Synthesis and electrophilic heterocyclization of 3-alkenylsulfanyl-5-phenyl-1,2,4-triazines under the action of iodine and bromine

Derivatives of 1,2,4-triazine-3-thione are interesting research objects due to a wide range of their applications: drugs, optical substances, precursors for synthesis of new pyridine systems by the Diels-Alder reaction. In the present study, using alkylation of 5-phenyl-2,3-dihydro-1,2,4-triazine-3-thione (1) by 3-chloro-2-methyl-propene, 2,3-dibromopropene-1, 1-bromo-3-methylbut-2-ene and 4-brombut-1-ene, we have obtained 3-(2-methylprop-1-enyl)-, 3-(2-bromoprop-1-enyl)-, 3-prenylsulfanyl- and 3-butenylsulfanyl-5-phenyl-1,2,4-triazines (2a-d), previously unknown. In the 1H NMR spectrum of compound 2b, the shifts of the signal of protons of the SCH2 group and the signal of protons of the vinyl group into a weak field by 0.48 ppm and 0.60 ppm can be observed, compared to the similar signals in the spectrum of 3-allylsulfanyl-5-phenyl-1,2,4-triazine. This may be due to the bromine atom in the allyl fragment. The weakest-field signal in the 13C NMR spectra of compounds 2a-d in the region of 171.86-173.68 ppm refers to the aromatic carbon atom of the triazine cycle in the third position (C-3), bound to a sulfur atom and two nitrogen atoms. New condensed heterocyclic systems of ionic type with a bridging nitrogen atom have been synthesized by electrophilic heterocyclization of methallyl-, bromallyl-, prenyl- and butenyl sulfides 2a-d. At the same time, [1,3]thiazolo[3,2- b ][1,2,4]triazinium halides have been obtained by heterocyclization of compounds 2a,2b, and [1,3]thiazino[3,2- b ][1,2,4]triazinium halides have been produced by heterocyclization of compounds 2c,2d. In the 1H NMR spectra of triazinium halides, a characteristic shift of the signal of the H-6 aromatic proton to the weak field region is observed in comparison with the similar signal in the spectrum of the initial 2a-d sulfides. In the 13C NMR spectra of triazinium halides, there is a shift in the signal of the aromatic carbon atom associated with a sulfur atom and two nitrogen atoms (in the region of 162.56-172.42 ppm), which can be explained by the appearance of a positively charged nitrogen atom in their structure.