A compound of formula (X) can be prepared by reacting a compound of formula (XI): ##STR13## wherein L1 and R1 are as defined above and L2 is a leaving group, for example a halogen atom wherein L1 and L2 are preferably the same, with a compound of formula (XII): ##STR14## wherein P2 and P3 are as defined above, in the presence of a base such as C1-6 -alkyl-M or MH wherein M is a metal ion, for exam