Catalytic Schmittel-Type [2+2] Cycloaddition of γ‑AlkynylDiazoacetates with Terminal Alkynes for Accessing Cyclobuta[<i>a</i>]indenes

Instead of the conventional [4+2] cycloaddition, a regioselective Schmittel-type [2+2] cycloaddition of yne–allene esters, generated <i>in situ</i> from copper-catalyzed dediazotized coupling of γ-alkynyl diazoacetates with terminal alkynes, is reported, enabling a bicyclization process to produce a diverse array of C₁-arylated cyclobuta[<i>a</i>]indenes in moderate to good yields. The protocol features wide functional group compatibility, mild reaction conditions, and experimental simplicity, holding significant potential for building new tricyclic cyclobutenes.