The structures and stabilities of biologically active 1-phenacyl- and 1-benzoylethyl-derivatives of the pyridinium cation

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• 作者：Bla啪enka Foreti膰^a ; ^{bforetic@mef.hr} ; Igor Picek^a ; Vladimir Damjanovi膰^a ; Danijela Cvijanovi膰^a ; Dalibor Mili膰^b
• 关键词：1-Phenacylpyridinium ion ; 1-Benzoylethylpyridinium ion ; Ionization constants ; Spectroscopy
• 刊名：Journal of Molecular Structure
• 出版年：2012
• 期刊代码：62_00222860
• 类别：ch
• 出版时间：18 July, 2012
• 卷：1019
• 期：Complete
• 页码：196-205
• 文件大小：1497 K

摘要

Chlorides of 1-phenacylpyridinium (1), 2-methyl-1-phenacylpyridinium (2), 1-benzoylethylpyridinium (3) and 1-benzoylethylpyridinium-4-aldoxime (4) were synthesized and characterized by X-ray diffraction and by electronic absorption and NMR spectroscopies. Although declared as pharmacologically active in extracellular fluids, their stability and ionization ability as well as predominant ionic forms in aqueous environments were not clarified. Comparative electronic absorption spectral studies in aqueous media at 25 掳C and I = 0.1 M performed in this work revealed the predominance of their keto-tautomeric forms and pronounced differences in stability and ionization ability. The ionization of 1-phenacylpyridinium ions 1 and 2 with pK_a values of 11.57 卤 0.04 and 11.66 卤 0.05, respectively produced enolates (i.e., ylides), while the subsequent base-catalyzed first-order decomposition occurred via hydrate zwitterion and produced the benzoate ion and the corresponding 1-methylpyridinium derivative. A different proximate cause of the ascertained instabilities of the 1-benzoylethylpyridinium compounds (3 and 4) was determined. The base-catalyzed establishment of the ketone to gem-diol equilibrium of 3 was found to have a hydration constant smaller than 0.01. Compound 4 underwent a base-catalyzed breakdown due to the instability of its enolate form which resulted in the formation of a pyridine-4-aldoxime and phenyl vinyl ketone. Ionization constants of 3 and 4 keto-forms were estimated as the lowest possible pK_a values of 12, while the pK_a value of pyridinium aldoxime group of 4 was found to be 8.51 卤 0.04. The identified stabilities and ionization abilities of these compounds were additionally supported by their presented coordination ability toward the iron(II) in the pentacyanoferrate(II) moiety.