13C-NMR spectra

Active Figure 13.3 la) The H NMR spectrum and (b) the 13C NMR spectrum of methyl acetate, CH3C02CH3. The small peak labeled "TMS" at the far right of each spectrum is a calibration peak, as explained in Section 13.3. Sign in afwww.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

Assign the resonances in the 13C NMR spectrum of methyl propanoate, CH3CPJ2C02CH3 (Figure 13.9). 

Figure 13.11 The 13C NMR spectrum of 1-methylcyclohexene, the E2 reaction product from treatment of 1-chloro-1-methylcyclohexane with base.

Carbon atoms of an aromatic ring absorb in the range 110 to 140 8 in the 13C NMR spectrum, as indicated by the examples in Figure 15.16. These resonances are easily distinguished from those of alkane carbons but occur in the same range as alkene carbons. Thus, the presence of l3C absorptions at 110 to 140 8 does not in itself establish the presence of an aromatic ring. Confirming evidence from infrared, ultraviolet, or 1H NMR is needed. 

Ether carbon atoms also exhibit a downfield shift in the 13C NMR spectrum, where they usually absorb in the 50 to 80 5 range. For example, the carbon atoms next to oxygen in methyl propyl ether absorb at 58.5 and 74.8 8. Similarly, the methyl carbon in anisole absorbs at 54.8 8. 

N-Methylcyclohexylamine, 13C NMR spectrum of, 954 1H NMR spectrum of, 953 Methylene group, 178 N-Methylguanine, electrostatic potential map of, 1121 6-Methyl-5-hepten-2-ol, DRPT-NMR spectra of. 451... 

Another interesting observation was made by Bagal et al. a year later (1992). In the reaction of 4-nitrobenzenediazonium ions with various 4-phenylazophenols, with or without substituents in the 2- and 3-positions of the phenolic ring and in the 4 -position of the phenylazo ring, in addition to azo coupling in the 6-position they obtained a product that had the same atomic composition as 2,4-bis(4 -nitrophenyl-azo)-phenol (Ci8Hi4N605), but whose 13C NMR spectrum clearly showed a tetrahedral and a carbonyl carbon in the 4- and 1-positions. This product must therefore be the compound 12.153. 

Fig. 12.13C-NMR spectrum of erythrodiisotactic poly(l,2-dimethyltetramethylene) at 75.47 MHz and 303 K. a) in solution of CDC13, b) CP-MAS spectrum of the semicrystalline polymer in the bulk. Chemical shifts given at the signals refer to TMS = 0 ppm. (Ref.20))...

These spectra not only confirm the primary structure of the repeat unit of the polymer but also strongly suggest that no side reactions are detectable within the limitations of the instrument. In the 13C NMR spectrum (vide infra) all resonances can be unequivocally assigned, demonstrating the clean nature of the... 

ADMET reaction. The 13C NMR spectrum also allows the scientist to distinguish between cis and trans internal sp2 carbons as well as the allylic carbons, which are adjacent to the internal vinyl position. Using quantitative 13C NMR analysis, the integration of the peak intensities between die allylic carbon resonances and diose of the internal vinyl carbons gives die percentage of trans/cis stereochemistry diat is present for the polymer.22 Empirically, the ratio of trans to cis linkages in ADMET polymers has typically been found to be 80 20. Elemental analysis results of polymers produced via ADMET demonstrate excellent agreement between experimental and theoretical values. 

The UV-Vis spectral detection of an intermediate in the catalytic reductive alkylation reaction provides only circumstantial evidence of the quinone methide species. If the bioreductive alkylating agent has a 13C label at the methide center, then a 13C-NMR could provide chemical shift evidence of the methide intermediate. Although this concept is simple, the synthesis of such 13C-labeled materials may not be trivial. We carried out the synthesis of the 13C-labeled prekinamycin shown in Scheme 7.5 and prepared its quinone methide by catalytic reduction in an N2 glove box. An enriched 13C-NMR spectrum of this reaction mixture was obtained within 100 min of the catalytic reduction (the time of the peak intermediate concentration in Fig. 7.2). This spectrum clearly shows the chemical shift associated with the quinone methide along with those of decomposition products (Fig. 7.3). 

FIGURE 7.4 13C-NMR spectrum of the crude product mixture from the reaction shown in Scheme 7.6. Each chemical shift represents the C-10 center of a reaction product. 

We succeeded in isolating the same adduct reported by Maliepaard et al. in trace quantities, but a 13C-NMR spectrum of the crude reaction mixture revealed the presence of hundreds of additional products (Fig. 7.4). The chemical shift of the guanosine 2-amino adduct (35.4 ppm) is not obvious in this spectrum. The bands of chemical shifts centered at 65 and 55 ppm correspond to structures with oxygen attachment to the C-10 center, but the band of products centered at 28 ppm represented unknown structures. 

FIGURE 7.5 13C-NMR spectrum of the pentamer showing four methylene linkages and a methyl terminus. 

The toxicity of 3-methylindole has been attributed to methyleneindolenine trapping of nitrogen and sulfur nucleophiles.57 60-62 Likewise, the ene-imine shown in Scheme 7.9 readily reacted with hydroquinone nucleophiles, resulting in head-to-tail products. Shown in Fig. 7.6 is the 13C-NMR spectrum of a 13C-labeled ene-imine generated by reductive activation. The presence of the methylene center of the ene-imine is apparent at 98 ppm, along with starting material at 58 ppm and an internal redox reaction product at 18 ppm. Thus, the reactive ene-imine actually builds up in solution and can be used as a synthetic reagent. 

C-NMR, COSY, HMQC (heteronuclear multiple quantum coherence), and HMBC (heteronuclear multiple bond correlation).48 Furthermore, the structure of trimer was confirmed by X-ray crystallography.48 The incorporation of 13C into the indole 3a position proved valuable in these structural determinations and in documenting the ene-imine intermediate. For example, the presence of a trimer was readily determined from its 13C-NMR spectrum (Fig. 7.7). 

FIGURE 7.7 13C-NMR spectrum of an ene-imine trimer. The 13C labels are designated with asterisks ( ). 

FIGURE 7.9 13C-NMR spectrum of 10a-13C-labeled WV-15 reductively alkylated d (GGGCCC). 

FIGURE 7.10 13C-NMR spectrum of d(ATGCAT) reductively alkylated with the 3a-13C-labeled cleaving agent shown in the inset of Scheme 7.12. 

The reductive activation reaction of the 13C-labeled pyrrolo[ 1,2-a]indole shown in Scheme 7.14 was carried out in methanol and a 13C-NMR spectrum was obtained for the crude organic extract. This 13C-NMR spectrum, shown in Fig. 7.14, reveals the presence of starting material as well as products with 13C-labeled alkene and alkane centers. We confirmed the 13C assignments shown... 

To assess the trapping of biological nucleophiles, the pyrido[l,2-a]indole cyclopropyl quinone methide was generated in the presence of 5 -dGMP. The reaction afforded a mixture of phosphate adducts that could not be separated by reverse-phase chromatography (Fig. 7.16). The 13C-NMR spectrum of the purified mixture shown in Fig. 7.16 reveals that the pyrido [1,2-a] indole was the major product with trace amounts of azepino[l,2-a] indole present. Since the stereoelec-tronic effect favors either product, steric effects must dictate nucleophilic attack at the least hindered cyclopropane carbon to afford the pyrido[l,2-a]indole product. Both adducts were stable with elimination and aromatization not observed. In fact, the pyrido [1,2-a] indole precursor (structure shown in Scheme 7.14) to the pyrido [l,2-a]indole cyclopropyl quinone methide possesses cytotoxic and cytostatic properties not observed with the pyrrolo [1,2-a] indole precursor.47... 

FIGURE 7.21 Enriched 13C-NMR spectrum of cyclopent[ ]indole reductive activation... 

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