The 1,3-dipolar cycloadditions of the cyclic nitrones 2-phenyl-3H-indole-3-one N-oxide (2-phenylisatogen, 1a) and 2-phenyl-3-phenylimino-3H-indole N-oxide (1b) to several electron-deficient alkenes 2a-d are reported. All the reactions studied are highly diastereoselective: the regioisomers with the ethoxycarbonyl group attached to the 4-position of the isoxazolidine moiety are the only products isolated, except in the reactions of nitrones 1a and 1b with ethyl acrylate 2a where mixtures of the two different regioisomers are obtained. Crystal structures of compounds 3b, 5a, 5b, 9a and 10b were determined by X-ray analysis, while the structure of the other cycloadducts was determined by spectroscopic analysis. Ab initio calculations were carried out showing that product distribution may be influenced by kinetic rather than thermochemical factors. Ab initio calculations of the H-1 NMR shielding tensor were performed on compounds 3b, 4b, 5a and 6b and the chemical shifts obtained are in perfect agreement with the experimental ones.
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