Acetic acid labelling

In the experiments with acetic acid labelled radioisotopically and fed to ani-mals, it has been established that the cholesterol carbon framework is made up entirely of the acetic acid carbon. 

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... 

PNA13). In a complementary experiment, incubation of [20-14C 2,7-mef/iy/-3H]sirohydro-chlorin gave radiolabelled acetic acid labelled exclusively in the carboxyl carbon (79UP10400, 81PNA11). 

B-6. What is the distribution of radioactive carbon (14C) in isopentenyl pyrophosphate biosynthesized from acetic acid labelled with 14C at its carboxyl carbon (CH3C02H) 14C is indicated by an asterisk ( ) in the structures. 

Electrical Conductivity, (a) Pure Substances. Note that the lamp does not glow when air fills the space between the electrodes. Then raise successively between electrodes B distilled water, alcohol, pure acetic acid (labeled glacial acetic acid ), and place in contact with electrodes A lumps of any two dry salts found in the laboratory, for example, common salt, NaCl, and blue vitriol, CuS04 5H20. 

Fill a test tube one-third full of one of the. following dilute acids sulphuric, hydrochloric, and nitric fill another test tube one-third full of concentrated acetic acid. Label the tubes in some distinguishing manner. 

Long has prepared oxalic acid in 50% yield by introducing C 02 into an evacuated flask containing molten potassium and sand. Acetic acid labelled with C in both carbons has been prepared by Barker and... 

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. 

Four reports describe investigations of the incorporation of various isotopo-mers of acetic acid labeled with stable isotopes into the tropane alkaloids. The first of these, by Sankawa and Noguchi [42], reported the regiochemistry of [ 1,2- C2] acetate incorporation into hyoscyamine (3) in Hyoscyamus albus. Surprisingly at the time, it was found that the resonances due C-2 and C-4 both showed coupling to the resonance assigned to C-3. The authors concluded that two labeled species were present in the sample, one in which an intact bond from acetate was introduced in such a fashion that C-2 and C-3 were bond-labeled and another one in which C-4 and C-3 were bond-labeled. The analysis of the C NMR spectra of other alkaloids, the 6)3- hydroxy derivatives of (3), isolated from this experiment indicated an identical pattern of incorporation of acetate into these substances as well. 

Newman lA (1965) Distribution of indolyl-3-acetic acid labelled with carbon-14 in Avena. Nature 205 1336-1337... 

Further experiments using acetic acid labeled in the carboxyl group with and in the methyl group with deuterium showed conclusively that both carbon atoms of acetate participate in cholesterol biosynthesis (Rittenberg and Bloch, 1944,1945). Subsequent experiments by Little and Bloch (1950) carried out with CHg COOH and iscHgi COOH showed that all 27 carbon atoms of cholesterol could be derived from acetate and that the ratio of incorporation of methyl/ carboxyl was 15/12. Separate analysis of nucleus and side chain showed... 

C]ALA, Kuhn-Roth oxidation should have yielded acetic acid labelled in the methyl group. Instead, the acid isolated contained only 8—9% of the expected label Therefore, the origin of the C-methyl group from either the 6-mcthine bridge of the a-d porphyrin rings or from methionine remained unestablished. 

Shanorellin (48). Previous studies with C-labelled precursors indicated that acetate and the S-methyl group of methionine were efficiently incorporated into shanorellin (48), which is a benzoquinone pigment obtained from Shanorella spirotricha Benjamin. Oxidation with chromic add of shanorellin labelled by [l- C]acetate yielded an inactive acetic acid, whereas quinone samples labelled by [Afe- C]methionine and [2- C]acetate gave, after oxidation, acetic acid labelled at the methyl and carboxy-groups, respectively. On this basis, the biosynthetic route in Scheme 14 was proposed. 

Most naturally occurring fatty acids have even numbers of carbon atoms. Therefore, it was natural for biochemists to postulate that they were formed by condensation from two-carbon units. This suggestion was confirmed in 1944 when Rittenberg and Bloch isolated fatty acids from tissues of rats which had been fed acetic acid labelled with C in the carboxyl group and in the methyl group. The two kinds of atoms were located at alternate positions along the chain, showing that the complete chain could be derived from acetic acid. 

Now, let s draw out the forward scheme. This multi-step synthesis uses three equivalents of ethylene (labeled A, B, C in the scheme below) and one equivalent of acetic acid (labeled D). Ethylene (A) is converted to 1,2-dibromoethane upon treatment with bromine. Subsequent reaction with excess sodium amide produces an acetylide anion which is then treated with bromoethane [made tfom ethylene (B) and HBr] to produce 1-butyne. Deprotonation with sodium amide, followed by reaction with an epoxide [prepared by epoxidation of ethylene (C)] and water workup, produces a compound with an alkyne group and an alcohol group. Reduction of the alkyne to the cis alkene is accomplished with H2 and Lindlar s catalyst, after which the alcohol is converted to a tosylate with tosyl chloride. Reaction with the conjugate base of acetic acid [produced by treating acetic acid (D) with NaOH] allows for an Sn2 reaction, thus yielding the desired product, Z-hexenyl acetate. 

Figure 26 8 traces the label from its origin m acetic acid to its experimentally deter mined distribution m citronellal... 

How many carbon atoms of citronellal would be radioactively labeled if the acetic acid used in the experiment were enriched with at C 1 instead of at C 2 Identify these carbon atoms... 

Commercial acetaldehyde has the following typical specifications assay, 99% min color, water-white acidity, 0.5% max (acetic acid) specific gravity, 0.790 at 20°C bp, 20.8°C at 101.3 kPa (1 atm). It is shipped in steel dmms and tank cars bearing the ICC red label. In the Hquid state, it is noncorrosive to most metals however, acetaldehyde oxidizes readily, particularly in the vapor state, to acetic acid. Precautions to be observed in the handling of acetaldehyde have been pubHshed (103). 

Synthetic chemical approaches to the preparation of carbon-14 labeled materials iavolve a number of basic building blocks prepared from barium [ CJ-carbonate (2). These are carbon [ C]-dioxide [ CJ-acetjlene [U— C]-ben2ene, where U = uniformly labeled [1- and 2- C]-sodium acetate, [ C]-methyl iodide, [ C]-methanol, sodium [ C]-cyanide, and [ CJ-urea. Many compHcated radiotracers are synthesized from these materials. Some examples are [l- C]-8,ll,14-eicosatrienoic acid [3435-80-1] inoxn. [ CJ-carbon dioxide, [ting-U— C]-phenyhsothiocyanate [77590-93-3] ftom [ " CJ-acetjlene, [7- " C]-norepinephrine [18155-53-8] from [l- " C]-acetic acid, [4- " C]-cholesterol [1976-77-8] from [ " CJ-methyl iodide, [l- " C]-glucose [4005-41-8] from sodium [ " C]-cyanide, and [2- " C]-uracil [626-07-3] [27017-27-2] from [ " C]-urea. All syntheses of the basic radioactive building blocks have been described (4). 

Nucleophilic substitution in cyclohexyl systems is quite slow and is often accompanied by extensive elimination. The stereochemistry of substitution has been determined with the use of a deuterium-labeled substrate (entry 6). In the example shown, the substitution process occurs with complete inversion of configuration. By NMR amdysis, it can be determined that there is about 15% of rearrangement by hydride shift accon any-ing solvolysis in acetic acid. This increases to 35% in formic acid and 75% in trifiuoroacetic acid. The extent of rearrangement increases with decreasing solvent... 

The occurrence and extent of rearrangement of the 2-butyl cation have also been investigated by solvolysis studies using isotopic labeling. When 2-butyl tosylate is solvolyzed in acetic acid, C-2/C-3 rearrangement occurs only to the extent of 9% in the 2-butyl acetate which is isolated.Thus, under these conditions, most of the reaction proceeds by direct participation of the solvent. 

The solvolysis of 2, 35-3-(4-methoxyphenyl)but-2-yl/>-toluensulfonate in acetic acid can be followed by several kinetic measurements (a) rate of decrease of observed rotation (k ) rate of release of the leaving group (k,) and, when 0-labeled sulfonate is used, the rate of equilibration of the sulfonate oxygens (k ). At 25°C, the rate constants are... 

Mechanism I was ruled out by an isotopic labeling experiment. The mixed anhydride of salicylic acid and acetic acid is an intermediate if nucleophilic catalysis occurs by mechanism 1. This molecule is known to hydrolyze in water with about 25% incorporation of solvent water into the salicylic acid. 

A reaction which is of considerable importance for the selective labeling of a position adjacent to a carbonyl group is the treatment of a-halo ketones (mainly bromides) with zinc in acetic acid-OD, or deuteriobromic acid at... 

To illustrate the value of the mass spectra of the labeled compounds, the peaks at m/e 129 in Figures 7 and 8 will be considered first. These peaks could be from the loss of acetic acid (60 mass units) from m/e 189, or the loss of water (18 mass units) from m/e 189 followed by loss of ketene (42 mass units) structure 15, containing C-1-C-2-C-3 less a rearranged hydrogen atom from C2, is another possibility. The composition of this ion could be important for confirming the presence of a 3-hydroxyl group. 

We said in Section 21.6 that mechanistic studies on ester hydrolysis have been carried out using ethyl propanoate labeled with lsO in the etherlike oxygen. Assume that Odabeled acetic acid is your only source of isotopic oxygen, and then propose a synthesis of the labeled ethyl propanoate. 

A kinetic evidence for reversibility of bromonium ion formation has been obtained in the reaction of tetraisobutylethylene and its Dg labeled derivative with Br2 in acetic acid (ref. 9). Owing to steric effects, the first formed bromonium ion cannot undergo backside attack to give the dibromide, but looses a proton to yield... 

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