N-Boc-4-aminopyrazole-5-carbaldehydes in Friendländer synthesis of pyrazolo[4,3-b]pyridines

N-Boc-4-aminopyrazole-5-carbaldehydes reacted with aryl, hetaryl, alkyl, and cycloalkyl ketones containing methylene groups. The reactions were accomplished in refluxing acetic acid in the presence of pyrrolidine and resulted in the formation of 5-substituted and carbo[b]fused pyrazolo[4,3-b]pyridines.