Anomers acetates

LiBF4, CH3CN, 70°, 3-8 h, 81-90% yield. This system of reagents also cleaves benzylidene acetals. Conventional reagents failed to cleave these glycosides. It is interesting to note that the /3-anomers are cleaved more rapidly than the a-anomers and that the furanoside derivatives are not cleaved. 

Several features of the rearrangement have been elucidated. Although in the treatment of the ester 26 with acetic acid the products were isolated in only 64% yield, evidence was obtained (22) that finally no 1,2-unsaturated compounds remained, since the noncrystalline portion on hydrogenation and deacetylation afforded only 3-deoxy-D-ribo and -d-arabino-hexose and no 1,5-anhydrohexitols. That the components of the final mixture were in equilibrium was indicated by the observation that the main component 27 underwent reaction in boiling acetic acid to give a solution with the same optical activity as that of the original reaction mixture. Thus the 2,3-unsaturated compounds are more stable than the hydroxyglycal derivatives and the a isomer 27 is more stable than its anomer 28. 

From the rearrangement of tetra-O-acetyl-2-hydroxy-D-galactal in boiling acetic acid it was possible to isolate l,2,4,6-tetra-0-acetyl-2,3-dide-h.ydro-3-deoxy-a-jy-threo-hexose (32) (58%) and a small amount of 1,2,3,4,6-penta-O-acetyl-jS-D-galactopyranose. In the reaction mixture the presence of some a-pentaacetate was demonstrated chromatographically but NMR spectroscopy indicated no resonances corresponding to the / anomer of compound 32. These spectral measurements indicate that compound 32 constituted 80% of the mixture of products. 

Mutarotation (Section 25.5) The change in optical rotation observed when a pure anomer of a sugar is dissolved in water. Mutarotation is caused by the reversible opening and closing of the acetal linkage, which yields an equilibrium mixture of anomcrs. 

Specifically, in reactions of 4 with acetic anhydride in the presence of zinc chloride58 or boron trifluoride17,35 as the catalyst, 1,2,5-tri-O-acetyl-/J-D-glucofuranurono-6,3-lactone (12) is the preponderant product in the presence of pyridine,58 however, acetylation of 4 by acetic anhydride leads to the favored formation of the corresponding a-D anomer. In contrast to acetylations, in benzoylation reactions, the ratio of anomers apparently changes. Thus, on treatment of 4 with benzoyl chloride in pyridine, Momose and coworkers17 isolated... 

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. 

An intramolecular acetal has also been introduced by the treatment of a mixture of a 1-thio-mannoside, having a methoxybenzyl protecting group at C-2 and an alcohol with DDQ [71] (Scheme 4.4c). Activation of the thioglycoside with methyl triflate gave a P-mannoside as the only anomer. This approach was employed for the synthesis of the core pentasaccharide of N-linked glycoproteins. 

The conformational anomeric effects design the contrasteric effects observed in acetals which render the more sterically encumbered gauche/ gauche conformers more stable than their anti/gauche and anti/anti conformers. Such effects were first evidenced by Jungins in 1905 and rediscovered by Edward in 1955 and by Lemieux and Chiu in 1958. They observed the higher stability of alkyl a-D-glucopyranosides in comparison with their (3-anomers (Fig. 5).8... 

The study of monosaccharides subjected to conditions of methanolysis is considered for two reasons. First, the decomposition of monosaccharides is indicative of the decomposition of monosaccharides liberated during methanolysis second, the ratio of methyl glycosides of a particular standard monosaccharide is the same for the same monosaccharide released during methanolysis (for a particular set of methanolysis conditions), provided that the concentration of sugars is relatively low. Up to four methyl glycosides (the a and )8 anomers of the pyranoside and furan-oside forms) of a particular monosaccharide may be formed the acyclic dimethyl acetal is an additional possibility. The ratios of methyl glycosides of 10 monosaccharides subjected to methanolysis with M hydrogen chloride for 24 h at 80° has been reported. Similar information is also available in another study."... 

In sucrose, fructose is present as the P anomer. Now, one of these sugars has acted as an alcohol to make a bond to the other sugar. We can look at this in two ways. Either frnctose acts as an alcohol to react with the hemiacetal glucose to form an acetal, or alternatively, glucose is the alcohol that reacts with the hemiketal fructose to form a ketal. In sucrose, the pyranose ring is an acetal, whilst the fnranose ring is a ketal. This all seems rather... 

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