Compounds of formula (III) in which L is a halogen atom may also be prepared by reaction of a compound of formula (V) as hereinbefore defined, particularly such a compound in which Y represents a carbonyl group and X represents --O--, with an aryl thioether of formula ##STR8## (where R1a and R2a are as hereinbefore defined and R11 represents an aryl group such as phenyl), e.g. in a polar solvent s