Time-Dependent Diastereodivergent Michael Addition Enabled by Phosphazenes Acting as Catalysts and Reactants

A switching of the step order within cascade process enabled design of a diastereodivergent approach to phosphazenopyrrolines with a 1,3‐relationship of stereocenters. The approach is based on a cascade transformation of readily accessible γ‐azidobutyronitriles that includes Staudinger, aza‐Wittig and Michael steps. Stereodivergence is achieved at the Michael step that is self‐catalyzed by phosphazene intermediates and provides construction of quaternary stereocenter. Depending on the type of phosphazene intermediates, generating either via Staudinger reaction or via subsequent intramolecular aza‐Wittig reaction with nitrile and then reacting with a Michael acceptor, either trans‐ or cis‐isomers of the target product can be produced. Thus, diastereoselectivity can be achieved by varying the time of addition for the Michael acceptor. Therefore, three distinct roles were revealed for phosphazenes in this process: catalysts, reactants and stereoselectivity controllers. This stereodivergent strategy was applied for the synthesis of all stereoisomers of the biorelevant tetrahydro‐7‐aza‐indoles.

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