Long-Range Heteronuclear Chemical Shift Correlation - HMBC

5 Long-Range Heteronuclear Chemical Shift Correlation - HMBC 

To date, the only inverse-detected heteronuclear chemical shift correlation experiment available for multiple-bond correlation is the HMBC experiment of Bax and Summers (1986). The pulse sequence is shown in Fig. 5 and 

An alternative to absolute value presentation has been proposed by Bax and Marion (1988). We have found mixed-mode processing, in which the data is absorptive in the (Fi) frequency domain and absolute-value-calculated in the proton (F2) frequency domain to provide a superior presentation of HMBC spectral data. Williamson et al. (1989) carried the idea of absorptive HMBC experiments a step further, demonstrating in the specific case of the antibiotic distamycin-A that fully phase-sensitive HMBC spectra may be recorded. 

There have been numerous applications of the HMBC experiment in the elucidation of alkaloid structures, which are discussed in the applications sections (see Sect. 9) of this chapter. One other application that bears mention at this point has yet to be applied to an alkaloid the application of long-range correlations between identical atoms in a molecule exhibiting C2 symmetry. An example of this type of application was recently reported for the C2-symmetric tetrastilbene hopeaphenol. The reader faced with this type of problem is referred to the excellent treatment of Kawabata et al. (1992b). In similar fashion, it is also possible to address stereochemical issues of this type (Kawabata et al. 1992a). For a discussion of an example of this type of application, the reader is directed to the section of this chapter dealing with HMQC-NOESY/ROESY. 

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Unlike HMBC /GHMBC and related long-range heteronuclear chemical shift correlation experiments, which have hundreds of reported applications in the published literature, there are considerably fewer reported applications of 1,1-ADEQUATE and related long-range variants in the literature. In part, the dearth of reported applications can be attributed to the considerably lower sensitivity of these experiments relative to, for example HMBC/GHMBC. Sensitivity concerns are largely ameliorated,... 

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. 

The HMBC spectrum of vasicinone displays long-range heteronuclear shift correlations between the various H/ C nuclei. These correlations are very helpful to determine the C-NMR chemical shifts of quaternary carbons and allow the interlinking of the different substructures obtained. 

COSY = homonuclear chemical shift correlation spectroscopy NOESY = 2D nuclear Overhauser effect (NOE) spectroscopy HMQC = heteronuclear mulltiple-quantum coherence RELAY = relayed coherence transfer COLOC = correlation via long-range coupling HMBC = heteronuclear long-range coupling INADEQUATE = incredible natural abundance double quantum transfer experiment. 

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