Pyranose–furanose and Anomeric Equilibria: Influence of Solvent and of Partial Methylation

Abstract

Sugars possessing the arabino (2,3,4-trans,cis) configuration exist a s furanoses to a greater extent in dimethyl sulfoxide than in water. Their 2,3-di-0-methyl derivatives show a n even stronger preference for a five membered ring structure in both solvents. This is most li~arl<ed for 2,3-di-0-methyl-D-arabinose and 2,3-di-0-methyl-D-dtrose, which are 657, and 50% furanose, respectively, in dimethyl sulfoxide. C o m p o~~n d s in the xylo (2,3,4-trans,trans) or ly.vo (2,3,4-cis,trans) series show little tende~icy to be furanoses in either solvent. However, apyranoses in the lyro series are relatively more stable in dimethyl sulfoxide than in water, whereas the anomeric composition for ~ilernbers of the xylo series is the same in both solve~its. Some of these variations in the equilibria are attributed to the preferential stabilization of pyranose forms in water. D-Lyxose and D-ribose show nuclear magnetic resonance spectral differences in the two solvents that appear to be due to conformational, a s well a s tautomeric, equilibrium changes.An eq~latorial a~lorneric hydroxyl proton of a given pair in dimethyl sulfoxide shows a larger spacing (6.5-5.0 c.p.s.) than an axial anomeric hydroxyl proton (4-5 c.p.s.). The signal for the latter proton occurs a t higher field than a n equatorial OH-1, except when OH-2 is axial.In a recent study of hydroxyl proton magnetic resonance spectra it was found that certain sugars tend to exist as furanoses to a greater extent in dimethyl sulfoxide than in water (1). D-Arabinose shows this difference very markedly. At equilibrium, virtually 110 furanose is detected in the aqueous solution, whereas in dimethyl sulfoxide its proportion is one-third of the total (Table I). Galactose behaves similarly, although the final percentage of furanose is not as high. D-Altrose and D-fructose, both of which are configurationally related to these sugars, exist as furanoses to a very considerable extent in ivater. I-Iowever, the tautomeric conlposition of the former (2) is altered significantly in dimethyl sulfoxide, in which it shows an even stronger preference for the five membered ring structure (Table I). The presence of mixed furanose and pyranose forms of D -~~L I C~O S~ in water is shown by the fact that, when an aqueous solution of the sugar is concentrated, the resulting syrupy material shoivs several strong singlets ascribable to OH-2 ivhen freshly dissolved in dimethyl sulfoxide. As equilibration proceeds in this solvent, the main change observed is a decrease in the relative intensity of the signal attributable to OH-2 of the P-pyranose form.One possible explanation (1) of these compositional differences in the two solvents is that water may preferentially stabilize the pyranoses through hydrogen bonding. This is based on evidence (3) that, \\illen water solvates a sugar inolecule possessing a pyranose chair forin, the structure of water itself suffers very little disorientation, and intermolecular association can take place readily. Although dinlethyl sulfoxide also is strongly hydr...

Stereochemical dependence of vicinal H—C—O—H coupling constants

“…In its most general form, such a relation can be written as a power series limited to the second power in cos + A similar angular dependence of the protonproton coupling (J,,,,) for the H-C-0-H fragment has been proposed by a number of authors (2)(3)(4)(5) and in several papers the relation between JHcoH and 4 has been discussed on empirical grounds (6)(7)(8). However, while a functional behavior of the type given by eq.…”

Microwave Spectroscopy of Biomolecular Building Blocks

“…Similar to other six-carbon monosaccharides, it may exist in either linear or cyclic, pyranose or furanose, forms. In aqueous solution, NMR studies have shown that the pyranose form is dominant [210,212,213]. The cyclization leads to the occurrence of two anomeric species, α and β, according to the position of the OH group (see Fig.…”

“…60 , À60 or 180 , respectively. Experimental observations in both the solid phase [211,[214][215][216][217]254] and solution [212,213,233] display approximately equal populations of G+ and GÀ conformers, with an almost complete absence of the T conformer. This propensity in glucopyranosides to adopt gauche conformations is known as gauche effect [255, 256 and references therein].…”

Relation between the Δ2 effect and the solution conformational entropy of aldohexoses and select methyl glycosides

“…For the monosaccharides, this difference in ranking could be due to a difference in the relative ratios of a to b pyranose anomers, and/or of pyranose to furanose structures, in DMAc or DMAc/LiCl versus the same ratios in water (Franks, 1977). For example, the percentage of glucose a pyranose anomer changes from 38% in water at room temperature to 44% in DMSO at 30°C (Bubb, 2003;Franks, 2000); the amount of furanose anomers of galactose in water at room temperature is 6%, but rises to $15% in DMSO (the temperature of the DMSO experiments does not appear to have been specified by Mackie and Perlin in the original publication) (Mackie & Perlin, 1966); and the amount of furanose anomers of glucose, negligible in both water and DMSO, rises to 4.5% in N,N-dimethyl formamide at 70°C (Reine, Hveding, Kjølberg, & Westbye, 1974). It is also known, from the classic infrared absorption work of Tipson and Isbell, that in water the equilibrium distribution of arabino sugars such as galactose, and of ribo sugars such as talose, is higher in furanose structures than are the equilibrium distributions of xylo (glucose) and lyxo (mannose and gulose) sugars (Tipson & Isbell, 1962).…”