The anomeric and exo-anomeric effects in 2-methoxytetrahydropyran

Abstract

Analysis of the ¹³C n.m.r. spectrum of 2-methoxytetrahydropyran [(1)⇌(2)] in the range 143 to 165 K, and of the ¹H 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, ΔH°ca. 0, and ΔS°[(1)→(2)]ca.–2.7 cal K^–1 mol^–1(–11.3 J K^–1 mol^–1) for the solvent CFCl₃–CDCl₃(85:15 v/v);the smaller entropy of the equatorial conformation is consistent with a stronger exo-anomeric effect in that conformation.