Ketoses acyclic forms

The basis for the name is the structure of the parent monosaccharide in the acyclic form. Charts I and IV (2-Carb-10) give trivial names for parent aldoses and ketoses with up to six carbon atoms. 2-Carb-8.2 and 2-Carb-10.3 describe systematic naming procedures. 

Replacement of a hydroxy oxygen atom of an aldose or ketose, or of the oxygen atom of the carbonyl group of the acyclic form of an aldose or ketose, by sulfur is indicated by placing the prefix thio , preceded by the appropriate locant, before the systematic or trivial name of the aldose or ketose. 

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... 

The rate of the interconversion may also be followed by measuring the change in volume or in refractive index. Such measurements give rate coefficients identical with those obtained by the polarimetric method. In Table XVIII, rate coefficients for the mutarotation of a number of sugars are listed. The rates of mutarotation of several sugars (for example, D-ri-bose, D-galactose, and all the ketoses) do not obey the first-order law. Their complex mutarotations result from the presence in solution, in appreciable concentrations, of more than two species. In addition to pyranoses, there must be present either furanoses or acyclic forms, or both. 

Acetamido-5-deoxy-D-threo-pentulose and 5-acetamido-5-deoxy-L-erythro-pentulose (204) are obtainable by the bacterial oxidation of 1-acetamido-l-deoxy-D-arabinitol and 1-acetamido-l-deoxy-D-ribitol, respectively. Both ketoses are crystalline, and show a strong carbonyl vibration in their infrared spectra. Accordingly, they exist in the acyclic form. Their isopropylidene acetals are likewise acyclic. Furthermore, 5-acetamido-5,6-dideoxy-L-xylo-hexulose (205), obtainable by bacterial oxidation, shows a great tendency to assume the acyclic structure. 5-Acetamido-5-deoxy-L-xi/io-hexulose (206) may be obtained by the bacterial oxidation of 2-acetamido-2-deoxy-D-glucitol it assumes, exclusively, the sterically more-favored pyranose form. ... 

The influence, on the reaction, of different substituents in the phenyl-hydrazine molecule and the specificity due to these molecular changes have been studied. The specificity in respect to certain aldose configurations was proved, and it was also shown that D-fructose and other 2-ketoses react with such weakly basic hydrazines as (p-bromo-, (p-carboxy-, (p-carbeth-oxy- and (p-nitro-phenyl)hydrazine. Correlations between the reaction rate and ease of interconversion between cyclic and acyclic forms were also determined. Studies with tritium-labeled D-fructose showed that the... 

The carbonyl stretching frequency of aldehydes and ketones is found at 1730-1665 cm For example, the acyclic form of some aldoses and ketoses (in a lyophili-zate of the equilibrium mixture of mutarotation) produces a very weak band (Tipson and Isbell, 1962) at 1718 cm . Kuhn (1950) attributed the band at 1613 cm exhibited by periodate-oxidized methyl a-D-glucopyranoside to aldehydic carbonyl. Periodate-oxidized cellulose has only a very weak band (Rowen et ai, 1951), and exists mainly as the hemialdal —CH(OH)—O—CH(OH)— (Spedding, 1960), formed by hydration of two aldehyde groups per oxidized residue. 

Monosaccharides are divided into two major groups depending on whether their acyclic forms possess an aldehyde or a keto group, that is, whether they are aldoses or ketoses. Monosaccharides can also be grouped by the size of the ring that they form, for example, furanoses or pyranoses. Both aldoses and ketoses may be designated as to size, for example, as aldopentoses or 2-hexuloses (El Khadem, 1988). Sug-... 

Any ring carbohydrate that exists as a hemiacetal will be in equilibrium with its acyclic form.The acyclic form will either be an aldose or a ketose, which under the conditions of theTollens test will be converted to an aldose (see 2 above). It is the aldose that is the actual reducing sugar ... 

Another common monosaccharide is D-fructose (6), a ketose. In aqueous solution, D-fructose also undergoes mutarotation to produce a complex equilibrium mixture of the acyclic form 6 (< 1%), the five-membered hemiacetals 7 (31%) and 8 (9%), which are called furanoses, and the cyclic pyranoses 9 (57%) and 10 (3%) this mixture exhibits an [a] = -92° (Scheme 23.1). 

Methyl Ethers.—6-0-Methyl-L-sorbose, 5-O-methyl-D-fructose and -L-sorbose, and 5,6-di-O-methyl-L-sorbose have been synthesized. " U.v. spectroscopy was used to determine the proportion of the acyclic form of these and other ketoses present in methanol solution the extinction coefficient (e = 34.7) for the carbonyl group was determined using 5,6-di-O-methyl-L-sorbose, which cannot cyclize. Of the other ketoses examined, 6-O-methyl-L-sorbose was found to contain the highest proportion (7.4%) of the acyclic form at equilibrium. 

The carbonyl absorption of several O-methyl ketoses e.g, 5- and 6-O-methyl-L-sorbose and 5-O-methyl-D-fructose) has been used to determine the proportion of the acyclic forms present in the tautomeric equilibria in methanol. "... 

Assuming that 1,2-enolization is rate-limiting, the stability of aldoses compared to ketoses could be due to a lower concentration of the acyclic, carbonyl form, and to differences in the basic strength of the carbonyl groups and in the acidity of the protons at C-1 and C-2. 

Since the carbonyl forms of aldoses or ketoses are acyclic, the enediols... 

The substrate specificity of glucose-phosphate isomerase illuminates additional stereochemical subtleties of the isomerase reaction (JOO). In the aldose to ketose direction, the enzyme potentially operates on an equilibrium mixture of substrate forms composed of two cyclic hemiacetals (the a- and /3-anomers, of glucose 6-phosphate) and trace quantities of the acyclic aldehyde form lEq. (17)] ... 

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