Ring transformations of heterocyclic compounds.XVI. Spiro[cyclohexadiene-dihydroacridines]. A novel class of spirodihydroacridines by ring transformation of pyrylium salts with 9-methylacridine and its quaternary salts

Abstract 2,4,6‐Triarylpyrylium salts 1 react with the in situ generated anhydrobase of 9,10‐dimethylacridinium methosulfate ( 2a ) in the presence of anhydrous sodium acetate in ethanol by a 2,5‐[C 4 +C 2 ] pyrylium ring transformation to give the hitherto unknown 6‐aroyl‐3,5‐diaryl‐10′‐methylspiro[cyclohexa‐2,4‐diene‐1,9′‐9′,10′‐dihydro‐acridines] 3 . When the pyrylium perchlorate 1a is treated under the same conditions with the N ‐ethyl, N ‐allyl or N ‐benzyl substituted acridinium salts 2b‐d a dealkylation of these salts occurs and the N ‐unsubstituted spiro[cyclohexadiene‐dihydroacridine] 4a is formed. The same compounds 4 can also be obtained by transformation of the pyrylium salts 1 with 9‐methylacridine ( 7 ) and triefhylamine/acetic acid in ethanol. Structure elucidation is performed by an X‐ray crystal structure determination of the spiro[cyclohexadiene‐dihydroacridine] 3a . Spectroscopic data of the transformation products and their mode of formation are discussed.