EXPERIMENTAL STUDIES OF THE ANOMERIC EFFECT. PART I. 2-SUBSTITUTED TETRAHYDROPYRANS.

Article abstract of DOI:10.1016/S0040-4020(01)86912-X

Variable temperature (1)H and (13)C n.m.r. studies in CFCl3/CDCl3 of equilibria in 2-substituted- and 2-substituted-4-methyl-tetrahydropyranes have given conformational enthalpy differences and conformational entropy differences for chloro, methoxy, hydroxy and methylamino substituents.For ΔH^o_a->e, the values obtained, in kcal /mol, were 1.67 (Cl), 0.03 (OCH3), -0.63 (OH) and -1.75 (NHCH3); for ΔS^o_a->e the values obtained, in cal K^-1mol^-1 were -1.69 (Cl), -2.52 (OCH3), -2.50 (OH) and -0.60 (NHCH3).The trend in ΔH^o values confirms the importance of antiperiplanar n-?^* stabilisation as a contributor to the explanation of the anomeric effect, and supports a suggestion that endo- and exo-anomeric effects which occur in the same conformation are competitive.A variable temperature (13)C n.m.r. study of (Me-(13)C)-4-methyl-tetrahydropyran in CD2Cl2 has given a conformational enthalpy difference (ΔH^o_a->e) of -1.86 kcal/mol and a conformational entropy difference (ΔS^o_a->e) of -0.07 cal K^-1mol^-1 for a methyl substituent at the 4-position of a tetrahydropyran.