Isotopic labelling carboxylic acid derivatives

He reacted water with carboxylic acid derivatives RCOX that had been labelled with an isotope of oxygen, 180. 

We don t expect you to be satisfied with the bland statement that tetrahedral intermediates are formed in these reactions of course, you wonder how we know that this is true. The first evidence for tetrahedral intermediates in the substitution reactions of carboxylic acid derivatives was provided by Bender in 1951. He reacted water with carboxylic acid derivatives RCOX that had been labelled with an isotope of oxygen,... 

Probably the most widely cited evidence for the existence of a tetrahedral intermediate comes from isotopic exchange reactions. For example, in the reaction of a carboxylic acid derivative, such as an ester, the two OH groups in the tetrahedral intermediate are equivalent (examine the T.L in Scheme 10.22). If the reaction were performed in 0-labeled water, one of these OH groups would be isotopically labeled. Reversal of the nucleophilic addition step would exchange the 0 into the carbonyl oxygen 50% of the time. As the reaction proceeds... 

Amino Acids and Peptides. - Wasserman s method of one-carbon homologation of carboxylic acids to give a-ketocarboxylates involves reaction with cyanomethylenetriphenyl-phosphorane followed by ozone (Scheme 24) and has been used as a key step in a chemo-enzymatic synthesis of isotopically labelled L-valine, L-isoleucine, and o/fo-isoleucine. Alkylation of the carbanion derived from the imino-substituted methylphosphonate diphenyl ester (186) with indol-3-ylmethyl bromide followed by appropriate deprotection has been used to prepare the phosphonate analogue (187) of tryptophan (Scheme 25). The deprotected analogue (188) and derived peptides show activity as inhibitors of chymotrypsin. Two approaches to solid phase Wadsworth-Enunons reactions which have applications in combinatorial chemistry have been reported. In one diethylphosphonoacetamide is bound to PEG-PAL resin via a peptide link, while... 

The formation of 2-furaldehyde on acid-catalysed dehydration, etc., of 1-benzyl-amino-l-deoxy-D-f/ireo-pentulose and l-benzylamino-l-deoxy-o-fructuronic acid has been studied in isotopically labelled water (either acidified deuterium oxide or tritiated water).The pattern of labelling of the 2-furaldehyde derived from the latter compound is consistent with a mechanism involving the decarboxylation of a Py-unsaturated carboxylic acid intermediate (see also p. 136). 

Administration203 to chicks of acetic acid labeled with C14 (in the carboxyl group) gave D-glucose (from the glycogen) labeled equally at C3 and C4, but the D-ribose (from nucleic acids) had more label at C3 than at C2, indicating that in vivo D-ribose does not arise exclusively from hexose by loss of Cl. The 20-30% isotope content of D-ribose formed from D-glucose-l-C14 by Escherichia coli indicates that direct conversion is the major pathway but that a part is probably derived from transketolase action.204 206... 

This mechanism was tested by use of C-labeled carbon dioxide (Barker, 1943 Buswell and Sollo, 1948 Stadtman and Barker, 1949, 1951 Pine and Barker, 1956 Baresi et al, 1978). Essentially none of the methane was found to be derived from carbon dioxide. Methane is derived entirely from the methyl carbon atoms and carbon dioxide is derived exclusively from carboxyl carbon atoms. Van Neil s mechanism is clearly not valid because the methyl carbon atom is not oxidized to carbon dioxide. Other work has been done to ascertain whether hydrogen atoms are removed during the fermentation of acetic acid, and whether the methyl group is incorporated intact into methane (Pine and Barker, 1954). Water and heavy water were used with deuterated and nondeuterated acetic acid. Acetic acid labeled in the methyl group, when used as the substrate, showed that the isotopic content of acetic acid and methane are the same. Unlabeled acetic acid fermented in the presence of heavy water indicated that about one atom of deuterium per molecule of methane formed is derived from heavy water. It was concluded that the methyl group is transferred from acetic acid to methane as a unit without the loss of attached hydrogen or deuterium atoms. 

The heavy carbon-isotope and the radioactive carbon isotope were used to trace the pathway of carbon atoms through the citric acid cycle. One such experiment created a great controversy over whether or not citric acid was the first tricarboxylic acid in the cycle. In this experiment, acetate labeled in the carboxyl carbon (CHj—C ooi was incubated aerobically with a tissue preparation. After incubation, a-ketoglutaric acid was isolated from the tissue preparation. This compound was then degraded through known chemical reactions in order to establish the positions of the radioactive carbon derived from the labeled acetate. In TCA cycle. 

Several tracer studies of the biosynthesis of (—)-shikimic acid in higher plants have been made and these all accord with the view that, as in micro-organisms, the seven-carbon skeleton of the alicyclic acid is derived by an initial condensation of D-erythrose-4-phosphate (7) and phosphoenolpyruvate (8), Figure 1.2. Uniform labelling of (—)-shikimic acid was obtained by exposure of living plant tissues to carbon dioxide for extended periods " . Short-term exposure gave (—)-shikimic acid with 25 per cent of the isotope in the carboxyl group . This was rationalist on the basis... 

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