Curti, Claudio and Rassu, Gloria Maria Rita and Zambrano, Vincenzo and Pinna, Luigi and Pelosi, Giorgio and Sartori, Andrea and Battistini, Lucia and Zanardi, Franca and Casiraghi, Giovanni (2012) Bifunctional cinchona alkaloid/thiourea catalyzes direct and enantioselective vinylogous Michael addition of 3-alkylidene oxindoles to nitroolefins. Angewandte Chemie, Vol. 124 (25), p. 6304-6308. eISSN 1521-3757. Article.
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3-Alkylidene oxindoles (methyleneindolinones), be they natural or man-made substances, occupy a preeminent position among the various classes of chemically and medicinally relevant small-molecule scaffolds. Their plural functional architecture featuring a lactam carbonyl flanked by a highly substituted exocyclic double bond renders them enabling intermediates to be elaborated into a myriad of useful nitrogen heterocycles of varied complexity. For example, 3-alkylidene oxindoles can be viewed as electrophilic Michael acceptors, which react with carbon-centered anions to give bsubstituted oxindoles of type A (Scheme 1a). In addition, they can act as electron-poor components in synchronous (Scheme 1b) or stepwise (Scheme 1c) cycloadditive functionalizations, thus opening the way to a wide range of highly valuable 3,3-spirocyclic structures of type B or C. Whereas these protocols have been largely pursued and formed the basis of many synthetic achievements, an “umpolung” option could also be envisaged (Scheme 1d), and capitalizes on the vinylogous pro-nucleophilic character of the alkyl group attached at the b-position of the ylidene. By reacting with the proper acceptors, these nucleophiles furnish olefinic oxindoles of type D, which are functionalized at the most distant point of the molecule (Cγ).
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