The conformational equilibria in a cyclohexene-like system: 3,6-dihydro-1,2-oxazine

Abstract

Low-temperature n.m.r. spectra of some 3,6-dihydro-1,2-oxazines show peaks due to cis- and trans-isomers. Peak-area measurements by planimetry show that 2-methyl and -ethyl substituents prefer to be equatorial by ca. 1.0 kcal mol^–1(1 cal = 4.184 J), a 2-isopropyl group has a larger equatorial preference (>1.3 kcal mol^–1), and a 6-phenyl group prefers to be equatorial by ca. 1.3 kcal mol^–1. There are substantial solvent effects upon the equilibria.