Aldose 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. 

In summary, it may be stated that the reaction of acyl esters of aldoses and aldobioses (see Section III, p. 92) with ammonia consists of a set of competitive pathways, including intramolecular O —> N migrations of acyl groups, deacylations, and transesterifications, with formation of aldose amides and variable proportions of the free sugar as the principal products. Significant proportions of basic or insoluble polymeric substances were not observed with aldose acetates or benzoates, although occurrence of extensive browning indicates the probable formation of soluble melanoidins. 

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. 

The importance of the methyl ethers of D-mannose lies largely in their occurrence in structural studies in the polysaccharide field. Since previous articles in this series14 have discussed the basic principles involved in the preparation and elucidation of the structure of partially methylated aldoses, it is not intended to give unnecessary details in cases where general procedures are already familiar. 

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. 

Figure 4.1 shows the formulas and names for D-aldoses using simplified Fischer projections. The occurrence of aldoses of importance in food is compiled in Table 4.1. Epimers are monosaccharides which differ in configuration at only one chiral C-atom. D-Glucose and D-mannose are 2-epimers. D-glucose and D-galactose are 4-epimers. 

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). 

Modified aldoses, of wide natural occurrence, are the uronic acids. In these molecules the terminal CH2OH group of a hexose has been oxidised to carboxyl. Polysaccharides consisting almost entirely of uronic acid units are pectic acid (D-galacturonic acid) (LXXVIII), the main component of pectin found in plant tissues, and alginic acid (D-mannuronic (LXXIX) and L-guluronic acid (LXXX)) synthesised by the brown algae. For a full description of these acids see Chapter II, p. 73. 

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