Glucose epimers

A non-specific bacterial acid phosphatase from Shigella flexneri (PhoN-Sf) has been screened for regioselective phosphorylation of primary alcohol(s) of more than 20 different cyclic and acyclic monosaccharides using pyrophosphate as the phosphate donor (O Scheme 61) [368]. These studies have shown that PhoN-Sf is capable of phosphorylating a range of hexoses (D-glucose epimers, glycosides, and C-2 derivatives), pentoses, heptoses, ketoses, and acyclic carbohydrates. 

The same behavior was observed for the glucose epimer [85]. On the contrary, starting from benzylated j0-o-gluco and /3-D-galactopyranosyl isothiocyanates, or from acetylated a-D-mannopyranosyl isothiocyanate 61 (Scheme 15), due to the trans disposition of substituents on Cl and C-2 no rearrangement took place after deprotection, and the corresponding 2-glycopyranosylamino-oxazolines were obtained [85]. The a-mannopyran-osyl-oxazoline 63 turned out to be a moderate inhibitor of a-mannosidase (Ki = 98 iM). 

Hlie chemistry of the conversion of the disaccharide sucrose ([a]o = +66.5) to the monosaccharide fructose ([a]D = -92.4) and an equilibrium mixture of a-D-glucopyranose (a-D-glucose) and its anomer, (i-D-glucopyranose (p-D-glucose) ([ajo = +52.7 for the mixture), the structures of which were unknown at the time, will be discussed in Chapter 11. The positive rotation of the mixture of glucose epimers is less than the high negative rotation of fructose. 

Glucose Epimers Or Fluoro Analogues Epimeric At Position of Receptor Binding Percent H-Bond Atom Stimulation Acceptor ... 

Glucose Epimer or Fluoro Analogs Epimeric At Binding Atom Percent Stimulation Position of Receptor Position of H-Bo Receptor Acceptor H-Bond Donor... 

In section 10.1, we saw that the first hexose to be formed in the fixation of CO2 was D-fructose-l,6-bisphosphate. This sugar phosphate is converted to D-fruc-tose-6-phosphate by a specific phosphatase (reaction 10.3). The keto group can be epimerized by two different enzymes, D-fructose-6-phosphate/D-glucose epimer-ase or D-fructose-6-phosphate/D-mannose epimerase, to form D-glucose-6-phosphate and D-mannose-6-phosphate, respectively (see Fig. 10.5). Both of these sugar phosphates can be converted into their 1-phosphates by reaction with specific mutase enzymes. a-D-Glucose-1-phosphate and a-D-mannose-1-phosphate can then react with UTP and GTP to form UDP-Glc and GDP-Man (see Fig. 10.5). 

We can also nse the term epimer to describe the relationship between isomers, where the difference is in the confignration at jnst one centre (see Section 3.4.4). This is shown for the four epimers of o-(- -)-glucose. An interesting observation with the 16 stereoisomers is that optical activity of a particular isomer does not appear to relate to the confignration at any particnlar chiral centre. 

The synthesis of " C-labelled o-glucose starts with the pentose o-arabinose and " C-labelled potassium cyanide, which react together to form a cyanohydrin (see Section 7.6.1). Since cyanide can attack the planar carbonyl group from either side, the cyanohydrin product will be a mixture of two diastereoisomers that are epimeric at the new chiral centre. The two epimers are usually formed in unequal amounts because of a chiral influence from the rest of the arabinose structure during attack of the nucleophile. 

On the other hand, borohydride reduction of the ketose o-fructose will give a mixture of o-glucitol and its epimer, D-mannitol. A better approach to D-mannitol would be reduction of the aldose D-mannose. o-Glucitol (sorbitol) is found naturally in the ripe berries of the mountain ash (Sorbus aucuparia), but is prepared semi-synthetically from glucose. It is half as sweet as sucrose, is not absorbed orally, and is not readily metabolized in the body. It finds particular use as a sweetener for diabetic products. o-Mannitol also occurs naturally in manna, the exudate of the manna ash Fraxinus ornus. This material has similar characteristics to sorbitol, but is used principally as a diuretic. It is injected intravenously, is eliminated rapidly into the urine, and removes fluid by an osmotic effect. 

The stereochemical isomerization or structural rearrangement resulting in the interconversion of epimers. For example, the conversion of jS-D-glucose to jS-D-galac-tose involves epimerization at the C-4 carbon atom, and the epimerization at the C-2 of jS-D-glucose results in the synthesis of jS-D-mannose. 

The carbonyl group (and adjacent alcohol) oxidizes with excess phenyl hydrazine (PhNHNH2) to form an osazone (see Figure 16-15). Osazone formation is very important in determining the relationship between various monosaccharides. For example, both D-glucose and D-mannose produce the same osazone, so they re epimers. Epimers differ by only one chiral center, which osazone formation destroys. 

Problem 22.8 Glucose is reduced to a single alditol fructose is reduced to two epimers, one of which is identical to the alditol from glucose. Explain in terms of configurations. ... 

In glucose no new chiral C is formed on going from CHO to CH,OH. In fructose the carbonyl C (C ) becomes chiral there are two configurations and we get epimers. 

Since mannose and glucose are C epimers, they are identical at the C, C. C, and C" portions of the molecule, which are unaltered during osazone formation. The chiral C , which differs in each hexose, loses chirality in the osazone and becomes identical for both mannose and glucose. 2-Ketohexoses give osazones in which the C CH,OH is oxidized. Since C, C, C and C of fructose and glucose are identical (Problem 22.8), fructose also gives the same osazone. Identical portions are in the boxes below. 

Problem 22.23 o-Allose, o-glucose and o-talose (the epimer of o-galactose) each give a me o-heptaglycaric acid after the sequence----— ------------ , Assign all structures. ... 

Problem 22.24 What monosaccharide is obtained from a Ruff degradation of o-mannose (Problem 22 11), the C epimer of o-glucose (Problem 22.23)7 ... 

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