Pyranose structures

The oxidation of N,N -diacetylkasugamine with bromine-water (11) yielded a 8-lactone (7), showing an absorption at 1752 cm.-1 (KBr) in the infrared spectrum. It suggested the pyranose structure of kasugamine (6). 

As already shown in Sect. 4.4, ribose-2,4-diphosphate is obtained in a base-catalysed condensation of glycolaldehyde phosphate in the presence of formaldehyde (Muller et al., 1990). The phosphate group in the 4 position of the sugar prevents the formation of a 5-membered furanose ring, but a 6-membered pyranose structure can be formed. 

One notices that the data from the oxidation of melezitose by per-iodic acid confirm the pyranose structure of the D-glucose unit in turanose, and therefore also in the case of maltose, in agreement with the original assignments for both of these disaccharides from methylation studies. 

Both methods of approach moreover proved that a series of methylated derivatives of D-glucosamine described by Cutler, Haworth and Peat,14 and most useful as reference compounds in this field, were all of the pyranose configuration. Further proof of the pyranose structure of methyl D-glucosaminide and methyl N-acetyl-D-glucosaminide was pro-... 

The pyranose structure of D-galacturonic acid, which lactonizes with great difficulty, has been confirmed by Levene and Kreider66 from a study of the methylated ester of the uronoside. Methyl a-D-galacturonoside (XXXIII) was methylated with silver oxide and methyl iodide and there was obtained methyl 2,3,4-trimethyl-a-D-galacturonoside methyl ester (XXXVI) which on oxidation was converted to d,l(2,3,4-trimethylmucic) acid (XXXVII) and h-arabo-trimethoxyglutaric acid (XXXVIII). 

The essentially nonreversible formation of D-fructose 1-phosphate in the muscle-aldolase system is probably attributable to thermodynamic stabilization. D-Fructose 1-phosphate can form a stable pyranose structure, whereas D-fructose 1,6-diphosphate can exist only in the less stable furanose or acyclic forms.72(,) Only when the cleavage products are removed is the monophosphate effectively split under the influence of aldolase. 

Fig. 13 Relative syn position of the two rings in furanose-pyranose structures bearing bulky R substituents.

It is noteworthy that D-fructose, which has a pyranose structure in the free crystalline state, assumes a furanose configuration whenever it combines with another sugar to form an oligosaccharide or polysaccharide. Apparently the ketohexose L-sorbose shows the same behavior. 

D-glucose and its lactone from 2,3,6-trimethyl-D-glucose provided conclusive proof that the ring system was not of the hexylene oxide type.142 188 The final evidence necessary to characterize the tetramethylglucose in question as a furanose derivative was provided by Haworth, Hirst and Miller,176 who demonstrated that oxidation of the tetramethylglucose with bromine water and of the resulting lactone with nitric acid yielded dimethoxysuccinic acid and oxalic acid, but not i-zyZo-trimethoxyglutaric acid, the absence of which ruled out a pyranose structure. 

Dextrose in solution or in solid form exists in the pyranose structural conformation. In solution, a small amount of the open-chain aldehyde form exists in equilibrium with the cyclic structures (1) and (2). The open-chain form is responsible for the reducing properties of dextrose. 

The first oxidative studies on N-acylglycosylamines were conducted on a preparative scale by Niemann and Hays.65 The N-acetyl-D-glucopyranosylamine (69) obtained by acetylation of D-glucopyranos-ylamine with ketene (or by acetylation of this amine, followed by ammonolysis) was oxidized by way of the dialdehyde 70 to the acid 71, isolated as the strontium salt. Hydrolysis of 71 led to D(+)-glyceric acid (72), thus showing the pyranose structure of 69. 

On the other hand, the favored formation (and the yields) of N-acetylglucofuranosylamines were explained on the basis of steric considerations.24,26 The formation of N-acylglycosylamines is a kinetically controlled, irreversible reaction, and the prevalence of the furanose over the pyranose structure has been shown in many irreversible, cyclization reactions of carbohydrates.73 This preponderance, as well as the favoring of a determined anomeric configuration, would be the result of a balance of relative ffee-energies of the transition states in the cyclization step, which would have virtually the same geometry as the final, cyclic structure.74 The five-membered... 

Figure 2. Pyranose structures showing axial-equatorial orientations that favor intramolecular hydrogen bonding between anionic oxygen and hydroxyl group...

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-... 

The predominant active sugar transport systems of intestine and kidney are specific for sugars which have the pyranose structure with a... 

Conclusive proof of the structures of the glycosides B and C was obtained by methylation, hydrolysis, oxidation and comparison of the rates of hydrolysis of the lactones so obtained. From the glycoside B a 1,5-lactone was obtained,141 whereas a 1,4-lactone was derived from C, showing that the glycoside B had a 1,5-pyranose lactol ring and C a 1,4-furanose lactol ring. Since A and B were a- and /3-anomers, it followed that A also had a pyranose structure. 

This sugar was isolated as its monohydrate from the hydrolysis products of methylated yeast mannan and its structure proved by Haworth, Heath and Peat.23 Methylation of the methyl glycoside gave the crystalline methyl 2,3,4,6-tetramethyl-a-D-mannopyranoside, indicating a pyranose structure. It seemed likely that the compound was the monohydrate of 2,4,6-trimethyl-D-mannose, as the 2,3,4- and 2,3,6-isomers were known as sirups that did not form hydrates, and the crystalline... 

Most, if not all, of the stable forms of crystalline aldose and ketose monosaccharides exist in the pyranose structure. Each in solution, as with D-glucose, exists as an equilibrium mixture of open chain and of a- and / -anomers of the cyclic forms. The cyclic five- and six-membered structures formulated below are an illustrative selection of monosaccharides. 

Pepinsky59 by the x-ray analysis of the strontium chloride complex. The specific rotation of this compound indicates the /S-D-pyranose structure and the only model of this structure which is in accord with the Fourier projection is the Sachse-Mohr trans (chair) form, with cis hydroxyls on C2 and C3. 

The acetylation of D-ribose in pyridine solution at low or ordinary temperatures leads to the formation of a crystalline tetraacetate melting at 110° and showing [ ]26D — 54.3° in chloroform.56-14 The ring structure of this substance is retained on conversion to triacetyl-D-ribosyl bromide since the latter compound may be reconverted to it by treatment with silver acetate.66 While indirect evidence early indicated94 that the bromide as well as the tetraacetate possessed a pyranose structure the question was not settled unequivocally until the bromide was used in the synthesis of nucleosides which were known to be pyranosides through quantitative periodate oxidation.95 While the configuration of the crystalline tetraacetate at carbon one will not be known with certainty until its anomer is obtained, the compound may provisionally be considered jS-D-ribopyranose tetraacetate because of its strong levorotation. 

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