Formation of δ‑Lactones by Cerium-Catalyzed, Baeyer–Villiger-Type Coupling of β‑Oxoesters, Enol Acetates, and Dioxygen

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%).