D-Glucopyranose anomers

But there are two chair conformations possible for a D-(+)-glucopyranose anomer 1 and 11 for /3-D-(+)-glucopyranose, for example. 

Glucose precipitates from concentrated solutions at room temperature to give crystals that melt at 146°C. Stmctural analysis by X-ray diffraction reveals that these crystals contain only the a-D-(+)-glucopyranose anomer (Figure 24-3). When crystalline a-D-(+)-glucopyranose is dissolved in water and its optical rotation measured immediately, a value = +112... 

Both anomers of o-glucopyranose can be crystallized and purified. Pure a-n-glucopyranose has a melting point of 146 °C and a specific rotation, lo-Jn, of +112.2 pure /3-D-glucopyranose has a melting point of 148 to 155 °C and a specific rotation of +18.7. When a sample of either pure anomer is dissolved in water, however, the optical rotation slowly changes and ultimately reaches a constant value of +52.6. That is, the specific rotation of the a-anomer solution decreases from +112.2 to +52.6, and the specific rotation of the /3-anomer solution increases from +18.7 to +52.6. Called mutarotation, this change in optical rotation is due to the slow conversion of the pure anomers into a 37 63 equilibrium mixture. 

Several 1 -phosphates of deoxyfluoro sugars were prepared, and their acid-catalyzed hydrolysis was studied. 2-Deoxy-2-fluoro- (580), 3-deoxy-3-fluoro- (582), 4-deoxy-4-fluoro- (583), and 6-deoxy-6-fluoro-a-D-gluco-pyranosyl phosphates (584) were prepared by treatment of the corresponding per-( -acetylated )9-D-glucopyranoses with phosphoric acid [the p anomer (581) of 580 was prepared by a different method]. The first and second ionization constants (pA a, and pA a2) of these compounds were determined potentiometrically, as well as by the F-n.m.r. chemical shifts at a series of pH values, and then the rate constants of hydrolysis for neutral (B) and monoanion (C) were decided. The first-order rate-constants (k) for 580-584 and a-D-glucopyranosyl phosphate (in Af HCIO4,25 °) were 0.068, 0.175, 0.480, 0.270, 1.12, and 4.10 (all as x lOVs), respectively. The rate... 

As generally expressed in Section I, derivatives of a-D-glucopyranose are thermodynamically more stable than the /3-d anomers, whereas, for the D-glucofuranoses, the opposite stability of the anomers is observed. This regularity also applies to the respective glycosyl bromides and chlorides. 

Stereospecificity was also encountered in long-range interactions. For 3-deoxy-3-fluoro-D-glucopyranose, the a anomer resonated to... 

Several methods were described for the selective de-S-acetylation of 0-acetyl protected 1-thioglycoses. Sodium methoxide in methanol at low temperature (below -20 °C) was known to afford mainly the de-S-acetylated compound [16] or exclusively this compound when the reaction was quenched at low temperature by adding H-l- resin [17]. Demercuration of tetra-O-acetyl-l-phenylmercury(II)-thio- -D-glucopyranose (12) (Scheme 4) obtained by treatment of (8e) with phenylmercury(II)acetate afforded a convenient synthesis of tetra-0-acetyl-l-thio-/3-D-glucose (8a) [18]. This sequence applied to the a-anomer (10a) (Scheme 3) led to the expected de-S-acetylated compound (10b) [19]. Chemoselective deprotection of thioacetate at the anomeric position of peracetylated 1-thioglycoses was also achieved in good yield by action of cysteamine in acetonitrile or hydrazinium acetate in DMF [20,11]. 

The surprising feature of such a structure is the possibility of the occurrence of the /3-d anomer in the crystal. The electron density can be explained by the presence of 22% as the /3-d anomer it is indicated by the height of an unexplained peak near C-l at the correct distance and angle for the /3-D structure. When once-recrystallized 2-acetamido-2-deoxy-a-D-glucopyranose is dissolved, its initial rotation is [a]D +56.5°, as compared to +82° for the a-D and —21.5°... 

The most important result, however, is the binding of the trisaccharide tri-N-acetylchitotriose, as the model proposed for substrate binding and enzymic hydrolysis is based on this compound. The three substituted /3-D-glucopyranose residues are labeled A, B, and C in Fig. 2. Two observations are the same as for 2-acetamido-2-deoxy-D-glucose residue C has the same binding as the /3-d anomer, and the resulting shift of amino acid residue 62 (L-tryptophan) is 0.75 A. The hydrogen bonds between lysozyme and carbohydrates A and B are shown in Fig. 2 and listed in Table IV. Residue A, which is located... 

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