EXPERIMENTAL STUDIES OF THE ANOMERIC EFFECT .6. RING INVERSION EQUILIBRIA IN CYCLOHEXANE, TETRAHYDROPYRAN AND PIPERIDINE RINGS SUBSTITUTED BY A CARBOMETHOXY OR A CYANO GROUP

The ring inversion equilibrium in carbomethoxycyclohexane is compared with those in 2-carbomethoxytetrahydropyran and 2-carbomethoxypiperidine through a variable temperature nmr study of the positions of equilibria. The derived DELTA-H-degrees(a-->e) values (kcal mol-1) of -1.24 (cyclohexane), -1.69 (tetrahydropyran) and -0.54 (piperidine) can be rationalised in terms of competition between steric effects and endo-anomeric effects of the ring heteroatom. Variable temperature studies have also given DELTA-H-degrees(a-->e) values (kcal mol-1) for ring inversions in cyanocyclohexane, 2-cyanotetrahydropyran and 2-cyanopiperidine as -0.18, +0.36 and +2.22. Steric effects are small for CN, and the trend in DELTA-H-degrees values is consistent with the expected increase in stabilising endo-anomeric effect along the series C, O, N.