DQF-COSY

Suppression of the tme diagonal peaks by double-quantum filtering (DQF-COSY) may resolve such problems. Finally, quantitative measurements of the magnitude of the coupling constants is possible using the Z-COSY modification, These experiments ate restricted to systems of abundant spins such as H, and which have reasonably narrow linewidths. 

The DQF (double-quantum filtered)-COSY spectrum of an isoprenyl coumarin along with H-NMR data are shown. Determine the H/ H homonuclear interactions in the DQF-COSY spectrum. 

Fi and F. The off-diagonal peaks (cross-peaks) represent the direct coupling interactions between protons. Working through cross-peaks, one can easily correlate protons that are coupled to each other. Several versions of the COSY experiment have been designed to get optimum performance in a variety of situations (such as DQF COSY, COSY-45°, and COSY-60°). 

In 2006, Milosavljevic and co-workers64 reported a study of the complete 4H and 13C NMR assignment of a new triterpenoid saponin, leucantho-side-A (13), from Cephalaria leucantha L. In the course of determining the structure and assigning the spectra, the authors made extensive use of the normal ensemble of 2D NMR experiments in use for the characterization of natural product structures HSQC, HMBC, DQF-COSY, TOCSY, and NOESY. The authors supplemented the aforementioned list of experiments with 2D /-resolved, DINE-(Double INEPT-Edited)-HSQC, and INADEQUATE spectra. The authors made no mention of the use of the connectivity information derived from the 1,1-ADEQUATE spectrum in the assembly of the triterpene nucleus of the molecule but reported extensive tabulations of the 1,1-ADEQUATE correlations that were used to sequence and assign the saccharide resonances of the tri- and di-saccharide sub-units, 14 and 15, respectively, linked to the triterpene nucleus. 

Fig. 10.10. DQF-COSY data of astemizole (5). The black bars indicate the contiguous spin system drawn with arrows on the relevant portion of astemizole.

Artifactual peaks are even more dangerous than noise since they may not always be immediately recognized and may lead to erroneous assignments. An important source of artifacts is instability in the steady-state condition, e.g. if the relaxation delay is set too short. A commonly encountered example is presented by peaks which occur at the double quantum frequencies in DQF-COSY spectra. For detailed treatments of aspects of noise and artifacts see [4, 5]. 

The data from H NMR studies of 63, which included double quantum filtered phase sensitive correlated spectroscopy (DQF-COSY) and rotating frame nuclear Overhauser effect spectroscopy (ROESY) experiments (Figure 12), are collected in Table 17. 

The coupling constants listed in Table 17 were assigned on the basis of the ID H NMR spectrum of 63, but for a few cases the analysis of phase structure of the cross-peaks in the DQF-COSY spectrum was carried out to attribute correct values to the appropriate protons. Also, the analysis of the NOE effects yields the same results (Figure 13). 

FIGURE 12. Spectra of vacidin A methoxycarbonylmethylamide 64. Spectral region 0.72-5.06 ppm of 300 MHz ROESY (upper triangle) and DQF-COSY (lower triangle) spectra of VacGlyOMe (15 mg ml-1, pyridine-d5-methanol-d4, 9 1) combined along the diagonal. Reproduced by permission of Japan Antibiotics Research Association from Reference 40... 

I h. correct values of coupling constants were attributed to the appropriate protons by the analysis of antiphase structures of cross-peaks in DQF-COSY spectrum. 

This programme turns DQF-COSY and HMBC spectra into bond constraints. Then it turns C DEPT spectra and the molecular formula into building blocks such as -CH3 and -CH2-. These are then assembled into as many complete structures as are compatible with the bond constraints. CISOC-SES is designed to be as compatible with real-world spectra with their attendant ambiguities as possible. CISOC-SES is a result of collaborative work with Bodenhausen et al. who had previously tackled the problem indepen-dently. CISOC-SES has since been commercialized as NMR-SAMS by Spectrum Research, EEC. 

Identification of constitutive monosaccharides two-dimensional homonuclear NMR techniques such as DQF-COSY and TOCSY are used to assign chemical-shift values for all C-bonded protons in each individual monosaccharide (96). One-dimensional NMR spectra provide useful information about the chemical shifts and scalar couplings of such well-resolved signals as methyl groups for 6-deoxy monosaccharides (fucose, quinovose, and rhamnose) at 6 1.1-1.3 ppm. 

The UV spectrum [7max 237, 250 (sh), 289 (sh), 299, 345, and 357nm] of carbazomadurin A (256) resembled that of carbazomadurin B (257), indicating a similar carbazole framework. The H-NMR spectrum was also similar to that of carbazomadurin B (257), except for the signals of the alkyl side chain. Detailed analysis of the DQF-COSY and HMBC spectra confirmed a 2,5-dimethylhex-l-ene side chain at C-1 with the same configuration of the double bond. Based on the spectral data, structure 256 was assigned to carbazomadurin A (229). 

Reddy, B.P. and Krupadanam, G.L.D., Chemical constituents of the leaves of Himalayan Taxus baccata use of DQF-COSY in the structure elucidation of biflavones, Indian J. Chem., 35B, 283, 1996. 

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