Krauze, A. A.; Liepin'sh, E. E.; Dubur, G. Ya. Chemistry of Heterocyclic Compounds (New York, NY, United States), 1989 , vol. 25, # 6 p. 650 - 657 Khimiya Geterotsiklicheskikh Soedinenii, 1989 , # 6 p. 787 - 794