Sonogashira‐Type Reactions with 5‐Chloro‐1‐phenyl‐1 H ‐pyrazole‐4‐carbaldehydes: A Straightforward Approach to Pyrazolo[4,3‐ c ]pyridines

The easily obtainable 5‐chloro‐1‐phenyl‐1 H ‐pyrazole‐4‐carbaldehydes 2 were used as precursors in Sonogashira‐type cross‐coupling reactions with various alkynes to provide the corresponding 5‐alkynyl‐1 H ‐pyrazole‐4‐carbaldehydes 3 . Treatment of these with tert ‐butylamine with microwave assistance afforded the corresponding 1‐phenylpyrazolo[4,3‐ c ]pyridines 4 . The oximes 5 – derived from the aldehydes 3 – were transformed into the corresponding 1‐phenylpyrazolo[4,3‐ c ]pyridine 5‐oxides 6 through silver‐triflate‐catalysed regioselective cyclisation. Ring closure of the oximes 5 in the presence of iodine in ethanol gave the 7‐iodo‐1‐phenylpyrazolo[4,3‐ c ]pyridine 5‐oxides 7 . Detailed NMR spectroscopic investigations were undertaken with all obtained products.