Cyclic β‐oxo esters are converted with enol acetates in a cerium‐catalyzed, oxidative Umpolung reaction to furnish 1,4‐diketones with up to 95 % yield. Atmospheric oxygen is the oxidant in this process, which can be regarded as ideal from economic and ecological points of view. Further advantages of this new C–C coupling reaction are its operational simplicity and the application of nontoxic and inexpensive CeCl3 ·7H2O as precatalyst.

Formation of δ-lactones is observed when cyclopentanone-2-carboxylates are converted in a cerium-catalyzed reaction with α-aryl vinyl acetates under oxidative conditions. The products of this transformation possess a 1,4-dicarbonyl constitution together with a quaternary carbon center. Atmospheric oxygen is the oxidant in this process, which can be regarded as ideal from economic and ecological points of view. Further advantages of this new C-C coupling and oxidation reaction are its operational simplicity and the application of nontoxic and inexpensive CeCl3·7 H2O as precatalyst. This so far unprecedented reaction is proposed to proceed via 1,2-dioxane derivatives, which decompose under formation of an oxycarbenium cation in a Baeyer-Villiger-type pathway. This mechanistic picture is supported by the observation that electron-rich (donor substituted or heteroaromatic) enol esters give higher yields than electron deficient congeners. Apart from 1,4-diketones and α-hydroxylated β-oxoesters formed as byproducts, the yields of δ-lactones range from moderate to good (up to 74%).

Herein, we report the first Pd‐catalyzed enantioselective arylation of α‐substituted γ‐lactams. Two sets of conditions were developed for this transformation, allowing for the use of either aryl chlorides or bromides as electrophiles. Utilizing a highly electron‐rich dialkylphosphine ligand we have been able to construct α‐quaternary centers in good yields (up to 91 % yield) and high enantioselectivities (up to 97 % ee).

Robinson-type cyclopentannulations of cyclic β-oxoesters possessing a 1,4-diketone moiety are accomplished under four different Brønsted basic reaction conditions. Using pyrrolidine/acetic acid in DMSO, an oxohexahydrocyclopenta[a]indene (42%) and an N-Boc-protected oxohexahydrocyclopenta[c]pyridine derivative (62%) are obtained with retention of the ester moieties. The latter compound defines an interesting new scaffold for medicinal chemistry with three positions allowing further derivatizations. The use of KOtBu in DMSO or NaH in toluene leads to cyclopentene derivatives with either partial ester saponification and decarboxylation or displacement of the ester moiety within the carbon skeleton. With aqueous KOH, the cyclopentannulations are successful in almost all cases, but with the ester moieties cleaved off. The respective bicyclic and tricyclic products are obtained in good to excellent yields. The 1,4-diketone starting materials are prepared by cerium-catalyzed oxidative coupling of β-oxoesters with isopropenyl acetate. Alternatively, a two-step sequence consisting of α-propargylation followed by palladium-catalyzed alkyne hydration is used.

The cerium-catalyzed, aerobic coupling of β-oxoesters with enol acetates and dioxygen yields δ-lactones with a 1,4-diketone moiety. In contrast to the Baeyer-Villiger oxidation (BVO), where the higher substituted residue migrates; in the case of this oxidative C-C coupling reaction, the less substituted alkyl residue undergoes a 1,2-shift. An endoperoxidic oxycarbenium ion comparable to the Criegee intermediate in the BVO is proposed as a reaction intermediate and submitted to conformational analysis by computational methods. As a result, the inverse regiochemistry is explained by a primary stereoelectronic effect. A Hammett analysis using different donor- and acceptor-substituted enol esters provides support for the oxycarbenium ion being the crucial intermediate in the rate determining step of the conversion. An overall mechanism is suggested with a radical chain reaction for the formation of endoperoxides from β-oxoesters, enol acetates and dioxygen with a cerium(IV) species as initiating reagent.