Synthesis of Spiro-oxindoles (Spiroindolones) via Oxidative Ring Contraction Approach

Ring contraction reaction is among the several vital strategies in organic synthesis that can provide very diversified and useful molecular architecture. Among these strategies, the oxidative ring contraction is of special interest as it can give a more straightforward way of designing and synthesizing several complex natural products, specifically the oxindole alkaloids and some newer oxindole analogs. Even this methodology can be expanded to prepare dispiro-bisoxindoles-type compounds besides the dipolar cycloaddition methodology. In this review, we have tried to collect the reports on oxidative ring contraction reactions using several oxidants, like tert-Butyl hypochlorite, oxaziridine, dimethyldioxirane, N-bromosuccinimide, etc., in different reaction conditions for the synthesis of spiro-oxindole natural products as well as newer synthetic analogs.