NOEs NOESY

Steady-state measurements are well suited to small-molecule configurational and conformational studies since the enhancements obtained will be intrinsically larger than those from transient measurements. Energy is continually fed into the spin system in the form of rf saturation, and this is able to drive the development of the NOE. Transient enhancements, 

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Dipolar Coupling Relaxation Times, Nuclear Overhauscr Effect (NOE), NOESY... 

Results similar to those shown in the slice of Fig. 8.22 can be obtained with the so-called NOE-NOESY sequence [36]. Here a hyperfine shifted signal, e.g. I2-CH3 of the above compound, is selectively saturated, and then the NOESY pulse sequence is applied. The NOESY difference spectrum obtained by subtracting a NOESY spectrum without presaturation of the I2-CH3 signal is shown in Fig. 8.23. Here, some more cross peaks are evident with respect to the 3D NOESY-NOESY experiment because secondary NOEs develop much more when the primary NOEs from the I2-CH3 signal evolve in a steady state experiment like the NOE-NOESY rather than in a transient-type experiment like the NOESY-NOESY. In Fig. 8.23, dipolar connectivity patterns are apparent among protons... 

Fig. 8.23. NOESY spectrum (A) and NOE-NOESY spectrum (B) of met-myoglobin cyanide [36], The latter spectrum is obtained by pre-irradiation of the I2-CH3 signal.

Other variants are NOE-COSY and NOE-TOCSY. However, these experiments are much less sensitive than NOE-NOESY. 

Although structural elucidation of lignans is not a difficult task, the similarities between the structures can create problems. In particular, the determination of stereochemistry at the chiral center requires NOE/ NOESY NMR experiments and/or X-ray analyses. The enantiomeric excesses of the known lignans (+)-lariciresinol, (-)-secoisolariciresinol and (+)-taxiresinol, isolated from Japanese yew T. cuspidata roots, were determined by chiral high-performance liquid chromatographic analyses [78] except for (+)-pinoresinol (77% enantiomeric excess), they were found to be optically pure by Kawamura et al. In an earlier study, the presence of taxiresinol in Taxus species was reported by Mujumdar et al. [69] after they had isolated it from the heartwood of T. baccata, although they did not study its stereochemistry. 

P-Epoxy-8-deoxy-cumambrin B C15H20O4 264 [alD -9° CHCl3 c0.86 HNMR, C NMR, DEPT, COSY, CLOCK, NOE, NOESY, MS,EIMS,C1MS 38... 

NOE and chemical exchange Selective water inversion in 2D NOE-NOESY WaterLOGSY... 

Usually the 2D NMR spectnun is either for the spin-spin interaction (COSY) or for the NOE (NOESY). Spectra of 2D NMR are plotted on two frequency axes, which show a crosspeak at the frequencies corresponding to each pair of nuclei that interact. The magnitude of the interaction effect is represented by the intensity of the crosspeak. It is shown by the number of contours (like a contour map) at the point of interaction the more contours, the higher the peak corresponding to the interaction. The diagonal represents ID spectrum of the contour line of peaks, which also plot along both axes. 

Table 141. Sequence dfCACA), dfTGTG) Molecular species sh Solvent DjO, nb, pH 7.0 Spectrometer frequency 300,500 MHz Method of assignment COSY, NOE, NOESY...

The 2D NOE (NOESY) experiment is formally equivalent to a large number of transient NOE experiments. Each cross-section of a 2D NOESY spectrum is equivalent to a ID selective transient NOE experiment, in which the peak corresponding to the diagonal is selectively inverted. The NOE transfer in both NOESY and ID transient NOE takes place in the absence of the RE field and, theoretically, the two experiments are identical (except for a factor of 2 in the intensities of ID experiments which will be explained later). The theory of the NOE, given in the following subsections is divided into three parts, (i) steady-state NOE, (ii) transient NOE and (hi) 2D NOE (NOESY). Later sections describe the NOE in the rotating frame, heteronuclear NOE and transferred NOE. 

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