Construction of 2,3-Dihydrofuran Cores through the [3+2] Cycloaddition of Gold alpha-Carbonylcarbenoids with Alkenes

Treatment of 2-epoxy-1-alkynylbenzenes with electron-rich alkenes and a [AuCl(PR3)]/AgX catalyst in CH2Cl2 led to the formation of 2-alkenyl-1-(2,3-dihydrofuran-4-yl)benzenes. This transformation comprises of a gold-catalyzed redox reaction to form a gold alpha-carbonylcarbenoid initially, which then reacts in situ with an alkene in a [3+2] cycloaddition. A range of alkenes are amenable to this tandem reaction, amongst them alpha-substituted styrenes, enol ethers, and 2,3-dimethvlbutadienes. Deuterium-labeling experiments suggest a stepwise mechanism for the alpha-carbonylcarbenoid/alkene [3+2] cycloaddition. The resulting 2,3-dihydrofuran products allow access to diverse oxacyclic compounds through a stereoselective eneoxonium reaction initiated by treatment with HOTf (1 mol %, Tf = trifluoromethanesulfonyl). A stepwise pathway is proposed for this reaction. The feasibility for direct transformation of 2-alkenyl-1-alkynylbenzenes into the desired 2,3-dihydroluran products through initial m-chloroperbenzoic acid oxidation, followed by the addition of gold catalyst and alkene, has also been demonstrated.