Construction of a 2H-1,4-selenazin-3(4H)-one framework via cyclization of pyrazole derivatives bearing chloroacetamide and selenocyanate fragments

Abstract

For the first time, we developed an efficient three-step method for the synthesis of substituted 1,7-dihydropyrazolo[4,3-b][1,4]selenazin-6(5H)-ones starting from readily accessible 5-aminopyrazoles. The presented approach is based on preliminary preparation of chloroacetamide derivatives followed by selenocyanation of the pyrazole ring and final reductive cyclization. The key selenocyanation stage includes the employment of commercially available SeO₂ and malononitrile for the generation of triselenium dicyanide in situ in a simple setup under open-air conditions. The elaborated general protocol allowed us to obtain a set of final pyrazoloselenazinones with yields up to 88%. ⁷⁷Se NMR-monitoring was applied for a detailed investigation of selenocyanation and intramolecular cyclization steps. Mechanistic studies showed that the conclusive reductive stage proceeds through the corresponding diselenide intermediate.