Heteronuclear multiple bond assignment

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. 

In order to assign the chemical shifts of the carbon atoms of the conjugated diene system of each CLA isomer, it was necessary to conduct INADEQUATE, HMBC (heteronuclear multiple bond correlation) and two-dimensional 1H-13C correlation spectroscopy (COSY) techniques on the carbon signals of the diene system of the ,Z-isomers. The results of these experiments for the CLA isomers are summarized in Table 13. 

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. 

The heteronuclear multiple bond correlation (HMBC) experiment correlates proton nuclei with carbon nuclei that are separated by more than one bond. In Fig. FI. 4.7 the 37CH and 27Ch couplings dominate. Major applications related to anthocyanins include the assignment of resonances of nonprotonated carbon nuclei of the... 

The structural assignment of the trithiepines 44-46 has been performed using H, 13C, heteronuclear multiple bond correlation (HMQC), heteronuclear multiple quantum correlation (HMBC), and variable-temperature NMR spectroscopic data. The 60MHz H NMR spectrum of trithiepine 44 exhibits a broad singlet at 3.05 ppm in CDC13, whereas a narrow ABCD multiplet was observed for all of the protons in a 300 MHz spectrum. The two 13C NMR signals at... 

A. Bax and M. F. Summer, and C assignments from sensitivity-enhanced detection of heteronuclear multiple-bond connectivity by 2D multiple quantum NMR,/Am. Chem. Soc. 108 (1986),2093-2094. 

Two-dimensional NMR H- C heteronuclear multiple bond correlation (HMBC) experiments were performed on both 38 and 39 to assign the structure of these two compounds and their regioisomers <2003BMC2175>. 

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. 

The two-dimensional, heteronuclear multiple quantum correlation (HMQC) and heteronuclear multiple bond correlation (HMBC) spectra were taken with standard Bruker pulse programs. The HMQC and HMBC spectra are given in Figures 8 and 9 (9). The chemical shifts and spectral assignments are provided in Table 2 (9,10). The effect of Al3+ on the carbon spectrum of lomefloxacin is shown in Figure 10 (9). 

The assignment of heteronuclei generally requires acquisition of a heteronuclear single quantum coherence (HSQC) experiment and a heteronuclear multiple bond correlation (HMBC) experiment, and these are described in more detail below. 

R74 R. Araya-Maturana, T. Delgado-Castro, W. Cardona and B. E. Weiss-Lopez, Use of Long-Range C-H ( J n>3) Heteronuclear Multiple Bond Connectivity in the Assignment of C NMR Spectra of Complex Organic Molecules , p. 253... 

Bax A, Summers MF (1986) Proton and Carbon-13 Assignments from Sensitivity-Enhanced Detection of Heteronuclear Multiple-Bond Connectivity by 2D Multiple Quantum NMR. J Amer Chem Soc 108 2093... 

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]. 

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... 

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