Conformations of Acyclic Sugars

Conformational Equilibria of Acylated Aldopentopyranose Derivatives and Favored Conformations of Acyclic Sugar Derivatives... 

Conformational depictions of acyclic sugar chains are conveniently expressed by locating certain atoms in the plane of the paper and orientating the remaining atoms or groups appropriately above and below that plane, as shown for D-arabinitol and xylitol (it should be recognized that the favoured conformation does not necessarily have all the carbon atoms in the same plane) ... 

D. Horton and J. D. Render, Conformation of acyclic detivtdves at sugars, Cartokydr. Res. 15 271 (197D). 

Andr6, C., Luger, R, Bach, R., Fuhrhop, J.-H. (1995). On the conformational and packing behaviour of acyclic sugar amphiphiles The crystal structures of iV-(l-octyl)-D-arabinonamide and A-(l-dodecyl)-D-ribonamide and the supramolecular assembly-forming properties of iV-(l-octyl)-D-pentonamides, Carbohydr. Res., 266 15. 

The importance of parallel 1,3-interactions in determining the favored conformational state of acyclic sugar derivatives in solution was also apparent from n.m.r.-spectral studies of polyhydroxyalkyl-quinoxalines in methyl sulfoxide, and of their acetates in carbon tetrachloride. 

For molecules in the solid state, it is possible that lattice forces may profoundly affect the process of adoption of a particular conformation, and thus the favored conformation of a molecule in the crystalline state is not necessarily the same as that adopted when the molecule is surrounded by a sheath of solvent molecules. Nevertheless, there is a high degree of correlation between the results of X-ray crystallography of alditols and their derivatives (see p. 69) and the conformations adopted by various types of acyclic sugar derivatives in solution. 

The pyranoid monosaccharides provide a wide range of asymmetric molecules for study by the c.d. spectroscopist. However, these compounds are not without their difficulties. In aqueous solution, these compounds exist in a complex equilibrium involving the two possible chair conformers of the pyranoses, the furanoses, a and p anomers, and the acyclic form, as well as septanoses for aldohexoses and higher sugars. 

Undoubtedly, the vicinal groups play a fundamental role in the outcome of this reaction, especially with the sugars, where the favored conformation of the molecule at equilibrium is controlled at the outset by groups that determine whether the molecule exists as a cyclic or acyclic structure. The deamination of cyclic and acyclic amino sugar derivatives by nitrous acid will be considered in turn. 

The conformations in solution of various acylated l,l-bis(acyl-amido)-l-deoxypentitols have been examined by p.m.r. spectroscopy.793 In line with general behavior observed78 with other acyclic-sugar derivatives, it was found that the arabino and lyxo derivatives adopt extended, zigzag conformations, whereas the ribo and xylo derivatives favor sickle conformations that result by rotation about one carbon-carbon bond of the backbone chain to alleviate a destabilizing 1,3-interaction of acyloxy substituents that would be present in the extended, zigzag conformation. 

T1he conformational studies (I) on acyclic sugar derivatives and on aldopentopyranose derivatives that have been conducted in our laboratories during the last few years are surveyed, and some of our more recent results in each of these areas are introduced. For each aspect the sugar derivatives were examined in solution by proton magnetic resonance (PMR) spectroscopy, and the data obtained were used to provide conformational information. Acyclic systems will be treated first. 

These results suggest that in the pentitols and hexitols, at least, the intermolecular H-bonds have only a secondary influence on the shape of the molecules in the solid state. The observed conformations in the solid state can be rationalized (16) in terms of the intramolecular interactions between hydroxyls which are believed to involve energies of the order of several kilocalories (27) (the interpretation of the NMR spectra of acetylated acyclic sugar derivatives in solution is based on the same type of hypothesis (29)). 

C20H26N4O8S2 (lR)-2,3,4,5-Tetra-0-acetyl-l-(l,6-dihydro-6-thioxopurin-9-yl)- 1-S -e thy 1- 1-thio-D-arab initol122 P2i Z = 4 Dx = 1.31 R = 0.063 for 3,955 intensities. The acyclic sugar residue attached to N-9 of the 6-thioinosine ring adopts the planar, zigzag conformation. 

---