13C-labelled compounds

Part of the biogenesis of cephalosporins was confirmed by isolating 13C-labeled compounds [1014]. Adding [2-13C]-acetate to cultures of Cephalosporium acremonium yielded an antibiotic labeled at C-l 1, -12, -13, -14 and -19 (marked with asterisks in the formula) [l-13C]-acetate, on the other hand, yielded a compound labeled at positions 10, 15 and 18 (symbolized by in the formula). 

C-labeled compounds can be easily prepared because reactive reagents with 13C-labeled carbonyl groups like l-13C-acetyl chloride are commercially available. 

Doubly charged ions of 1-heptene were observed in its field ionization mass spectrum by Tecon, Stahl and Gaumann267. By using 13C-labeled compounds they were able to prove that these ions are formed in an absorption process on the emitter surface, partly by ion-molecule reactions. 

The main source of conformational information for biopolymers are the easy-to-obtain chemical shifts that can be translated into dihedral restraints. In addition, for fully 13C labeled compounds, proton-driven spin diffusion between carbons [72] can be used to measure quantitatively distances between carbons. The CHHC experiment is the equivalent of the NOESY in solution that measures distances between protons by detecting the resonances of the attached carbons. While both techniques, proton-driven spin diffusion and CHHC experiment [73], allow for some variation in the distance as determined from cross-peak integrals, REDOR [74] experiments in selective labeled compounds measure very accurate distances by direct observation of the oscillation of a signal by the dipolar coupling. While the latter technique provides very accurate distances, it provides only one piece of information per sample. Therefore, the more powerful techniques proton-driven spin diffusion and CHHC have taken over when it comes to structure determination by ss-NMR of fully labeled ligands. 

Recently, it was shown that ss-NMR spectroscopy can be used to determine the conformation of EpoB in the solid state [116]. The method relies on the measurement of intramolecular short H-H distances (in the range 1.8-3.0 A) from 2D CHHC correlations under MAS [117]. Regarding the sample preparation, a small amount of 13C labeled compound was diluted with EpoB with natural abundance of carbon isotopes. This reduces the signal to noise but, on the other hand, it excludes contributions from intermolecular H-H polarization transfer. Under these conditions, all CHHC cross-peaks result from intramolecular polarization transfer and reflect intramolecular interproton distances. 

Figure 9.19 Variations in the concentration ((Jig g 1) of five cell-associated 13C-labeled compounds oxic (I open) and anoxic (II open, and VI closed) incubations. Straight lines and curves were fit to a first-order degradation model of concentration data. (Modified from Sun et al., 2002.)...

We saw in Chapter 2 (Fig. 2.18) that the intensities of these two peaks (singlet for H-12C and doublet for H- C) are reversed for 13C-labeled compounds The wide doublet dominates, and the residual 12C shows up as a tiny central singlet. Not only are the protons directly attached to 13 C split into doublets, but also those two or three bonds away (13C-CH and... 

C satellites are usually lost in the background noise of the spectrum and need concern us no further. You do, however, see coupling with 13C labelled compounds where the 13C abundance now approaches 100%. The same Wittig reagent we saw a moment ago shows a 3H doublet of doublets... 

Reaction of 4.6 with CO has been investigated by spectroscopy (IR and NMR) and product analyses. The final products of this reaction are 4.1 and CH3COI. In this conversion of 4.6 to 4.1, there is spectroscopic evidence to suggest the formation of an intermediate complex like 4.4 in solution. In fact, by carrying out low-temperature NMR studies on 13C-labeled compound, the sequence as shown by 4.4, has been suggested. 

Ethyl 4-hydroxy[1-13C]benzoate can be converted into other ring-13C-labeled compounds like 3,4-dihydroxy[1-13C]benzoic acid, 1,3,4-trihydroxy[1-13C]benzene, and D,L-[1 -13C]tyrosine.4b... 

Using Complex Mixtures of Universally 13C-Labeled Compounds as Precursors -... 

Although it is possible to measure one-bond 13C-13C coupling constants in unenriched samples (13C natural abundance is 1-108%), (28, 29) such measurements are not routine on most spectrometers. Most of the reported data on C-C coupling in the past two years have been obtained from 13C labelled compounds. 

The proton NMR spectra for methyltriphenylphosphonium halide and its carbon-13 analogue are shown in this problem. Concentrate your attention on the doublet at 3.25 ppm ( Jhp = 13.2 Hz) in the first spectrum and the doublet of doublets at 3.25 ppm in the 13C-labeled compound. The coupling constants in the second spectrum are 135 and 13.2 Hz. Interpret the two spectra. You may need to refer to Section 7.12 and Appendix 9. Ignore the phenyl groups in your interpretation. 

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