Proton magnetic resonance studies of compounds with bridgehead nitrogen. Part 39. Stereochemistry of perhydropyrido[3,2,1-ij][3,1]benzoxazines and the conformational equilibrium for perhydropyrido[1,2-c][1,3]oxazine

Abstract

The four diasteroisomeric perhydropyrido[3,2,1-ij][3,1]benzoxazines have been synthesised and their configurations assigned by ¹H n.m.r. and i.r. spectroscopy and by kinetic studies of N-methylation. Comparisons of the ¹H n.m.r. chemical shifts of protons α to nitrogen in these isomers suggested an equilibrium (CDCl₃, 25°) for perhydropyrido[1,2-c][1,3]oxazine containing ca. 90% of the trans-fused conformer.