Nitrogen bridgehead compounds. Part 46. Synthesis and stereochemistry of 9-formyl-6,7,8,9-tetrahydro-2-oxo-2H- and 4-oxo-4H-pyrido[l,2-a]pyrimidine-3-carboxylates and their homologues

The structures of the title compounds, bearing five-, six-, seven-, or eight-membered rings, have been investigated by u.v. and 1H n.m.r. spectroscopy. In the 4-oxo derivatives the enol–enamine–imine tautomerism depends greatly on the ring size, whereas in the 2-oxo series the enamine tautomer always predominates, independently of the ring size. In the 4-oxo series the five-membered homologues are mainly in the enol form, and the six-membered ones in the enamine form. The seven-membered derivative (21) exists as a mixture of the enamine and imine tautomers. In the eight-membered homologue (22) the imine form is preferred. The enol tautomer of the five-membered 4-oxo derivatives has the Z-configuration in chloroform, and the E-configuration in dioxane, whereas in ethanol and dimethyl sulphoxide E–Z isomeric mixtures are present, with concentration-dependent isomer ratios.