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Molecules 2002, 7(5), 456-464; doi:10.3390/70500456
Article

Niobium Pentachloride Activation of Enone Derivatives: Diels-Alder and Conjugate Addition Products

* ,  and
Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, Av. Bandeirantes 3900, 14040-901 – Ribeirão Preto – SP, Brazil
* Author to whom correspondence should be addressed.
Received: 19 March 2002 / Revised: 15 May 2002 / Accepted: 17 May 2002 / Published: 31 May 2002
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Abstract

Niobium pentachloride has proven to be a powerful activating agent for Diels-Alder or conjugate addition reactions of cycloenones. The Diels-Alder product was obtained only with an unsubstituted enone (cyclohexenone) and the highly reactive diene cyclopentadiene; substituents in the b-position of enones seem to prevent Diels-Alder reaction: oxygenated substituents favor the formation of vinyl chlorides (ethyl ether or dichloromethane as solvents) or enol ethers (ethyl acetate as solvent), while a methyl substituent prevents any kind of transformation with NbCl5. Less reactive dienes, furan and 2-methylfuran gave the conjugate addition products of the furan ring to the enone system.
Keywords: Niobium pentachloride; Lewis acid; Diels-Alder reaction; vinyl chlorides Niobium pentachloride; Lewis acid; Diels-Alder reaction; vinyl chlorides
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Constantino, M.G.; Júnior, V.L.; Da Silva, G.V.J. Niobium Pentachloride Activation of Enone Derivatives: Diels-Alder and Conjugate Addition Products. Molecules 2002, 7, 456-464.

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