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.