Addition reactions of heterocyclic compounds. Part LXV. Synthesis, tautomerism, and rearrangement of some 2H- and 4H-quinolizine esters
Abstract
Tetramethyl 4H-quinolizine-1,2,3,4-tetracarboxylates on selective alkaline hydrolysis and decarboxylation give tri-methyl 2H- and 4H-quinolizine-1,2,3-tricarboxylates, which can be interconverted and which were identified from their u.v. and n.m.r. spectra. The non-equivalence of the 4-protons in the 4H-isomers at low temperatures is associated with an sp2-hybridised nitrogen atom and restricted rotation of the ester groups. The formation of indolizines from all these quinolizines with nitric acid or with phenol has been investigated, and mechanistic schemes are proposed. Some 2H-quinolizines have been synthesised from ethyl 2-(2-pyridyl)cinnamate and acetylene-carboxylates.