Analysis of the I3C n.m.r. spectrum of 2-methoxytetrahydropyran [(I ) + (2)] in the range 143 to 165 K, and of the H n.m.r. spectrum of the equilibrated mixture of cis-and trans-2-methoxy-4-methyltetrahydropyran at 245 and 273 K gives [(2)]/[(1)] ca. 0.26, AH0 ca. 0, and ASo[(l) -+ (Z)] ca.-2.7 cal K-I mol-1 ( -1 1.3 J K-I mol-l) for the solvent CFCI3-CDCI3 (85:15 v/v); the smaller entropy of the equatorial conformation is consistent with a stronger exo-anomeric effect in that conformation.

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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 (AAHO)' of +2.12, +0.75, +0.61, and -0.33 kcal mol-1 (1 cat = 4.184 J) for substituents CI, OMe, OH, and NHMe respectively; this sequence confirms the importance of antiperiplanar n-a* stabilisation but supports the suggestion that endo-anomeric and exu-anomeric effects which occur in the same conformation are competitive.Although the anomeric effect1.2 has generally'-5 been evaluated as (AG01-,2 -AGOh4) there are advantages in using AH"

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