Heteronuclear Correlation HMBC

FIGURE 6.17. Expanded view of the DQF-COSY of caryophyllene oxide (see text for discussion of the correlations shown). 

Are there other theoretical or experimental considerations to distinguish these two heteronuclear correlation experiments More bluntly, should we prefer one or the other experiments First, there are different hardware requirements for the two-experiments some instruments are incapable of running the inverse de- 

8 Proton-Detected, Long-Range —13C Heteronuclear Correlation HMBC 

For the HMQC described above, we wanted an experiment that eliminated long-range (i.e., two and three bond) proton-carbon couplings while preserving the directly attached (i.e., one-bond) couplings, which we correlated in a 2-D experiment. The HMBC (Heteronuclear Multiple Bond Coherence) experiment, on the other hand, which is also proton detected, capitalizes 

FIGURE 6.18. The 300-MHz (F2) HMQC (also called inverse detected HETCOR ) spectrum of caryophyllene oxide. 

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Long-Range Heteronuclear Shift Correlation HMBC... 

LONG-RANGE HETERONUCLEAR SHIFT CORRELATION HMBC AND RELATED EXPERIMENTS... 

Key Words NMR, Heteronuclear Long-range correlation, HMBC, CT-HMBC, ACCORD-HMBC, IMPEACH-MBC, BIRD-HMBC, G-BIRD-HMBC, CIGAR-HMBC, IMPACT-HMBC, TS-HMBC, Broadband-HMBC, H2BC, edited HMBC, 10-ppm HMBC, /CH artefacts suppression, Spectral editing... 

In the case of the 7-hydroxy-substituted compounds 44 (Scheme 2), 54 different derivatives were investigated by 13C NMR spectroscopy and, in some cases, also by 1SN NMR spectroscopy <1995JST(335)273>. With the help of proton-coupled 13C NMR spectra, semi-selective INEPT (insensitive nuclei enhanced by polarization transfer) experiments, and heteronuclear multiple bond correlation (HMBC) two-dimensional 2D-NMR spectra, all shifts could be unequivocally assigned. While the C-7 shifts did not allow the existing tautomeric situation to be determined, a clear decision could be made by H NMR spectroscopy in this respect. The 1SN NMR spectra revealed an equilibrium between the N(4)H and N(3)H tautomeric forms, which is fast on the NMR timescale. 

Additional heteronuclear long-range experiments include the BIRD-HMBC experiment and the broadband HMBC experiment [52]. Experimental variants capable of simultaneously recording direct and long-range heteronuclear correlations include the HMSQ and MBOB experiments [53]. 

Heteronuclear multiple bond correlation (HMBC) [1, 2] is one of the most powerful NMR techniques and is now being widely used for structural analysis of complicated molecules. Since it detects long range... 

There are a number of techniques stemming from the basic two-dimensional technique, which for example allow correlation between carbon atoms and the protons attached to them and correlations of carbon atoms with protons one or two bonds removed from them, heteronuclear multiple bond correlation (HMBC). 

NMR experiments. Heteronuclear NMR experiments are represented by H C HSQC (Figure 2.1) and heteronuclear multiple bond correlation HMBC) (Figure 2.2)... 

Product elucidation of trimethyloxonium tetrafluoroborate and methyl iodide reactions with [l,2,3]triazolo[4,5- -pyridazines (Schemes 1-3) using mainly 2-D heteronuclear multiple bond correlation (HMBC). NMR... 

The structural assignment of both 29 and 30 was accomplished through extensive two-dimensional (2-D) NMR heteronuclear multiple quantum correlation (HMQC) and heteronuclear multiple bond correlation (HMBC) spectroscopic studies <2004T8189>. In the HMBC spectrum of 29, the proton at 8.64p.p.m. shows a strong correlation Jq-h with the carbonyl carbon (C-10) at 180.9 ppm and the proton at 8.82p.p.m. with the carbonyl carbon (C-5) at 181.7 ppm. The HMBC spectrum of 30 shows a significant strong correlation Vq h of the C-5 carbonyl carbon with the H-6 proton at 8.52 ppm and the H-4 proton at 8.52p.p.m. 

Complete N NMR chemical shifts together with H- N coupling constants (referenced externally to NH4C1 in DMSO-1 6 DMSO - dimethyl sulfoxide) have been determined for toxoflavin 7 and fervenulin 8 in CDCI3 using pulsed field gradient heteronuclear multiple bond correlation (HMBC) techniques <2000H(52)811>, and are presented in Table 1. 

The structures of the compounds were elucidated by a combination of NMR techniques (lH-, 13C-, and 13C-DEPT NMR) and chemical transformation, enzymatic degradation, and as well as mass spectrometry, which gives information on the saccharide sequence. A more recent approach consists of an extensive use of high-resolution 2D NMR techniques, such as homonuclear and heteronuclear correlated spectroscopy (DQF-COSY, HOHAHA, HSQC, HMBC) and NOE spectroscopy (NOESY, ROESY), which now play the most important role in the structural elucidation of intact glycosides. These techniques are very sensitive and non destructive and allow easy recovery of the intact compounds for subsequent biological testing. 

Nuclear magnetic resonance (NMR) has proved to be a very useful tool for structural elucidation of natural products. Recent progress in the development of two-dimensional 1H- and 13C-NMR techniques has contributed to the unambiguously assignment of proton and carbon chemical shifts, in particular in complex molecules. The more used techniques include direct correlations through homonuclear (COSY, TOCSY, ROESY, NOESY) [62-65] and heteronuclear (HMQC, HMBC) [66. 67] couplings. 

Heteronuclear (X, H) Multiple Bond Correlation (HMBC) spectroscopy is a modified... 

Inverse-detected experiments have had the greatest effect in making 15N NMR experiments feasible for small samples. These experiments take advantage of the higher sensitivity of NMR to facilitate the observation of insensitive nuclei like 13C and 15N. The H-13C heteronuclear multiple quantum coherence (HMQC) and the related heteronuclear multiple-bond correlation (HMBC) experiments are important in contemporary natural products... 

Record the 2-D H-13C heteronuclear multiple bond correlation (HMBC) spectrum (Braun et al., 1998, pp. 489-492). 

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