Method for producing bisbenzoxazoles

A process for preparing a bisbenzoxazole bonded to one another via a system of conjugated double bonds, of the formula

• wherein
• R1, R2, R3 and R4 are each independently selected from the group consisting of hydrogen, halogen, hydroxyl, nitro, amino, sulfonic acid, carboxyl, carboxamide acylamino, C1-C12-alkyl, C2-C12-alkenyl, C1-C12-alkoxy, phenoxy, C7-C12-alkylaryl, C1-C12-alkylsulfonyl, arylsulfonyl, C1-C12-carboxyalkyl, C1-C12-carboxamidoalkyl and sulfonic ester, where the alkyl and aryl radicals mentioned may be substituted by functional groups, and in which two adjacent radicals may form an optionally substituted, fused-on cycloaliphatic hydrocarbon ring or a mono- or polycyclic aromatic hydrocarbon ring, and
• Z is —CH═CH— or a hydrocarbon radical whose ends are bonded via a system composed of conjugated double bonds, comprising the steps of reacting at least one o-aminophenol with at least one dicarboxylic acid whose carboxyl groups are bonded to one another via a double bond or via a system of conjugated double bonds to give an ammonium salt and subsequently converting this ammonium salt in the presence of at least one dehydrating catalyst and at least one solvent with low dielectric loss while irradiating with microwaves to give the bisbenzoxazole.