Ribopyranose conformations

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. 

A-6. A wedge-and-dash representation of a form of ribose (called /3-D-ribopyranose) is shown here. Draw the most stable chair conformation of this substance. 

In some cyclic sugars, ring conformations may be stabilized by intramolecular hydrogen bonding. The configuration of the a-ribopyranose molecule is such that there is syndiaxial alignment of C-OH bonds in both the C4 and 4Q ring conformations. In consequence, there is believed to be an equilibrium in solution as shown below. 

In the isolated molecule or in nonpolar solvents, the intramolecular hydrogen bonding would be between 0(2)H and 0(4)H in the C4 conformation and between 0(1 )H and 0(3)H in the 4Q form. The a-ribopyranose molecule therefore provides an interesting example of the influence of intramolecular hydrogen bonding on ring conformation. Unfortunately, neither a- nor / -ribopyranose has been... 

The synthesis of 3-azido-3-deoxy-5-tliio-/i-D-ribopyranose (288) was realized" from the tosylate 286 via thioacetate 287 in five steps (Scheme 76). Treatment of287 with acidic methanol gave a septarable 1 2 mixture of the methyl a-and / -pyranosides. The a anomer 288, whose H NMR. spectrum suggested an equilibrium of the C4 and " Q conformations, was transformed into 289 by conventional reactions. l,2,3-Tri-6 -acetyl-3-azido-3-deoxy-5-thio-D-xylopyranose was prepared via the xylo analogue of 286, starting form D-glucose."" " ... 

Other workers have studied conformational and configurational equilibria by nuclear magnetic resonance spectroscopy. Horton and coworkers295ribopyranose tetraacetate exists in acetone-d6 at room temperature in the CE and CA conformations, in rapid equilibrium, in the ratio of 9 11. At —84°, the ratio of the CE and CA conformers was 2 1 at —60°, the rate of interconversion of the two conformers was found to be 130 times per second. The equilibrium... 

Also in D-ribopyranose derivatives there appears to be a conformational dependence of the geminal coupling. The numerical... 

An example of a sugar derivative held rigidly in a skew conformation is 3-0-benzoyl-l,2,4-0-benzylidene-a-D-ribopyranose (10). 

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. ... 

By this method, it was found that a solution of j3-D-ribopyranose tetraacetate in acetone-de at room temperature contains 45% of the 1C(d) conformer and 55% of the C1(d) conformer, thus indicating that the equilibrium constant for the 1C(d) C1(d) process... 

The nature of the solvent and its polarity do not appear to affect in any regular way the position of the conformational equilibria for example, /3-D-xylopyranose tetrabenzoate, which exists in ace-tone-de (dielectric constant, e, = 20.7) as a 1 1 mixture of the CJ(d) and 1C(d) conformations, shows practically the same conformational population in a range of solvents, including benzene-rie = 2.3), toluene-dg ( = 2.4), chloroform-d (e = 4.8), pyridine-dg (e = 12.3), hexachloroacetone, and methyl sulfoxide-dg ( = 48.9). Similar results, showing negligible dependence of conformational populations on the polarity of the solvent, were obtained with jS-D-ribopyranose tetraacetate, tri-0-acetyl-/3-D-xylopyranosyl chloride, and some simpler derivatives of tetrahydropyran. In contrast, as the polarity of the solvent was increased, there was observed with methyl 2,3,4-tri-0-benzoyl-/3-D-xylopyranoside a broad trend in favor of that conformation having the C-1 substituent equatorially attached. 

This conformational change has also been detected in a pento-pyranoid system. Thus, although l-0-acetyl-3-benzamido-2,4-di-0-benzoyl-3-deoxy-3-C-(ethoxycarbonyl)-/3-D-ribopyranose (22) adopts the 1C(d) conformation in chloroform solution, the corresponding nucleoside derivative 23 exists preponderantly in the CJ(d) conformation. 

Increment rules that predict the chemical shifts of protons in the H n.m.r. spectra of aldohexopyranoses to within 0.09p.p.m. have been presented. 300 MHz H n.m.r. spectroscopy has shown that both a- and p-D-ribopyranose exist as a mixture of C4 and C, conformations in deuterium oxide, while P-d-ribo- and P-D-xylo-pyranosylamines prefer the C, conformation. The con-... 

As expected, a-o-ribopyranose and a-D-ribofiiranose [41] signals in NMR are shifted downfield due to the addition of caldnm chloride, sinee they have acquired the arrangement of three hydroxyl groups. In addition p-o-ribopyranose signals has also been shifted due to the change of conformational equilibrium from C, to C (Fig. 9.8), since only the later has the sequence ax q-ax. 

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