For the fluorescence imaging of biologically active small compounds, the development of compact fluorophores that do not perturb bioactivity is required. Here we report a compact derivative of fluorescent 1,3a,6a-triazapentalenes, 2-isobutenylcarbonyl-1,3a,6a-triazapentalene (TAP-VK1), as a fluorescent labeling reagent. The reaction of TAP-VK1 with various aliphatic thiols proceeds smoothly to afford the corresponding 1,4-adducts in high yields, and nucleophiles other than thiols do not react. After the addition of thiol groups in dichloromethane, the emission maximum of TAP-VK1 shifts to a shorter wavelength and the fluorescence intensity is substantially increased. The utility of TAP-VK1 as a compact fluorescent labeling reagent is clearly demonstrated by the labeling of Captopril, which is a small molecular drug for hypertension. The successful imaging of Captopril, one of the most compact drugs, in this study demonstrates the usefulness of compact fluorophores for mechanistic studies.

The substituent effect at the C6-position of 1,3a,6a-triazapentalenes was elucidated. Synthesis of C6-substituted-1,3a,6a-triazapentalene was established by an epoxide-opening reaction of the 1,2,3-triazoles followed by elimination of the resulting hydroxy group. The electron-donating substituent induced a longer-wavelength shift of the fluorescence maximum and change of the fluorescence quantum yield depending on the C2-substituents. On the other hand, the electron-withdrawing group quenched the fluorescence of 1,3a,6a-triazapentalenes.

Reusable heterogeneous bifunctional silica-supported Ag ₂ O/Im ⁺ Cl ⁻ catalyst in carboxylative cyclization reaction of propargyl alcohols by the efficient utilization of CO ₂ under ambient conditions.