Lewis Acid-Catalyzed Malonate Addition onto 3-Hydroxy-2-oxindoles: Mechanistic Consideration and Synthetic Approaches to the Pyrroloindoline Alkaloids.

Metal triflate-catalyzed reactions of 3-hydroxy-2-oxindoles with a variety of malonates have been developed under mild conditions. The reaction afford a variety of 2-oxindoles sharing a C-3 quaternary center at the pseudobenzylic position in an operationally simple procedure. Control experiments using enantioenriched 3-hydroxy/methoxy 2-oxindoles (91% ee) afforded a malonate addition product in racemic form, clearly suggesting that the reaction proceeds through an in situ generated 2 H-indol-2-one intermediate (4a). Synthetic potential of this methodology has been shown by approaching the cyclotryptamine alkaloids linked with the aryl group at the pseudobenzylic position.