The H-1 and C-13 NMR spectra of 26 2,3-dihydro-[1,5] benezoxazepin-4(5H)-one derivatives and four thiono analogues are interpreted in terms of conformational equilibria. Similar to 1,3,4,5-tetrahydro-2H-1,5-benzodiazepin-2-ones, these compounds favour boat conformations of the seven-membered ring. In the lactams the conformational energy difference is very low, whereas a difference of 0.3 kcal mol-1 is recorded for the thionolactams, the conformation with the quasi-axial methyl group also being preferred.
