Direct Access to 3,4-Fused Spirooxindoles Through Visible-Light-Driven Diasterioselective [4+2] Cyclization of 3-Methylene Oxindoles

Abstract

A visible light-mediated strategy has been developed to synthesize diastereoselective 3,4-fused spirooxindoles via formal [4+2] cycloaddition of 3-methylene oxindoles without the need of pre-functionalization. The 3-methylene oxindoles, when exposed to visible light in the presence of catalytic amount of CeCl3 under alkaline conditions, afforded the complex 3,4-fused spirooxindole framework with four contiguous chiral centers. The protocol proved to be highly chemo- and regio-selective by affording 3,4-fused spirooxindoles in moderatetogood yields. The protocol also works smoothly under the direct exposure of sunlight. It has been demonstrated that the method is also scalable on gram quantity.