Ketose occurrence

The parent that includes the functional group most preferred by general principles of organic nomenclature [13,14], If there is a choice, it is made on the basis of the greatest number of occurrences of the most preferred functional group. Thus aldaric acid > uronic acid/ketoaldonic acid/aldonic acid > dialdose > ketoal-dose/aldose > diketose > ketose. 

The synthesis of cellulose by A. xylinum from various polyalcohols14 is accompanied by the formation of carbon dioxide, formic acid, nonvolatile acids, ketoses and sometimes ethanol. The much greater variety of substrates suitable for cellulose synthesis, as compared with the small number for dextran or levan, may account for the widespread natural occurrence of cellulose. 

A similar series of reactions was performed on a 6-azido-6-deoxy-2-ketose.246,252 In most of the foregoing syntheses, the piperidine formed is accompanied by the tautomeric furanose. The tautomers can, however, be separated without occurrence of equilibration (which can be induced by changing the pH of the medium). For the ketose derivative 183, the equilibrium mixture includes246 compound 184 and a dehydration product (182). The conformation and anomeric effect in these piperidines have been studied by n.m.r. spectroscopy,253 and their fragmentation pathways have been determined by mass spectrometry.254... 

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. 

Occurrence. D-Psicose (D-rifeo-hexulose, 30) is produced in the hydrolysis of the antibiotic psicofuranine.147,148 Although the sugar was initially isolated only as the phenylosazone, its identity was proved by synthesis of psicofuranine from tetra-0-acetyl-D-psicosyl chloride. The ketose also occurs in Itea plants, from which it has been isolated as the diisopropylidene acetal.149... 

Of common occurrence in biochemistry are the D forms of the aldoses, namely glucose (the most important), mannose and galactose, and the ketose called fructose. 

Of considerable importance for the preparation of the ketose sugars is the oxidation of the sugar alcohols by bacteria. It was first observed that the occurrence of L-sorbose in the juice of the mountain ash Sorhus aucu-paria) is due to the bacterial oxidation of the precursor, sorbitol. The bacteria responsible were isolated and their oxidizing action on a series of poly-hydric alcohols was extensively studied by Bertrand 238). The active organism was shown to be identical with Acetobacter xylinum previously... 

The fact that sucrose can be synthesized from -D-glucose 1-phosphate and D-fructose is consistent with the evidence presented by Isbell and Pigman 172) and Gottschalk 173) that D-fructose exists as an equilibrium mixture of the pyranose and furanose forms. The total system of sucrose synthesis can be represented by the equation (Fig. 4) which includes the equilibrium reaction of the two ring forms of D-fructose. The occurrence of D-fructose in the sucrose molecule as D-fructofuranose is an indication that sucrose phosphorylase is specific for the furanose configuration of that ketose 17A, 175),... 

Data are provided in Table 4.2 on the occurrence of ketoses of interest in food. 

In comparison to the synthesis of analogous compounds from simple aldoses or ketoses, the situation for sialic acid is more complicated, due to the occurrence of several different functional groups. Synthetic studies on sialo-compounds have been reviewed by Box and Jeanloz (1969), Tuppy and Gottschalk (1972), Holmquist (1975), and Van der Vleugel (1981). 

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