Some 8-substituted derivatives of the protoberberine alkaloids palmatine (1a) and berberine (1b) have been prepared and investigated by 1D and 2D NMR spectroscopy (H-1, C-13, N-15). Complete NMR data for the 8-hydroxy (2), 8-methoxy (3), 8-ethoxy (4), and 8-trichloromethyl (5) 7,8-dihydro derivatives of 1a and 1b, as well as X-ray data for 8-methoxydihydroberberine (3b), 8-trichloromethyldihydropalmatine (5a), and 8-trichloromethyldihydroberberine (5b), are presented. The physicochemical data for all of these compounds are reviewed and compared with previously published values.

Purine bases such as purine, adenine, hypoxanthine, and mercaptopurine are known to exist in several tautomeric forms. Characterization of their tautomeric equilibria is important not only for predicting the regioselectivity of their N‐alkylation reactions, but also for gaining knowledge of the patterns with which these compounds of significant biological activity form hydrogen bonds with their biological targets. The tautomeric equilibria of purine and some purine derivatives in methanol and N,N‐dimethylformamide solutions were investigated by low‐temperature 1H and 13C NMR spectroscopy. The N(7)H and N(9)H tautomeric forms were quantified by integrating the individual 1H NMR signals at low temperatures. The Gibbs free energy differences were calculated and the effects of substitution on the N(7)H/N(9)H ratio discussed. A previously published theoretically predicted mechanism of the tautomeric exchange is compared with our measurements in deuteriated solvents. The influence of concentration on the temperature of coalescence indicates that supramolecular clusters play a significant role in this proton transfer process. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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The free bases of eight quaternary benzo[c]phenanthridine alkaloids (chelerythrine, sanguilutine, chelirubine, chelilutine, sanguirubine, sanguinarine, nitidine and fagaronine) were investigated by 1 H and 13 C 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-d 6 solution. This observation differs from the measurements of preparative free bases in CDCl 3 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 (C 6 D 6 , CDCl 3 ), the formation of the bimolecular aminoacetal structure is preferred.