Conformation of Some Anomeric O-Benzylated D-gluco-, L-Ido-, and Hex-4-enopyranoside derivatives

By use of ¹H-NMR. spectroscopy it is shown that in the anomeric tri-O-benzyl derivatives of D -gluco- and L -idopyranosides the C(5) (aliphatic) substituent has a strong tendency to assume an equatorial position. The conformer (⁴C₁ or ¹C₄) with C(6) in equatorial position is favoured, even if other bulky substituents are forced to occupy an axial position; the same effect is observed in some anomeric O-benzylated 6-deoxy-L -idopyranoside derivatives. If, however, the tetragonal configuration of the saturated C(5) is changed into a trigonal (sp²) configuration, e.g. in the anomers of 1,2,3-tri-O-benzylated L -threo-hex-4-enopyranosiduronates, the C(5)–C(6) bond no longer determines the equilibrium of (distorted) pyranoid conformations; the anomeric (‘Eduard-Lemieux’) effect then becomes important in the stabilization of the corresponding half-chair (¹H₂ or ²H₁) conformations.