ROESY phase-sensitive

The data from H NMR studies of 63, which included double quantum filtered phase sensitive correlated spectroscopy (DQF-COSY) and rotating frame nuclear Overhauser effect spectroscopy (ROESY) experiments (Figure 12), are collected in Table 17. 

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. 

Since the pulse sequence is the same for EXSY and NOESY, NOESY (or ROESY) cross peaks might be mistaken for EXSY cross peaks. They can be distinguished in the phase-sensitive experiment, since EXSY and ROESY peaks have opposite phases, as do EXSY and NOESY peaks in the fast motion regime. For example, two resolved OH or NH resonances may exhibit EXSY cross peaks from slow proton exchange. These peaks could be mistakenly taken to be NOESY peaks and interpreted incorrectly in terms of stereochemistry. 

Figure 27 provides an example of one two-dimensional NMR spectrum. It depicts a phase-sensitive two-dimensional ROESY spectrum where only negative levels are shown. It was recorded for a pure sample of standard material ginsenoside Re This figure was drawn from reference [30] and the readers are strongly encouraged to review the discussion relating to the interpretation of that spectrum as 2-D spectra are, at best, difficult to interpret despite their nice pictures appearance. 

Figure 27. Typical phase-sensitive two-diemnsional ROESY spectrum of ginsenoside Re. Only negative levels are show. Reprinted from reference [30], with permission.

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