Labelling experiment

Spectrometer settings all spectrometers were operated under quadrature (or single side-band) detection, PW pulse width expressed in term of flip angle T pulse repetition time 

Chemical shifts are relative parameters which can be measured with a high degree of reproductibility providing that the same reference substance is used in the same experimental conditions. Unfortunately, this goal was not achieved in the last decade, which saw the sudden rise of 

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The aldehyde or ketone needed for this reaction is not always readily available. TM 133, labelled with radioactive in one carboxyl group, was needed for a biochemical labelling experiment. How would you make it ... 

Neither ground-state ethynol (hydroxyacetylene) (80) nor carbenaoxirane (81) appears to be a viable point of ingress to the oxirene-methanoylcarbene system, as both can isomerize to ketene by lower-energy pathways. The limited experimental information available on carbenaoxirane (Section 5.05.6.3.4(f/)) indicates that it is indeed largely isolated from the oxirene-methanoylcarbene manifold (but note the photolysis of ketene in Section 5.5.6.3.4(ff)) appropriate labelling experiments with (the unknown) ethynol have not been performed. 

Formation of diazomethane from diazirine was also observed in a solid nitrogen matrix on irradiation (64JCP(41)3504). Labeling experiments demonstrated that elimination and uptake of nitrogen occurred. 

Isotopic labeling experiments have established that C—O bond rupture occurs between the carbonyl carbon and oxygen substitution at the alcohol C—O bond is not involved. 

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. 

Below — 140°C, the EPR spectrum observed was that of the cyclopropylmethyl radical. If the photolysis was done above — 140°C, however, the spectmm of a second species was seen, and above — 100°C, this was the only spectmm observed. This second spectmm could be shown to be that of the 3-butenyl radical. This study also established that the 3-butenyl radical did not revert to the cyclopropylmethyl radical on being cooled back to — 140°C. The conclusion is that the ring opening of the cyclopropyl radical is a very facile process and that the lifetime of the cyclopropyl radical above — 100°C is very short. Even though the equilibrium favors the 3-butenyl radical, the reversible ring closure can be detected by isotopic labeling experiments, which reveal the occurrence of deuterium migration ... 

The mechanism outlined above is supported by experimental findings. An intermediate 5 has been isolated, " and has been identified by and N-nuclear magnetic resonance spectroscopy. Side-products have been isolated, which are likely to be formed from intermediate 4. N-isotope labeling experiments have shown that only the nitrogen remote from the phenyl group is eliminated as ammonia. 

Much work has been invested to reveal the mechanism by which propylene is catalytically oxidized to acrolein over the heterogeneous catalyst surface. Isotope labeling experiments by Sachtler and DeBoer revealed the presence of an allylic intermediate in the oxidation of propylene to acrolein over bismuth molybdate. In these experiments, propylene was tagged once at Ci, another time at C2 and the third time at C3. 

Evidence in support of the mechanism shown in figure 21.4 comes from isotope-labeling experiments. When 180-labeled methanol reacts with benzoic acid, the methyl benzoate produced is found to be l80-labeled but the water produced is uniabeled. Thus, it is the C-OH bond of the carboxylic acid that is broken during the reaction rather than the CO—H bond and the RO-H bond of the alcohol that is broken rather than the R-OH bond. 

Problem 29.5 Evidence for the role of acetate in fatty-acid biosynthesis comes from isotope-labeling experiments. If acetate labeled with 13C in the methyl group ( CFtyCC H) were incorporated into fatty acids, at what positions in the fatty-acid chain would you expect the, 3C label to appear ... 

One is the concerted decomposition of a dioxetanone structure that is proposed for the chemiluminescence and bioluminescence of both firefly luciferin (Hopkins et al., 1967 McCapra et al., 1968 Shimomura et al., 1977) and Cypridina luciferin (McCapra and Chang, 1967 Shimomura and Johnson, 1971). The other is the linear decomposition mechanism that has been proposed for the bioluminescence reaction of fireflies by DeLuca and Dempsey (1970), but not substantiated. In the case of the Oplopborus bioluminescence, investigation of the reaction pathway by 180-labeling experiments has shown that one O atom of the product CO2 derives from molecular oxygen, indicating that the dioxetanone pathway takes place in this bioluminescence system as well (Shimomura et al., 1978). It appears that the involvement of a dioxetane intermediate is quite widespread in bioluminescence. 

A similar reaction the with rran.v-isomer 3b gave c -3,5-dimethylcyclohexene (4) with very high diastereoselectivity. Accordingly, the stereochemistry of this substitution is anti. Deuterium labeling experiments using the 1-deuterio or 3-deuterio derivative of 3 a showed that the ratio of SN2 /SN2 with lithium dimethylcuprate was about 50 50, while the ratio with lithium cyano(methyl)cupratc was >96 4. 

Hydrazino compounds can react with one or two equivalents of arenediazonium ions. In reactions of arylhydrazines without substituents at the P-nitrogen in mineral acid media the initial product, the 1,4-diaryltetraz-1-ene (6.24), disproportionates rapidly into aryl azide and amine (Scheme 6-17). As shown by 15N labeling experiments (Clusius and Craubner, 1955), equal amounts of all the products shown in Scheme 6-17 are obtained. In acetate buffer the reaction is regiospecifically different. The diazonium ion attacks the a-, not the P-nitrogen, and a 1,3-diaryltetraz-l-ene is formed (6.25). 

There are two mechanisms that are consistent with the experimental data. They are summarized in Scheme 6-23, which shows first of all the sequence diazonium ion - diazoazide - phenyl azide for the major process (M). The second process involves the phenylpentazole as intermediate. There are two possible pathways for this process. Both are consistent with the kinetics and the results of 15N labeling experiments. One was proposed by Huisgen (Huisgen and Ugi, 1957 Huisgen 1984, p. 152), the other by Ugi (1963). In Scheme 6-23 the letters H and U indicate the two... 

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