Ketohexoses D-fructose

Figure 4.17 The trioses D-glyceraldehyde (aldose) and dihydroxyacetone (ketose), the pentose D-ribose, the hexoses D-galactose and D-glucose (aldoses) and the ketohexose D-fructose in their open chain forms. The configuration of the asymmetrical hydroxyl group on the carbon, the furthest away from the aldehyde or ketone group, determines the assignment of D- or L-configuration.

Epimerization. In weakly alkaline solutions, glucose is in equilibrium with the ketohexose D-fructose and the aldohexose D-mannose, via an enediol intermediate (not shown). The only difference between glucose and mannose is the configuration at C-2. Pairs of sugars of this type are referred to as epi-mers, and their interconversion is called epimerization. 

Hausoul et al. [60] also reported on telomerization with aldopentoses (D-xylose, L-arabinose), aldohexoses (D-glucose, D-mannose, D-galactose), ketohexoses (d-fructose, L-sorbose) and the disaccharides D-sucrose and cellobiose, using Pd/ TOMPP as catalyst without the addition of base in /V,/V-di methyl acetamide as the solvent (Fig. 15). The Pd/TOMPP combination had previously been shown to be highly active in the telomerization of various polyols (vide supra). Good conversion... 

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 add in aqueous solution, the 2-ketohexoses, D-fructose, L-sorbose and D-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-D-xylose, respectively (see Petrus, Petrusov4 and Hricovmiova, this voL). 

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

From a structural point of view, the carbohydrate template can have either furan or pyran rings although in some cases open chain structures can be formed. A large variety of aldopentoses (e.g. d- and L-arabinose, D-ribose, D-xylose), aldohexoses (e.g. D-glucose, D-mannose, D-galactose) as well as ketohexoses (e.g. D-fructose, L-sorbose) can be used as scaffolds. 

The in vitro bioassays allowed to determine the inhibition constant of D-fructose transport by the CHO cells. This measure is carried out by competition with radioactive D-fructose. The study put in evidence that pentose-OZT derivatives are not recognized by the protein transporter. Only the ketohexose-OZT derivatives expressed some inhibition of GLUT5. These inhibition constants showed to be much effective with L-Sor derivatives than with D-Fru derivatives and even better than D-fructose itself (Kt = 15.5 mM) (Table 2). 

These blocked structures gave a picture of the interaction of D-fructose with the protein. It is assumed that a ketohexose C-l additional arm (absent in pentose series), is required in order to achieve some inhibition, but a certain freedom of substitution is possible (OBn, OA11 or OH). Comparison of D-Fru- and L-Sor-OZTs showed that substituted derivatives are better... 

It is noteworthy that D-fructose, which has a pyranose structure in the free crystalline state, assumes a furanose configuration whenever it combines with another sugar to form an oligosaccharide or polysaccharide. Apparently the ketohexose L-sorbose shows the same behavior. 

Phosphoric acid esters of the ketopentose D-ribulose (2) are intermediates in the pentose phosphate pathway (see p.l52) and in photosynthesis (see p.l28). The most widely distributed of the ketohexoses is D-fructose. In free form, it is present in fruit juices and in honey. Bound fructose is found in sucrose (B) and plant polysaccharides (e.g., inulin). 

The ketohexoses identified as components of bacterial polysaccharides include D-fructose, found in Vibrio lipopolysaccharide167 and Hemophilus influenzae capsular polysaccharide,252 and D-xylulose (d- riireo-2-pen-tulose), identified in Pseudomonas lipopolysaccharide.253 Activated forms of the monosaccharides were not determined. 

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. 

A mixture of ketohexoses, from which D-sorbose and D-fructose were isolated in crystalline condition,29 was also obtained by barium hydroxide treatment of D-glyceraldehyde alone, or with dihydroxyacetone. 

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). 

Sugars are further classified by the number of carbons. Thus, aldoses include aldotrioses (C3 D-glyceraldehyde, HO—CH2—GHO), aldotetroses (Gy D-erythrose) aldopentoses (C-, D-ribose, D-arabinose, D-xylose) and aldohexoses (Ce D-glucose, D-mannose, D-gulose, D-galactose). Ketoses include ketotrioses (Gy dihydroxyacetonephosphate, HO-CH2-CO— CH2OH), ketotetroses (C D-erythrulose), ketopentoses (C3 D-ribulose, D-xylulose) and ketohexoses (C6 D-fructose). 

The family of D-ketoses, shown in Figure 27.5, is formed from dihydroxyacetone by adding a new carbon (bonded to H and OH) between C2 and C3. Having a carbonyl group at C2 decreases the number of stereogenic centers in these monosaccharides, so that there are only four D-ketohexoses. The most common naturally occurring ketose is D-fructose. 

The same procedure can be used to draw the furanose form of D-fructose, the most common ketohexose. Because the carbonyl group is at C2 (instead of Cl, as in the aldoses), the OH group at C5 reacts to form the hemiacetal in the five-membered ring. Two anomers are formed. 

Dihydroxyacetone is the simplest ketose. The stereochemical relation between d-ketoses containing as many as six carbon atoms are shown in Figure 11.3. Note that ketoses have one fewer asymmetric center than do aldoses with the same number of carbons. d-Fructose is the most abundant ketohexose. 

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. 

D-Fructose, a ketohexose, can potentially form either a five-membered (furanose) or a six-membered (pyranose) ring involving formation of an internal hemiketal linkage between C2 (the anomeric carbon atom) and the C5 or C(, hydroxyl group, respectively. The hemiketal linkage introduces a new asymmetrical center at the C2 position. Thus, two anomeric forms of each of the fructo-furanose and fructopyranose ring structures are possible (Figure 9-10). In aqueous solution at equilibrium, fructose is present predominantly in the )3-fructopyranose form. 

Natural 2-ketohexoses, with the relative configurations of the three asymmetric centres corresponding to all four aldopentoses are known, and have trivial names D-psicose (= D-rifto-hexulose), D-fructose (= D-arabino-hexulose), L-sorbose (= L-xy/o-hexulose) and D-tagatose (= D-/yxo-hexulose) (Figure 1.7). 

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