Asymmetric [4 + 2] Annulation of Cyclobutenones and Pyrazolone 4,5-Diones: Access to Novel δ‑Lactone-Fused Spiropyrazolones

Although numerous chiral pyrazolones
with a six-membered spirocyclic
center at the C4 position have been developed, the asymmetric construction
of six-membered oxa-spiropyrazolones is still a challenging task in
organic synthesis. Herein, we describe the [4 + 2] annulation of cyclobutanones
and pyrazoline-4,5-diones for the efficient synthesis of δ-lactone-fused
spiropyrazolone derivatives with generally high yields and good enantioselectivities
under mild conditions. The successful scale-up synthesis and further
transformation of the final product highlight the practicality and
reliability of this reaction.