1. Concise Chemical Encyclopedia [in Russian], Moscow: Soviet Encyclopedia, Vol. 3 (1964), p. 1050.
2. R. Elderfield (editor), Heterocyclic Compounds, Vol. 1, Wiley.
3. G. Joule and H. Smith, Heterocyclic Chemistry, Van Nostrand Reinhold (1972).
4. J. Mathier and A. Halle, Principles of Organic Synthesis [Russian translation], Inostr. Lit., Moscow (1962), p. 58.
5. G. Roberts and M. Casserio, Basic Principles of Organic Chemistry, 2nd edn., Benjamin (1977).