Studies of tautomerism and protonation in 2-aryl-1H-imidazo[1,2-a]imidazole derivatives using1H and13C NMR
作者:Maili Liu、R. Duncan Farrant、Robert C. Glen、John C. Lindon、Paul Barraclough、Steven Smith
DOI:10.1002/mrc.1260291112
日期:1991.11
The tautomerism and protonation of the putative inotropic 2-(2′,4′-dimethoxy)phenyl-1H-imidazo[1,2-a]imidazole (2) has been studied in several solvents by comparing its 1H and 13C chemical shifts with those of its 1- and 7-methyl derivatives 3 and 4, respectively, and acid salts. Tautomer and rotamer populations were also estimated from measurements of proton relaxation rates and NOE effects. Heterocycle 2 exists predominantly as the 1H-tautomer in CDCl3, but as the 7H-tautomer in DMSO-d6 and methanol-d4–D2O solutions. In CDCl3 solution, 2 appears to exist with the N-1-H and 2′-OMe groups adjacent, but in DMSO-d6 the conformation is the rotated form with N-7-H and H-6′ adjacent; 3 exists as a mixture of rotamers in CDCl3 and in DMSO-d6 whereas 4 is in the form with the N-7-Me and H-6′ adjacent in both solvents. The observed conformational preferences have been compared with the results of semi-empirical molecular orbital calculations and found to be in broad agreement. Protonation of 2 occurs mainly at N-1 in DMSO-d6 and at N-7 in CDCl3, as expected from observed tautomeric ratios in the free base.
已研究假定的正性肌力药物2-(2′,4′-二甲氧基)苯基-1H-咪唑[1,2-a]咪唑(2)的互变异构和质子化,比较了其1H和13C化学位移与其1-和7-甲基衍生物3和4的化学位移,以及酸盐的化学位移。还通过测量质子弛豫速率和NOE效应估算了互变异构体和旋转异构体的分布。异环化合物2在CDCl3中主要以1H-互变异构体的形式存在,但在DMSO-d6和甲醇-d4–D2O溶液中则以7H-互变异构体的形式存在。在CDCl3溶液中,2似乎以N-1-H和2′-OMe基团相邻的形式存在,而在DMSO-d6中则是N-7-H和H-6′相邻的旋转构型;3在CDCl3和DMSO-d6中存在于旋转异构体的混合物中,而4在这两种溶剂中均以N-7-Me和H-6′相邻的形式存在。观察到的构象偏好与半经验分子轨道计算的结果进行比较,发现大致一致。在DMSO-d6中,2的质子化主要发生在N-1,而在CDCl3中则发生在N-7,这与在游离碱中观察到的互变异构体比例相符。