Synthesis of naphtho- and pyrazolo-fused systems by an intramolecular Friedel–Crafts approach

Efficient access to the tetracyclic pyrazolo-fused carbo- and N-heterocyclic systems: naphtho[2′,1′:3,4]cyclohepta[2,1-c]pyrazolones, naphtho[2′,1′:5,4]azepino[6,7-c]pyrazolones, naphtho[2′,1′:5,4]azocino[6,7-c]pyrazolones and naphtho[2′,1′:6,5]azonino[7,8-c]pyrazolones is described involving intramolecular Friedel–Crafts cycliacylations of synthesized pyrazole-based carboxylic acid or ester derivatives aided by treatment with Lewis and Brønsted acids. The required starting carbaldehyde was obtained by the Vilsmeier–Haack reaction of the corresponding 2-acetylnaphthalene hydrazone. Our developed strategy, involving a three-step route, offers easy access to tetracyclic pyrazole-fused systems in moderate to good yields.