0-Methyl glycopyranosides

It is puzzling that the chlorine and other halogen substituents are not known to enhance the sweetness of other sugars, such as methyl -d-glycopyranosides, a,a-trehalose, maltose, or lactose. On the contrary, all of the deoxyhalo sugar derivatives tasted were extremely bitter. The high sweetness of the deoxyhalosucroses is clearly inexplicable in terms of either... 

In their study of Bronsted acid induced cleveage of a [with axial C(l)-OMe] and (3 [with equatorial C(l)-OMe] glycopyranosides, Fraser-Reid et al.46 demonstrated that the (3/a rate ratios for hydrolysis of methyl pyranosides (Table 5) can be explained by the different intermediates and transition-state structures through which proceed the heterolysis of a and [3 isomers (Fig. 18). 

Reports on solvent effects on rate constants for aquation of diimine complexes include those on [Fe(5Brphen)3] + and [Fe(4,7-Me2phen)3] " " in methanol- and ethanol-water, [Fe(bipy)3] +, [Fe(phen)3] +, and [Fe(5N02phen)3] + in aqueous methyl D-glycopyranosides, and... 

The 18-crown-6 derivatives (obtained from the methyl 4,6-0-benzylidene-a-D-glycopyranosides) such as a-D-206 with both KI (206) and KSCN (207), aa-DD-115 (Figure 18) in its fiee form (150), and a-o-103 (Figure 17), a-d-105 (Figure 17), and aa-DD-119 (Figure 18) with BH3NH3 (208). 

J. M. Williams and A. C. Richardson, Selective acylation of pyranosides I. Benzoylation of methyl a-D-glycopyranosides of mannose, glucose and galactose, Tetrahedron, 23 (1967) 1369-1378. 

A parameter that has been useful for defining the anomeric stereochemistry of glycopyranosides is the chemical shift of the anomeric carbon atom, C-l. Similarly, the 13C NMR shifts of C-l in methyl septanosides also diagnostic for their anomeric configuration. In general, the Sc.i values for a-septanosides are slightly upfield (99-104 ppm in completely deprotected septanosides) relative to (S-septanosides (104-111 ppm) (Table I). This distribution of chemical shifts mirrors the trends for pyranosides namely, the C-l chemical shifts for a-pyranosides lie upfield from those of p-pyranosides. This observation suggests that the anomeric effect is operative in... 

R. U. Lemieux, A. A. Pavia, J. C. Martin, and K. A. Watanabe, Solvation effects on conformational equilibria. Studies related to die conformational properties of 2-methoxytetrahydropyran and related methyl glycopyranosides, Can. J. Chem., A1 (1969) 4427-4439. 

The first synthesis of methyl 4-thiomaltoside (36b) (34%), 4-thiocellobioside (39d) (52%) and 4-thiodigalactobioside (86) (56%) was reported 18 years ago by using the Sn2 displacement of 1,2-cis- or 1,2-trans- glycosyl halides (9 a), (7 a) and (7c) with methyl 4-thio-a-D-glycopyranoside sodium salts generated from the acetylated methyl glycosides (84) and (85a) (Scheme 25) [14a]. 

In the absence of this information, early attempts at rationalization of the experimental results were based on a detailed investigation of enzymatic substrate specificity. For instance, acylation of enantiomeric methyl glycopyranosides by different lipases was focused on the characterization of the reaction outcomes (percentage of the formed regioisomers after the complete disappearance of starting materials or after a fixed reaction time), and the results obtained were interpreted on the basis of the relative orientation of hydroxyls at C-2, C-3, and C-4 [97]. 

One common structural feature is the presence of a xylose unit attached to C-3 of the aglycone and substituted at C-2 with a quinovose unit. As shown in Table 2, carbons involved in the interglycosidic linkages show chemical shifts at 5 ca. 82-88 ppm, shifted downfield from those expected for the corresponding methyl glycopyranosides. 

The characteristic feature of the mass spectra of methyl glycopyranosides is the presence of peaks M+l. 

The reaction of methyl glycopyranosides with sulfuryl chloride has also provided a route to 6-chloro (and other monochloro, dichloro, and trichloro) derivatives. 44 Schlubach and Wagenitz 44 observed that reaction of /3-d-galactopyranose pentaacetate with hydrogen bromide affords a 1,6-dibromo derivative which they converted into crystalline D-fucose in two steps. 

The first application of GLC in carbohydrate chemistry was to separate fully methylated methyl glycopyranosides of simple pentoses and hexoses (10), and much literature now covers these derivatives (II, 12, 13, 14). Shortly after this work, acetate derivatives were examined by various workers (15, 16), and a little later another development was the use of these acetates with thin-film columns containing liquid phases of high thermal stability such as silicone polymers (e.g., SE-30) and fluoro-alkyl silicone polymers (e.g., QF-1) (17). 

Molecular Rotations of Anoniers of Methyl Glycopyranosides and of Methyl 2-Desoxyglycopyranosides... 

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