The conformational analysis of perhydropyrido[1,2-c][1,3]oxazines, perhydropyrido[1,2-c][1,3]thiazines, and perhydropyrido[1,2-c] pyrimidines

Abstract

Dipole moments of perhydropyrido[1,2-c][1,3]oxazine, 2-t-butylperhydropyrido[1,2-c]pyrimidine, and syn-perhydrodipyrido[1,2-c : 2′,l′-f] pyrimidine indicate their exclusive existence in the trans-fused conformation. Perhydropyrido[1,2-c][1,3]thiazine and 2-methylperhydropyrido[1.2-c]pyrimidine also exist to a significant extent in this conformation. The effects of cis(4a-H, 5-H)-5-methyl, cis(4a-H, 6-H)-6-methyl and cis and trans(4a-H, 7-H)-7-ethyl groups in the perhydropyrido[1,2-c][1,3] oxazine system are discussed, and the conformational behaviour of this series is rationalized. These results are compared with estimates of the positions of conformational equilibria based on n.m.r. data.