Synthesis of pyrazolo[3,4‐d]pyrimidine derivatives using ketene dithioacetals

The cyclization of 5‐amino‐3‐methylthiopyrazole‐4‐carbonitriles or 4‐carboxamides 3a‐j, which were prepared by the reaction of ketene dithioacetals 1a,b [1a: bis(methylthiomethylenemalononitrile; 1b: bis(methylthio)methylenecyanoacetamide] with hydrazines (hydrazine hydrate, phenylhydrazine, p‐chlorophenylhydrazine, p‐nitrophenylhydrazine), with formamide or carbon disulfide proceeded to give the corresponding 4‐amino‐ or 4‐hydroxy‐3‐methylthiopyrazolo[3,4‐d]pyrimidines 6a‐h in good yields. 3‐Aminopyrazolo[3,4‐d]pyrimidine derivatives 6i‐1 were also obtained by the application of the cyclization reaction of 3,5‐diaminopyrazoles with formamide.