The present invention provides a process for preparing a compound of the general formula (I): wherein R1 to R5 are independently selected from hydrogen, halogen, C1-6 alkyl, C2-6 alkenyl, C2-6 alkynyl or C1-6 alkoxy, all optionally substituted by one or more of halogen, hydroxy and aryl groups, as well as being independently selected from amino, mono- or disubstituted amino, alkenyloxy, acyl, acyloxy, cyano, nitro, aryl and alkylthio groups, which process comprises reacting a compound of the general formula (II): wherein R1 to R5 are as defined in formula (I), with aminoguanidine bicarbonate in a mixture of a water-soluble solvent and polyphosphoric acid.