Synthesis of Tri- or Tetrasubstituted Pyrimidine Derivatives through the [5+1] Annulation of Enamidines with either N,N-Dimethylformamide Dialkyl Acetals or Orthoesters and Their Application in a Ring Transformation of Pyrimidines to Pyrido[2,3-d]pyrimidin-5-one Derivatives

The [5+1] annulation of enamidines, which were prepared from functionalized silanes, organolithium compounds and two nitriles, with N,N-dimethylformamide dialkyl acetals as the C1 unit is described, leading to the synthesis of tri- and tetrasubstituted pyrimidine derivatives under catalyst-and solvent-free reaction conditions. Furthermore, the [5+1] annulation of enamidines by using orthoesters as the C1 unit is described, in which catalytic amounts of ZnBr2 catalyze the annulation to produce polysubstituted pyrimidines under toluene or xylene reflux conditions. Moreover, the combination of a reductive ring-opening reaction with [Mo(CO)(6)] and a subsequent intramolecular cyclization with tBuOK effectively causes a skeletal transformation from the pyrimidines containing an isoxazolyl and an ethoxy substituent to form pyrido[2,3-d]pyrimidin-5-one frameworks in excellent yield.