Unusual Formal [4 + 2] Cycloaddition of Ethyl Allenoate with Arylidenoxindoles: Synthesis of Dihydropyran-Fused Indoles

An unusual DABCO-catalyzed formal [4 + 2] cycloaddition of ethyl allenoate, as a surrogate of a “1,2-dipole”, with various arylidenoxindoles has been developed for the synthesis of dihydropyran-fused indoles. The DFT mechanistic study indicates that the cycloaddition takes place stepwise and the essential role of the catalyst is to raise the HOMO of allenoate.