Regioselectivity Control in the Reaction of Cross-Conjugated Enynones with Monosubstituted Hydrazines: Original Synthesis of Luminescent 5-Styrylpyrazoles with a 1-Benzo[d]thiazole Ring.

Direct interactions of 1-arylpent-1-en-4-yn-3-ones with 2-hydrazinylbenzo[d]thiazoles lead to the formation of the mixtures of 3-styryl- and 5-styryl-1-(benzo[d]thiazol-2-yl)pyrazoles. Under the same conditions, 1,5-diarylpent-1-en-4-yn-3-ones give 5-aryl-3-arylethynyl-1-(benzo[d]thiazol-2-yl)-4,5-dihydro-1H-pyrazoles. We have described and estimated the highly regioselective one-pot synthetic route to luminescent 5-styrylsubstituted pyrazoles containing thiazole and benzo[d]thiazole fragments on the basis of pent-1-en-4-yn-3-ones (cross-conjugated enynones) and 2-hydrazinylbenzo[d]thiazoles. The synthetic protocol involves a preliminary addition of piperidine via the triple bond of pointed cross-conjugated enynones resulting in 1-(piperidin-1-yl)penta-1,4-dien-3-ones that in turn readily form 5-styryl-1-(benzo[d]thiazol-2-yl)pyrazoles with high regioselectivity in excellent yields. The proposed methodology is applicable to a wide range of 1-mono- and 1,5-disubstituted cross-conjugated enynones and monosubstituted hydrazines containing aryl and benzothiazole substituents. The obtained substances exhibit strong luminescence activity (absolute quantum yield reaches 0.7). Structural and regiochemical aspects of the newly synthesized pyrazoles were confirmed by spectral and analytical data.