One-pot synthesis of fluorescent substituted 2,5-Dihydro-1,2,3-Triazines from a cycloaddition, rearrangement, ring-expansion cascade of 1,2,3-Triazolium-1-amide 1,3 dipoles with propiolate esters

The reactions of 1,2,3-triazolium-1-aminides 1 ( readily available from benzil bishydrazones) with propiolate esters leads to fluorescent 2,5-dihydro-1,2,3-triazine derivatives 2, 3 in one pot. These synthetic reactions can be carried out in acetone, in water, or under solvent-free conditions. The reactions involve a Huisgen cycloaddition followed by a sequence of rearrangements. The final ring-expansion step was blocked by linking a six-methylene hydrocarbon chain between the prospective 1,2,3-triazine C-4 and C-6 atoms, using substrate 8 which gave the fused tricyclic azapropellane product 9 exclusively. X-ray crystal structures were determined for two 2,5-dihydro-1,2,3-triazine derivatives and for compound 9. The UV absorption of the 1,2,3-triazine derivatives showed a dual absorption at ca. 310 and ca. 390 nm with fluorescent emission at ca. 480 and 528 nm ( for excitation at 317 nm). The significant Stokes shift of ca. 200 nm shows the potential for biological fluorescent labeling experiments.