Multiplicity-edited HSQC

The order in which various NMR data are acquired is largely one of user preference. Acquisition of the proton reference spectrum will invariably be undertaken first. Whether a user next seeks to establish homo- or heteronuclear shift correlations is where individual preferences come into play. Many spectro-scopists proceed from the proton reference spectrum to either a COSY or a TOCS Y spectrum next, while others may prefer to establish direct proton-carbon chemical shift correlations. This author s preference is for the latter approach. From a multiplicity-edited HSQC spectrum you obtain not only the carbon chemical shifts, which give an indication of the location of heteroatoms, the degree of unsaturation and the like, but also the number of directly attached protons, which eliminates the need for the acquisition of a DEPT spectrum [51, 52]. The statement in the prior sentence presupposes, of course, that there the sensitivity losses associated with the acquisition of multiplicity-edited HSQC data are tolerable. 

NMR (Fig. 17) spectrum for 58 revealed 15 resonances, interpreted from multiplicity-edited HSQC data as eight quaternary, two methine, two methylene, and three methyl carbons. In the H NMR spectrum (Fig. 17)... 

Figure 7-26 Expansions HSQC spectrum (left) and multiplicity-edited HSQC spectrum (right) of T-2 toxin.

Boyer, R. D., Johnson, R., and Krishnamurthy, K. (2003) Compensation of refocusing inefficiency with synchronized inversion sweep (CRISIS) in multiplicity-edited HSQC. 

Figure 6.19. The multiplicity-edited HSQC spectrum of the disaccharide 6.6 in which positive CH correlations (red) are distinguished from negative CH2...

D CRISIS-HSQC Offset compensation in multiplicity-edited HSQC experiments with adiabatic pulse inversion. 58... 

GIC of the simplified models of the COSY and the multiplicity-edited HSQC. The spectrum resulting for A=1 resembles the UIC spectrum. Yet, the signal intensities are different when the square root operation >1 = 0.5 is applied, see Fig. 5.1 G. The positive effect of the GIC concerning the minimization of artefacts will be summarized in due course. It is not visible in this model. Fig. 5.1H symboHzes the transformation of a multiplicity-edited HSQC with an 1,1-ADEQUATE to yield a C—C correlation map, whose interpretation scheme will be given in Fig. 5.14. The construction of a 3R cube from two 2D spectra is illustrated in Fig. 5.11. It is the only covariance transformation among the examples that does not result from matrix multiphcation but from reconstruction according to Eq. (5.24). 

Figure 5.13 Expansion of the aliphatic region of the 40 Hz HSQC-1,1-ADEQUATE spectrum of Dinaciclib 10. A convenient starting point is provided by the piperidine methine carbon resonance at 47 ppm enclosed in the black box. Two correlation pathways can be traced beginning from the resonance. One coupling pathway defines the pendant p-hydroxy ethyl moiety (C26, C27). The other connectivity pathway sequences and assigns the resonances of the remainder of the piperidine (C16-C13). Responses enclosed in red boxes are artefacts that arise due to resonance overlaps proton spectrum in the multiplicity-edited HSQC and 1,1-ADEQUATE spectra used in the GIC calculation of the HSQC-1,1-ADEQUATE spectrum. Reprinted from Martin et al. [96]. Copyright 2011, with permission from Elsevier.

Very recently, the signal assignment of the antifungal agent posaconazole 12 and the structure elucidation of its degradants was reported [97,98]. Both relied heavily on the GIC HSQC-1,1-ADEQUATE derived from multiplicity-edited HSQC and 1,1-ADEQUATE spectra. 

The H—IMPEACH spectrum of (—)-ebumamonine (7) is shown in Fig. 13A. The multiplicity-edited HSQC spectrum is shown in Fig. 13B. Both spectra were acquired with identical proton spectral widths F and were equivalently processed to afford spectra that were 2048 x 512 points (F2,Fi). The C— chemical shift correlation spectrum calculated using the unsym-... 

C) from component multiplicity-edited HSQC (B) and IMPEACH... 

Kobakyashi and coworkers [82] reported the isolation and characterization of three bromotyrosine alkaloids isolated from an Okinawan marine sponge, Pseudoceratina sp. Two of the three alkaloids contained 4-quinolone moieties in the structures, which provides the basis for the present categorization. It is quite common for spectroscopists working with families of compounds to fingerprint them using, for example, and shifts obtained from multiplicity-edited HSQC spectra. Kobakyashi s group extended this idea further to include the diagnostic H7—N9 correlation of ceratinadin-A, which resonates quite far downfield at 384.2 ppm. It should also be noted... 

The DEPT experiment, or distortionless enhanced polarization transfer, is a carbon selectivity experiment. ° ° ° Depending on the pulse length selected, one can selectively observe different types of carbon entities. We recommend setting the DEPT proton pulse length to 135°. In this case, quarternary carbons are suppressed, methylenes are inverted, and methine and methyl carbons appear upright. Methines and methyls are distinguished based on chemical shift and 2D proton correlations. Methines usually appear downfield of methyls. Alternatively, if time permits, the entire series of DEPT experiments can be performed to conclusively distinguish methine from methyl resonances. One second is a reasonable default value for the recycle time. The spectrum should be set to capture the resonances of interest with 32-64 K data points and four dummy scans. While this is an easily interpreted data set, the multiplicity edited HSQC provides much more robust information, both multiplicity as well and the one-... 

Once the information is gleaned from the ID data, the 2D data should be analyzed. A reasonable approach to take is to begin with the multiplicity edited HSQC (see Figure 32) to get the information concerning the numbers of protons attached to a respective carbon. This allows one to get a handle on the types of carbon that are present and get an idea of chemical environment base of the multiplicity as well as the chemical shift. For example, a methyl group with a carbon and proton chemical shift of approximately 70 and 3.5 ppm, respectively would represent a methyl ether moiety. 

Once the protonated carbons and nonprotonated carbons are determined (quaternary carbons can be deduced through a combination of the multiplicity edited HSQC and the ID C data) a good approach is to begin the analysis of the COSY data (see Figure 33). This will allow the spectroscopist to look for the presence or absence of contiguous IH spin systems. While much of this can be deduced from a detailed understanding of the ID H NMR data the use of the COSY data will both confirm previous data as well as enable one to look at spin system that have non-first-order spin systems. 

FIGURE 32 Multiplicity edited HSQC spectrum of ibuprofen. The boxes are coior-matched to the arrows indicating the Jch heteronuciear scaiar coupiing. 

At this point it is useful to remind the reader that the bottleneck of the PANACEA experiment is the low sensitivity INADEQUATE step. Thus, several of the more sensitive H-1 detected spectra can be recorded while the required sensitivity for the INADEQUATE spectrum is achieved. The H-1 detected experiments start with recording of multiplicity edited HSQC spectra. The first three 14 increments are chosen to be of duration 1/4 J(CH), 2/4 J(CH) and 3/4 J(CH). Thus the first 2D HSQC spectrum contains all CH, CH2 and CH3 correlations, the second HSQC spectrum contains only CH correlations and the third HSQC spectrum contains positive CH and CH3 peaks while the CH2 peaks appear negative. 

Fig. 12 Selected regions of the PANACEA spectra of quinine recorded in a single experiment using the pulse sequence of Fig. 11. C-13 spectrum, multiplicity edited HSQC spectra and three-dimensional J-HMBC spectrum are all recorded in parallel with the C-C INADEQUATE spectrum. The experiment was recorded on a Varian 600 MHz NMR system equipped with two receivCTs...

Fig. 20 Selected PANACEA spectra of a cholesterol sample (ca. 30% in CDCI3) recorded on a Varian 600 MHz NMR system equipped with two receivers and a cryogenic HCN probe optimized for C-13 detection. The experiment time was 23 min. A fourfold undersampling (aliasing) was used with optimized offset and spectral window in the Fi (SQ) dimension, (a) Inadequate spectrum (b) multiplicity edited HSQC spectrum (the negative peaks are shown in red). Reproduced from Ref. 36. Elsevier, 2010...

---