Aldoses labeled

Isotopic Labeling and Substitution at the Anomeric Center of Aldoses by Reduction of Aldonolactones... 

A sequence known as the Kiliani-Fischer synthesis was developed primarily for extending an aldose chain by one carbon, and was one way in which configurational relationships between different sugars could be established. A major application of this sequence nowadays is to employ it for the synthesis of " C-labelled sugars, which in turn may be used to explore the role of sugars in metabolic reactions. 

Among the classic methods for the extension of the aldose chain by one carbon atom from the reducing end [9J, the Kiliani-Fischer cyanohydrin synthesis [10] is a milestone in carbohydrate chemistry. However after 110 years from discovery and numerous applications [11], including the preparation of carbon and hydrogen isotopically labeled compounds for mechanistic and structural studies [12], there are still several drawbacks that make the method impractical. These are the low and variable degree of selectivity and the harsh reaction conditions that are required to reveal the aldose from either the aldonic acid or directly from the cyanohydrin. Synthetic applications that have appeared in recent times confirmed these limitations. For instance, a quite low selectivity was registered [13] in the addition of the cyanide ion to the D-ga/acfo-hexodialdo-l,5-pyranose derivative 1... 

Two free-radical chain reactions, in addition to the ionic enolate mechanism, seem reasonable for the oxidation of the sugars by oxygen. With an aldose-2-f one of the free-radical mechanisms would yield non-labeled formic acid and the next lower aldonic acid the other would yield labeled formic acid and the same aldonic acid. 

Heparin has been labeled by this method85 and also by platinum-catalyzed exchange with tritium gas,58 in both cases with retention of biological activity, and it is used commercially for labeling C-l of aldoses. 

The aldehyde group of aldoses can either be oxidized or reduced and ketoses can be reduced. The best laboratory reagent for reduction is sodium boro-hydride which acts rapidly in neutral aqueous solutions (see Eq. 4-2). Since both NaB H4 and NaB H4 are available, radioactive or heavy isotope labels can be introduced in this way. The aldehyde groups can be oxidized by a variety of agents to the corresponding aldonic acids, a fact that accounts for the reducing properties of these sugars. In alkaline solution aldoses reduce Cu + ions to cuprous oxide (Eq. 4-3), silver ions... 

In one modification (Serianni et al. 1979), cyanide is added keeping the pH around 7.5, subsequently lowered to 4.2 at the end of the reaction. Nitriles are stable under these conditions. They are reduced to imines by hydrogenation with palladium over barium sulfate. These imines are rapidly hydrolysed into aldoses. This is the method recommended for the preparation of C-1-labelled sugars. 

This title compound, aldose reductase inhibitor CP-73,850 (zopolrestat) 189, has been " C-labelled (equation 75) for use in metabolism and pharmacokinetic studies, since... 

Non-enzymic aldose-ketose isomerisations that are acid catalysed appear to involve a 1,2-hydride shift. During acid-catalysed rearrangement of glucose to fructose, the label of [2- H]glucose substrate is retained in the [l- H]fructose product, distributed equally between the proR and proS positions." In the reverse sense retention of the label of tritiated fructose in the glucose and mannose products was not complete. Similar observations were made for the xylose-xylulose interconversion." With an appropriate sugar configuration (ribose), even the base-catalysed reaction proceeds partly with retention of label, presumably by the same mechanism as with trioses. 

An enzymic counterpart of these complex base-catalysed rearrangements of sugars may be the reaction catalysed by 4-phospho-3,4-dihydroxy-2-butanone synthetase. The enzyme catalyses the formation of the eponymous intermediate in secondary metabolism from ribulose 5-phosphate. Labelling studies indicated that C1-C3 of the substrate became C1-C3 of the product, that H3 of the substrate derived from solvent and that C4 was lost as formate. X-ray crystal structures of the native enzyme and a partly active mutant in complex with the substrate are available. The active site of the enzyme from Met ha-nococcus jannaschii contains two metals, which can be any divalent cations of the approximate radius of Mg " or Mn ", the two usually observed. Their disposition is very reminiscent of those in the hydride transfer aldose-ketose isomerases, but also to ribulose-5-phosphate carboxylase, which works by an enolisation mechanism, so the enolisation route suggested by Steinbacher et al. is repeated in Figure 6.14, as is the Bilik-type alkyl group shift, for which an equivalent reverse aldol-aldol mechanism cannot be written. 

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... 

A secondary KIE is any KIE on an atom not involved in the bond making or breaking steps of a reaction. Secondary KIEs are designated in terms of the number of bonds separating the isotopic label from the site where chemistry occurs thus, in aldoses, the I - H KIE is an a-secondary KIE, the 2 - H KIE is a /3-secondary KIE. Because secondary KIEs are generally smaller in magnitude than primary KIEs, it is most commonly hydron KIEs that are measured. However, secondary and KIEs have been measured, and are large in... 

USE Reagent for labeling a terminal amino acid group in modified Wohl degradations of aldoses. As hapten, Caution,- Vesicant. For proper handling see J. S, Thompson, O. P. Edmunds, Ann. Occup. Hyg, 23. 27 (1980). 

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