"The invention relates more especially to compounds of formula I and Ia in which R1 is C1 -C4 alkyl, such as methyl, or ??,??,??-trifluoro-C1 -C4 alkyl, such as trifluoromethyl, R2 is C1 -C4 alkyl, such as propyl, C2 -C4 alkenyl, such as allyl, ??,??,??-trifluoro-C1 -C4 alkyl, such as 3,3,3-trifluoropropyl, or, less preferably, hydrogen, X is C1 -C3 alkylene, such as methylene, oxy or thio, alk is" . . . .