Extension of the Biginelli-Type Reaction: One-Pot Synthesis of Pyrimido-pyrimidines and Spirobi[pyrimidine]s Using Potassium Hydrogen Sulfate as a Catalyst - Shi, Feng; Jia, Runhong; Zhang, Xiaojing; Tu, Shujiang; Yan, Shu; Zhang, Yan; Jiang, Bo; Zhang, Junyong; Yao, Changsheng (Synthesis, 2007)