Formal [2+2+2] Cycloaddition Strategy Based on an Intramolecular Propargylic Ene Reaction/Diels-Alder Cycloaddition Cascade
Author(s)Robinson, Julia M.; Sakai, Takeo; Okano, Katsuhiko; Kitawaki, Takafumi; Danheiser, Rick Lane
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A formal, metal-free, [2 + 2 + 2] cycloaddition strategy is described based on a cascade of two pericyclic processes. The first step involves an intramolecular propargylic ene reaction of a 1,6-diyne to generate a vinylallene, which then reacts in an inter- or intramolecular Diels−Alder reaction with an alkenyl or alkynyl dienophile. Reactions involving unsymmetrical alkenyl and alkynyl dienophiles proceed with good to excellent regioselectivity, and the diastereoselectivity in the Diels−Alder step is also high, with endo cycloadducts produced as the exclusive products of the reaction. In the case of alkynyl dienophiles, [4 + 2] cycloaddition initially generates an isotoluene-type intermediate that isomerizes to the isolated aromatic product upon exposure to a catalytic amount of DBU at room temperature. The mechanism of several earlier fully intramolecular related transformations have been shown to involve an analogous process rather than the diradical-mediated pathways proposed previously.
Experimental procedures, characterization data, and [superscript 1]H and [superscript 13]C NMR spectra for all new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.
DepartmentMassachusetts Institute of Technology. Department of Chemistry
Journal of the American Chemical Society
American Chemical Society
Robinson, Julia M. et al. “Formal [2 + 2 + 2] Cycloaddition Strategy Based on an Intramolecular Propargylic Ene Reaction/Diels−Alder Cycloaddition Cascade.” Journal of the American Chemical Society 132 (2010): 11039-11041. Web. 16 Dec. 2011. © 2011 American Chemical Society
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