Pyrazol-3-ones. Part II: Reactions of the Ring Atoms

Abstract

This chapter examines the reactions of the ring atoms of pyrazol-3-ones, in particular 1,2-dihydro-3 H -pyrazol-3-one and 2,4-dihydro-3 H -pyrazol-3-one. The chapter describes reactions of pyrazol-3-ones with alkyl halides, esters of sulfonic and carbonic acids, trimethyloxonium tetrafluoroborate, diazo compounds, Mannich or Mitsunobu conditions or miscellaneous reagents. Pyrazol-3-ones are known to be ambient compounds and depending on the solvent in which they are dissolved can exist in several tautomeric forms. Usually in less polar solvents such as chloroform pyrazol-3-one tautomers are dominant, whereas in more polar solvents such as dimethylsulfoxide 3-hydroxypyrazole tautomers are dominant. It is noted that pyrazol-3-ones are weak acids and can be titrated with strong bases. 2,4-Dihydro-3H-pyrazol-3-ones are stronger acids than 1,2-dihydro-3H-pyrazol-3-ones, which are very weak. The outcome of acylation of the ring atoms of pyrazol-3-ones with acid chlorides depends on the type of substitution on the pyrazol-3-one ring. Both C- and N acylation are possible. With 2-aryl-4-(or 5)-substituted pyrazol-3-ones and reaction with acid chlorides occurs in 1,4-dioxane with calcium hydroxide.