Synthesis of 2-alkenylidene-3-oxoindoles by gold-catalyzed cascade reaction

Homogeneous gold-catalyzed cascade reactions are important and useful process for the synthesis, inter alias, of highly functionalized heterocycles and natural products. In this context, we recently investigated the use of functionalized 4H-furo[3,2-b]indoles for the synthesis of more complex structures. Thus, 4H-furo[3,2-b]indoles were explored for the synthesis of 2-spirocyclopentane-1,2-dihydro-3H-indolin-3-ones through the addition of gold-activated allenes to the C2 furan moiety. Herein we present the evolution of this work involving the C2-C3 bond of 4H-furo[3,2-b]indoles in the reaction with a gold(I)-carbene generated in situ from propargyl esters. In particular, reaction between 4H-furo[3,2-b]indoles and propargyl esters, performed in presence of gold(I) catalysts, leads to the formation of 2-alkenylidene-3-oxoindoles in a cascade process that includes a double ring opening step. A series of 2-alkenylidene-3-oxoindoles were obtained in good to excellent yields. Optimization of the reaction conditions, scope and proposed reaction mechanism will be illustrated in the poster, together with preliminary photophysical studies.