We introduce 3-(alkoxycarbonyl-2-alkyliden)-2-oxindoles as pronucleophilic donors in the direct, vinylogous Michael addition to nitroolefins orchestrated by a chiral, bifunctional cinchona-thiourea organocatalyst. This reaction displays excellent levels of gamma-site-, diastereo- and enantioselectivity delivering valuable enantioenriched functionalized oxindoles. Of note, the C-gamma enolization of these pronucleophiles by the organocatalyst generates a multidentate, captodative dienolate that delivers vinylogous adducts with an unprecedented Z-selectivity through a peculiar interaction with the catalyst and the nitroolefin. The optimized procedure is operatively simple: the reaction is conducted in air, at room temperature, with low catalyst loading (up to 1mol%). The synthetic versatility of these Michael adducts is demonstrated by several transformations leading to a valuable quaternary oxindolyl proline analogue and a chiral spirocyclic furoindolone structure. Finally, a mechanistic rationale and a suitable transition state accounting for the observed selectivities are proposed, which are supported by DFT calculations.
(E)-3-(Alkoxycarbonyl-2-Alkyliden)-2-Oxindoles: Multidentate Pronucleophiles for the Organocatalytic, Vinylogous Michael Addition to Nitroolefins / Curti, Claudio; Battistini, Lucia; Sartori, Andrea; Rassu, Gloria; Pelosi, Giorgio; Lombardo, Marco; Zanardi, Franca. - In: ADVANCED SYNTHESIS & CATALYSIS. - ISSN 1615-4150. - 360(2018), pp. 711-721. [10.1002/adsc.201701164]
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