Shift DEPT-HMQC

Kessler, H., Schmieder, R, and Kurz, M. (1989) Implementation of the DEPT sequence in inverse shift correlation The DEPT-HMQC. J. Magn. Reson. 85(2), 400-405. 

Heteronuclear chemical shift correlation methods establish the direct link between protons and the respective, directly attached carbons (or nitrogens). In the case of methylenes with inequivalent (anisochronous) protons, the "multiplicity of the carbon in question is irrefutably obvious. For isotropic methylenes and other resonances, the multiplicity of the resonance (CH, CH2 or CH3) in question may be less obvious. Early work by Kessler and co-workers addressed this issue via the development of the DEPT-HMQC experiment. [120] Multiplicity editing is also available for experiments such as GHSQC. An extra pair of delays and pulses, with the flip angle of the proton pulse being adjustable, allow the acquisition of data in... 

At present, several experiments are available for inverse-detected one-bond heteronuclear shift correlation. The HMQC experiment described by Bax and Subramanian (1986) has probably been most widely employed. Alternatives, however, are available in the form of DEPT-HMQC (Kessler et al. 1989b) and the HSQC or so-called Overbodenhausen experiment (Boden-hausen and Ruben 1980). For alkaloids with highly congested proton spectra, DEPT-HMQC may be a useful alternative to HMQC, because it allows the acquisition of edited correlation spectra. For investigators interested in correlation of protons to alkaloidal nitrogen atoms via one or two bonds, HSQC or a doubly refocused variant may be the preferred choice. 

There are only three cases possible for each carbon atom. If a line drawn encounters no cross peaks, then the carbon has no attached hydrogens. If the drawn line encounters only one cross peak, then the carbon may have either 1,2, or 3 protons attached if 2 protons are attached, then they are either chemical shift equivalent or they fortuitously overlap. If the horizontal line encounters two cross peaks, then we have the special case of diastereotopic protons attached to a methylene group. Much of this information will already be available to us from DEPT spectra (see Section 4.6) indeed, the HMQC spectrum should, whenever possible, be considered along with the DEPT. 

A better approach for solving structures relies less on the 1-D spectra and taps the wealth of information in the 2-D spectra. We obtain the molecular formula as we did above, noting also the presence of the alcohol function from the IR with confirmation in the l3C/DEPT and H NMR spectra. Next, we turn to the 2-D data. Evidence of diastereotopic protons is quickly ascertained in the HMQC by noting if there are two protons with different chemical shifts that are correlate to the same 13C peak. No such diastereotopic correlations are seen. 

Later we will see how these couplings can be exploited in experiments that enhance the sensitivity of 13C spectra (INEPT), measure the number of hydrogens attached to each carbon (APT and DEPT), and correlate 13C chemical shifts with H chemical shifts using a second dimension (2D-HETCOR, -HMQC, -HSQC, and -HMBC). But for detecting a simple 13C spectrum, we need a way to suppress these 13C- H couplings so we can observe a single line (singlet) for each 13 C resonance. 

The DEPT pulse sequence is illustrated in Fig. 4.31. To follow events during this, consider once more a H- C pair and note the action of the two 180 pulses is again to refocus chemical shifts where necessary. The sequence begins in a similar manner to INEPT with a 90 (H) pulse after which proton magnetisation evolves under the influence of proton-carbon coupling such that after a period 1 /2J the two vectors of the proton satellites are antiphase. The application of a 90 (C) pulse at this point produces a new state of affairs that has not been previously encountered, in which both transverse proton and carbon magnetisation evolve coherently. This new state is termed heteronuclear multiple quantum coherence (hmqc) which, in general, cannot be visualised with the vector model, and without recourse to mathematical formalisms it is... 

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. 

NMR spectroscopy has been extensively used for determining the carbon framework of Amaryllidaceae alkaloids, and the major contributions are due to Crain et al (203), Zetta and Gatti (210), and Frahm et al. (143). The assignments are made on the basis of chemical shifts and multiplicities of the signals (by DEPT experiment). The use of 2D NMR tecniques such as HMQC and HMBC allow the assignments to be corroborated. Table 5 shows a compilation of the different NMR spectra classified according to the different types. 

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