Proton magnetic resonance studies of compounds with bridgehead nitrogen. 37–a comparison of the positions of conformational equilibria in 1‐methylperhydro‐oxazolo[3,4‐a]pyridines and in 1‐methylperhydrothiazolo [3,4‐a]pyridines

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Abstract
cis(1‐H, 8a‐H)‐1‐Methylperhydro‐oxazolo[3,4‐a]pyridine and cis(1‐H, 8a‐H)‐1‐methylperhydrothiazolo[3,4‐a]pyridine both adopt exclusively the trans‐fused conformation in carbon tetrachloride solution at room temperature. Both parent unsubstituted systems exist under similar conditions as equilibria containing c. 67% (oxazolo compound) and 64% (thiazolo compound) of the trans‐fused conformation. In marked contrast to these similar positions of conformational equilibria in both systems the trans(1‐H,8a‐H)‐1‐methylperhydrooxazolo[3,4‐a]pyridine exists as c. 73% trans‐fused in equilibrium with a cis‐fused conformation whereas the trans(1‐H, 8a‐H)‐1‐methylperhydrothiazolo[3,4‐a]pyridine exists almost exclusively in a cis‐fused ring conformation. These differences in conformational equilibria are explained in terms of changes in non‐bonded interactions.