Chapter 16 - Five-membered heterocyclic compounds with two nitrogen atoms in the ring

Abstract

This chapter discusses the synthetic methods and properties of five-membered heterocyclic compounds with nitrogen atoms in the ring. Syntheses, in which the final step is a cyclization phase, are still the most widely used routes to pyrazoles. The reaction of hydrazine with β -diketones, a widely used method for pyrazole synthesis, has been used to prepare derivatives with substituted phenolic groups at the 5-position of the pyrazole ring. A group of pyrazole derivatives of quinazolinone, which show anti-inflammatory activity, has been synthesized by condensation of hydrazine derivatives with 3-aryl-2-(3-aryl-3-oxopropenyl)-4(3 H )-quinazolinones. It is also possible to prepare pyrazoles from other heterocycles by ring contraction. Thiadiazines, with hydrazine, readily undergo ring contraction to pyrazoles in good yield. The reaction is useful as it provides an efficient route to N -methylsulfamide a key intermediate in the synthesis of N-SO 2 -N containing heterocycles. Regioselective alkylation of pyrazoles by tertiary carbocations is catalyzed by sulfuric acid, under quite mild conditions, or by boron trifluoride.