Tandem methodology for heterocyclic synthesis
A. Padwa
Department of Chemistry, Emory University, Atlanta, GA 30322, USA
Abstract: Tandem methodology for heterocyclic synthesis represents
a powerful approach for the rapid buildup of molecular complexity from
potentially simple starting materials. Work from our laboratory has
shown that the rhodium(II)-catalyzed cyclization cascade of alpha-diazo
imides represents an effective method for the synthesis of a variety
of heterocyclic systems. As an extension of these studies, we became
interested in using a linked Pummerer/N-acyliminium ion cyclization
sequence since we felt that this combination offers unique opportunities
for the assemblage of complex target molecules. A synthetic method that
combines transformations of different reaction types significantly broadens
the scope of such procedures in synthetic chemistry. Alpha-thiocarbocations
generated from the Pummerer reaction of beta-phenylsulfinylmethyl-alpha,
beta-unsaturated amides can be intercepted by the adjacent amido group
to produce transient amino-substituted furans which undergo subsequent
Diels�Alder cycloadditions. Using this domino amido Pummerer/Diels�Alder
cascade, we were able to assemble novel polycyclic systems in a single
operation. The key step in the process involves the generation of a
reactive N-acyliminium ion by fragmentation of an amino-substituted
[4+2]-cycloadduct. The successful synthesis of a number of alkaloids
by this sequence of reactions reveals the usefulness and importance
of this unique domino cascade. Application of the process for the preparation
of the stenoma alkaloid stenine was recently carried out in our laboratory.
* Plenary lecture presented at the 5th Florida Heterocyclic
Conference, Gainesville, FL, USA, 7-10 March 2004. Other presentations
are published in this issue, pp. 1933-1983.
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