Synthesis of Some amino‐4,5‐dihydropyrazolo[3,4‐ a ]acridines as potential cholinesterase inhibitors

A synthesis of the 4,5‐dihydro derivatives of the previously known pyrazolo[3,4‐ a ]acridine ring system is described. The reaction of a 3,4‐dihydroacridin‐1(2 H )‐one with N,N ‐dimethylformamide dimethyl acetal gave a reactive enamino ketone, which yielded the desired heterocycle upon reaction with hydrazine. Using this chemistry, 11‐amino‐4,5‐dihydro‐2 H ‐pyrazolo[3,4‐ a )acridine ( 3 ) and a number of its 2‐substituted derivatives 4a‐k were synthesized and evaluated as acetylcholinesterase inhibitors, based on their relationship to 1,2,3,4‐tetrahydro‐9‐acridinamine (THA). 1‐Amino‐4,5‐dihydro‐1 H ‐pyrazolo[3,4‐ a ]acridine ( 11a ) and 2‐amino‐4,5‐dihydro‐1 H ‐pyrazolo[3,4‐ a ]acridine ( 11b ) were also synthesized and investigated as potential cholinesterase inhibitors.