A new
photocatalyst-free visible-light-enhanced strategy for the synthesis of pyrazolo[1,5-a][1,3,5]triazine-2,4-diamines via the formation of electron
donor–acceptor (EDA) complexes is reported. The in
situ generated pyrazolthiourea intermediates from 1H-pyrazol-3-amines and isothiocyanates undergo formal [4
+ 2] annulation with 1,1,3,3-tetramethylguanidines (TMG) to deliver
the corresponding products involved in three C–N bond formations
in a one-pot protocol. The formation of EDA complex from pyrazolthiourea
and TMG is confirmed by UV–vis spectroscopy and 1H NMR experiments. Moreover, this mild reaction proceeds in the absence
of any external transition metals, oxidants, bases, and ligands. This
efficient methodology for the synthesis of purine analogues pyrazolo[1,5-a][1,3,5]triazine-2,4-diamines provides potential synthetic
applications in the field of drug research and development.