Thus, for example, a compound of formula (VI) wherein each group X is hydrogen or C1 -C2 alkyl may be reacted with an halogenating agent such as, e.g., an halide, e.g. chlorine or bromine, or a thionyl halide, e.g. thionylchloride, to give a compound of formula (I) wherein R is a group ##STR37## wherein each R6 and R7 is C2 -C4 alkyl 2-substituted by halogen, e.g. chlorine or bromine.