Alternatively, intermediates of formula (3) may be prepared by alkylation of a corresponding compound of formula (4) ##STR5## using a compound R2 Hal where Hal is a halogen atom such as a bromine or chlorine atom! where necessary in the presence of a base such as caesium or potassium carbonate or an alkoxide such as potassium t-butoxide, in a dipolar aprotic solvent such as an amide, e.g. a substi