Congested polycyclic compounds frequently pose difficulty in structural elucidation by NMR spectroscopy (for example, proton and carbon NMR signal assignment problems) even when the structures have been established previously by X-ray crystallography. Often the source of complication derives from several nuclei being in very similar chemical environments therefore making it difficult to identify an unequivocal entry point based which other assignments can be made. For example, the compound 1 contains four spin systems of the A-D rings and the 1H NMR spectrum in CDCl3 solution revealed a high degree of overlap of B-ring (H-29, H-30, H-32), C (H-20), D (H-13) and Phenyl (H-39, H-40) protons. In contrast, the 1H NMR spectrum in C6D6 solution gave readily interpretable patterns for protons on A-C rings, thereby permitting a confirmation structure of 1 based on the 2D NMR spectra. The coupling values between protons (JHH) of the A-C ring were calculated on the basis of first order approximation in contrast to coupling values for D-ring protons (H-11, H-12, H-13 and H-14). The D-ring protons constitute complex spin system and the coupling values were deduced from the simulated gNMR spectra. The Figs (see below) represents simulated spectra obtained in C6D6.
Ring-A
Ring-B
Ring-C
Ring-D
