The total syntheses of (+)-crotanecine, (-)-hastanecine and (-)-rosmarinecine utilizing the tandem (4+2)/(3+2) cycloadditions of nitroalkenes
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 90 and chiral vinyl ether (+)-28 afforded a nitronate which underwent facile (3+2) cycloaddition with dimethyl maleate. The resulting nitroso acetal (${-}$)-96 had all the required stereocenters for (${-}$)-hastanecine. The critical unmasking of the nitroso acetal (${-}$)-96 employed a hydrogenolytic cleavage to give the 1-azabicyclo (3.3.0) octane skeleton of (${-}$)-hastanecine.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign