A-pyranosides

With this ortho ester good selectivity for the axial alcohol is achieved in the acidic hydrolysis of a pyranoside derivative." ... 

This ether, developed for the protection of a pyranoside anomeric hydroxyl, is prepared via a Konigs-Knorr reaction from the glycosyl bromide and 2-(ben-zylthio)ethanol in the presence of DIPEA. It is cleaved, after oxidation with dimethyldioxirane, by treatment with LDA or MeONa. ... 

The /7-nitrophenyl ether was used for the protection of the anomeric position of a pyranoside. It is installed using the Konigs-Knorr process and can be cleaved by hydrogenolysis (Pd/C, H2, AC2O), followed by oxidation with ceric ammonium nitrate (81-99% yield). ... 

HCOOH, 60°, 1 h, 93% yield. This method can be used to protect selectively only the primary alcohol of a pyranoside. ... 

Deoxy-L-galactose (L-fucose) is common, and has only been found as the a- or )3-pyranoside. The rare D-fucose has, however, been found both as a-pyranoside, in the LPS frorn Pseudomonas cepacia serotypes B and E, and as a-furanoside, in the cell-wall antigen from Eubacterium saburreum L 452 and the O-antigens from different strains of Psuedomonas syrin-gae The a-furanoside, as in 3, has a cis relationship between the aglycon and OH-2. The corresponding P form has not yet been found. 6-Deoxy-o-and -L-talose are components of the extracellular polysaccharides from some strains of Butyrivibrio fibrisolvens and of the LPS from some strains of E. coli respectively. 

Y. pseudotuberculosis VB, however, the same sugar is a-furanosidic. The LPS from other species of Yersinia contain 6-deoxy-o-gulose, which is a-pyranosidic in the LPS from Y. enterocolitica serovar 0 6,31. 

Pikho and co-workers50 found that nonanomeric [6,5]-spiroketals (having a pyranoside moiety with an equatorial CO acetal bond) can be formed under conditions of kinetic control. For instance, 62 undergoes acid catalyzed spiroacetalization giving the anomeric (most stable) acetal 63... 

As these results and Fig. 2 show, three structural components may be defined in lipid A (/) the lipid A backbone consisting of a pyranosidic HexN disaccharide and phosphate groups, (ii) substituents of the backbone phosphate residues (polar head groups), and (iii) fatty acids. Therefore, lipid A of different bacteria may be classified according to the nature of the backbone constituents (GlcpN or GlcpN3N), the type and nature of the polar head groups, and features of the acylation pattern. In a few instances, other backbone substituents have been encountered. These will be described later in conjunction with individual lipid A forms. 

The implications of the above observations may be important, especially if similar trends are observed in pyranose anomers. For example, with respect to the mechanism of acid-catalyzed hydrolysis of pyranosides, endocyclic C-0 bond cleavage (preceeded by 05 protonation) may be assisted in P-anomers in which the Cl-01 bond is equatorial, since the 04-Cl bond may already be extended in these anomers. By a similar argument, exocyclic C-0 scission (preceeded by 01 protonation) may be assisted in the hydrolysis of a-pyranosides in which the Cl-01 is axial and extended, thus resembling the transition state. Post and Karplus have recently suggested that enzyme-catalyzed glycoside hydrolysis of P-pyranosides may indeed take place by ring oxygen protonation, followed by endocyclic C-0 bond scission. 

Acetamido-2-deoxy-D-glucose (General Alcoholysis) a-Pyranosides prevalent at equilibrium P-pyranosides present to large extent before final equilibration or if ferric sulphate used as catalyst Ferric suphate may act as a catalyst with valuable specificities in glycosidations 18)... 

Thio-D-ribose (Methanolysis) a-Pyranoside formed first followed by P-pyranoside then p-furanoside and oc-furanoside (furanosides only minor products) Mechanism probably quite different to that of D-ribose first step may be direct displacement at C-1 of the six-membered, sulphur-containing ring 20)... 

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... 

Treatment of a cyclic 23- or 3,4-ortbo-ester in a pyranoside under mild acidic conditions will cause regioselective cleavage so that a free equatorial OH is produced adjacent to an axial O-acyl group [14] (Scheme 1). The reasons for this have engendered considerable theoretical treatment based on considerations of stereoelectronic effects [IS]. 

R. Tsang and B. Fraser-Reid, Serial radical cyclization via a vinyl group immobilized by a pyranoside. A route to bis-annulated pyranosides, J. Am Chem Soc. 108 2116 (1986). 

M. A. Rahman, D. R. Kelly, R. M. Srivastava, and B. Fraser-Reid, Second generation a-enones from a pyranosidic a-enone, Carbohydr. Res., 136 (1985) 91-99. 

The product gave 5-methyl-L-rhamnose phenylosazone and formed a furanoside, but not a pyranoside, with methanolic hydrogen chloride. 

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