Controllable Access to Furans and Dihydrofurans through Cyclization/Coupling of Internal Acetylenic β‐Ketoesters with Aryl Bromides

Abstract Pd‐catalyzed tandem cyclization/coupling of internal β‐propargylic‐β‐ketoesters with (hetero)aryl bromides is described. Two protocols are established which selectively lead to 2‐benzylidene‐dihydrofurans, when isomerization is inhibited under mild conditions (temp. ≤40 °C and weak base – K 2 CO 3 ), or to 2‐benzyl‐furans when base‐induced aromatization is enabled. A catalytic cycle involving oxidative addition, substitution of bromide with alkyne, rate limiting anti‐selective 5‐exo‐dig oxocyclization followed by fast deprotonation of the oxonium intermediate, and reductive elimination is proposed based on detailed experimental and theoretical studies. magnified image