HMBC correlations

The HMBC spectrum of vasicinone along with the H-NMR assignments are shown. Determine the H/ C long-range heteronuclear shift correlations based on the HMBC experiment, and explain how HMBC correlations are useful in chemical shift assignments of nonprotonated quaternary carbons. 

Partial structures I and II thus obtained were connected through the carbonyl group by the strong HMBC correlations of H-7 (8 4.05ppm) and H-9 (8 3.18ppm) to C-8 carbonyl carbon (8 208.5 ppm). Thus, the total planar structure of 46, including the partial relative stereostructure of the decalin system, was elucidated as shown in Figure 4. 

The UV spectrum (/Imax 224, 243, 298, and 327 nm) of mahabinine-A (212) resembled that of bismahanine (208) (see Scheme 2.49) indicating a similar dimeric carbazole framework. The H- and C-NMR spectra were also similar to those of bismahanine, except for the signal of one additional aromatic methyl group indicating the presence of an isomahanine (138) unit (see Scheme 2.26). The H-NMR spectrum showed two, orf/zo-coupled (/=8.3Hz) aromatic protons at S 6.71 and 7.70. The HMBC spectrum with cross-peaks between the aromatic proton at 5 6.71 and C-8 at 5 104.8, as well as C-4b at 5 117.9, suggested that the aromatic proton was located at C-6. The HMBC correlation of the other ortho-coupled aromatic proton at 5 7.70 and the aromatic carbon atoms at 5 136.0 (C-8a) and at 6 150.9 (C-7) indicated that the aromatic proton at 6 7.70 was located at C-5. Moreover, HMBC... 

The H- and C-NMR spectra of dihydroarcyriarubin C (353) were very similar to those of arcyriarubin C (351), except for the presence of one sp methine proton at 6 4.44, which was assignable to the C-8 proton. The HMBC spectrum showed a cross peak from H-8 to C-8 (5c 48.2), and this HMBC correlation may be assigned to H-8 to C-8 (or H-8 to C-8), indicating the symmetrical structure of this alkaloid. This structure was further supported by its H- H COSY and HMQC spectra (334). Comparison of the H-NMR spectral data of synthetic cis- and frans-dihydroarcyr-iarubin A (339) indicated the frans-relative stereochemistry for the natural dihydroarcyriarubin C (353). Based on these spectral data, and comparison with arcyriarubin C (351), as well as with synthetic fraMS-dihydroarcyriarubin A, the structure 353 was assigned to dihydroarcyriarubin C (334,339) (Scheme 2.90). 

The HMBC correlates proton nuclei with carbon nuclei that are separated by more than one bond (Figure 2.2). The experiment is normally optimized for Vch and VcH couplings however, the intensity of the crosspeaks generated by this experiment... 

Compound 57 had molecular formula C28H32O16. Its H NMR and I3C spectra compared with those of 56 each revealed one more signal respectively at 5 3.76, (3H, s) and 51.9 ppm assigned to one methoxyl group. Its position is established from HMBC correlation data. The methoxylation shift observed at C-6 and C-8 supported the position of the methoxyl group at C-7. 

The structure of a compound separated from the antifungal constituents of the Chinese liverwort Marchantia polymorpha L. (Marchantiaceae) was identified by H and 13C NMR spectroscopy and established to be 13,13 -0-isopropylidene-riccardin 17 <2006MI34>. The acetonide structure of 17 emerged from the 13C NMR spectra and HMBC correlations. For the structural elucidation of a new . 

Fig. (5). Clave HMBC correlations to discern between a and P-cycled naphthoquinones...

A table of predicted and observed HMBC correlations confirms that many crosspeaks are missed, even in a fairly concentrated sample ... 

In this example, we had already assigned all of the carbon peaks in the 13C spectrum, so there was no mystery. In the most difficult case of a complete unknown, the peaks in the 13C spectrum are numbered arbitrarily from upfield to downfield, and the protons are numbered correspondingly according to the carbon they correlate to in the HSQC/HMQC spectrum. Then the HMBC correlations are tabulated (e.g., C-5-H-16a) and the puzzle solving begins. 

Figure 11.60 shows a portion of the DQF-COSY spectrum of LGJC3 with part of the HSQC spectrum aligned above it and, turned sideways, to the left side. There are correlations from hl8 (1.56 ppm) to h8b (1.70 ppm, strong) and h8a (2.42 ppm, weak). Since these cannot be geminal couplings, we can assume that c8 is attached to cl8 and the relationship is vicinal (three bonds) between hi8 and h8b and between hl8 and h8a. We can now connect the hl/h2 fragment to the hlO fragment (Fig. 11.61). Likewise, the h6a COSY crosspeak to hl2a (Fig. 11.60) allows us to connect the h9 fragment to the hlO fragment. Because of the shared HMBC correlations from hi (CH3) and h9 (CH3) to...

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