13C labelling studies

By using an elegant 13C-labeling study that involved incubation of Phoma sp. with 1-13C and 1,2-13C acetate, Oikawa et al. [9], were able to isolate the proposed biosynthetic intermediate phomactatriene (or Sch 49026), with 13C incorporation from singly labeled acetate units as indicated by in Fig. 8.3. Phomactatriene is strikingly reminiscent of taxadiene, a biosynthetic intermediate for Taxol . The net biosynthesis for both involves geranylgeranyl diphosphate (GGDP) cyclization [9]. It is noteworthy that prior to isolation of phomactins, the only known related structure is cleomeolide, a diterpene from the herb Cleome viscosa [10] that remarkably resembles phomactin H. 

The scope and mechanism of carboxylic acid homologation is examined here in relation to the structure of the carboxylic acid substrate, the concentrations and composition of the ruthenium catalyst precursor and iodide promoter, synthesis gas ratios, as well as 13C labelling studies and the spectral identification of ruthenium iodocarbonyl intermediates. 

Anthramycin, Pyrrolnitrin, and Tomaymycin.—An important piece of evidence on the biosynthesis of anthramycin (116) is that methionine provides C-14. In part this evidence comes from a 13C-labelling study,96 and the conclusion has been supported by the use of 13C-JH long-range coupling in analysing the H n.m.r. spectrum of anthramycin (116) derived from [Afe-13C]methionine.97 Similar application to the study of pyrrolnitrin (117) biosynthesis has confirmed97 the mode of tryptophan incorporation98 [a 13C label at C-3 of the amino-acid appeared at C-3 of (117)]. 

Definitive assignment of the 13C resonances has been possible only in few cases where 13C labeling studies have been carried out249 or when additional information is provided by coupling to other spin-active nuclei associated with the supporting metal-ligand fragments. In this respect, /(CP) values... 

Based on a 13C labeling study, the /3-acetylenic carbon atom C3 is more deshielded than the a-acetylenic carbon atom C2.l7a... 

A 13C-labeling study showed that an lH-azirine is also involved in the formation of 3-phenylindole by flash pyrolysis of l,5-diphenyl-l,2,3-triazole such an intermediate is not required, however, for the formation of the same product from 1,4-diphenyl-1,2,3-triazole (Scheme 14).75... 

Crow et a/.201,204 found that also 3- and 4-pyridylcarbenes are converted to phenylnitrene. A 13C-labeling study of 4-pyridylcarbene by Thetaz... 

The structures of PMs are trimeric, consisting of three molecules of PS and two molecules of methylamine that are condensed together (Table 9.5). In the structures of PMs, however, the bonds created by the condensation lack adjacent hydrogen atoms, making the connectivity assignment in 1H-NMR studies virtually impossible (see the 9-membered rings in Fig. 9.11). To circumvent this problem, a model compound of panal, K-l, having 13C-labeles at the C12 and C13 positions, has been synthesized at Kishi s laboratory (Harvard University) to make a model compound of PM (Stojanovic, 1995). 

MMA polymerization is one of the most studied systems and was thought to be explicable, within experimental error, in terms of Bemoullian statistics. Moad et ai.jb have made precise measurements of the configurational sequence distribution for PMMA prepared from 13C-labeled monomer. It is clear that... 

Our studies of quinone methides and related species using 13C labeling and spectral global fitting started in the late 1990s and have continued to the present with two papers on prekinamycins slated for publication in 2009. In this section, we summarize what we learned from both our published and unpublished studies. 

In a recent study, we showed that the more flexible pyrido[l,2-a]indole-based cyclopropyl quinone methide is not subject to the stereoelectronic effect.47 Scheme 7.17 shows an electrostatic potential map of the protonated cyclopropyl quinone methide with arrows indicating the two possible nucleophilic attack sites on the electron-deficient (blue-colored) cyclopropyl ring. The 13C label allows both nucleophile attack products, the pyrido[l,2-a]indole and azepino [l,2-a]indole, to be distinguished without isolation. The site of nucleophilic is under steric control with pyrido [1,2-a]indole ring formation favored by large nucleophiles. 

We postulate that the double13C -labeling technique presented in this chapter could be used to study adducts on large pieces of DNA and even follow the chemical details cellular metabolic processes in real time. The double 13C-labeling technique is currently being developed to solve problems in metabolism and toxicology. 

Although limited by sensitivity, chemical reaction monitoring via less sensitive nuclei (such as 13C) has also been reported. In 1987 Albert et al. monitored the electrochemical reaction of 2,4,6-tri-t-butylphenol by continuous flow 13C NMR [4]. More recently, Hunger and Horvath studied the conversion of vapor propan-2-ol (13C labeled) on zeolites using 1H and 13C in situ magic angle spinning (MAS) NMR spectroscopy under continuous-flow conditions [15]. 

In collaboration with Professor Collum and coworkers, 6Li NMR (including 13C-labeled acetylene 37 and 15N-labeled chiral modifier 46 experiments) and Li aggregation studies were implemented to assist in the understanding of some of the factors responsible for the stereoselective nature of this chemistry [35],... 

The structure of the major aggregate was identified by labeling studies. Since the major set has two equal intensity 6Li signals, these signals could be assigned as a 1 1 complex 68 of lithium acetylide and lithium alkoxide or a dimer (such as 69) of the 1 1 complex 68 shown in Figure 1.9. Both structures have two different Li species. In order to discriminate between 68 and 69, a terminal acetylene carbon of 37 was labeled with 13C. In the case of 68, both lithium signals will be a doublet... 

Exchange reactions between bulk and adsorbed substances can be studied by on-line mass spectroscopy and isotope labeling. In this section the results on the interaction of methanol and carbon monoxide in solution with adsorbed methanol and carbon monoxide on platinum are reported [72], A flow cell for on-line MS measurements (Fig. 1.2) was used. 13C-labeled methanol was absorbed until the Pt surface became saturated. After solution exchange with base electrolyte a potential scan was applied. Parallel to the current-potential curve the mass intensity-potential for 13C02 was monitored. Both curves are given in Fig. 3.1a,b. A second scan was always taken to check the absence of bulk substances. 

A wide range of other isotopic labels, e.g. 3H (or T), 13C, I4C, 15N, 32P, 35S, 37C1, 1311, etc., have also been used to provide important mechanistic information. The major difficulties encountered in such labelling studies have always been (a) ensuring that the label is incorporated only into the desired position(s) in the test compound and (b) finding exactly where the label has gone to in the product(s) after the reaction being studied has taken place. 

The basic reactions of Tetralin and derivatives have been extended to the use of 1-13C labels and 1,2-dihydronaphthalene, with and without a source of free radicals. The studies with Tetralin were monitored equally well with C-NMR and GLC techniques. The rate constant for the conversion of Tatralin to methyl indan in the presence of dibenzyl at 450°C was 6.4 x 10 min i which is consistent with that previously reported [2]. 

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