Abstract: A process for preparing a compound of formula (I): ##STR1## wherein R is C.sub.1 -C.sub.6 alkyl, C.sub.3 -C.sub.6 cycloalkyl or benzyl, and wherein the phenyl moiety of said benzyl group is optionally substituted with one or two substituents each independently selected from halo, nitro, C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 alkoxy, amino and trifluoromethyl, which comprises treating a solution of a compound of formula (II): ##STR2## wherein R is as defined for formula (I), and a halonitromethane in a first organic solvent, with a quaternary ammonium fluoride of formula (III):R.sup.1 R.sup.2 R.sup.3 F.sup.4 N.sup.+ F.sup.- (III)wherein R.sup.1,R.sup.2,R.sup.3 and R.sup.4 are each independently selected from C.sub.1 -C.sub.8 alkyl and benzyl, optionally in solution in a second organic solvent.