CONFORMATIONAL-ANALYSIS .30. A H-1 AND C-13 NMR STEREOCHEMICAL STUDY ON N-METHYL-SUBSTITUTED CIS-FUSED AND TRANS-FUSED OCTAHYDRO-2H-1,3-BENZOXAZINES AND OCTAHYDRO-3,1-BENZOXAZINES

The H-1 and C-13 NMR spectra of cis- and trans-fused N-methyloctahydro -2H-1,3- and -3,1-benzoxazines and their 2-methyl derivatives were ana lysed, All trans-fused derivatives studied existed in biased double ch air conformations with roughly equal contributions of the equatorial a nd axial N-methyl groups, Of the eis-fused isomers, both N-methyl-octa hydro-2H-1,3-benzoxazine and its 2-methyl derivative attain exclusivel y the O-in conformation, whereas N-methyloctahydro-2H-3,1-benzoxazine is not conformationally homogeneous but a ca, 3:1 mixture of the N-in and N-out forms at 198 K and a 7:3 mixture at ambient temperature, Sim ilarly, the 2-methyl derivative of N-methyloctahydro-2H-3,1-benzoxazin e was found to be a cn, 3:2 mixture of C-2 epimers with the N-in and N -out conformations, respectively.