1. V. V. Titov and L. F. Kozhokina, Khim. Geterotsikl. Soedin., 1423 (1971).
2. E. Steinbauer and E. Waldman, Monatshefte, 570 (1958).
3. H. S. Broadbent and R. G. Anderson, J. Org. Chem.,27, 2699 (1962).
4. P. E. Fanta and C. Wang, J. Heterocycl. Chem.,3, 367 (1966).
5. W. Ried and W. Kunstmann, Ber.,102, 1418 (1969).