Anomeric ratio

In view of the great preponderance of the 3-d anomer in crystalline D-glucofuranurono-6,3-lactone (4), the results obtained from its acetylation with acetic anhydride-pyridine disagree with observations59 suggesting that anomeric ratios are not altered during acylation in pyridine. 

A method to form nucleosides from chlorofuranoses was shown to be highly /3-selective (Equation 5) <2000TL8207>. For example, the reaction of 6-(benzyloxy)purine with 41 provided nucleoside 42 with a >99 1 ratio of [3- to a-anomers. Product distribution was found to be independent of the anomeric ratio of the chlorofuranose starting materials. 

To a solution of phenylthio disaccharide 3 (56 mg, 0.11 mmol) in CHjC (2 mL) at —15°C was added DAST (21 pL, 0.16 mmol), followed by Af-bromosuccinimide (25 mg, 0.14 mmol). After stirring at — 15°C for 15 min, the reaction mixture was poured onto saturated aqueous NaHC03 solution (5 mL) and extracted with ether (3 x 10 mL). The organic extracts were combined, washed with brine (5 mL), and dried (MgS04). Evaporation of the solvent followed by flash chromatography (silica, ether-petroleum ether) gave the disaccharide fluoride 4 (40 mg, 85%) with an anomeric ratio of 5 1 (oc/p). 

Fig. 6. Limited stereoselectivity in NeuA-catalyzed aldol additions, observed with aldopyranose substrates (including known anomeric ratios)...

With FAPY-A derived from dAMP, where the pyranose forms cannot play a role, an anomeric ratio of p/a = 1.33 develops after 6 h, while the deglycosylation half-life is 103 h at 37 °C (Greenberg et al. 2001). The deglycosylation of FAPY-G derived from dGMP is even slower. 

In general, a six-membered pyranose form is preferred over a five-membered furanose form because of the lower ring strain, and these cyclic forms are very much favoured over the acyclic aldehyde or ketofte forms. As can be seen in Table 1.3, at equilibrium, the anomeric ratios of pyranoses differ considerably between aldoses. These observations are a direct consequence of differences in anomeric and steric effects between monosaccharides. The amount of the pyranose and furanose present in aqueous solution varies considerably for the different monosaccharides. Some sugars, such as D-glucose, have undetectable amounts of furanose according H-NMR spectroscopic measurements whereas others, such as D-altrose, have 30% furanose content under identical conditions. 

As for hydrolysis of disaccharides maltose and cellobiose, the (Ha anomeric ratios are in agreement in terms of stereoelectronic effects which are, according to the... 

The H-l signals of the a and fi anomer of GlcNAc-2 are found at 8 5.209 and 4.718, respectively. The intensity of these signals reflects the anomeric ratio. The same ratio is found for the signals of theN-ace-tyl group of GlcNAc-2, at 8 2.043 (a anomer) and 2.041 (j3 anomer), respectively. 

As discussed for 9, H-3 of Man-4 can be conceived of as being a structural-reporter group typical for the presence of the 7-8 branch. Its signal is doubled the intensities reflect the anomeric ratio. 

The H-l atom of Man-4 resonates, as one doublet with relatively broad lines, at a position that is rather low-field for an asialo upper branch (8 5.127). The H-2 signal of Man-4 appears as two doublets of doublets in the anomeric ratio. The shift increment (A8 0.12 p.p.m.) of H-2 of Man-4 in comparison to 5 reflects the substitution at 0-2 of Man-4 by GlcNAc-5, as already described for the step from 5 to 6. 

The Anomeric Ratio op Products op the Hilbert-Johnson Reaction op Dimethoxyfyrimidines (54 and 55) with Halogenoses (56, 57, and 58) in Acetonitrile (Procedure A) and in Benzene (Procedure B)... 

Schnupf, U. Willett, J. L. Momany, F. DFTMD studies of glucose and epimers Anomeric ratios, rotamer populations, and hydration energies, Carbohydr. Res. 2010, 345, 503-511. 

FIGURE 2.36 The a,p-anomeric ratio of some D-glucopyranose derivatives. 

Glycosyl fluorides, already addressed as substrates for cyanations and allylations, are excellent substrates for radical-mediated C-glycosidations. As shown in Scheme 7.80, Nicolaou et al. [43] demonstrated such reactions in the coupling of 2,3,4,6-tetra-O-benzyl-D-glucopyranosyl fluoride with acrylonitrile. The reaction gave a 61% yield with an anomeric ratio of greater than 10 1 favoring the a isomer. 

Tetra-O-benzylglucose (Scheme 16) exists at room temperature in benzene or THF in an a P anomeric ratio of This ratio is presumably not much changed on 1-0-deprotonation. How-... 

N(CH2)2 group, the polarity of the solvent has little effect on the anomeric ratio, and the proportion of the a anomer is the same in dimethyl sulfoxide and water solution. ... 

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