Acridines, Bemthsen acridine synthesis

Diarylamines will also usually require strong acylating agents to undergo acylation. If Lewis acids such as ZnCl2 are used as catalysts the formation of an acridine can compete with N-acylation (Bemthsen acridine synthesis last reaction, Scheme 7.10). 

Bemthsen s synthesis of 9-phenyl aeridine in 1884 is the first reported method for making aeridines and was used as proof of their structure. At the turn of the 19th eentury, there were several syntheses of acridines reported following Bemthsen s procedure with a variety of substitutions, mainly in the 9 position through use of a variety of earboxylic acids. Early in the 20th century, acridines were of interest beeause it was found that 3,6-diamino-2,7,10-trimethylacridinium chloride and 3,6-diamino-10-methylacridinum... 

A modification of the Bemthsen reaction uses polyphosphoric acid instead of zinc chloride however, the conditions are also quite vigorous with respect to the temperature (200 °C 15 min) in addition, the yields of 9-substituted acridines were low (1962JOC2658). In 2002, two synthetic procedures have been published, which use microwave conditions and zinc chloride as catalyst to prepare 125 from 124 (2002SC729 2002MI390 Scheme 44). Reaction conditions for the synthesis of 9-substituted acridines have been optimized for microwave conditions in the presence of 2,2-dime-thoxypropane as water scavenger very recendy (2014H(89) 1877). 

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