Furanose structures

D-ribose, CjHioOj. M.p. 87 0. The sugar of ribonucleic acid it is therefore present in all plant and animal cells. It has the furanose structure shown. 

The pyranose or furanose structures shown for the free sugars are arbitrary assignments.)... 

L-arabinose molecules combine together in the furanose form to produce an araban. Since enzymes capable of hydrolysing pectic acid to n-galacturonic acid are known to be present in certain plant juices, and since the comparatively low temperature and slightly acid condition of plant materials would tend to favor the transformation of arabinose into the furanose form, the occurrence of the furanose structure in the polysaccharide derived from arabinose is not altogether unexpected. 

It is interesting to note that the formation of the dioxane ring stabilizes the anhydrides as to their pyranose or furanose structures, difructose anhydride I as 2,1 l,2 -di-D-fructofuranose and dihetero-levulosan as 2,1 l,2 -di-D-fructopyranose. 

Asano N, Oseki K, Kizu H, Matsui K. (1994) Nitrogen-in-the-ring pyra-noses and furanoses Structural basis of inhibition of mammalian glycosidases. J Med Chem 37 3701-3706. 

In addition, while it is difficult to extrapolate the data in Figure 14 to conditions in aqueous solution, it is nevertheless clear that the nature of conformational averaging, which affects the interpretation of NMR parameters, depends on ring configuration, and that a singular approach to NMR parameter interpretation cannot be applied to all furanose structures. 

Both six-membered pyranose and five-membered furanose structures are encountered, a particular ring size usually being characteristic for any one sugar. Thus, although glucose has the potential to form both six-membered and five-membered rings, an aqueous solution consists almost completely of the six-membered hemiacetal form five-membered rings... 

The diacetamide compounds were regularly obtained in all degradations employing ammonia with or without silver oxide until Hockett and Chandler applied the method to hexaacetyl-D-gluco-D-flruZo-heptono-nitrile (XLIX) and obtained a monoacetamide derivative that was identified as iV-acetyl-D-glucofuranosylamine (L). The furanose structure of L was established by lead tetraacetate oxidation. They... 

In 1911, when our first investigations of the action of a- and /3-d-glucose on boric acid were made, the furanose structure was assumed for the monosaccharides having four and more carbon atoms, so that the establishment of an equilibrium between a- and /3-D-glucose was represented by the following formulas, written now according to present conventions. 

Similar products (but having R = /3-D-galactopyranosyl) were obtained from octa-O-acetyllactobiononitrile56 in 25% aqueous ammonia. The furanose structure of the monoamido derivative 61 (and that of its analog N-acetyl-3-0-/3-D-galactopyranosyl-D-arabinofuranosyl-amine56) was ascertained by methylation, hydrolysis, and identification of 2,5-di-O-methyl-D-arabinose. 

When applied to N-acylglycofuranosylamines, the periodate oxidation showed an abnormal uptake of oxidant ( overoxidation ). For example, when oxidized with lead tetraacetate12 and with periodate,1065 N-acetyl-a-D-glucofuranosylamine (15) afforded formaldehyde (indicating a furanose structure), and it consumed more than 5 moles of oxidant per mole. This result can be attributed to subsequent oxidation of the formic acid produced,69 or to the formation,10 by hydrolysis, of the intermediate 2-hydroxypropanedial (tartron-aldehyde) (77) that would then be oxidized. This tendency to undergo overoxidation has been found common for the furanoid N-acyl-gly cos y lam ines.24,25... 

However, if the reactive form of D-glucose is not the a-pyranose structure but the a-furanose structure, then the implications of this reac-... 

High-held H and 13C NMR spectra of aqueous solution of D-xylo-hexos-5-ulose (242) have provided evidence for the presence of at least six isomeric forms and one anhydro form. The dominant isomeric form was the /f-pyranose 242a (67%) with the next most abundant form being the anhydro structure 242c (18%). The a- and /My4-furanoses (242d,e) and 1-aldehydrol /1-5,2-furanose structure (242f) were also observed.459... 

Bell114 has suggested that the very ready hydrolysis of a -diisopro-pylidene-D-fructose might possibly indicate a furanose structure, namely, that a-diisopropylidene-D-fructose is l,2 4,6-diisopropylidene-D-fructo-furanose. Hydrolysis would have to be accompanied by change in the size of the ring since the structure of the mono-isopropylidene derivative is, without doubt, pyranose. In this connexion, however, it should be... 

Answer Straight-chain fructose can cyclize to yield either the pyranose or the furanose structure. Increasing the temperature shifts the equilibrium in the direction of the furanose form, reducing the sweetness of the solution. The higher the temperature, the less sweet is the fructose solution. 

Condensation of D-ribose with two molar equivalents of phenyl-boronic acid in 2-methoxyethanol gave a diester to which was assigned the l,5 2,3-/3-furanose structure (25) on the basis of as-yet-unreported n.m.r. data.26... 

---