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Soft Lewis Acid-Catalyzed Cycloisomerization of Epoxy Alkynes
ZEZHOU WANG, GARRETT GIBBONS, SHIYUE FANG*
Department of Chemistry, Michigan Technological University, Houghton, MI
Some transition metals have been found to act as soft Lewis acids to activate alkynes toward nucleophilic attack. The “soft” character of these large and polarizable cations makes them to display significant affinity to the Π electrons of alkynes. For example, Pt (II), Au(I), Pd(II), and Bi(III) catalysts can catalyze similar reactions to achieve a variety of cycloisomerization reactions. In one of our projects, PtCl2 was used as the catalyst to catalyze the intramolecular reaction between an alkyne and an epoxide; the alkyne is activated by the metal, then the lone pair on the oxygen atom of the epoxide attacks the alkyne, which gives a new bridged-bicyclic product. The bicyclic product contains a vinyl ether function, which can be hydrolyzed to give products that contain a seven-membered ring. This method has been applied to several different substrates and similar results have been obtained. The methodology may find application in the synthesis of bioactive natural products. |