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■ Mn(III)-Based Oxidative Radical Ring-Expansion Reaction Using Squarate Derivatives: Selective Synthesis of Bis(butenolide)s and the Acetate Monomers

Jun-ichi Sasaki, Makoto Kobayashi, Yûsuke Ibe, and Hiroshi Nishino*

*Department of Chemistry, Graduate School of Science, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan

Abstract

The Mn(III)-based oxidation of phenyl- and alkyl-substituted hydroxycyclobutenones selectively produced the bis(butenolide)s or the acetate monomers via the 5-endo radical cyclization depending upon the concentration of the reaction. A similar reaction of hydroxycyclobutenones bearing an alkenyl and alkynyl substituent did not produce any bis(butenolide)s or acetate monomers, but the 5-exo and 6-endo radical cyclization products including the unsaturated group. The oxidation of the hydroxycyclobutenones having an unsaturated substituent in the presence of alkenes afforded radical coupling products during the 5-exo radical cyclization. The reaction details, structure determination of the products, and the mechanism for the formation of the products are described.