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3. H. D. Hartough, Thiophene and Its Derivatives, Interscience, New York (1952).
4. O. Meth-Kohn in: P. G. Sammes (ed.), Comprehensive Organic Chemistry: Heterocyclic Compounds, Vol. 4, Pergamon Press, Oxford (1979), p. 229.
5. A. R. Katritzky and C. A. Rees (eds.), Comprehensive Heterocyclic Chemistry, Vol. 4, Pergamon Press, Oxford (1984), p. 713.