P-Anomer

Because six membered rings are normally less strained than five membered ones pyranose forms are usually present m greater amounts than furanose forms at equilib rium and the concentration of the open chain form is quite small The distribution of carbohydrates among their various hemiacetal forms has been examined by using H and NMR spectroscopy In aqueous solution for example d ribose is found to contain the various a and p furanose and pyranose forms m the amounts shown m Figure 25 5 The concentration of the open chain form at equilibrium is too small to measure directly Nevertheless it occupies a central position m that mterconversions of a and p anomers and furanose and pyranose forms take place by way of the open chain form as an inter mediate As will be seen later certain chemical reactions also proceed by way of the open chain form... 

Trimethylsilyl triflate—silver perchlorate (1 1) also cataly2es the highly stereoselective glycosylation of P-naphthol to provide predominantiy the P-anomer. 

Early fractions were mixtures of a/(3 anomers while later fractions contained pure P-anomer. The... 

The Noyori procedure was applied to a total synthesis of baiyunoside, a sweet principle, using 2,3,4-tri-<9-benzyl-D-xylopyranosyl fluoride (18 see Table 1), and a synthesis of glycotriosyl ceramide. A model experiment for the synthesis, using 18, showed a solvent dependence for the a ratio of the products. In this case, the use of acetonitrile, oxolane, or ether gave the a anomer (1,2-a.v), and the use of toluene or hexane gave the P anomer (1,2-trans), preponderantly. 

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... 

The pyranoid monosaccharides provide a wide range of asymmetric molecules for study by the c.d. spectroscopist. However, these compounds are not without their difficulties. In aqueous solution, these compounds exist in a complex equilibrium involving the two possible chair conformers of the pyranoses, the furanoses, a and p anomers, and the acyclic form, as well as septanoses for aldohexoses and higher sugars. 

The carboxyl chromophore is axial for the a anomer and equatorial for the p anomer. The sugar was studied as the carboxylate anion as it has a (low) piC of 2.6, and the compound is degraded in acidic solution. The c.d. spectrum of this compound contains contributions from the carboxylate n-jr at 217 nm, the amide n-tr at 210 nm, and the amide 7T7r at 190 nm. Apparently, all of these bands are positive, giving rise to a c.d. spectrum (see Fig. 29) having " a maximum at 199 nm and a shoulder at 210 nm. The c.d. spectra of a number of derivatives confirmed these assignments. 

The cleavage mode of pentaGalU-ol by PGII is essentially the same as found for PGI. Only the secondary hydrolysis reaction of the primary product triGalUA proceeds much more slowly. Spectra are not shown. The time course of the relevant resonances depicted in Fig. 2 demonstrates that the P-anomer of the triGalUA is initially formed. Thus, like PGI, PGII is an inverting enzyme. 

Reaction of 3-amino-l-propanol and 5-bromo-5-deoxy-r/-furanoxylose 469 in DzO was monitored by H NMR spectroscopy (Scheme 43). The a-anomer of trihydroxypyrido[2,l- ][l,3]oxazine 470 formed 20 times faster, but the P-anomer 471 was more stable (Ai /p 7.3). The faster formation of the a-anomer is a consequence of a kinetic anomeric effect that destabilizes the transition state for equatorial N-alkylation and formation of the P-anomer 471 <2000JOC889>. 

This enzyme oxidizes a- and P-anomers of D-glucose to the same extent and shows excellent stability and sensitivity about twice that for the methods with immobilized glucose oxidase. In this approach, pyranose oxidase is immobilized on tresylate-poly(vinylalcohol) beads and packed into a stainless column and peroxidase is immobilized on tresylate-hydrophylic vinyl polymer beads and packed into a transparent FITE tube that is used as the CL flow cell. The H202... 

In summary, for O-glycosides, a-anomers (a-D and oc-l) are thermodynamically more stable than the p-anomers (p-D and p-L) due to the anomeric effect. Therefore l,2-c -a-D- or a-L-glycosides are preferentially obtained, in general. On the other hand, the lower stability of glycosides such as p-D-arabinosides and p-D-mannosides requires special methods for their preparation, thus promoting the development of new synthetic methodologies. 

Deoxy-a-D-ribosyl-l-phosphate 20, a key substrate in the preparation of 2 -deoxynucleosides, was stereoselectively prepared by crystallization-induced asymmetric transformation in the presence of an excess of ortho-phosphoric acid and tri( -butyl)amine under strictly anhydrous conditions (Scheme 2).7 Initial Sn2 displacement of Cl in ot-glycosyl chloride 16 by phosphoric acid resulted in a 1 1 a/p anomeric mixture of 17 and 18 due to the rapid anomerisation of the a-chloride in polar solvents. Under acidic conditions, in the presence of an excess of H3P04, an equilibration between the a and p anomers gradually changed in favour of the thermodynamically more stable a-counterpart. By selective crystallization of the mono tri( -butyl)ammonium salt of the a-phosphate from the mixture, the equilibrium could be shifted towards the desired a-D-ribosyl phosphate 18 (oc/p = 98.5 1.5), which was isolated as bis-cyclohexylammonium salt 19 and deprotected to furnish compound 20. 

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