Abstract: | An efficient synthesis of 1‐arylisochromeno3,4‐d]1,2,3]triazol‐5(1H)‐ones, involving the diazotization of 3‐amino‐4‐arylamino‐1H‐isochromen‐1‐ones in weakly acidic solution, has been developed and the spectroscopic characterization and crystal structures of four examples are reported. The molecules of 1‐phenylisochromeno3,4‐d]1,2,3]triazol‐5(1H)‐one, C15H9N3O2, (I), are linked into sheets by a combination of C—H…N and C—H…O hydrogen bonds, while the structures of 1‐(2‐methylphenyl)isochromeno3,4‐d]1,2,3]triazol‐5(1H)‐one, C16H11N3O2, (II), and 1‐(3‐chlorophenyl)isochromeno3,4‐d]1,2,3]triazol‐5(1H)‐one, C15H8ClN3O2, (III), each contain just one hydrogen bond which links the molecules into simple chains, which are further linked into sheets by π‐stacking interactions in (II) but not in (III). In the structure of 1‐(4‐chlorophenyl)isochromeno3,4‐d]1,2,3]triazol‐5(1H)‐one, (IV), isomeric with (III), a combination of C—H…O and C—H…π(arene) hydrogen bonds links the molecules into sheets. When compound (II) was exposed to a strong acid in methanol, quantitative conversion occurred to give the ring‐opened transesterification product methyl 2‐4‐hydroxy‐1‐(2‐methylphenyl)‐1H‐1,2,3‐triazol‐5‐yl]benzoate, C17H15N3O3, (V), where the molecules are linked by paired O—H…O hydrogen bonds to form centrosymmetric dimers. |