Access to 4-(pyridin-2-yloxy)-1,2,4-thiadiazol-5(4 <i>H</i> )-ones <i>via</i> an amidoxime O-heteroarylation/cyclization reaction sequence

A synthetic method was developed for the preparation of <i>O</i>-pyridylamidoximes in good preparative yields (up to 99%) by a reaction of the corresponding amidoximes with pyridine/quinoline <i>N</i>-oxides in the presence of PyBroP as an activator. Substituents in the used arylamidoxime had little effect on the yield of the target products, whereas the <i>N</i>-oxide structure had the greatest effect. <i>O</i>-Pyridyl-substituted amidoximes undergo an unusual condensation with (chlorocarbonyl)sulfenyl chloride. This transformation provides access to previously unreported 1,2,4-thiadiazol-5(4<i>H</i>)-ones functionalized at the N4 atom of the heterocycle by a pyridine moiety. Structural analysis of the products, combined with DFT theoretical calculations, allowed us to propose a reaction mechanism involving a key acylpyridinium intermediate.