Regioselectivity of the reaction of 4H-1,2,4-triazole-3-thiol derivatives with chloroethynylphosphonates and structure of the products

Chloroethynylphosphonates reacted with 4H-1,2,4-triazole-3-thiols in anhydrous acetonitrile to afford fused heterocyclic compounds, 6-(dialkoxyphosphoryl)-3H-thiazolo3,2-b]1,2,4]triazol-7-ium chlorides, with high regioselectivity. The products were converted into inner salts (zwitterions) of the corresponding phosphonic acids or their monoesters with the positive charge localized on N⁷. A probable reaction mechanism implies initial formation of sulfonium ion via attack by the thionic sulfur atom on the acetylenic carbon atom linked to chlorine, followed by intramolecular cyclization involving attack on the other acetylenic carbon atom by N² of the triazole ring.