Proton magnetic resonance studies of compounds with bridgehead nitrogen. Part XXIX. Configurational and conformational studies with derivatives of perhydro-oxazolo[4,3-c][1,4]thiazine

Abstract

Configurational and conformational assignments have been made to a range of substituted perhydro-oxazolo[4,3-c][1,4]thiazines on the basis of n.m.r. and i.r. spectroscopic data. In contrast to perhydro-oxazolo[4,3-c][1,4]oxazine which exists in solution at room temperature as ca. 86%cis-fused conformation in equilibrium with 14%trans-fused conformation, perhydro-oxazolo[4,3-c][1,4]thiazine is shown to exist as ca. 84% of the trans-fused conformation. This difference in conformational preference is discussed in terms of ring fusion strain. cis-(6-H-8a-H)-6-Methyl-, 6-ethyl-, and 6-phenyl-perhydro-oxazolo[4,3-c][1,4]thiazine exist in solution at room temperature as cis-fused ⇌trans-fused conformational equilibria containing ca. 44, 34. and 63%cis-fused conformers respectively.