endo-Anomeric and exo-anomeric effects in 2-substituted tetrahydropyrans

Determinations by variable temperature 13C and/or 1H n.m.r spectroscopy of equilibrium constants for inversion of 2-substituted tetrahydropyrans and comparisons with substituted cyclohexanes have given ‘anomeric effects (ΔΔH°)’ of +2.12, +0.75, +0.61, and –0.33 kcal mol–1(1 cal = 4.184 J) for substituents Cl, OMe, OH, and NHMe respectively; this sequence confirms the importance of antiperiplanar n-σ* stabilisation but supports the suggestion that endo-anomeric and exo-anomeric effects which occur in the same conformation are competitive.