Schmidt Andreas, Habeck Tobias, Kindermann Markus Karl, Nieger Martin
Technical University of Clausthal, Institute of Organic Chemistry, Leibnizstrasse 6, D-38678 Clausthal-Zellerfeld, Germany.
J Org Chem. 2003 Jul 25;68(15):5977-82. doi: 10.1021/jo0344337.
Pyrazolium-3-carboxylates were examined as relatives of the betainic alkaloid Nigellicine and as new examples of the sparsely populated class 16 of heterocyclic pseudo-cross-conjugated mesomeric betaines (PCCMB). The title compounds were prepared in a 4-step procedure starting from beta-diketo compounds 8 which were cyclized with substituted hydrazines. The resulting isomeric pyrazole esters 9 and 10 were separated and subsequently quaternized with dimethyl sulfate in the presence of nitrobenzene to pyrazolium esters 11 and 12. Saponification was best accomplished in diluted sulfuric acid, which resulted in the formation of the pseudo-cross-conjugated mesomeric betaines 13 and 14 in one step. Protonation to the corresponding carboxylic acids required the treatment of the betaines with tetrafluoroboric acid in dichloromethane. The effect of negative solvatochromism proves the charge separation in the ground state of the molecules. X-ray crystallographic analyses, semiempirical calculations, and ESI mass spectrometric measurements were performed to gain knowledge about the phenomenon of pseudo-cross-conjugation.
对吡唑 -3 - 羧酸盐作为甜菜碱生物碱黑种草碱的类似物以及作为杂环假交叉共轭内消旋甜菜碱（PCCMB）中数量稀少的第16类新实例进行了研究。标题化合物采用四步程序制备，起始原料为β - 二酮化合物8，其与取代肼环化。将得到的异构吡唑酯9和10分离，随后在硝基苯存在下用硫酸二甲酯季铵化得到吡唑鎓酯11和12。皂化反应在稀硫酸中最易完成，一步生成假交叉共轭内消旋甜菜碱13和14。质子化生成相应的羧酸需要在二氯甲烷中用四氟硼酸处理甜菜碱。负溶剂化显色效应证明了分子基态中的电荷分离。进行了X射线晶体学分析、半经验计算和电喷雾电离质谱测量以了解假交叉共轭现象。
