1. T. P. Glushchenko, V. I. Goncharov, and A. V. Aksenov, Khim. Geterotsikl. Soedin., 409 (2008). [Chem. Heterocycl. Comp., 44, 313 (2008)].
2. H. Weidel, Monatsh., 2, 491 (1881).
3. A. V. Aksenov, I. V. Magedov, and Yu. I. Smushkevich, J. Chem. Soc., Perkin Trans. 1, 759 (1992).
4. A. V. Aksenov, A. Yu. Polykarpov, Yu. I. Smushkevich, and I. V. Magedov, J. Chem. Res. (S), 402 (1994).
5. M. Ishikura, I. Oda, and M. Terashima, Heterocycles, 2375 (1985).