Catalytic Diastereoselective Hetero-Diels–Alder Reaction of α‑Haloacroleins with Alkenes: Construction of 3,4-Dihydropyran

In this Letter, a catalytic diastereoselective hetero-Diels–Alder reaction of α-haloacroleins with less polarized alkenes was developed, and the resulting 3,4-dihydropyrans were produced in high yields with a broad substate scope. Mechanism studies showed that 3,4-dihydropyran was produced from the ring expansion of cyclobutane, which was generated in the ring contraction of the initially formed unstable 3,4-dihydropyran conformer.