Synthesis of pyrazolo[4′,3′: 3,4]pyrido[1,2-a]benzimidazoles and related new ring systems by tandem cyclisation of vic-alkynylpyrazole-4-carbaldehydes with (het)aryl-1,2-diamines and investigation of their optical properties

Abstract The synthesis of 2H- and 3H-pyrazolo[4′,3′:3,4]pyrido[1,2-a]benzimidazole derivatives from 3-alkynyl- or 5-alkynylpyrazole-4-carbaldehydes and benzene-1,2-diamines was carried out using copper-free tandem cyclisation. When 2,3-diaminopyridine was used as the diamine component in this type of tandem cyclisation, 3H-pyrazolo[4,3-c]imidazo[1,2-a:5,4-b′]dipyridine derivatives were obtained. Copper catalysis and microwave activation were required for the reaction of 5-alkynylpyrazole-4-carbaldehydes and 1,8-naphthalenediamine, affording the corresponding 13,13a-dihydro-3H-pyrazolo[4′,3′:3,4]pyrido[1,2-a]perimidines. The structure assignments were based on data from ¹H, ¹³C and ¹⁵N spectroscopy and single-crystal X-ray diffraction analyses. The optical properties of the obtained new heterocyclic derivatives were studied by UV-vis and fluorescence spectroscopy.