Friedel-Crafts chemistry: Part 41. A new facile synthesis of indeno[1,2-c] pyrazoles, 2H-benzo[g]indazoles and benzo[6,7]cyclohepta[1,2-c]pyrazoles via Friedel-Crafts ring closures

Abstract

Expedient and novel alternative synthesis of some fused heteropolycycles containing pyrazole moiety is described. A series of indeno[1,2-c]pyrazoles, 2H-benzo[g]indazoles and benzo[6,7] cyclohepta[1,2-c]pyrazoles were prepared by Friedel-Crafts ring closure of suitable synthesized carboxylic acids and alkanols in the presence of AlCl₃/CH₃NO₂ or P₂O₅ or PPA (polyphosphoric acid) catalysts. The precursor acids were obtained by utilizing KMnO₄ oxidation of the corresponding aldehyde, alkylations of diethyl malonates and Perkin type approaches, whereas starting alkanols were smoothly obtained by reaction of carboxylic acid ester with Grignard reagent. To exemplify, cyclizations of 5-chloro-1,3-diphenyl-1H-pyrazole-4-carboxylic acid (6) were accomplished using the above catalysts afforded 3-chloro-2-phenylindeno[1,2-c]pyrazol-4(2H)-one (1b). Similarly, cyclization of the 2-(5-chloro-1,3-diphenyl-1H-pyrazol-4-yl)propan-2-ol (8) by a Friedel–Crafts-type ring closure afforded the 3-chloro-2,4-dihydro-4,4-dimethyl-2-phenylindeno[1,2-c]pyrazole (1a).