Substituent effects in various alkyl derivatives of 9aH-quinolizine-1,2,3,4-tetra-carboxylate studied by 13C NMR spectroscopy and X-ray analysis

Abstract

The ¹³C NMR spectra of 22 alkyl-substituted 9aH-quinolizine-1,2,3,4-tetracarboxylates have been obtained and X-ray analyses have been performed for three of them. The chemical shift differences between the parent 9aH-quinolizine and the methyl-substituted compounds can only be interpreted in terms of the usual α and β effects for 8-methyl-9aH-quinolizine. 6-, 7-, 9-, and 9a-methyl substituents cause not only a very large deshielding of the carbon at the position of substitution together with shielding changes at adjacent atoms, but also influence the shieldings of the other carbons in both rings of the compounds under study. The observed changes are interpreted in terms of steric hindrance between the methyl groups of ring B and the ester groups of ring A, and hyperconjugative effects introduced by the methyl groups.