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.