Idopyranose chair conformers

N.m.r. spectroscopy at 220 MHz has shown that the Ci conformation is adopted by fully benzylated a- and jS-D-glucopyranoses inversion of the configuration at C-5 to give the corresponding L-idopyranose derivatives results in an inversion of conformation also. Two groups have shown that derivatives of methyl or benzyl (methyl 4-deoxy-L-rAreo-hex-4-enopyranosid)uronate [e.g. (447)] adopt half-chair conformations that place the anomeric methoxy-group in... 

Since the formation of the anhydrides in acid solution involves an equilibrium, it is more correct to examine the conformations of the end-products of the reaction. An explanation is then apparent. In 1,6-anhydro-/3-D-idopyranose (LXIX), the three free hydroxyl groups are all equatorial in the chair ring known to be present in these anhydrides, and two equatorial hydroxyl groups are present in 1,6-anhydro-(3-o-altropyranose (LXX) and in 1,6-anhydro-/3-D-gulopyranose (LXXI). The known stable types of... 

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. 

Starting with the assumption that all pyranoses exist in a chair form, a hexopyranose will try to adopt a conformation, which allows an equatorial rather than an axial position for the hydroxymethyl (primary hydroxyl) group. Thus in the D-series, a Ci conformation will be preferred, except idopyranose in which an equatorial position for the hydroxymethyl group, Cl, must be at the expense of three axial hydroxyl groups. 

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