Hexopyranoses conformation

Chemical shifts and chemical-shift differences between pairs of ano-mers in the monodeoxymonofluoro-D-hexopyranose series were observed to be strongly dependent on the stereochemistry of electronegative substituents in the molecule. In consequence, no a priori distinction between anomers can be made on the sole basis of their respective fluorine shifts. The use of fluorine shifts as a probe for structural and conformational... 

Conformational Preferences for Solvated Hydroxymethyl Groups in Hexopyranose Structures... 

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). 

D. Horton and J. D. Wander, Calculation of molecular rotation by summation of partial conformational contributions. Part 2. Rotation of the l,6-anhydro-deoxy-/i-D-hexopyranoses, 2,7-anhydro-/i-D-heptulopyranoses, and their acetates, Carbohydr. Res., 14 (1970) 83-94. 

T. Tmka, M. Cerny, M. Budesinsky, and J. Pacak, The synthesis of 3-amino-3-deoxy-D-glucose (Kanosamine) and its 1,6-anhydro derivative. Conformation of amino derivatives of l,6-anhydro-/S-D-hexopyranoses, Collect. Czech. Chem. Commun., 40 (1975) 3038-3045. 

Most aldohexopyranoses exist in a chair form in which the hydroxymethyl group at C(5) assumes an equatorial position. All the P-D-hexopyranoses exist predominantly in the 4Ci form since the alternative C4 conformer involves a large unfavourable xyn-diaxial interaction between the hydroxymethyl and anomeric group (Figure 1.8). Most of the a-D-hexopyranosides also adopt the 4Ci conformation preferentially. Only ot-idopyranoside and a-D-altropyranose show a tendency to exist in the C4 conformation, and they coexist with the alternative 4Ci conformations according to H-NMR (hydrogen nuclear magnetic resonance) spectroscopy studies. 

The conformational equilibrium in hexopyranoses is considerably influenced by the configuration of the various centers of chirality. In / -D-glucopyranose (LXXXVIa) all substituents can simultaneously adopt an equatorial position, hence the considerable stability of this conformer. In contrast, a-D-isopyranose (LXXXVII)... 

In contrast, the primary 0(6) -H hydroxyl group can rotate about the exocyclic C(5)-C(6) bond. As with hexopyranoses discussed in Part II, Chapter 13, there are two preferred conformations + sc and - sc for the 0(6) - C(6) - C(5) - 0(5) torsion angle which are determined by the perio- or 1,3-diaxial or Hassel-Ottar effect [499, 501, 502, 560]. The ap conformation has never been found in the cyclodextrin... 

Anhydro- 8-D-hexopyranoses are in equilibrium with the D-hexopyranoses and water when the latter compounds are heated in aqueous acid [181]. On formation of a 1,6-anhy-dro ring, the normal Ci conformer is forced into a C4 conformation. The amount of the... 

On the other hand, pentopyranoses derivatives in solution do not exhibit the conformational effects observed with hexopyranose derivatives. In a chloroform solution of the g-D-ribo derivative, the conformation with an axial chlorine is... 

Scheme 2 Schematic representation of a hexopyranoses (a) and a 1 —>6 linked disaccharide (b) showing the uj torsion angle. Schematic diagram of the gt, tg, and gg staggered conformers around the C5-C6 bond.

More broadly speaking, we can anticipate the following four orientations around the cyclic oxygen of a D-pyranose I and III for trans derivatives, and II and IV for cis (Fig. 2.9). With the exception of idose (and perhaps altrose), all of the trans derivatives, in this case the monocyclic a-D-hexopyranoses and their derivatives, exist under the only observable conformation, d- Ci, which corresponds to the local conformation I, doubly stabilized by the anomeric and coplanar effects. 

In fact, the conformation of pyranoses is dominated by two effects, not present in the cyclohexane, which appear at positions 2 and 6 of the oxane. One of them is characteristic of hexopyranoses and I propose that we call this the coplanar effect in order not to imply a particularly restrictive structure by using the name of an effect already present in methoxyethane. The other effect, present in all pyranoses, is referred to as anomeric. This name, taken from the nomenclature of sugars because it was first recognized in this family, in fact disguises its general nature since it is also present in methyl chloromethyl ether. The consequences of these effects can be modulated by cyclohexane-type interactions, but not to the point where more than a qualitative discussion is necessary. 

The vigorous anchoring of nearly all the hexopyranoses in D- C (l- C4) conformations by the coplanar effect brings about a certain rigidity of the oligosaccharide chains. It is possible that the introduction of ido residues with a flexible conformation into certain sites creates the flexibility indispensible for certain functions. 

A characteristic feature of these compounds is their steric rigidity in the crystalline state,4 as well as in solution,5 the l,6-anhydro-/3-D-hexopyranoses adopt exclusively the 4C4(d) conformation (la), whereas the corresponding D-hexoses and their glycosides generally occur in the 4Ci(d) conformation. These different conformations determine the chemical and physical properties of the l,6-anhydro-/3-D-hexopyra-noses. A similar situation exists with the 1,6-anhydroaldohexofuranoses. 

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