The present invention relates to a process for the preparation of a compound of the general formula (I), which comprises converting a compound of the general formula (II), in an aprotic organic solvent in the presence of a catalytically active amount of a strong acid (catalyst) or of an agent that liberates a strong acid or of an ammonium salt of a strong acid, it also being possible for the catalyst to be part of the starting material/product, into a compound of formula I, wherein R5, R6 and R8 are each independently of the others a hydrogen atom, a nitro group, a sulfo group, a halogen atom, a pseudohalogen, a group COOR1 or CONHR2 or a C1-8 alkyl, C1-8 alkoxy or aryloxy radical, an amide group, a thioalkyl or thioaryl radical, an alkyl- or aryl-sulfonyl radical, an alkyl- or aryl-sulfinyl radical, a trifluoromethyl group or a phosphono group, R1 and R2 being a hydrogen atom or a C1-8 alkyl radical or an aryl or aralkyl radical, R10 is a group —C(O)CH2C(O)CH3 and R11 is a hydrogen atom or an acyl radical, or R10 and R11 are a group —C(O)CH2C(O)CH3. The process according to the invention is simple to perform and results in products of high chemical purity and high isomeric purity in a high yield