Panda Kausik, Suresh J R, Ila H, Junjappa H
Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India.
J Org Chem. 2003 May 2;68(9):3498-506. doi: 10.1021/jo026786w.
A highly efficient and regioselective annulation protocol for a series of linearly 2,3- and angularly 1,2-substituted and annulated pyrido[1,2-a]benzimidazoles involving [3 + 3] cyclocondensation of the dianions generated from 2-methyl (2A) and 2-cyanomethyl (3A) benzimidazoles with a variety of alpha-oxoketene dithioacetals has been reported. Thus the dianion 2A derived from 2-methylbenzimidazole has been shown to undergo regioselective 1,2-addition with various alpha-oxoketene dithioacetals derived from acyclic (4a-d) and cyclic ketones (13a,b, 20, 29 and 32) to afford various carbinol acetals which on intramolecular cyclocondensation in the presence of phosphoric acid furnish the corresponding 1-methylthio-2,3-substituted (5a-c) and 2,3-fused linear polycyclic (14a,b,21, 30, and 33) pyrido[1,2-a]benzimidazoles in high yields. Similarly the dianion 3A from 2-cyanomethylbenzimidazole undergoes one-pot conjugate addition-elimination and cyclocondensation with these alpha-oxoketene dithioacetals to give 4-cyano-3-(methylthio)-1(or 1,2-)-substituted (6a-d) and the corresponding angularly 1,2-fused (16a,b, 23, 31, and 34) polycylic analogues of pyrido[1,2-a]benzimidazoles in excellent yields.
报道了一种高效且区域选择性的环化反应方案，用于一系列线性2,3 - 以及角型1,2 - 取代和稠合的吡啶并[1,2 - a]苯并咪唑的合成，该方案涉及由2 - 甲基（2A）和2 - 氰甲基（3A）苯并咪唑产生的二阴离子与多种α - 氧代烯酮二硫代缩醛的[3 + 3]环缩合反应。因此，已证明由2 - 甲基苯并咪唑衍生的二阴离子2A与源自无环（4a - d）和环状酮（13a,b、20、29和32）的各种α - 氧代烯酮二硫代缩醛发生区域选择性1,2 - 加成反应，得到各种甲醇缩醛，这些甲醇缩醛在磷酸存在下进行分子内环缩合反应，以高产率提供相应的1 - 甲硫基 - 2,3 - 取代（5a - c）和2,3 - 稠合的线性多环（14a,b、21、30和33）吡啶并[1,2 - a]苯并咪唑。同样，来自2 - 氰甲基苯并咪唑的二阴离子3A与这些α - 氧代烯酮二硫代缩醛进行一锅法共轭加成 - 消除和环缩合反应，以优异的产率得到4 - 氰基 - 3 - （甲硫基） - 1（或1,2 - ） - 取代（6a - d）以及相应的角型1,2 - 稠合（16a,b、23、31和34）的吡啶并[1,2 - a]苯并咪唑多环类似物。
