Arabinopyranose conformation

The importance of the configuration and conformation of the saporous group cannot be overstressed. This is demonstrated by the correct prediction that /3-L-arabinopyranose tastes sweet because of its similar configuration... 

A few examples of the Cl (l), that is, the 1C (d) conformation have been described. /3-D-Arabinopyranose and its enantiomorph crystallize in this conformation (Ia2e3e4a),1(i as illustrated in formula 20 for the /3-D compound. The Cl (d) conformer is shown, for comparison, in formula 21. As may be seen, neither form has any large nonbonded interactions. Both forms have one axial hydroxyl group on... 

A melting point of 82°-83°C and [ ]D20 —85° - —100° were reported for 2,3-di-O-methyl-D-arabinose by Verheijden and Stoffyn (20). These authors preparation must have been largely or wholly an a-anomer, whereas ours is preponderantly ft. In view of the rarity of crystalline furanoses, it is presumed to be the / -pyranose, and in the PMR spectrum run at 0°C on a sample freshly dissolved in DoO the anomeric proton gave a signal, with the appearance of an unresolved doublet, at t 4.50. This chemical shift and a small value of Jit2, would be expected for H-l of a / -arabinopyranose in the 1C conformation. 

Figure 3. Hydroxyl PMR signals for p-o-fructopyranose (5) (upper left) and ct-L-gala-heptulose (6) (lower left) in methyl sulfoxide-dtf. 13C NMR spectra of 5 and 6 (in water). The diagonal line relating the 13C-6 resonances of 5 and 6 reflects the large downfield shift attributable to replacement of one H-6 with the 7-carbinol group. 13C chemical shifts for a- and B-o-arabinopyranose (a and b, respectively) (10, 11) are inserted to illustrate the close conformational affinity between 5 and / -, though not a-, d-arabinose (ppm relative to downfield...

G. Yang and F. Kong, Synthesis, conformational analysis and the glycosidic coupling reaction of substituted 2,7-dioxabicyclo[4.1. OJheptanes 1,2-an hydro-3,4-di-0-bcnzyl-/f I -arabinopyranose, J. Carbohydr. Chem., 13 (1994) 909-921. 

Y. Du and F. Kong, Synthesis, conformation and glycosylation reaction of substituted 2,6-dioxabicyclo[3.1. IJhcptancs 1,3-anhydro-2,4-di-O-bcnzyl-0(-i, -arabinopyranose, Carbohydr. Res., 275 (1995) 259-273. 

The conformational preferences of the aldopentoses, which have no hydroxymethyl group at C(5), are mainly governed by minimising steric repulsion between the hydroxyl groups. Thus, D-arabinopyranose favours the C4 conformer, and a-D-lyxopyranoside and a-D-ribopyra-noside are conformational mixtures and the other aldopentoses are predominantly in the C4 form. 

On hydrolysis with sulfurous acid, 5-amino-5-deoxy-l,2-0-isopro-pylidene-jS-D-arabinofuranose gives an acyclic bisulfite adduct which, on treatment with barium hydroxide, afiFords a solution of 5-amino-5-deoxy-D-arabinopyranose that behaves similarly to the analogous D-xylose compound. The Amadori rearrangement proceeds with somewhat more difiBculty. The equilibrium between the pyranose form and its dehydration product is recognizable by the presence of a positive Cotton efiFect (300 nm), a result predictable by theory, as 42 should represent the most favored conformation. 

The strong anomeric effect should be operative in 5-[(benzyloxy-carbonyl)amino]-5-deoxy-j8-L-arabinopyranose. In fact, this compound shows a strong tendency to assume only the /3-l form, since this compound exists in the Cl (l) conformation. The steric situation at C-1... 

The aldopentopyranoses are not constrained to the Ci conformation by the presence of a hydroxymethyl group. As a result, the equitibria between the Ci and conformers are more closely balanced, with the D-xylopyranoses present in aqueous solution mainly as " Ci conformers (25), the D-arabinopyranoses mainly as 4 conformers (26), and the others as mixtures [136,137,138]. 

Now the calculated values may be applied to additional, isomeric pentose derivatives. In the a-D-xylopyranose derivative the equilibrium is practically totally on the side of the 4Ci form (l ). Similarly, the p-D-arabinopyranose derivative totally adopts the 1C4 conformation (13). Consequently, neither... 

The i"C chemical shift thus reveals the relative configuration of substituents in molecules with a definite conformation, e.g. the axial position of the OH group in trans-3-methylcyclohexanol, cis-4-methylcyclohexanol, -D-arabinofuranose and a-D-xylopy-ranose (Table 2.12). It turns out, in addition, that these compounds also take on the conformations shown in Table 2.12 (arabinopyranose, C the others, CJ if they occurred as the other conformers, then the OH groups on C-l in these molecules would be equatorial with the result that larger shifts for C-l, C-3 and C-5 would be recorded. A ring inversion (50 50 population of both conformers) would result in an average C shift. 

The conformations adopted in solution by the eight aldopentopy-ranoses is determined by the relative configurations of the hydroxyl groups. Thus, a- and j8-D-arabinopyranose favor the iC conformation, a-D-lyxopyranose and a-i>ribopyranose appear to contain substantial proportions of each chair form, and jS-D-lyxopyranose, jS-D-ribopy-ranose, a-D-xylopyranose, and jS-D-xylopranose exist preponderantly in the Cl conformation. In methyl sulfoxide, however, a-D-lyxopy-ranose exists almost entirely in the Cl conformation, and in this solvent, jS-D-ribopyranose appears to contain an appreciable proportion of the alternative, 1C conformer. ... 

From a consideration of detailed results on the conformational equilibria of aldopentopyranose derivatives, it has been pointed out92- 9 that a more sophisticated model is required before conformational populations can be reliably predicted, at least with acylated derivatives. Even with adjustment of the original parameters in order to take revised values for the anomeric equilibrium of D-lyxopyranose tetraacetate and the conformational equilibrium of /3-D-arabinopyranose tetraacetate into account, the observed data cannot be accommodated within the framework of this model, except on a very broad, qualitative basis. Other possible factors that should be considered " include polar contributions from substituents other than that on C-1, attractive interactions between syn-diaxial acyloxy groups, non-bonded interactions between atoms that have unshared pairs of electrons,repulsive interactions between gauche-vicinal groups, the effect of solvent pressure, and differences between the molar volume of conformers. 

The high susceptibility of pentoses to the secondary process, with the proposed interface of the interconversion D-xylopyranose- Ci D-arabinopyra-nose- C4 between their two epimeric pairs, is probably connected with the conformational readiness of both aldoses for the reaction [42]. This susceptibility is less pronounced for higher aldoses because of a lower population of C4 con-formers of both D-altropyranose and D-idopyranose when compared with that of D-arabinopyranose. 

Other Anhydrides. - Stereoselective chemical and chemoenzymatic approaches to the preparation of 1,6-anhydrohexopyranoses have been reviewed, and base treatment of pentabromophenyl glycosides has been investigated as an improved procedure for the synthesis of 1,6-anhydro sugars. Treatment of O-benzylated-3-O-acetyl-glycosyl chlorides with base (KO Bu, THF) has allowed syntheses of l,3-anhydro-2,4,6-tri-0-benzyl-P-D-talopyranose, l,3-anhydro-2,4-di-C -benzyl-a-L-arabinopyranose and 1,3-anhydro-2,4-di-0-benzyl-6-deoxy-P-L-talopyranose. The theoretical solution conformation of methyl 3,6-anhydro-a-D-galactopyranoside has been studied using ab initio calculations. The computed conformation was similar to that adopted in the crystal (X-ray analysis). ... 

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