Synthesis and Reactions of 2-Aminotropone-imines Directed toward Synthesis of Tropones Having a Carbon Side-chain

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Friedel-Crafts reaction is an excellent method for introducing a carbon side chain onto an benzenoid aromatic ring. However, the reaction is not applicable to tropolone derivatives because tropolones form metal complexes with catalyst such as aluminum chloride. In order to overcome this defect, we studied the synthesis and reactions of N,N'-dialkyl- and N,N'-diaryl-2-aminotroponeimines.(1) Synthesis of 2-Aminotroponeimines : Seven N,N'-dialkyl-2-aminotroponeimines were obtained by the reactions of cyclohepta[b]-[1,4]benzoxazine with aliphatic amines, while the reactions with aromatic amines did not give the corresponding 2-aminotroponeimines. The N,N'-diaryl-2-aminotroponeimines were prepared by the reactions of 1,7,7-trichlorotropilidene with anilines. A new synthon, 1,2-di-chlorotropylium salt, was also obtained.(2) Reactions of 2-Aminotroponeimines : N,N'-Dialkyl-2-aminotroponeimines reacted with bromine and nitric acid to yield 5-bromo-and 5-nitro-N,N'-dialkyl-2-aminotroponeimines, respectively, but Friedel-Crafts acylation was not achieved. The reactions of 2,3-di-hydro-1H-cyclohepta[b]pyrazine, which is cyclic analog of 2-aminotroponeimines, with acyl chloride gave N-acylated products and pyrazine ring-opened ones. On the other hand, 5-bromo-N,N'-dibenzyl-2-amino-troponeimine reacted with ethyl acetate and methyl formate in the presence of buthyllithium to afford 5-acetyl- and 5-formyl-2-aminotroponeimine, respectively. The former was hydrolyzed to yield 5-acetyl-tropolone via 5-acetyl-2-benzylaminotropone.