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

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Abstract
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(2H)‐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‐2H‐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‐1H‐pyrazolo[3,4‐a]acridine (11a) and 2‐amino‐4,5‐dihydro‐1H‐pyrazolo[3,4‐a]acridine (11b) were also synthesized and investigated as potential cholinesterase inhibitors.