This invention provides a process for preparing a compound of formula III ##STR1## wherein R.sup.1 is bromo, iodo or COOR.sup.2 ; R.sup.2 is H, C.sub.1 -C.sub.4 alkyl, phenyl which may be substituted or benzyl; and A is a 5- or 6-membered aromatic residue which may contain up to three hetero atoms and which may optionally be substituted with one or two groups selected from the group consisting of halo, hydroxy, C.sub.1 -C.sub.4 alkyl, and C.sub.1 -C.sub.4 alkoxy; or a salt thereof, which comprises (a) reacting a compound of formula I ##STR2## wherein R is C.sub.1 -C.sub.4 alkyl or phenyl which may be substituted; and R.sup.1, R.sup.2, and A are as defined above; or a salt thereof, with a sulfurization agent; (b) cyclizing the reaction product from step (a) with guanidine; and (c) optionally salifying the reaction product from step (b).
