Chiral Lewis Acid Catalyzed Asymmetric Cycloadditions of Disubstituted Ketenes for the Synthesis of β‑Lactones and δ‑Lactones

Highly
diastereo- and enantioselective [2 + 2]- and [4 + 2]-cycloadditions
of disubstituted ketenes were realized by chiral Lewis acid catalysis.
A series of arylalkylketenes underwent the reaction smoothly with
isatins and β,γ-unsaturated α-ketoesters, providing
optically active β-lactones and δ-lactones with vicinal
chiral centers in excellent yields (up to 99%) and enantioselectivities
(up to 99% ee), as well as exclusively high diastereoselectivities
under 0.2–2 mol % catalyst loading.