Anomers glucopyranose

Barrows S E, J W Storer, C J Cramer, A D French and D G Truhlar 1998. Factors Controlli Relative Stability of Anomers and Hydroxymethyl Conformers of Glucopyranose. Journal Computational Chemistry 19 1111-1129. 

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. 

Maltose and cellobiose are both reducing sugars because the anomeric carbons on the right-hand glucopyranose units have hemiacetal groups and are in equilibrium with aldehyde forms. For a similar reason, both maltose and cellobiose exhibit mutaiotation of a and /3 anomers of the glucopyranose unit on the right. 

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. 

The most common activator for the glycosyl sulfoxides is trifluoromethanesulfonic anhydride (triflic anhydride), which, in the absence of nucleophiles, rapidly and cleanly converts most sulfoxides into the corresponding glycosyl triflates in a matter of minutes at —78 °C in dichloromethane solution [86,280,315,316]. In the more extensively studied mannopyranose series, only the a-mannosyl triflate is observed by low-temperature NMR spectroscopy (Scheme 4.35) [280]. In the glucopyranose series, mixtures of a- and (1-triflates are observed, in which the a-anomer nevertheless predominates (Scheme 4.36) [280],... 

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... 

Problem 22.30 (a) Draw the chair conformations for a- and /3-D-(-t-)-glucopyranose. The bulky CHjOH is usually assigned an equatorial position. In flat Fischer formulas. OH s on the right are tram to CHjOH, and those on the left are as. (b) Why are most naturally occurring glycopyranosides /3-anomers -4... 

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