Convenient access to 5-membered cyclic iminium ions: evidence for a stepwise [4 + 2] cycloaddition mechanism
Jared L Freeman1, Margaret A Brimble, Daniel P Furkert
1School of Chemical Sciences, The University of Auckland, Symonds Street, Auckland 1010, New Zealand. m.brimble@auckland.ac.nz d.furkert@auckland.ac.nz.
Abstract:
In situ generation and reaction of novel 5-membered N-tosyl cyclic α,β-unsaturated iminium ions from readily prepared stable precursors is demonstrated. Formal iminium Diels-Alder cycloaddition proceeded in good yield via a stepwise rather than concerted cycloaddition process, confirmed through the isolation of a Mukaiyama-Michael type intermediate. Relative stereochemistry was determined upon subsequent intramolecular cyclisation under Lewis acid catalysis to afford formal endo 5,6-spirobicyclic adducts, as confirmed by crystallography. Further synthetic elaboration towards complex molecular scaffolds based on the dinoflagellate metabolite portimine, a potent apoptosis inducer, were also developed.
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