PIDA/BF3·OEt2-Mediated Cyclization of Bis(indoylmethyl)benzene Sulfonamides: Assembly of Di-indoloazepines, Spiro Carbolines, and Heterotriquinane Frameworks.

This study presents a metal-free, hypervalent iodine-(III)-promoted oxidative cyclization of N,N-bis(indol-2-ylmethyl)benzenesulfonamide to access isomeric di-indoloazepine under mild conditions. During the synthetic exploration of di-indoloazepine, we observed several unexpected transformations, including the rearrangement and oxidation of the diindoloazepine framework, that generate isomeric spiro β/γ-carbolines and unusual bi-indole-2-carboxaldehydes. In contrast, the oxidative cyclization of an isomeric N,N-bis(indol-3-ylmethyl)benzenesulfonamides yielded typical di-indoloazepines and highly strained heterotriquinane frameworks involving unconventional cyclization. This protocol exhibits a broad substrate scope and reasonable functional group forbearance. Reasonable mechanisms for the formation of these unusual heterocycles have been proposed. The structure of representative heterocyclic frameworks was confirmed by single-crystal X-ray diffraction analyses.