NOE difference spectrometry

The utility of NOE difference spectrometry for stereochemical problems is demonstrated. 

The same report that clarified the structures of the calafatine A -oxides (Section II,C,10) also described berbibuxine (87), C38H42N207, amorphous, [8]D -228° (c 0.19, CHC13). Diazomethane converted the alkaloid to calafatine (88, Section II,C,9). The location of the phenolic function was revealed by an NOE difference spectrometry (NOEDS) study (54, see Section VI,A,2,b). 

FIGURE 3.60 NOE difference spectrometry for compound III. a shows the H NMR spectrum, b shows enhancement of the H-4 proton on irradiation of the 3-methyl group, c shows enhancement of both protons on irradiation of the 5-methyl group. 300 MHz in CDCl3. 

NOE difference spectrometry determined the substitution pattern of a natural product, whose structure was either I or II. The readily available dimethyl homologue III was submitted to NOE difference spectrometry (Figure 3.60). 

Distinguishing between a trisubstituted ( )- and a (Z)-double bond is not a trivial assignment. The use of NOE difference spectrometry for this purpose is illustrated in Problem 7.4. 

There are still two remaining tasks assignment of stereochemistry of the C-2, C-3-double bond and assignment of the C-8 and C-9 methyl groups. NOE difference spectrometry is described in Section 3.16. It is a 1-D experiment that reveals H— H proximity through space because of enhancement by the nuclear Over-hauser effect. The difference spectrum is obtained by subtracting a standard H spectrum from the NOE spectrum this leaves only the enhanced peak(s). 

NOE difference spectrometry is described in Section 4.20. It is a 1-D experiment that determines JH—JH proximity through space. The difference spectrum is obtained by subtracting a standard H spec-... 

Several physical methods have been employed to ascertain the existence and nature of ICs infrared (IR) absorption spectroscopy nuclear magnetic resonance (NMR) spectroscopy,14 including JH nuclear Overhauser effect (NOE) difference spectroscopy, H 2-D rotating-frame Overhauser effect spectroscopy (2-D ROESY),15 and solid-state 13C cross-polarization/magic angle spinning (CP/MAS) spectroscopy 16 induced circular dichroism (ICD) absorption spectroscopy 17 powder and singlecrystal X-ray diffraction 18 and fast atom bombardment mass spectrometry (FAB MS). 

The traditional nuclear Overhauser difference spectroscopy can be replaced by DPFGSE-NOE (Double Pulsed Field Gradient Spin Echo method-NOE spectrometry) experiment. The desired NOE effects can be recorded without interference from other signals and without phase distortion [66]. 

From the roots of Zizyphus mucronata, in addition to the known alkaloid mucronine D (23), two new cyclopeptide alkaloids 24 and 25 were isolated. The structures were elucidated by ID- and 2D-NMR techniques (48). A new cyclopeptide alkaloid, named mucronine J (53), was isolated together with known alkaloids abyssenine A and mucronine D (23) from the methylene chloride extract of the root bark of Z. mucronata (49). The structure of mucronine J was elucidated by mass spectrometry and ID and 2D-NMR techniques, and the solution conformation is proposed on the basis of NOE experiments in combination with MM2 calculations. Leaves of Zizyphus hysodrica were extracted with methanol and the different alkaloidal fractions gave a new compound, hysodricanine B, which is a cyclopeptide alkaloid (50). 

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