Structural studies of benzophenanthridine alkaloid free bases by NMR spectroscopy

The free bases of eight quaternary benzo[c]phenanthridine alkaloids (chelerythrine, sanguilutine, chelirubine, chelilutine, sanguirubine, sanguinarine, nitidine and fagaronine) were investigated by ¹H and ¹³C NMR spectroscopy. The pseudobases (the 6‐hydroxy‐5,6‐dihydro derivatives) were found to be the only products of the alkalization of these alkaloids using sodium carbonate in DMSO‐d₆ solution. This observation differs from the measurements of preparative free bases in CDCl₃ solution, where the structures of the bimolecular aminoacetals were unequivocally confirmed. According to the results obtained, in biological systems, in the presence of excess of water and ions, free bases of the investigated alkaloids are assumed to adopt the pseudobase (6‐hydroxy derivative) structure. In contrast, in less polar media (C₆D₆, CDCl₃), the formation of the bimolecular aminoacetal structure is preferred. Copyright © 2001 John Wiley & Sons, Ltd.