A process for preparing a compound of formula (I): wherein X represents O or S； R 1 represents C 1-6 alkyl, C 3-8 cycloalkyl, C 3-8 cycloalkylmethyl or C 3-8 cycloalkenyl any of which optionally may be substituted by one or more methyl groups or halogen atoms or R 1 represents aryl, 4-(diethylamino)sulphonylphenyl, 2,6-difluorophenyl, 4-methoxyphenyl, 3-difluoromethylthiophenyl, 4-cyanophenyl, heteroaryl, 5-chloro-4-methoxy-thiophene-3-yl, 2-isopropyl-1,3-thiazol-4-yl, 5-trifluoromethylfuran-2-yl, 5-methylsulphonyl-thiophene-2-yl, 5-methylthio-thiophene-2-yl or 5-ethyl-isoxazol-3-yl； R 2 represents hydrogen, methyl, which may be in either the ± or ² configuration, or methylene； R 3 and R 4 are the same or different and each independently represents hydrogen, halogen or a methyl group； and represents a single or a double bond； or a physiologically acceptable salt or solvate thereof； comprising the reaction of a carboxylic acid (X=O) or carbothioic acid (X=S) of formula (II) wherein R 1 , R 2 , R 3 , R 4 , X and are as defined for formula (I) above； with a compound of formula L-CH 2 CN wherein L represents a leaving group.
