Novel products of formula (I), their preparation, the pharmaceutical compositions which contain them and their use in the preparation of medicaments. ##STR1## In the formula (I), the following substituents are among the preferred: Ar represents a substituted or condensed phenyl radical or a polycyclic or heterocyclic aromatic radical, R represents a radical of general formula --(CH.sub.2).sub.m --X.sub.1 --(CH.sub.2).sub.n --Z in which X.sub.1 =single bond, O, S; m=0, 1; n=0, 1, 2; it being possible for the CH.sub.2 radicals to be substituted; Z represents carboxyl, COOR.sub.6 (R.sub.6 =alkyl), CON(R.sub.7)(R.sub.8) (R.sub.7 =hydrogen or alkyl and R.sub.8 =hydrogen, hydroxyl, arylsulphonyl, heterocyclyl, optionally substituted amino, optionally substituted alkyloxy or optionally substituted alkyl), PO(OR.sub.9).sub.2 (R.sub.9 =hydrogen or alkyl), an --NH--CO--T (T=hydrogen or optionally substituted alkyl) radical, or else--a ##STR2## radical, R.sub.1 and R.sub.2 =hydrogen or halogen or alkyl, alkyloxy, which is optionally substituted, alkylthio, alkyloxycarbonyl or else R.sub.1 and R.sub.2, at the ortho position with respect to one another, form an optionally substituted heterocycle containing 1 or 2 heteroatoms, R.sub.3 and R.sub.4 =hydrogen or halogen or alkyl, alkenyl, alkyloxy, alkylthio, carboxyl or alkyloxycarbonyl, R.sub.5 =hydrogen, alkyl, alkylthio, X=O or S or --NH--, --CO--, methylene, vinyldiyl, alkene-1,1-diyl or cycloalkane-1,1-diyl, and Y=O or S, in the racemic form, as well as the optical isomers (diastereomers and enantiomers) and the salts of the product of formula (I). The compounds of formula (I) are farnesyl transferase inhibitors which exhibit notable antitumor and antileukemic properties.