Phosphine-Promoted Tandem Intermolecular Diels–Alder Reactions with Pentadienyl 4‑Nitrobenzoate as a Diene Precursor

A phosphine-promoted tandem Diels–Alder reaction using pentadienyl 4-nitrobenzoate (α-vinyl MBH adduct) as a diene precursor with 3-olefinic oxindoles or CF3-activated ketones as dienophiles has been developed. The reaction proceeds through the formation of a pentadienyl phosphonium intermediate via SN2′′ addition, which acts as both a D–A diene and a precursor for the exomethylene moiety. This method offers a metal-free and step-efficient approach for synthesizing exomethylene-bearing spirooxindoles and dihydropyrans, which are privileged structures found in natural products.