Flexible BODIPY Platform That Offers an Unexpected Regioselective Heterocyclization Reaction toward Preparation of 2‑Pyridone[a]‑Fused BODIPYs

We have explored the synthetic routes for regioselective formation of 2-pyridone­[a]- and 2-pyridone­[b]-fused BODIPYs using 1,3,5,7-tetramethyl-2,6-dicarbethoxy-BODIPY as the universal starting platform. While heterocyclization of the 3-(dimethyl­aminovinyl)-BODIPY and 3,5-bis­(dimethyl­amino­vinyl)-BODIPY results in the formation of mono-2-pyridone- and bis-2-pyridone­[b]-fused BODIPYs, respectively, similar heterocyclization of the 1,3-bis­(dimethylaminovinyl)-BODIPY leads to the regioselective formation of the 2-pyridone­[a]-fused BODIPY core, which is the first example of heterocycle­[a]-fused BODIPYs. The regioselective formation of the 2-pyridone­[a]-fused BODIPY was further confirmed by X-ray crystallography and explained on the basis of the DFT and TDDFT calculations that are suggestive of the energy-favorable out-of-plane rotation of the dimethylaminovinyl group located at first position, which accelerates the reaction with n-butylamine. Trends in the UV–vis and fluorescence spectra of the BODIPYs 1–17 were discussed on the basis of DFT and TDDFT calculations.