“…[5 + 4] cycloaddition, as an efficient and flexible approach, provides a significant advantage of directly coupling two building blocks (serving as dipoles and dipolarophiles) to produce nine-membered heterocycles. 5–21 In 2017, the Zhao group 5,6 reported the first [5 + 4] cycloaddition between α,β-unsaturated imines and vinylethylene carbonates (VECs) for the construction of nine-membered heterocycles via palladium catalysis. Since then, a series of excellent studies about palladium-catalyzed [5 + 4] cycloaddition involving 1,5-dipoles and α,β-unsaturated imines have been reported by Zhao, 8,14 Archambeau, 15 Guo, 16 Hu 17,18 and Li 21 groups.…”