Rassu, Gloria Maria Rita and Zambrano, Vincenzo and Pinna, Luigi and Curti, Claudio and Battistini, Lucia and Sartori, Andrea and Pelosi, Giorgio and Casiraghi, Giovanni and Zanardi, Franca (2014) Direct and enantioselective vinylogous Michael addition of α-alkylidenepyrazolinones to nitroolefins catalyzed by dual Cinchona alkaloid thioureas. Advanced Synthesis & Catalysis, Vol. 356 (10), p. 2330-2336. ISSN 1615-4150. eISSN 1615-4169. Article.
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While several protocols exist for the asymmetric functionalization of pyrazolinones at the α-position relying on nucleophilic addition or annulation procedures, use of α-alkylidene electron-rich analogues in asymmetric vinylogous coupling to carbon electrophiles is substantially an uncharted domain. We now report, for the first time, that alkylidenepyrazolinones carrying an enolizable carbon at the γ-position efficiently participate in direct and asymmetric, catalytic vinylogous Michael-type additions to nitroolefins providing the expected adducts in high yields, with complete γ-site selectivity and with extraordinary levels of enantio-, diastereo-, and geometrical selectivities. Both enantiomeric adducts were equally accessed by employing a quasi-enantiomeric quinine- or quinidine-based thiourea catalyst pair.
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