The Composition of Keto Aldoses in Aqueous Solution as Determined by NMR Spectroscopy

Abstract

Keto aldoses usually form complex mixtures of equilibrating isomers in solution. This is due to the two different positions that may be used for ring closure in dicarbonyl sugars. The composition of various 2-keto aldoses 1 ± 5 and 8, the 3-keto aldose 2-deoxy-d-erythro-hexos-3-ulose (9), and the ketose 1-deoxy-d-ribulose (10) in aqueous solution has been determined by NMR spectroscopy. The investigated keto aldoses form equilibria containing three to fifteen isomers. Among various furanose and pyranose ring structures stemming from 1,4-, 1,5-, 2,5-, and 2,6-cyclization, bicyclic forms were also found in several cases. The 2-keto aldoses mainly exist as hydrated isomers in H 2 O. Therefore, these forms and their proportions were compared to forms found in two homomorphous aldoses and one homomorphous ketose as model compounds. Besides the NMR data, also the composition of the 2-keto aldoses agreed with the average of forms found in the model compounds, a finding that might eventually be useful for deducing the composition of other keto aldoses.1. Introduction. ± The composition of carbohydrates in solution is of major importance for their chemical, physical, and biological properties. Common reducing and nonreducing sugars have been studied extensively by NMR spectroscopy, and their equilibria in H 2 O are well-documented [1] [2]. Aldoses and ketoses mainly form two pyranose and two furanose anomers, which are rarely accompanied by open-chain or other forms. The analysis of the NMR spectra of these common sugars is, therefore, relatively straightforward. This is not the case for carbohydrates that have a second CO function. In keto aldoses, ring closure can take place at two different positions, and, accordingly, these compounds exist as complex mixtures of isomeric forms. For this reason, the composition of keto aldoses has been analyzed thoroughly in only a few cases [3 ± 14].Keto aldoses are important intermediates in biological and chemical processes. In mammals, the keto aldose d-glucosone (1) is believed to be formed by auto-oxidation of glucose [15] or by other processes [16], and 3-deoxy-d-glucosone (5) occurs in blood and urine [17] in concentrations that are elevated in diabetic patients [18]. The hexosulose 5 also plays an important role in the Maillard reaction [19]. The chemical interest in keto aldoses stems from the fact that they combine the high number of functional groups and the inherent information on chirality of carbohydrates that have a higher degree of diversity than normal hexoses. They are consequently very interesting synthons for a variety of chemical procedures [20 ± 22], and some of them have been effectively used for the synthesis of antibiotics [23 ± 26] and amino sugars [27]. Therefore, the composition in solution is of interest for both the chemistry and

A C4-oxidizing Lytic Polysaccharide Monooxygenase Cleaving Both Cellulose and Cello-oligosaccharides

“…6B, showing an overlay of the 13 C HSQC and the 13 C HMBC spectra of products obtained in the reaction with NcLPMO9C and CDH. The overlay shows correlations from H/C-5 and H/C-3 to peaks with a carbon chemical shift at C4 of 95.9 and 175.2 ppm, corresponding well to the chemical shift of a gemdiol (29) and keto group, respectively. The gemdiol and keto groups account for ϳ80 and 20% of the signal intensity, respectively.…”

“…Because the glycosidic bond in cello-oligosaccharides (and cellulose) links the C1 of one glucose to the C4 of the adjacent glucose, it is logical to suggest that the oxidation carried out by NcLPMO9C takes place at the C4 position of the nonreducing end moiety. The resulting keto sugar will be in equilibrium with the C4 gemdiol in water solution (a feature that is common to keto saccharides (29)). Generally, it is not straightforward to prove the position of LPMO generated oxidations using mass spectrometry because the masses of various possible products are identical and because the mass difference between sodium and potassium adducts equals the mass of an oxygen atom.…”

“…The gemdiol and keto groups account for ϳ80 and 20% of the signal intensity, respectively. Although documentation on the keto:gemdiol ratio in literature is limited, the gemdiol would be expected to dominate at pH 6.0 because the keto group is easily hydrated (29) in aqueous medium. The overlay also shows a correlation from the H/C-2 to a peak with a carbon chemical shift of 181.1 ppm that corresponds well to the shift expected when a carboxylate group is present at position C1 (12).…”

Ammoxidation of Lignocellulosic Materials: Formation of Nonheterocyclic Nitrogenous Compounds from Monosaccharides

“…7,15 As compared to the 3-deoxyhexonic acid epimers, the observed stereoselectivity in the formation of the 3-deoxypentonic acids was much smaller. This is likely due to the higher equilibrium concentration of the furanose form of the respective precursor molecule 3-deoxy-pentosone in comparison to 3-deoxyhexosone 29,30 which leads to less stereochemical induction.…”

Catabolism of 1,5-Anhydro- d -Fructose in Sinorhizobium morelense S-30.7.5: Discovery, Characterization, and Overexpression of a New 1,5-Anhydro- d -Fructose Reductase and Its Application in Sugar Analysis and Rare Sugar Synthesis

“…Due to structural similarities between AF and the osones listed in Table 3, these compounds were accepted as substrates by AFR. One requirement seems to be the pyran ring structure with the C-2 carbonyl group, as in AF (16,23). The best structural match to AF (1-deoxy-D-glucosone) is obviously D-glucosone, which explains the high stereoselectivity of its conversion to D-mannose by AFR.…”