Idopyranose conformation

Problem 22.32 Draw the more and less stable conformations of a) /3-o-mannopyranose (Problem 22.11), (f>) a-D-idopyranose (idose is the C epimer of gulose see Problem 22.27), (c) /3-L-glucopyranose (/3-l- and /3-Dare enantiomers). Explain each choice. ... 

The equilibrium compositions of aqueous solutions of some aldoheptoses are listed in Table III. Because the additional carbon atom in the side chain does not introduce additional steric interactions, the composition of solutions of heptoses is similar to that of the homomorphous hexoses, with only one exception, namely, n-glycero-n-ido-heptose, 92 a-D-Idopyranose in solution is a mixture of the 4Ci and 1C4 conformant) S. J. Angyal and R. J. Beveridge, Carbohydr. Res., 65 (1978) 229-234. 

Exercise 20-5 Make a sawhorse drawing of what you believe to be the favored conformations of a- and /3-d -ribopyranose and of a- and /3-d-idopyranose. 

All stereocenters in 1,6-anhydrohexopyranoses are of inverted orientation compared to those in the parent 4Ci(d) or 1C4(l) conformations of the corresponding hexopyranoses for example, see 21, 23, and l,6-anhydro-/J-D-glucopyranose (22). In chemical properties, these compounds resemble to a certain degree the methyl /f-D-hexopyranosides. They are relatively stable in alkaline media, but are readily hydrolyzed by acids. In aqueous acid solution, an equilibrium is established between the 1,6-anhydrohexo-pyranose and the corresponding aldohexose, whose composition correlates with expectations from conformational analysis and calculations from thermodynamic data.121 Extreme values, 0.2 and 86%, are observed respectively with 1,6-anhydro-/f-D-glucopyranose (22) and l,6-anhydro-/f-D-idopyranose (the latter has all hydroxyl groups in equatorial disposition). 

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. 

In an extension to aldohexose peracetates, the favored form of a-D-idopyranose pentaacetate in solution in acetone or chloroform was found to be the CJ(d) conformer, having four substituents axially and one equatorially attached. a-D-Altropyranose pentaacetate had previously been reported also to adopt the CJ(d) conformation, having three substituents axial and two equatorial. Consideration of the observed proton spin-couplings allowed the exclusion of the boat or skew forms as major contributors to the conformational populations for both compounds near room temperature. 

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. 

---