A process for the preparation of a compound of formula (I) or a salt thereof wherein R is an amino, nitro or -NHCOR 1 group, wherein R 1 is C 1 -C 6 alkyl or phenyl optionally substituted by a -O-(CH 2 ) n -phenyl group, wherein n is an integer of 1 to 6; comprising reacting a compound of formula (II) or a salt thereof, both as the individual isomer and as an isomeric mixture, wherein R is as defined above; with a dehydrating agent, if necessary in the presence of an organic solvent, and optionally in the presence of a basic agent; and, if desired, converting a compound of formula (I) to another compound of formula (I) or a salt thereof.