Mechanistic Insights into the Formation of δ‐Lactones by Cerium‐Catalyzed Aerobic Coupling of β‐Oxoesters with Enol Acetates

Abstract

Abstractδ‐Valerolactone derivatives are formed by the cerium‐catalyzed, aerobic coupling of β‐oxoesters with enol acetates and dioxygen. The products possess a 1,4‐diketone moiety, thus, the conversion can be regarded as an Umpolung since the β‐oxoesters are oxidized to electrophilic α‐radicals. The transformation has similarities to the Baeyer‐Villiger oxidation (BVO) where the higher substituted residue migrates. An endoperoxidic oxycarbenium ion comparable to the Criegee intermediate in the BVO is proposed as a reaction intermediate in this case of the oxidative C−C coupling reaction, but in contrast to the BVO, the less substituted alkyl residue migrates. It was demonstrated by the conversion of β‐oxoesters with two stereocenters that this 1,2‐alkyl shift proceeds with retention of configuration. A radical chain mechanism of the coupling reaction was furthermore evidenced by the conversion of enol acetates and β‐oxoesters with cyclopropyl substituents. Isolation and characterization of products with opened cyclopropane rings established the constitution of radical intermediates.