Selective INEPT

The selective INEPT experiment also requires a separated proton resonance for excitation and also uses rectangular soft pulses (Bax and Freeman, 1982). Magnetization is selectively transferred by the soft proton pulses to the corresponding carbons to which the protons are coupled. 

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. 

For the new C-hydroxybenzyl glycoflavones from Citnillus colocynthis," the complete C NMR data were assigned and additionally confirmed by 2D-COSY, HETCOR, and selective INEPT experiments. 

During the structural elucidation of the first flavone-xanthone dimer (swertifranche-side) through a series of ID or 2D NMR techniques including COSY, phase-sensitive ROESY, reversed-detected HMQC, HMBC, and selective INEPT experiments, H and C signals appeared as two peaks indicating two conformers and classically, the coalescence of the split signal was observed as the temperature was increased. 

Selective excitation and selective detection have been recently reviewed by the recognized leaders in this field135 who viewed 29Si NMR applications from a broader perspective. In this review we focus on those selective experiments that have found wider use in studies of organosilicon compounds, selective INEPT and HEED as representatives of the two classes (excitation and detection) of selective experiments. Some other selective experiments are mentioned in other sections (e.g. SPT, selective decoupling, SPINEPTR, V>-BIRD HMQC, DQF COSY). 

Selective INEPT can be run without decoupling and refocusing, and it can be further combined with selective decoupling as described for 13C and 15N NMR by Uhrin and... 

FIGURE 43. Assignment of the two lines in the 29Si NMR spectmm of methyl 3o, 12a-bis(trimethyl-siloxy)-5/i-cholenate, 37, by selective INEPT. Top trace 29Si INEPT spectmm two middle traces selective INEPT spectra measured with selective excitation of H lines indicated by arrows in the bottom trace with partially assigned ll NMR spectmm (25 mg of the sample in 0.7 ml of CDCI3, H frequency 200 MHz, 29Si frequency 39.7 MHz, 5 mm broad-band probe, selective pulse by DANTE train, r = 70 ms, A = 149 ms). Reproduced with permission of Collection of Czechoslovak Chemical Communications from Reference 304... 

Fig. 1. Pulse sequences for ID- X, "Y H) polarization transfer experiments. If not stated otherwise, narrow and wide bars denote 90° and 180° hard pulses, narrow and wide ellipsoids 90° and 180° shaped pulses. Essential phase cycles for selection of the polarization transfer signal are given on top of the pulses, if no phase is indicated, pulses are applied along the x-axis A denotes a fixed delay of length (n/(X,Y))" (a) UPT (, 0 = 90°), (b) unrefocused INEPT, (c) unrefocused selective INEPT with soft pulses, (d) INEPT with selective excitation via H, "Y cross-polarization si denote WALTZ-17 spinlock pulse trains which were applied for a period t— (2/( H,Y)) ...

Figure 5.77. The gradient-selected INEPT-INADEQUATE sequence for identifying carbon-carbon connectivities through proton observation (time delays in the sequence have been removed for clarity). The sequence selects for H- C- C fragments for which the gradient ratios are 1 —1 1. Despite the complexity of the sequence, its operation may be understood by recognising discrete, simpler segments within it that have defined roles to play, which in this case are INEPT and INADEQUATE steps.

Fig. 5.16 The partial DC selective INEPT J-resolved spectrum of a spin system...

The structural elucidation of these sesquiterpenes, given the difficulty in determining the linking sites of the respective ester groups when more than three kinds of acid are involved as esters in the molecule, necessitates the use of nmr experiments (selective INEPT [10], Coloc [11] or HMBC 2D [12]), selective hydrolysis processes or X-ray crystallographic methods [13], From comparison of the chemical shifts of similar compounds, it is not easy to establish the exact position of each ester. 

If long-range heteronuclear couplings to only one single proton are required, it may be sufficient to record one-dimensional selective irradiation experiments, such as selective INEPT (8). 

The structures of all new compounds were determined by application of MS, H-NMR, C-NMR, DEPT, H- C HETCOR, HMQC and HMBC techniques. Structure elucidation and stereochemical assignments including absolute stereochemistry of the furanonaphthoquinone 96 involved the use of the selective INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) NMR technique and comparison of the CD (Circular Dichroism) spectra with related compounds whose absolute stereochemistry has been determined by x-ray crystallography [75]. 

A new alkaloid (7) is the first example of a Gelsemium alkaloid having an A a-methoxyindole moiety in the molecule. This alkaloid may be an early biogenetic intermediate to the A a-methoxyoxindole alkaloids and their related compounds. Full assignments of the IH- and l C-NMR spectra of A7 a-methoxy-19(Z)-anhydrovobasinediol (7) were conducted mainly by CSCM ID (decoupled selective population transfer experiment) (18) and selective INEPT (insensitive nuclei enhanced by polarization tansfer) (19) experiments. The structure was finally determined by single crystal X-ray analysis (11). 

In the selective INEPT technique (ref. 5), the proton is irradiated with a soft pulse and the dwell time prior to observation is varied depending on the anticipated (or preferably known) coupling constant. Typically, the experiment is utilized for the selective enhancement of carbons three bonds away from the irradiated proton where J values in the range 6-8 Hz are used. There are two important exceptions to this range of J. values. In the case of... 

In this brief review, the importance of difference nOe and selective INEPT spectroscopy in elucidating the structure of a novel naphthoquinone from Larrea tridentata and a red pigment from Dalbergia candenatensis will be presented and the powerful combination of CSCM ID and selective INEPT... 

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