Anomers, a and

A new NMR method for the determination of the anomeric configuration in mono- and disaccharides has been described.18 The protocol is based on the different cross-correlated relaxation between proton chemical shift anisotropy (CSA) and dipolar relaxation for the a and (3 anomers of sugars. Only the ot-anomers show the presence of CSA (HI or Hl )-proton dipole (H1-H2 or Hl -H2 ) in the longitudinal relaxation of the anomeric protons. The method is of special interest for cases in which vicinal coupling constants between HI and H2 in both anomers a and (3 are similar and small, such as D-mannose, and the non-ambiguous description of the anomeric configuration needs additional measurements. 

Mutarotation produces two types of cyclic forms called anomers (a and P), which differ in their arrangement about the anomeric carbon atom (originally the carbonyl carbon atom). If the -OH on the anomeric carbon atom is down, then the structure represents the a anomer if it s up, the structure represents the p anomer. Due to the equilibrium present, one anomer rapidly converts to the other. 

By modifications of known procedures, the following compounds have been prepared l-P-D-arabinofuranosylth3miine,2 -deoxy-5-(trifluoromethyl)-uridine and its a-anomer, a- and p-5-trifluoromethyl-6-aza-2 -deoxyuri-dlnes,l-p-D-arabinofuranosyl-5-fluorouracil,5-fluoro-2 deoxycyti-dine,24 3 -deoxynucleosides of pyrimidines, pyrimidine and purine nucleosides of D-glucuronlc acid, 9-D-mannofuranosyladenlne, 3-P-D-ara-... 

Anomers also appear in cyclic form as five-membered rings, for instance in the monosaccharides fructose and ribose. These cyclic molecules are named furanoses because of their similarities to the cyclic ether furan. Starting from D-fructose and D-ribose, the cyclization yields anomers a- and p-D-fructofuranose, as well as a-and P-D-ribofuranose, respectively. 

The anomers a- and /3-glucopyranose should form in equal amounts because they are enantiomers. True or false Explain your answer. 

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... 

Fmctose is sweeter than sucrose at low temperatures (- S C) at higher temperatures, the reverse is tme. At 40°C, they have equal sweetness, the result of a temperature-induced shift in the percentages of a- and P-fmctose anomers. The taste of sucrose is synergistic with high intensity sweeteners (eg, sucralose and aspartame) and can be enhanced or prolonged by substances like glycerol monostearate, lecithin, and maltol (19). 

The syn-anti conformational problem of a- and /3-pyrazofurins (756 one of the rare naturally occurring pyrazole compounds, see Section 4.04.4.4.3), which involves a rotation around a pyrazolic sp carbon atom and a sugar sp carbon atom, has been studied theoretically using the PCILO method (81MI40403). In agreement with the experimental observations, the /3 anomer is energetically more favourable than the a anomer, the preferred conformations being anti and syn, respectively. 

The rules previously mentioned for assignment of a- and /3-configurations can be readily applied to Haworth projection formulas. For the D-sugars, the anomeric hydroxyl group is below the ring in the a-anomer and above the ring in the /3-anomer. For L-sugars, the opposite relationship holds. 

This synthesis appears to be quite general for the preparation of 1-substituted nucleosides and was used with small modifications for the synthesis of l-ribofuranosyl-6-azathymine and 2 -deoxyribo-furanosyl-6-azauracil and -6-azathymine. In the case of 2 -deoxy-ribofuranosyl a mixture of a- and )8-anomers is produced, their ratio depending on the reaction conditions. In the preparation of 2 -deoxy-ribofuranosyl-6-azathymine only one anomer was obtained having probably the )8-configuration, ... 

In similar manner 3,6-dideoxyhexoses have been prepared from esteri-fied 6-deoxy-2-hydroxyglycals. 2,3,4-Tri-0-acetyl-6-deoxy-2-hydroxy-D-glucal was converted into the a and / forms of l,2,4-tri-0-acetyl-2,3-didehydro-3,6-dideoxy-D-en/t/iro-hexose. The a anomer was the main product (77%, 55% isolated crystalline) and, in addition to the ft anomer (19%), a small amount (4%) of saturated products was obtained. On hydrogenation, the major product also suffered some hydrogenolysis but afforded two tri-0-acetyl-3,6-dideoxyhexoses which were shown by NMR spectroscopy to be present in the ratio 12 13 and to have the a configuration. Deacetylation of the reduction products gave... 

Although the Koenigs-Knorr reaction appears to involve a simple backside S 2 displacement of bromide ion by alkoxide ion, the situation is actually more complex. Both a and /3 anomers of tetraacetyl-o-glucopyranosyl bromide give the same /3-glycoside product, implying that they react by a common pathway. 

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. 

In solution, most simple sugars and many of their derivatives occur as equilibrium mixtures of tautomers. The presence of a mixture of two anomers of the same ring size may be indicated in the name by the notation a,P-, e.g. a,P-D-gIucose. In formulae, the same situation can be expressed by separating the representation of the ligands at the anomeric centre from the a and P bonds [see examples (a) and (c)], or by use of a wavy line [(b) and (d)] (particularly if hydrogen atoms are omitted). 

Each of these compounds, 53-56, was shown to be a very effective competitive inhibitor of the enzyme with respect to the fructose 1,6-diphosphate, whereas several other analogs, including acyclic structures, had no effect. These and other results suggest that the furanose form of the sugar diphosphate is the active form in the enzymatic reaction (105). More recent studies using rapid quenching techniques and C-nmr measurements have confirmed this hypothesis and indicate that the enzyme uses the a anomer 52 much more rapidly than the 3 anomer 50 and probably uses the a anomer exclusively (106). 

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. 

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