PYRIDO ANNELATION REACTION BY A TANDEM AZA WITTIG ELECTROCYCLIC RING-CLOSURE STRATEGY - PREPARATION OF PYRAZOLO[4,3-C]PYRIDINE AND PYRAZOLO[3,4-C]PYRIDINE DERIVATIVES

The aza Wittig-type reaction of iminophosphorane 3, prepared from 5-formyl-1-phenylpyrazole by sequential treatment with ethyl azidoacetate and triphenylphosphine, with isocyanates, ketenes, aldehydes and carbon disulfide leads to the functionalized pyrazolo[4,3-c]pyridines 5, 7, 9 and 11 respectively. Iminophosphorane 15, prepared from 4-formyl-1-phenylpyrazole, undergoes pyrido annelation by reaction with ketenes to give the isomeric pyrazolo[3,4-c]pyridines 22 in modest yields.