Fructose a 2-ketohexose

D Fructose (a 2 ketohexose also known as levulose it IS found in honey and IS signficantly sweeter than table sugar)... 

Glucose is an aldohexose, where aldo- indicates that it is an aldehyde, -hex- designates the number of carbons, and -ose is the suffix used for carbohydrates. Some other common monosaccharides are fructose, a 2-ketohexose that is isomeric with glucose, and ribose, an aldopentose that contains one fewer carbons than glucose. 

In this reaction, a-D-glucose 6-phosphate, an aldohexose, is converted to a-D-fructose 6-phosphate, a 2-ketohexose ... 

Problem 22.34 (a) Write shorthand structures for the three o-2-ketohexoses other than o-fructose. (b) Which one does not give a meso alditol on reduction (c) Which one gives the same osazone as does D-galactose (Problem 22.22)7 <... 

Besides D-fructose, there are three D-2-ketohexoses D psicose, D-sorbose and t>-tagatose, (a) Draw the possible configurations for these three ketoses, (b) Given the configurations of all aldohexoses, tell how you could assign definite configurations to the ketoses. 

Ans. The previous discussion was an oversimplification. Hemiketal formation in o-fructose and other ketohexoses results in reaction at both C-5 and C-6 to give a mixture of 5- and 6-membered rings. Pure fructose in solution consists of a mixture of the a- and /3-anomers of both furanose and pyranose structures, as well as a very small amount (< 0.2 percent) of the open-chain form. However, all combined forms of o-fructose, e.g., fructose combined with glucose in sucrose, involve only the 5-membered ring. For this reason, most texts simplify matters for the student by ignoring the pyranose structures. The exclusive use of the 5-membered ring in combined forms of o-fructose is not understood. 

Step 2 is the isomerization of glucose to fructose. This reaction involves the conversion of the aldohexose into the 2-ketohexose. Retro-aldol reaction of the aldohexose leads to a C4 and C2 sugar, whereas the ketohexose leads to the two trioses, dihydroxyacetone (DHA) and glyceraldehyde (GLY). As the pathway to LA involves the trioses, selective glucose isomerization is essential, its conversion being limited by equilibrium in the operational temperature window. The isomerization of aldo- to ketoses can proceed via an acid-catalyzed hydride shift, a base-catalyzed mechanism with a proton shift (and intermediate enol), or via a concerted 1,2-hydride shift in neutral media [96, 97]. The latter isomerization mechanism occurs at mild temperatures (100°C) in the presence of Lewis acid catalysts, first... 

The three most abundant hexoses in the biological world are D-glucose, D-galactose, and D-fructose. The first two are D-aldohexoses the third is a D-2-ketohexose. Glucose, by far the most common hexose, is also known as dextrose because it is dextrorotatory. Other names for this monosaccharide are grape sugar and blood sugar. 

In this context, it appears to be of relevance to briefly mention some comparable reaction systems for ketose epimerization. Petrus and his group, as well as Petrus, Serianni and co-workers, recently reported the stereospecific molybdic acid catalyzed isomerization of 2-hexuloses to branched-chain aldoses [55-57]. Upon treatment with a catalytic amount of molybdic acid in aqueous solution, the 2-ketohexoses, o-fructose, L-sorbose and o-tagatose, underwent a stereospecific intramolecular rearrangement to give the corresponding 2-C-hydroxyme-thylaldoses, 2-C-hydroxymethyl-D-ribose (o-hamamelose), 2-C-hydroxymethyl-L-lyxose and 2-C-hydroxymethyl-o-xylose, respectively (see Petrus, Petrusova and Hricovfniova, this vol.). 

In addition to forming its C-2 epimer in a basic solution, o-glucose also undergoes an enediol rearrangement, which forms o-fructose and other ketohexoses. 

A kinetic determination of 2-ketohexoses has been based on their reactions with the cysteine-sulphuric acid reagent sorbose gave a distinct absorption spectrum, whereas o-fructose, psicose, and tagatose were distinguished from each other by differences in the rate of increase of the absorbance at 412 nm. Procedures used in colorimetric ultramicroassays of reducing sugars, and... 

Fructose, a ketohexose, is also a reducing sugar. Usually a ketone cannot be oxidized. However, in a basic Benedict s solution, a rearrangement occurs between the ketone group on carbon 2 and the hydroxyl group on carbon 1. As a result, fructose is converted to glucose, which produces an aldehyde group with an adjacent hydroxyl that can be oxidized. 

The substantial amounts of this ketohexose are mainly prepared by base-catalyzed isomerization of starch-derived glucose, yet may also are generated by hydrolysis of inulin, a fructooligosaccharide. An aqueous solution of fructose—consisting of a mixture of all four cyclic tautomers (Figure 2.5), of which only the (3-D-pyranose ((3-p) form present to about 73% at room temperature is sweet — about 1.5 times sweeter than an equimolar solution of sucrose hence, it is widely used as a sweetener for beverages ( high fructose syrup ). 

The common six-carbon sugars (hexoses) are D-glucose, D-fructose, D-galactose, and D-mannose. They all are aldohexoses, except D-fructose, which is a ketohexose. The structures of the ketoses up to Cf) are shown for reference in Figure 20-2. The occurrence and uses of the more important ketoses and aldoses are summarized in Table 20-1. 

Five-carbon sugars, such as D-ribose (Topic Gl) and D-deoxyribose (Topic FI), and six-carbon ketose sugars (ketohexoses), such as D-fructose, form rings called furanoses (Fig. 6a) by comparison with the compound furan (Fig. 6b). Again furanoses can exists in both a and (5 forms (Fig. 6a) except here the nomenclature refers to the hydroxyl group attached to C-2 which is the anomeric carbon atom. 

In the sorbose series 3-methyl-D-sorbo8e14 has been synthesized, as well as the aforementioned 4-methyl-8 derivative the latter compound was obtained in a series of reactions from D-fructose derivatives through l,2-isopropylidene-3,4-anhydro-D-psicose. In the earlier study8 the configuration of the 4-methyl ketohexose was shown to be that of a D-sorbopyranose by methylation of l,2-isopropylidene-4-methyl-D-sorbose (VI) to the trimethyl derivative (LXV) which was hydrolyzed to 3,4,5-trimethyl-D-sorbose (LXVI). The latter compound, on oxidation, gave derivatives of levorotatory dimethoxysuccinic acid (LXVIII) and xylo-trimethoxyglutaric acid (LXVII). 

The C-2 keto group in the open-chain form of a ketohexose, such as fructose, can form an intramolecular hemiketal by reacting with either the C-6 hydroxyl group to form a six-membered cyclic hemiketal or the C-5 hydroxyl group to form a five-membered cyclic hemiketal (Figure 11.5). The five-membered ring is called a furanose because of its similarity to... 

Fructose is a ketohexose found in honey and a wide variety of fruits and vegetables. Combined with glucose in an a( 1 2)j8 linkage, it forms sucrose (Chapter 9). It makes up one sixth to one third of the total carbohydrate intake of most individuals in industrialized nations. Inulin, found in some plants, is a polymer containing about 40 fructose residues connected in (2 1) linkages with some )S(2 —> 6) branch points. It cannot be hydrolyzed by any human enzyme and has been used for measurement of renal clearance. 

For a ketohexose such as fructose, the C-2 keto group in the open-chain form of fructose reacts with a hydroxyl group within the same molecule to... 

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