Trapentsier, P. T.; Kalvin'sh, I. Ya.; Liepin'sh, E'. E'.; Lukevits, E'. Ya. Chemistry of Heterocyclic Compounds (New York, NY, United States), 1983 , vol. 19, # 4 p. 391 - 394 Khimiya Geterotsiklicheskikh Soedinenii, 1983 , # 4 p. 481 - 485