1. U.S. Patent No. 2734082; Chem. Abstr.,50, 13085 i (1956).
2. British Patent No. 970414; Chem. Abstr.,61, 15981 d (1964).
3. Japanese Patent No. 7428187; Chem. Abstr.,82, 125136u (1975).
4. Japanese Patent No. 6926099; Chem. Abstr.,72, 12335k (1970).
5. British Patent No. 1129437; Chem. Abstr.,70, 19616 x (1969).