A Genuine Four‐Component Spirocyclization Reaction: Synthesis of Spiro[pyrimido[1,2‐ b ]indazole] Ring Derivatives

A [3 + 1 + 1 + 1] four‐component spirocyclization reaction has been developed for the efficient synthesis of spiro[pyrimido[1,2‐ b ]indazole] skeletons from commercially available aryl methyl ketones, 3‐aminoindazoles, aromatic aldehydes, and cyclic β ‐diketones. This novel four‐component reaction is a self‐organizing long‐range cascade process: imine and dienophile intermediates, generated through two independent parallel tandem reaction sequences, undergo cross‐capture followed by an aza‐Diels‐Alder reaction to afford the target spirocyclic skeletons. This method features metal‐free catalysis, broad substrate compatibility, and simple operation. The reaction involves the formation of two CC bonds and two CN bonds, while constructing a congested spiro quaternary carbon center.