3-hydroxy-3H-benzofuran-2-ones afford when treated successively with thionyl chloride and triethylamine the otherwise difficult to make (E)-[3,3′]bibenzofuranylidene-2,2′-diones (isoxindigos). Access to the starting 3-hydroxy-3H-benzofuran-2-ones is easily achievable by reaction of properly substituted phenols with glyoxylic acid.