Aldohexose isomers

The first line of Table 3.1 shows seven aldohexose isomers of D-glucose. D-Galactose is the only other isomer identified in wine, but in very small quantities (0.1 g/1). Each of these D isomers has an L stereoisomer that is of no major biological importance. Similarly, ketohexoses, with three asymmetrical carbons, have eight isomers. D-Fruc-tose is the only one present in grapes and wine. 

Each series is built up by adding additional CHOH units between carbon 1 and carbon 2 to produce increasing numbers of isomers, the actual number being 2", where n is the number of additional chiral centres. Thus there are eight stereoisomers of the D-aldohexoses (Figure 9.3), eight stereoisomers of... 

The carbonyl group in glucose and ribose is an aldehyde such compounds are termed aldoses. Fructose, by contrast, has a ketone group and is therefore classified as a ketose. Glucose could also be termed an aldohexose and fructose a ketohexose, whereas ribose would be an aldopentose, names which indicate both the number of carbons and the nature of the carbonyl group. Another aspeet of nomenclature is the use of the suffix -ulose to indicate a ketose. Fructose could thus be referred to as a hexulose, though we are more likely to see this suffix in the names of specific sugars, e.g. ribulose is a ketose isomer of the aldose ribose. 

Williams and J. K. N. Jones have shown that the dithioacetals of aldopentoses and aldohexoses may be separated as their acetates.472 In certain cases, however, isomers were not separated (for example, D-altrose and D-talose), but reduction of the dithioacetal to the 1-de-oxyalditol permitted resolution of the acetates. The method is not suitable for quantitative determinations, but it does preserve the... 

Anhydrofuranoses derived from all eight diastereoisomeric aldohexoses were described in the literature [6]. These compounds are formed as side products in the synthesis of 1,6-anhy-dropyranoses from free (or partially protected) sugars. Treatment of free sugars with toluene-p-sulfonic acid in DMF solution affords the furanose and pyranose 1,6-anhydrides with the furanose form up to 33% for the galacto-, alio-, and talo- isomers [3] the example is shown in O Scheme 9. 

Deoxy-S-O-methyl-aldohexoses.—In this group, representatives of only six pairs are known. They may be differentiated unequivocally by using two systems of paper chromatography, but the evaluation must be made cautiously, since the isomers having the alio and gulo configurations are not known. 

Ans. Constitutional isomers. Ketohexoses differ from aldohexoses in connectivity. 

For a sugar having n stereocenters, there are 2" isomers, so that there are 2 =16 aldohexoses, 8 in the d series and 8 in the l series. The names of the D-aldoses and their stereochemical relationships are illustrated in Scheme 5. These names form the basis of the group-configurational designators alio, altro, gluco,... 

By means of 250 MHz H n.m.r. spectra, the solution conformations of stereoisomers of peracetylated aldohexose dimethyl acetals and diethyl acetals have been determined. With the exceptions of the T>-galacto- and D-man/io-isomers, which were extended planar, all isomers were in the sickle conformation. The results were interpreted on the basis of avoidance of parallel 1,3-substituent interactions. 

Now we know the structures of all the aldohexoses.The structures of the eight L isomers are simply the mirror images of the D series, and we have at last completed our task. 

---