Simple Ketoses

Structurally, the parent compound of the simple ketoses is dihydroxyacetone, a structural isomer of glyceraldehyde. 

Although dihydroxyacetone does not possess a chiral carbon atom, the simple ketoses are related to it structurally by the introduction of hydroxylated chiral carbon atoms between the keto group and one of the hydroxymethyl groups. Thus there are two ketotetroses, four ketopentoses, and eight ketohexoses. 

The most common ketose, d-fructose, is shown below. Compare the configuration of the chiral carbon atom most remote from the keto group (C-5) with D-glyceraldehyde. 

Fructose (shown in Example 2.7) was named long before its structure was known. The same is true for most aldoses. Names like glucose, mannose, ribose, and fructose are called trivial names i.e., they are nonsystematic. However, ketoses, which are isomers of aldoses, were isolated or synthesized after the corresponding aldoses and were given names based on the names of the isomeric aldoses. Such names are misleading because they do not reflect the structural elements present in the ketoses. 

The ketose shown below is known universally as d-ribulose because it is an isomer of D-ribose. The name is incorrect the compound has only two chiral centers, not three as the prefix rib- would imply. The compound is related to D-erythrose, and its correct name is D-eryt/iro-pentulose. 

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FIGURE 7.1 Structure of a simple aldose (glyceraldehyde) aud a simple ketose (dihy-droxyacetoue). 

The simple sugars or monosaccharides are polyhydroxy aldehydes or ketones, and belong to Solubility Group II. They are termed tetroses, pentoses, hexoses. etc. according to the number of carbon atoms in the long chain constituting the molecule, and aldoses or ketoses if they are aldehydes or ketones. Most of the monosaccharides that occur in nature are pentoses and hexoses. 

Both aldoses and ketoses reduce Fehling s solution (for details, see under 4). This fact may appear surprising when it is remembered that Fehling s solution is one of the reagents for distinguishing between aldehydes and ketones (see 4). The explanation lies in the fact that a-hydroxyketones are much more readily oxidised than simple ketones, perhaps because the hydroxy ketone allows its isomerisation, in the presence of alkali, into an aldehyde. For example, fructose, a keto-hexose, might Isomerlse thus ... 

In addition to the common carbohydrates mentioned in previous sections, there are a variety of important carbohydrate-derived materials. Their structural resemblance to sugars is clear, but they aren t simple aldoses or ketoses. 

Note. For simple aldoses up to aldohexoses, and ketoses up to hept-2-uloses, the anomeric reference atom and the configurational atom are the same. 

In aqueous solution the rate law for oxidation of simple a-ketols, such as acetoin and benzoin and various aldoses and ketoses o. sog-sioj markedly different from that of Wiberg and Nigh (vide supra), viz. 

The reaction of ethyl acetoacetate with simple hydroxy ketones has been compared with the corresponding reactions of the ketoses. The results obtained with l-hydroxy-2-propanone and 3-hydroxy-2-butanone, under the same experimental conditions as with D-fructose, establish a parallel between these reactions. However, as in the case of the aldoses, the yield is greater for these simpler hydroxy ketones than for the ketoses.9 The resultant esters, (XV and XVI), were obtained in the form of sirups, but the free acids, (XVII and XVIII), and their phenacyl esters are crystalline. The acids were shown to be identical with those of known structure described in the literature.9... 

The Maillard condensation is one of the most extensively studied reactions within the field of degradation chemistry, particularly in the area of food and nutritional science. Louis Mallard reported in 1912 that some amines react with reducing carbohydrates to produce brown pigments. The condensation typically yields a simple glycosylamine, which then readily undergoes the Amadori rearrangement to produce 1 -amino-1 -deoxy-2-ketoses [95]. Reducing carbohydrates... 

Oxidizing the aldehyde group present in aldoses is easy oxidizing the carbonyl group in a ketose is far more difficult. The susceptibility (or lack thereof) to simple oxidation is a useful method of distinguishing between aldoses and ketoses. The next sections explore the various types of monosaccharide oxidation reactions that can occur. 

For ketoses, this simple method of calculation does not give the correct values for the a / -pyranose ratio. Apparently, the conformational interactions have different values in this case it is considered15 that both the st/n-axial and the gauche interactions are larger on the anomeric carbon atom, which carries two substituents, than they are on that in the aldoses. Owing to these larger interactions, the pyranose forms of the ketoses are less stable than those of the aldoses, and, consequently, for the ketoses... 

The proportion of the acyclic form also increases with increasing temperature this is true for aldoses and ketoses,16,31 as well as for simple hydroxyketones.74 This would be expected from considerations of entropy, as the acyclic form has a greater degree of freedom, but studies on y- and d-hydroxyketones show that change in enthalpy contributes even more to the changing position of the equilibrium with increasing temperature. Evidently, cyclization of hydroxyketones is exothermic, and is favored by lower temperatures.74... 

Monosaccharides (simple sugars) containing an aldehyde group are called aldoses and those with a keto group are called ketoses. Disaccharides, oligosaccharides, and polysaccharides consist of monosaccharides linked by glycosidic bonds. 

The simple sugars, or monosaccharides, are poly-hydroxyaldehydes (aldoses) or polyhydroxyketones (ketoses).1-5 All have the composition (CH20)n, hence the family name carbohydrate. A typical sugar, and the one with the widest distribution in nature, is glucose. 

The simplest carbohydrates, sometimes referred to as monosaccharides, or sugars, are either polyhydroxyaldehydes (aldoses) or polyhydroxyketones (ketoses). They can be derived from polyalcohols (polyols) by oxidation of one carbinol group to a carbonyl group. For example, the simple three-carbon triol, glycerol, can be converted either to the aldotriose, glyceraldehyde, or to the ketotriose, dihydroxyacetone, by loss of two hydrogens (fig. 12.1). 

A carbohydrate contains many V f hydroxyl groups and either an aldehyde or ketone functional group. A simple carbohydrate (monosaccharide or sugar) is an aldose if it contains an aldehyde group, or a ketose if it contains a ketone group. 

Monosaccharides, also called simple sugars, are classified by the number of carbon atoms (triose, tetrose, pentose, and so on) and by the nature of the carbonyl group (aldose or ketose). 

Simple radicals derived from aldoses have been considered exclusively in the preceding sections, from the point of view of structure and transformation. Application of radical reduction to tertiary anomeric radicals, produced from ketoses, higher sugars and other substituted analogues, and having generally alkoxy and... 

The intermediate is thought to be an enediol formed by a simple hydrogen shift.133 The reaction was applied to disaccharides by Montgomery and Hudson134 when they treated lactose with a weak solution of calcium hydroxide and isolated a crystalline ketose, 4-(/3-D-galactopyranosyl) -D-fructose, which they named lactulose. Since these workers oxidized... 

Monosaccharides, or simple sugars, are carbohydrates that cannot be hydrolyzed to simpler compounds. Figure 23-1 shows Fischer projections of the monosaccharides glucose and fructose. Glucose is a polyhydroxyaldehyde, and fructose is a polyhydroxy ketone. Polyhydroxyaldehydes are called aldoses (aid- is for aldehyde and -ose is the suffix for a sugar), and polyhydroxyketones are called ketoses (ket- for ketone, and -ose for sugar). 

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