all the required stereocenters for (-)-hastanecine. The critical unmasking of the nitroso acetal 19 employed a hydrogenolytic cleavage to give the 1-azabicyclo-[3.3.0]octane skeleton of (-)-hastanecine.