δ‐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.

The cyanide-catalyzed ring-transformation of α-hydroxy-β-oxoesters to δ-lactones with exocyclic ester moiety is the key transformation for the preparation of racemic 5-hexadecanolide (15 % over nine steps) and 6-acetoxy-5-hexadecanolide (22 % over eight steps), two insect pheromones. The benzyloxycarbonyl moiety is transformed via the acid chloride to the undecanoyl side chain. After reduction of the ketone to the secondary alcohol, this functional group was either reduced via the chloro compound to furnish the 5-hexadecanolide. On the other hand, stereoselective reduction of the ketone gave secondary alcohol with the wrong relative configuration, which was established by X-ray single-crystal structure. Inversion was achieved by reaction of the methanesulfonate with cesium acetate yielding (R*,S*)-6-acetoxy-5-hexadecanolide with the relative configuration also present in the natural product.

The Cover Feature shows the excited cuckoo dashing out of his (radical) clock, because the mechanism of the cerium‐catalyzed radical coupling reaction of β‐oxoesters, enol acetates, and dioxygen was finally elucidated by the introduction of cyclopropyl substituents as radical clocks. Furthermore, the introduction of stereocenters in the starting materials causes a stir, since the 1,2‐alkyl shift within the endoperoxidic oxycarbenium intermediate was proved to proceed with retention of configuration. More information can be found in the Full Paper by J. Christoffers et al.