Aldopyranoses orientation

Fig. 2.—The Orientation Around the O-S-C-1 Bond in the Axial and Equatorial Forms of Aldopyranoses.

The conformations of the aldopyranoses in aqueous solution have been investigated by n.m.r. spectroscopy " (see Table IV). The chair form favored by the various aldohexopyranoses appears to be controlled by the tendency of the 5-(hydroxymethyl) group to assume the equatorial orientation. Hence, all the /3-D anomers exist preponderantly in the CJ(d) conformation, as the alternative, 1C(d) conformation appears to be highly destabilized by the si/n-diaxial interaction between the 1-hydroxyl and 5-(hydroxymethyl) groups. Although this interaction is not found in the 1C(d) conformation of the a-D anomers, most of the a-D anomers nevertheless adopt the C1(d) conformation, presumably because of a favorable anomeric effect (see p. 103). Solutions of a-D-idopyranose and a-D-altropy-ranose appear to contain appreciable proportions of each chair con-former at equilibrium. 

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