Two Dimensional J-Resolved Proton NMR Spectroscopy

The two-dimensional J-resolved spectrum is extremely useful for resolving signals that are severely overlapped in a normal one-dimensional spectrum, allowing determination of both chemical shift frequencies and coupling constants that may otherwise be arduous to ascertain (15, 46, 136, 278, 280). 

The two-dimensional spectrum may be plotted as a stacked plot, as shown for the insect antifeedant trichilin A, isolated from Trichilia roka (Fig. 2.39) (282, 284). Individual multiplicities were examined as cross-sections of the stacked plot 

2D-correlation experiments are almost invariably presented in a contour plot. Homonuclear correlation spectroscopy, in the form of COSY (proton correlation spectroscopy) (25, 28) or SECSY (2D-spin echo-correlated spectroscopy) (279) pulse sequences, is routinely employed in modern natural products structure determination (198, 328). 

The recent example of talaromycins A and B (Fig. 2.40) produced by the tox-icogenic fungus Talaromyces stipitatus illustrate the strategic utilization of sophisticated spectroscopic techniques in structure determination (239). These two toxins were isolated as a mixture and were extremely difficult to resolve. Due to the loss of water in the positive CI-MS, the molecular ions of these diastereomers were detected only by NCI-MS, with deuterium exchange NCI-MS confirming the presence of two exchangeable protons. The coupling connectivities of the individual diastereomers were mapped with a COSY spectrum (Fig. 2.40). The purified compounds were ultimately separated after formation of their phenyl boronic acid esters (Fig. 2.41). 

These long-range couplings were not detected in the COSY spectrum without the delay A inserted in the pulse sequence (Fig. 2.42b). 

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