Long-range HSQMBC

Figure 36 Pulse sequences for the CBC-HSQMBC experiment. Narrow and thick bars represent 90° and 180° RF pulses, respectively. Pulse phases are along x unless indicated otherwise. Phase cycles are (pn = x, x

R. T. Williamson, B. L. Marquez, W. H. Gerwick, and K. E. Kover, One- and two-dimensional gradient-selected HSQMBC NMR experiments for the efficient analysis of long-range heteronuclear coupling constants, Magn. Reson. Chem., 38 (2000) 265-273. 

In 2001, Meissner and Sprensen described methods for the measurement of Jhh anO "Jch couplings employing the broadband excitation approach (see Section 3.3.7) with the report of the broadband XLOC and broadband J-HMBC experiments, respectively. In a further extension of the idea of accordion optimization, Williamson ef reported the development of the J-IMPEACH-MBC pulse sequence. This approach is similar to the EXSIDE experiment previously described by Krishnamurthy and the J-resolved HMBC experiments of Furihata and Seto"- in its use of J-scaling to render small, long-range heteronuclear couplings conveniently measurable. Finally, Williamson e/ also described a new method, G-BIRDr-HSQMBC, that was reported in their survey of the available methods. 

Long-range heteronuclear coupling constants determined from HSQMBC spectra were also used in the J-based conformational analysis - of the aliphatic stereocenters at C7 through CIO of the neurotoxin kalkitoxin (28) from the marine cyanobacterium Lyimgba majuscula ... 

Finally, Moore and co-workersreported using a combination of HETLOC and HSQMBC experiments to measure the long-range heteronuclear coupling constants of apratoxin A (30), a cytotoxin from the marine cyanobacterium Lyngba majuscula. 

R.T. WiUiamson, A.V. Buevich, G.E. Martin, T. Parella, LR-HSQMBC a high sensitivity technique to probe very long-range heteronuclear coupling pathways, J. Org. Chem. 79 (2014) 3887-3894. 

A broadband. Pure In-Phase HSQMBC (PIP-HSQMBC) experiment yielding pure IP multiplet patterns for all long-range cross-peaks has been recendy reported (Fig. 27) [111]. It is an extension of the PIP-HSQC experiment described in Section 5.1.4, where the interference due to undesired J(HH) evolution during the longer heteronuclear transfer periods is more pronounced than the HSQC counterpart because the size of and... 

HMBC and HSQMBC are traditionally used to trace out long-range proton-carbon correlations. Most of the observed cross-peaks correspond... 

It has been reported that the 2-Hz optimized LR-HSQMBC experiment acquired a provided a major number of very long-range correlations than analogs HMBC and D-HMBC experiments. These data could be obtained in 2-3 h from a 4.9 mg sample of strychnine (10.5 pmol) dissolved in 40 pL of CDCI3 in a 600-MHz spectrometer equipped with a 1.7-mm MicroCryoProbe. Data were also compared with an equivalent dual optimization inverted J(CC) 1, -ADEQUATE spectrum acquired in about 24 h [171]. In comparison, 29 of the ADEQUATE correlations were missing in the LR-HSQMBC data (23.6%), but additional 62 new correlations (50.4%) not observed in the inverted J CC) 1,k-ADEQUATE data were visualized in the LR-HSQMBC experiment. While drawing comparisons, the sole negative attribute of the LR-HSQMBC experiment relative to dual optimization inverted J(CC) 1, -ADEQUATE is that the former does not allow the unequivocal differentiation of J(CC) from J(CC) CJ CH) vs. 7(CH) where > 3) afforded by the latter experiment [172-175]. 

The recently reported LR-HSQMBC experiment has been optimized for H—long-range heteronuclear couplings [176]. Several previously unreported four-bond correlations, consistent with the predicted by DFT calculations (0.2—0.3 Hz "J/(NH) couplings), have been observed for strychnine using 2 Hz optimization of the LR-HSQMBC experiment. The... 

J. Sauri, T. PareUa, Efficient measurement of the sign and the magnimde of long-range proton-carbon coupUng constants from a spin-state-selective HSQMBC-COSY experiment, Magn. Reson. Chem. 50 (2012) 717—721. 

S. GU, J.F. Espinosa, T. PareUa, IPAP—HSQMBC measurement of long-range heteronuclear coupling constants from spin-state selective multiplets, J. Magn. Reson. 207 (2010) 312-321. 

A number of papers devoted to extraction of small long-range proton-couplings from the spectra of natural products have been published by Parella and co-workers.The effects of phase modulation due to homonuclear proton-proton couplings in HSQMBC-IPAP and... 

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