Anomeric position

The stereochemistry and points of connection of glycosidic bonds are commonly designated by symbols such as a(l 4) for maltose and (3(1 4) for cellobiose a and (3 designate the stereochemistry at the anomeric position the numerals specify the ring car bons involved... 

This reaction has been used m an imaginative way to determine the ring size of glycosides Once all the free hydroxyl groups of a glycoside have been methylated the glycoside is subjected to acid catalyzed hydrolysis Only the anomeric methoxy group IS hydrolyzed under these conditions—another example of the ease of carbocation for matron at the anomeric position... 

The anomeric position of a carbohydrate was protected as its trichloroethyl ether. Cleavage is effected with Zn, AcOH, AcONa (3 h, 92%). ... 

The incorporation of heteroatoms can result in stereoelectronic effects that have a pronounced effect on conformation and, ultimately, on reactivity. It is known from numerous examples in carbohydrate chemistry that pyranose sugars substituted with an electron-withdrawing group such as halogen or alkoxy at C-1 are often more stable when the substituent has an axial, rather than an equatorial, orientation. This tendency is not limited to carbohydrates but carries over to simpler ring systems such as 2-substituted tetrahydropyrans. The phenomenon is known as the anomeric ect, because it involves a substituent at the anomeric position in carbohydrate pyranose rings. Scheme 3.1 lists... 

Pyridone also catalyzes epimerization of the anomeric position of the tetramethyl ether of glucose. The mechanism involves two double proton transfers. The first leads to a ring-opened intermediate, and the second results in ring closure to the isomerized product ... 

Structurally, 0-glycosides are mixed acetals that involve the anomeric position of furanose and pyranose forms of carbohydrates. Recall the sequence of intermediates in acetal formation (Section 17.8) ... 

A point to be emphasized about glycoside fonnation is that, despite the presence of a number of other hydroxyl groups in the carbohydrate, only the anomeric hydroxyl group is replaced. This is because a carbocation at the anomeric position is stabilized by the ring oxygen and is the only one capable of being fomned under the reaction conditions. 

Electron pair on ring oxygen can stabilize carbocation at anomeric position only. 

Glycoside (Section 25.13) A carbohydrate derivative in which the hydroxyl group at the anomeric position has been replaced by some other group. An O-glycoside is an ether of a carbohydrate in which the anomeric position bears an alkoxy group. 

Reducing sugar (Section 25.19) A carbohydrate that can be oxidized with substances such as Benedict s reagent. In general, a carbohydrate with a free hydroxyl group at the anomeric position. 

Sucrose (Section 25.14) A disaccharide of glucose and fructose in which the two monosaccharides are joined at their anomeric positions. 

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). ... 

When the reaction was run in CH3CN, migration of the EtS group to the 2-position was observed. This is attributed to episulfonium salt formation, with the resultant addition of acetate at the anomeric position. ... 

If it is assumed that the Mitsunobu glycosidation reaction described above proceeds through an SN2-type process with inversion of configuration at the anomeric position, then it follows that the desired / -glycoside can be formed selectively if pure a-lactol 17 is used in the reaction. Unfortunately, the /Mactol isomer of 17 is thermodynamically more stable than the a-diastereoisomer and is formed almost exclusively if the system is allowed to fully equilibrate. In the protic medium used for the Luche reduction, a signifi-... 

When the amino group is at the anomeric position, the compound is normally named as a glycosylamine (see 2-Carb-33.6). 

The ending -yl without locants signifies loss of OH from the anomeric position (see 2-Carb-31.1). Loss of H from the anomeric OH is indicated by the ending -yloxy , without locant. For examples see 2-Carb-33. 

This is the form employed by the carbohydrate databank CarbBank, and is preferred for most purposes. Each symbol for a monosaccharide unit is preceded by the anomeric descriptor and the configuration symbol. The ring size is indicated by an italic / for furanose or p for pyranose, etc. The locants of the linkage are given in parentheses between the symbols a double-headed arrow indicates a linkage between two anomeric positions. In CarbBank, omission of a/p, D/L, or ftp means that this structural detail is not known. 

Combinatorial solution-phase cycloadditions of (1Z,4R, 5R )-4-benzoylamino-5-phenylpyrazolidin-3-on-l-azomethine imines 320 to 3-keto esters afforded a library of 26 highly pure bicyclic pyrazolidinones 713 in 6-89% yields and in 14-100% de. Most of the products were isolated as mixtures of the major (13, 23, 3/ , 5/ , 6/ )-epimers 713 and the minor (1R, 2A, 3R, 5R, 6R )-epimers 714 (Scheme 113). Epimerization of these cycloadducts at the anomeric position in solution was confirmed by H NMR spectroscopy <2007MI717>. 

Fig. 18. A variant of the STARFISH 10-mer in which the Pk antigen is linked to the central glucose core at the anomeric position.166...

The isomerism existing between the pairs of nucleotides was attributed to the different locations of the phosphoryl residues in the carbohydrate part of the parent nucleoside,49 63 since, for instance, the isomeric adenylic acids are both hydrolyzed by acids to adenine, and by alkalis or kidney phosphatase to adenosine. Neither is identical with adenosine 5-phosphate since they are not deaminated by adenylic-acid deaminase,68 60 and are both more labile to acids than is muscle adenylic acid. An alternative explanation of the isomerism was put forward by Doherty.61 He was able, by a process of transglycosidation, to convert adenylic acids a" and 6 to benzyl D-riboside phosphates which were then hydrogenated to optically inactive ribitol phosphates. He concluded from this that both isomers are 3-phosphates and that the isomerism is due to different configurations at the anomeric position. This evidence is, however, open to the same criticism detailed above in connection with the work of Levene and coworkers. Further work has amply justified the original conclusion regarding the nature of the isomerism, since it has been found that, in all four cases, a and 6 isomers give rise to the same nucleoside on enzymic hydrolysis.62 62 63 It was therefore evident that the isomeric nucleotides are 2- and 3-phosphates, since they are demonstrably different from the known 5-phosphates. The decision as to which of the pair is the 2- and which the 3-phosphate proved to be a difficult one. The problem is complicated by the fact that the a and b" nucleotides are readily interconvertible.64,64... 

Polyhydroxylated indolizidines have attracted considerable interest due to their potent activity as glycosidase inhibitors. Some analogues of these molecules bearing a sulfur instead of a carbon atom at the anomeric position such as 430-432 have been prepared (Scheme 112). Compound 431 was shown to give a 3/7 mixture of epimers at the anomeric position in DzO. 

Carbohydrate derivatives, in which one or more of the oxygen atoms bonded directly to the carbon skeleton have been replaced by sulfur, are termed thiosugars. The placement of the sulfur atom at the anomeric position constitutes a special case, because thioglycosides, alkyl, aryl and heterocyclic, occupy a very important place as versatile glycosyl donors in glycosidation methodology. Anomeric thiocarbonyl compounds, on the contrary, have been less explored, although their potential and scope is likely to be similar. 

This methodology applied to aldohexoses differing in amine position on saccharidic template (anomeric position, C-6, C-2, C-3 or C-5) will be discussed. 

Saccharidic templates bearing an amino group in C-2, C-6, C-3, C-5. Generally, an amine located on non-anomeric position reacts similarly through the condensation reaction with carbon disulfide or thiophosgene, the preliminary formed isothiocyanate leads to spontaneous or base-induced cyclization into 1,3-oxazolidine- or l,3-oxazine-2-thiones, as previously mentioned. Several extensive studies are reported about the selective introduction of thionocarbamate moieties on carbohydrate scaffolds.3 22 23 320 34 38... 

It is worth mentioning that in d-manno series, the 2,3-m-fused OZT is formed exclusively, with the 1,2-cz s-regioisomer remaining undetected. This can be explained by a weaker nucleophilicity of the hydroxyl group in the anomeric position. 

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