A Visible Light Driven Diasterioselective (4+2) Cyclization of 3-Methylene Oxindole at Unactivated C4 Position: Direct Access to 3-4 Fused Spiroox-indoles

A visible light-mediated strategy has been developed for the unactivated C4 position of 3-methylene Oxindole to access 3,4-fused spirooxindoles via formal [4+2] cycloaddition without the need of pre-fucntionalization. The 3-methylene oxindoles, when exposed to visible light in the presence of catalytic amount CeCl3 under alkaline conditions, afforded the complex 3,4- fused spiro oxindole framework with four contiguous chiral centers. The protocol is highly chemo- and regio-selective by affording 3,4-fused spirooxindoles in moderate and good yields. The protocol also works smoothly under direct exposure to sunlight. It has been demonstrated that the method is also scalable on gram quantity. The synthetic strategy provides a new avenue for constructing complex molecular architectures with precision and efficiency.