Heteronuclear multiple bond connectivity HMBC

Because of the complexity of the polyether antibiotics tittle progress has been made in stmcture determination by the chemical degradation route. X-ray methods were the techniques most successfully applied for the early stmcture elucidations. Monensin, X206, lasalocid, lysocellin, and salinomycin were included in nineteen distinct polyether x-ray analyses reported in 1983 (190). Use of mass spectrometry (191), and H (192) and nmr (141) are also reviewed. More recently, innovative developments in these latter techniques have resulted in increased applications for stmcture determinations. Eor example, heteronuclear multiple bond connectivity (hmbc) and homonuclear Hartmann-Hahn spectroscopy were used to solve the stmcture of portimicin (14) (193). East atom bombardment mass spectrometry was used in solving the stmctures of maduramicin alpha and co-factors (58). 

Heteronuclear Multiple-Bond Connectivity (HMBC) Spectra... 

H homonuclear correlation spectroscopy (COSY). The connection between the benzyltetraisoquinoline and pavine moieties in 66 was located at C-10 and C-7 through an ether bridge as a result of the unambiguous assignments of H and 13C NMR signals by the heteronuclear multiple-bond quantum coherence (HMQC) and heteronuclear multiple-bond coherence (HMBC) NMR techniques. The EIMS of 66 confirmed the presence of a hydroxybenzyl moiety due to the observed complementary peaks at m/z 545 and 107 [11]. Furthermore, the structure of 66 was substantiated by the formation of herveline C (68) after 66 was treated with diazomethane in ethyl ether solution overnight [11]. 

NMR is the tool most widely used to identify the structure of triterpenes. Different one-dimension and two-dimension techniques are usually used to study the structures of new compounds. Correlation via H-H coupling with square symmetry ( H- H COSY), homonuclear Hartmann-Hahn spectroscopy (HOHAHA), heteronuclear multiple quantum coherence (HMQC), heteronuclear multiple bond correlation (HMBC), distortionless enhancement by polarisation transfer (DEPT), incredible natural abundance double quantum transfer experiment (INADEQUATE) and nuclear Overhauser effect spectroscopy (NOESY) allow us to examine the proton and carbon chemical shift, carbon types, coupling constants, carbon-carbon and proton-carbon connectivities, and establish the relative stereochemistry of the chiral centres. 

A H(detected)- C shift correlation spectrum (common acronym HMQC, for heteronuclear multiple quantum coherence, but sometimes also called COSY) is a rapid way to assign peaks from protonated carbons, once the hydrogen peaks are identified. With changes in pulse timings, this can also become the HMBC (heteronuclear multiple bond connectivity) experiment, where the correlations are made via the... 

Two-dimensional C//correlations such as C//COSY or HC HMQC and HSQC provide the Jqh connectivities, and thereby apply only to those C atoms which are linked to H and not to non-protonated C atoms. Modifications of these techniques, also applicable to quaternary C atoms, are those which are adjusted to the smaller Jqh and Jqh couplings (2-25 Hz, Tables 2.8 and 2.9) Experiments that probe these couplings include the CH COLOC (correlation via long range couplings) with carbon-13 detection (Fig. 2.16) and HC HMBC (heteronuclear multiple bond coherence) with the much more sensitive proton detection (Fig. 2.17)... 

A Ci-symmetric cis-1 bis-adduct with an additional bridge between the addends was obtained in the reaction between two molecules of ethyl propio-late and C6o in the presence of triphenylphosphane.318 Its structure (( )-162, Figure 1.37) was established with the help of the HMBC- (heteronuclear multiple bond correlation) NMR technique and includes stereogenic centers in the addend moiety as well as a noninherently chiral addition pattern. The latter can be related to the head-to-tail connectivity of the two propiolate units. 

The structural connectivity derived from examination of the 111, 13C/DEPT, DQF-COSY, HMQC, and HMBC data (DEPT = distortionless enhancement by polarization transfer DQF = double quantum filtering COSY = correlation spectroscopy HMQC = heteronuclear multiple quantum correlation HMBC = heteronuclear multiple bond correlation) resulted in global reevaluation of sclerophytin B structure and demonstrated that this compound and the related alcohol are not composed of two ether bridges as in the originally formulated structure 37, but share the structural features depicted as 38 <20000L1879>. Comparison of 13C and 111 NMR data of Norte s... 

A modification of the HMQC referred to as the HMBC (Heteronuclear Multiple Bond Coherence) experiment can be optimized for transfer through multiple bonds based on the value of the multiple bond couplings, which is typically about 8 Hz for a three-bond coupling. The HMBC spectrum of dutasteride shown in Fig. 9 illustrates the utility of this experiment for assigning quaternary carbons, connecting isolated spin systems (e.g., Hig and Hig) to other spin systems in the molecule, and confirming assignments made from the COSY and HMQC spectra. 

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