Brindani, Nicoletta and Rassu, Gloria Maria Rita and Dell'Amico, Luca and Zambrano, Vincenzo and Pinna, Luigi and Curti, Claudio and Sartori, Andrea and Battistini, Lucia and Casiraghi, Giovanni and Pelosi, Giorgio and Greco, Daniela and Zanardi, Franca (2015) Organocatalytic, asymmetric eliminative [4+2] cycloaddition of allylidene malononitriles with enals: rapid entry to cyclohexadiene-embedding linear and angular polycycles. Angewandte Chemie. International Edition, Vol. 54 (25), p. 7386-7390. eISSN 1521-3773. Article.
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A direct aminocatalytic synthesis has been developed for the chemo-, regio-, diastereo-, and enantioselective construction of densely substituted polycyclic carbaldehydes containing fused cyclohexadiene rings. The chemistry utilizes, for the first time, remotely enolizable π-extended allylidenemalononitriles as electron-rich 1,3-diene precursors in a direct eliminative [4+2] cycloaddition with both aromatic and aliphatic α,β-unsaturated aldehydes. The generality of the process is demonstrated by approaching 6,6-, 5,6-, 7,6-, 6,6,6-, and 6,5,6-fused ring systems, as well as biorelevant steroid-like 6,6,6,6,5- and 6,6,6,5,6-rings. A stepwise reaction mechanism for the key [4+2] addition is proposed as a domino bis-vinylogous Michael/Michael/retro-Michael reaction cascade. The utility of the malononitrile moiety as traceless activating group of the dicyano nucleophilic substrates is demonstrated.
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