Anomeric separation

In extending this direct method of synthesis, we next investigated the possibility of preparing similarly constituted halides from 2-deoxy-D-arabino-hexose (2-deoxy-D-glucose) (21). The hexose was subjected to a partial anomerization procedure described by Bergmann and co-workers (1). The solid material obtained by this procedure is a mixture of the anomeric forms of 2-deoxy-D-arabino-hexose low temperature p-nitro-benzoylation of the latter in pyridine resulted in a mixture of crystalline, anomeric tetrakis-p-nitrobenzoates in a ratio of approximately 1 1. They were readily separable by fractional recrystallization, and treatment of either with an excess of hydrogen bromide in dichloromethane, or with... 

The 3-deoxyhexoses were obtained crystalline after separation on a column of cellulose, a rather tedious operation recently separation of these sugars by fractional crystallization has been reported (41). From the major unsaturated ester 27, 3-deoxy-D-ribo- and -arabino-hexose were shown to have been produced respectively in the ratio 66 34, and from ester 28 in the ratio 24 76. Saturation of the enol grouping of the esters 27 and 28 gives, therefore, the 1,2-cis products preferentially. However, some hydrogenolysis of the anomeric acetate group accompanies simple saturation of the double bond and poses another separation problem. 

In solution, most simple sugars and many of their derivatives occur as equilibrium mixtures of tautomers. The presence of a mixture of two anomers of the same ring size may be indicated in the name by the notation a,P-, e.g. a,P-D-gIucose. In formulae, the same situation can be expressed by separating the representation of the ligands at the anomeric centre from the a and P bonds [see examples (a) and (c)], or by use of a wavy line [(b) and (d)] (particularly if hydrogen atoms are omitted). 

The second separation method involves n.O.e. experiments in combination with non-selective relaxation-rate measurements. One example concerns the orientation of the anomeric hydroxyl group of molecule 2 in Me2SO solution. By measuring nonselective spin-lattice relaxation-rat s and n.0.e. values for OH-1, H-1, H-2, H-3, and H-4, and solving the system of Eq. 13, the various py values were calculated. Using these and the correlation time, t, obtained by C relaxation measurements, the various interproton distances were calculated. The distances between the ring protons of 2, as well as the computer-simulated values for the H-l,OH and H-2,OH distances was commensurate with a dihedral angle of 60 30° for the H-l-C-l-OH array, as had also been deduced by the deuterium-substitution method mentioned earlier. 

Hultiple products are frequently observed for the separation of TMS-sugar derivatives. At equilibrium reducing sugars can exist in more than one isomeric form known as anomers. Formation of their THS derivatives followed by gas chromatography will result in multiple peaks corresponding in composition to the equilibriue anomeric mixture [436]. 

Unlike the behavior of 81, treatment of 2,3,5-tri-O-benzyl-D-ribofuranosyl bromide91 (110) with mercuric cyanide gave an ano-meric mixture of cyanides (111), which was reduced to an epimeric mixture of amines.92 Separation of this mixture by column chromatography gave the D-alio isomer (112), the D-altro isomer (113), and the glycal derivative (114). Compound 112 was, however, formed in moderate yield (18%), presumably because of a preponderance of the a anomer in the mixture of anomeric cyanides (111). Compound 112 was converted92 into the 1-ureido derivative (92) by treatment... 

These model reactions were of great value in the extension of the C-glycosylation reaction with malonic esters to five-membered ring-systems. Treatment of 2,3 5,6-di-O-isopropylidene-a-D-manno-furanosyl bromide122,123 (150) with diethyl sodiomalonate led to an anomeric mixture of C-glycosyl compounds that could be separated by column chromatography, with the a (151) and /B (152) anomers in... 

For 2,3 5,6-di-O-isopropylidene-a-D-mannofuranose (183), separation of the anomeric products (184 and 185) was possible,135 and their structures were elucidated by periodate-oxidation studies. The configurational assignment for these products was based on the greater value of the specific rotation for the a anomer, as compared with that of the j3 anomer. The higher chromatographic mobility of one of the... 

Since 2-deoxy-2-fluoro derivatives of aldopyranoses and furanoses are far more stable towards hydrolytic conditions than their 2-deoxy counterparts, the synthesis of the former entails primary preparation of the anomeric phosphotriesters 31, which are chromatographically separated into individual diastereomers and subsequently deprotected on phosphorus... 

The turning of the key once the complex has formed is a separate issue. In this regard, Lemieux (47) has pointed out that rotation about the glycosidic bond must weaken the exo-anomeric effect and thereby importantly activate the anomeric carbon to nucleophilic attack. Therefore, it seems likely that the role of the key hydroxyl group of the aglycon is to accommodate the rotation prior to the attack by water to form / -D-glucopyranose, which is the first product of the reaction. 

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