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■ Tautomerism in Pyrazino[2,3-c]-1,2,6-thiadiazine 2,2-Dioxides

Pilar Goya, Angela Herrero, M^a. Luisa Jimeno, Carmen Ochoa,* Juan Antonio Páez, Félix Hernández Cano, Concepción Foces-Foces, and Martín Martínez Ripoll

*Instituto de Química Médica, C. S. I. C., Calle Juan de Cierva, 3, 28006 Madrid, Spain

Abstract

Tautomerism of pyrazino [2,3-c]-1,2,6-thiadiazine 2,2-dioxides has been studied, in solution, by uv, ¹³C-, and natural abandance ¹⁵N-nmr spectroscopiea and in the solid state by X-ray crystallography. The 1-NH tatutomer is present in solid state and in the majority of solvents. Depending on the 6,7-substituent, the 8-NH tautomer is preferred in water solution. An improved synthesis of 6,7-diaryl substituted pyrazinothiadiazine derivatives is described.