Proton magnetic resonance studies of compounds with bridgehead nitrogen atoms—XX: The NMR spectra and stereochemistry of some hexahydropyrido[2, 1-c] [1, 4] oxazin-3(4H)-ones and related compounds

A series of substituted hexahydropyrido [2,1-c] [1,4] oxazin-3(4H)-ones has been synthesised, and the configurations of these bicyclic lactones assigned utilising chemical and spectral data. All the compounds adopt trans-fused conformations and the conformation of the lactone ring is discussed with reference to the magnitude of the geminal coupling constant of the N? CH₂? C(O)? O protons, and the vicinal couplings between the angular proton and the methylene protons adjacent to the ring oxygen atom. The lactone ring conformation is shown to differ slightly from the half chair conformation described for some monocyclic δ -lactones. The synthesis and NMR spectra of some related compounds possessing the bridgehead N? CH₂? C(O)? O system are discussed and these compounds are also shown to adopt a trans-fused ring conformation.