Process for preparing compounds of the formula (I) in which R 1 and R 2 represent methyl and R 3 represents chlorine, characterized in that in a first step compounds of the formula (II) in which R 1 , R 2 and R 3 have the definitions given above, are reacted with magnesium in the presence of a solvent to give compounds of the formula (III), in which R 1 , R 2 and R 3 have the definitions given above; in a second step the compounds of the formula (III) are reacted with compounds of the formula (IV) in which R 4 and R 5 represent methyl, to give a compound of the formula (V) in which R 1 , R 2 , R 3 , R 4 and R 5 have the definitions given above; in a third step the compounds of the formula (V) are reacted by hydrolysis under acidic conditions to give compounds of the formula (VI) in which R 1 , R 2 and R 3 have the definitions given above; in a fourth step compounds of the formula (VI) are hydrogenated to give compounds of the formula (VII) in which R 1 , R 2 and R 3 have the definitions given above; in the presence of a catalyst; in a fifth step compounds of the formula (VII) are reacted to give compounds of the formula (VIII) in which R 1 , R 2 and R 3 have the definitions given above and Y represents chlorine or bromine; in a sixth step the compounds of the formula (VIII) are reacted with a cyanide of the formula (IX) MCN (IX), in which M represents sodium, to give compounds of the formula (X) in which R 1 , R 2 and R 3 have the definitions given above; in a seventh step the compounds of the formula (X) are hydrolysed under acidic or basic conditions to give compounds of the formula (I) in which R 1 , R 2 and R 3 have the definitions given above. R 1 and R 2 represent methyl and R 3 represents chlorine, characterized in that in a first step compounds of the formula (II) in which R 1 , R 2 and R 3 have the definitions given above, are reacted with magnesium in the presence of a solvent to give compounds of the formula (III), in which R 1 , R 2 and R 3 have the definitions given above; in a second step the compounds of the formula (III) are reacted with compounds of the formula (IV) in which R 4 and R 5 represent methyl, to give a compound of the formula (V) in which R 1 , R 2 , R 3 , R 4 and R 5 have the definitions given above; in a third step the compounds of the formula (V) are reacted by hydrolysis under acidic conditions to give compounds of the formula (VI) in which R 1 , R 2 and R 3 have the definitions given above; in a fourth step compounds of the formula (VI) are hydrogenated to give compounds of the formula (VII) in which R 1 , R 2 and R 3 have the definitions given above; in the presence of a catalyst; in a fifth step compounds of the formula (VII) are reacted to give compounds of the formula (VIII) in which R 1 , R 2 and R 3 have the definitions given above and Y represents chlorine or bromine; Y represents chlorine or bromine; in a sixth step the compounds of the formula (VIII) are reacted with a cyanide of the formula (IX) MCN (IX), in which M represents sodium, M represents sodium, to give compounds of the formula (X) in which R 1 , R 2 and R 3 have the definitions given above; in a seventh step the compounds of the formula (X) are hydrolysed under acidic or basic conditions to give compounds of the formula (I) in which R 1 , R 2 and R 3 have the definitions given above. <EOS>