Configurations and conformations of the oximes, O-methyl oximes, and N-methylnitrones derived from chalcones

Abstract

The E- and Z-oximes, O-methyl oximes, and N-methylnitrones of (E)-chalcone and its 4-MeO, 4-NO₂, 4′-MeO, and 4′-NO₂ derivatives have been synthesised and examined by u.v. and ¹H and ¹³C n.m.r. spectroscopy. It is concluded that all these compounds exist in solution in the antiperiplanar (s-trans) conformation about the Cα–CN bond, with the aryl ring B (on the CN carbon) approximately orthogonal to the plane of the rest of the molecule. Electron densities and total energies have been calculated by CNDO/2 methods for the E- and Z-nitrones, and ¹³C chemical shifts for the αβ-unsaturated nitrone system are discussed in the light of these electron densities and are compared with similar treatments for the parent chalcones. The effects of substituents at C-4 or -4′ on the electronic structure of the nitrone group are discussed in terms of resonance contributors. Rates of thermal isomerisation of E,Z-nitrones to E,E-nitrones, and the position of the equilibrium between them, are reported. The λ^d value for the E-styryl group was found to be ca. 1.1.