Synthesis, structure, and optical properties of 3-(1H-1,2,3-triazol-1-yl)-substituted 4H-pyrido[1,2–a]pyrimidin-4-ones and 4H-quinolizin-4-ones

Copper-catalyzed cycloadditions of terminal alkynes to 3-azido-4 H -pyrido[1,2– a ]pyrimidin-4-one and 3-azido-1-cyano-4 H -quinolizin-4-one, prepared in four steps from methyl 2-benzoylamino-3-(dimethylamino)propenoate, yielded the corresponding 3-(1 H -1,2,3-triazol-1-yl)-substituted 4 H -pyrido[1,2– a ]pyrimidin-4-ones and 4 H -quinolizin-4-ones in 44–90% isolated yields. The structures of the cycloadducts were determined by 1 H NMR, 13 C NMR, and 2D NMR techniques (COSY, HSQC, HMBC). The optical properties of the luminescent intermediates and final products were determined. In methanol, the compounds showed absorption at 350–400 nm, emission at 411–526 nm, and quantum yields of ∼5×10 –2 . Optical properties depended on the heterocyclic core (pyrido[1,2– a ]pyrimidinone or quinolizinone), while the effect of substituents on optical properties was weak. • Novel probes for fluorescence imaging by "clicking" to alkyne-tagged molecules using CuAAC or SPAAC reaction. • Increased fluorescence intensity after cycloaddition to alkyne with typical λ ex 350–400 nm and λ em 410–525 nm. • Facile preparation, available reagents, and broad substrate scope warrant accessibility of structurally diverse probes.