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Synthesis of ferrocenylidene cyclopentenediones

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2005
Köktürk, Mustafa
2-Arylidine-4-cyclopentene-1,3-diones are known to be antitumor agents. Incorporation of the essential structures of such compounds with a ferrocene moiety instead of an aryl group could provide subtances with enhanced antitumor activities since some ferrocene derivatives have already proved to be active against a number of tumors. Thus, we have investigated the squarate-based synthesis of 2-ferrocenylidene-4-cyclopentene-1,3-diones. Upon thermolysis, 4-hydroxy-4-ferrocenylethynyl-2-cyclobutenones, prepared from 3-cyclobutene-1,2-diones and lithioethynylferrocene, produced exclusively 2-ferrocenylidene-4-cyclopentene-1,3-diones. In some cases, ferrocenyl quinone derivatives are obtained in very minor amounts. Moreover, the heating of a mixture of 4-ferrocenylethynyl-4-hydroxy-2-cyclobutenones and silica gel in oven at 120 oC without using any solvent provided the same type of products. More importantly, the stirring of a solution of 4-ferrocenylethynyl-4-hydroxy-2-cyclobutenone (37A-C) derivatives in ethyl acetate at the room temperature yielded the same type products, as well. It appears that 4-hydroxy-4-ferrocenylethynyl-2-cyclobutenones are more reactive than corresponding phenyl analogs. For the formation of ferrocenyl-substituted cyclopentenediones, a mechanism involving first electrocyclic ring opening of ferrocenylethynyl-substituted cyclobutenones to corresponding ketene intermediate and then ring closure, has been proposed. The exclusive formation of cyclopentenediones is attributed to the radical stabilizing ability of the ferrocenyl moiety during the course of the reaction.