Structures of the Aldoses and Ketoses

d-Aldoses are related to d-glyceraldehyde as can be illustrated by their chemical synthesis from d-glyceraldehyde (Fig. 1-1). l-Aldoses are similarly related to l-glyceraldehyde. In this synthetic scheme, a nucleophilic cyanide ion (:CN) adds to the carbonyl double bond (C-1) of glyceraldehyde, giving two cyanohydrin products. Selective reduction and hydrolysis of the CN group to an aldehyde completes the conversion of the triose d-glyceraldehyde to the pair of aldotetroses having a new chiral carbon at C-2. A chiral carbon atom is one that is bonded to four different groups. Accordingly this reaction scheme lengthens the carbon chain from the carbonyl end, giving two new aldoses whose last three carbons derive from glyceraldehyde. Thus, the configuration of the hydroxyl on the highest numbered chiral carbon of an aldose or ketose determines d or l status. For a d-sugar, this hydroxyl is to the right of the carbon chain in the Fischer formula. Each cycle of the synthesis creates a new chiral center at carbon 2 and a pair of stereoisomers. Accordingly, there are two tetroses, four pentoses, and eight hexoses in the d- (Fig. 1-1) and also in the l-series; not all of these occur commonly in nature. l-Sugars are mirror images of the d-sugars, with the configuration of all chiral carbons reversed (e.g., d-glucose and l-glucose). Sugars that vary in their configuration at only one carbon are epimers. Thus, glucose and galactose are C4 epimers, and enzymes that catalyze this conversion are epimerases.

In the common ketoses (Fig. 1-2), the carbonyl group is usually at carbon 2, but other ketoses occur as well. A few ketoses, such as fructose, are known by their trivial names, but others are named systematically with the suffix "-ulose" denoting a ketose sugar. In this nomenclature, a group of up to four consecutive chiral carbons is named after the aldose (triose, tetrose, pentose, or hexose) possessing this chiral group, and the number

HC=0

hcoh hcoh

hcoh ch2oh D-Ribose

CH2OH

D-Glyceraldehyde (D-Glycerose) 'CN~ Reduction

Hydrolysis_

ch2oh D-Erythrose

HC=0

HOCH

HCOH

HCOH

CH2OH D-Arabinose ch2oh D-Ribose

HC=0

HC=0

HCOH

HOCH

HCOH

HCOH

HCOH

HCOH

HCOH

HCOH

ch2oh D-Allose ch2oh D-Altrose ch2oh D-Allose ch2oh D-Altrose hc=0 hcoh

hoch

hcoh hcoh

ch2oh

4l ch2oh

D-Threose hc=0

hcoh hoch

hcoh

ch2oh

D-Xylose

hc=0

hoch

hoch

hcoh

ch2oh

D-Lyxose

HC=0 HOCH

HOCH

HCOH

HCOH

CH2OH D-Mannose

HC=0

HCOH

HCOH

HOCH

HCOH

CH2OH D-Gulose

HC=0

HOCH

HCOH

HOCH

HCOH

CH2OH D-ldose

HC=0

HCOH

HOCH

HOCH

HCOH

CH2OH D-Galactose

HC=0

HOCH

HOCH

HOCH

HCOH

CH2OH D-Talose

Figure 1-1. Structures of the d-aldoses (tetroses, pentoses and hexoses), showing their derivation from D-glyceralde-hyde by chemical synthesis. At each application of this reaction scheme, the :CN anion adds to the carbonyl carbon, lengthening the carbon chain by one carbon and creating a new chiral center at C-2 and a new pair of isomers. Reduction and hydrolysis of the :CN group restore the aldehyde function.

of carbon atoms is designated also. d-Fructose is the most common ketose, and its systematic name is d-arafomo-hexulose, showing that the three chiral carbons in d-fructose have the same configuration as those in d-arabinose. Frequently, the names ribulose and xylulose are used for the two ketopentoses. Their correct systematic names, however, are D-erythro-pentulose and d-iftreo-pentulose, respectively, showing that they have only two chiral carbons.

Although the common monosaccharides are pentoses and hexoses, sugars with more than six carbon atoms also occur naturally Some have trivial names, but most sugars having more than six carbons and possessing four or more chiral carbons are named systematically, as described above for the ketoses. Sedo-heptulose (d-a/fro-heptulose), a seven-carbon sugar, and the other ketoses shown in Figure 1-2 are involved as phosphorylated intermediates in carbohydrate metabolism. /V-Acetyl-

ch2oh c=o

HCOH

HCOH

CH2OH D-Ribulose

(D -erythro-Pentulose)

CH2OH

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  • frances
    How to systematically name aldoses?
    5 months ago

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