Belen'kaya, I. A.; Shulla, T. A. Chemistry of Heterocyclic Compounds (New York, NY, United States), 1989 , vol. 25, # 11 p. 1303 - 1307 Khimiya Geterotsiklicheskikh Soedinenii, 1989 , # 11 p. 1555 - 1558