New Triheterocyclic Ring System. Synthesis of Functionalized Pyrazolo[3,4‐d]Pyrimido[1,2‐b][1,2,4,5]Tetrazine Derivatives

New functionalized pyrazolo[3,4‐d]pyrimido[1,2‐b][1,2,4,5]tetrazine derivatives 6a‐q were synthesized via reaction of the hydrazonoyl halides 1a‐q with 5‐amino‐1‐phenyl‐6‐thioxo‐pyrazolo[3,4‐d]pyrimidin‐4‐one 2 and its 2‐methylthio derivative 3 . The mechanism and the regioselectivity of the studied reactions have been discussed.