Development of iminium ion cascade methodologies and their application to the synthesis of complex molecules
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In the interests of synthetic efficiency several cascade reactions involving iminium ion intermediates have developed to allow for the rapid assembly of complex molecules from relatively simple starting materials. A cascade methodology for the rapid, diastereoselective synthesis of quinolizidine ring systems has been expanded to encompass a greater range of substrates. The utility of this extension was demonstrated in the efficient syntheses of the tricyclic core structures found in halichlorine and the cylindricine family of alkaloids. Additionally a second cascade sequence has been developed that progresses through an S[subscript N]1 followed by nucleophilic attack on an N-acyl iminium ion to deliver the products of a formal [4+3] cycloaddition. This methodology has been successfully employed to synthesize a [3.3.2] bridgehead system that serves as a key intermediate in a proposed synthesis of (±)-actinophyllic acid. Synthesis of this compound represent progress in the ongoing effort to synthesize this natural product.