Organocatalytic Asymmetric [4 + 4] Annulation Reaction of Ynones with Unsaturated Pyrazolones: Synthesis of Eight-Membered Ether-Embedded 4,5-Fused Pyrazoles

An aminocatalytic asymmetric (4 + 4) annulation of ynones and unsaturated pyrazolones is reported. This study is the first to describe an enantioselective synthesis of eight-membered ether-embedded 4,5-fused pyrazoles. The scope of the reaction is broad, and cyclic ether-containing 4,5-fused pyrazole products were isolated in moderate to good yields with good to high enantioselectivities.