1. H. Grimmel and H. Freyermuth, US patent no. 2460745, 1949; C. A., 43, 4302, 1949.
2. V. P. Petrov, V. A. Barkhash, G. S. Shchegoleva, T. D. Petrova, T. I. Savchenko, and G. G. Yakobson, DAN, 178, 864, 1968.
3. V. P. Petrov and V. A. Barkhash, KhGS [Chemistry of Heterocyclic Compounds], 6, 381, 1970.
4. W. Emerson and H. Mohrman, J. Am. Chem. Soc., 62, 69, 1940.
5. A. Dornow and H. Theidel, Chem. Ber., 88, 1267, 1955.