Structural Characterization of Isomeric 2,3,5‐Substituted Tetrahydropyrrolo[3,4‐d]isoxazole‐4,6‐diones Prepared by Cycloaddition of N‐Methyl‐C‐arylnitrones to N‐Phenyl‐ or N‐Methylmaleimide.

Abstract

1,3-Dipolar cycloaddition reactions of N-methyl-C-arylnitrones with N-phenyl-or N-methylmaleimide were studied. The reaction of p-dimethylamino-, 4-benzyloxy-3-methoxy-, p-nitro-and p-chloro-substituted phenylnitrones with N-phenylmaleimide gave c i s and t r a n s cycloadducts but that of the corresponding phenylnitrones with N-methylmaleimides only the cis adducts in the case of p-dimethylamino and 4-benzyloxy-3-methoxy substitution. All cis adducts attain a biased conformation whereas the trans forms are shown (by 1 H NMR at 233 K and 13 C NMR at 208 K) to be mixtures of two invertomers, namely o-(N-lone pair antiperiplanar to 3H; minor) and i-conformations (3H-C-C-3aH dihedral angle close to 90°; major). PM3 and DFT calculations at the B3LYP/6-31G(d) level of theory prove qualitatively that these two conformers of the trans adduct are of comparable stability and represent energy minima.