Synthesis of the new pyrazolo[4,3-c]pyrrolizine skeleton via intramolecular nitrilimine cycloaddition

Starting from the appropriate 1-substituted pyrrol-2-yl-(1) or indol-2-ylcarboxylic acid (6), the hydrazonyl chlorides (3) or (8) have been synthesised respectively. Their base treatment promoted the in situ generation of the corresponding nitrilimines (4) or (9), whose intramolecular 1,3-dipolar cycloaddition gave the title compounds in nearly quantitative yields.