Studies on synthetic approaches to 1H- and 2H-indazolyl derivatives

Synthetic approaches designed to provide 1H- and 2H-indazolyl derivatives of potential biological interest are reported. Special emphasis has been placed on the characterization of indazolylpyridinium products generated from reactions between indazole and 4-chloro-1-methylpyridinium iodide under various conditions. A stable mixture consisting of 3 parts of the 1H-isomer 9 to 1 part of the 2H-isomer 10 was obtained at room temperature in the presence of the base 2,2,6,6-tetramethyl-piperidine (TMP). The same reaction at 60 degreesC gave only the 1H-isomer 9. At 100 degreesC in the absence of TMP only the W-isomer 10 was formed. The isomerization of 10 to 9 was found to proceed quantitatively at 60 degreesC but only in the presence of TMP. The effects of temperature and base on the course of these reactions are rationalized in terms of kinetic and thermodynamic parameters.