Ethyl 3‐ and 5‐Triflyloxy‐1 H ‐pyrazole‐4‐carboxylates in the Synthesis of Condensed Pyrazoles by Pd‐Catalysed Cross‐Coupling Reactions

The easily obtainable title compounds ethyl 5‐ and 3‐triflyloxy‐1 H ‐pyrazole‐4‐carboxylates ( 2 and 5 ) have been used as precursors in Sonogashira‐type cross‐coupling reactions with various alkynes to obtain the corresponding 5‐ and 3‐alkynyl‐4‐(ethoxycarbonyl)pyrazoles 3 and 6 . Cyclization of the latter ortho ‐difunctional synthons afforded different condensed pyrazoles such as pyrano[4,3‐ c ]pyrazol‐4(1 H )‐ones and ‐4(2 H )‐ones 7 and 10 as well as 1,5‐dihydro‐ and 2,5‐dihydro‐4 H ‐pyrazolo[4,3‐ c ]pyridin‐4‐ones 9 and 12 , respectively. Suzuki coupling of triflate 2 with phenylboronic acids gave the corresponding 5‐arylpyrazoles. Heating of 5‐(3‐chlorophenyl)‐1‐phenyl‐1 H ‐pyrazole‐4‐carboxylic acid ( 16 ) with trifluoromethanesulfonic acid led to the formation of the unexpected tetracyclic system 18 . Detailed NMR spectroscopic investigations were undertaken on all obtained products.