Abstract
The acid catalyzed reaction of several 1,2-di-substituted cyclopenten-3,4,5-triones with benzene is examined. It is found that the substituents on the cyclopentene ring affect the reaction pathway and the structure of the products. When the both substituents are C 6H 5 or CH 3O, they react only at their central CO group to give 1,2-di-substituted-4,4-diphenylcyclopenten-3,5-diones. When the substituents are OH (croconic acid), condensation with three molar equivalents of benzene occurs to give 1,4,-triphenyl-2-hydroxycyclopenten-3,5-dione. In this reaction, the intermediacy of 2-hydroxy-1-phenylcyclopenten-3,4,5-trione and the 1,4-diphenyl-allyl cation 16 is proposed. When the substituents are piperidyl, the compound does not condense with benzene, but rearranges intramolecularly to give 1,3-di-piperidylcyclopenten-2,4,5-trione.
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