A process for the preparation of a compound of the general formula (I): wherein R1 is a 4-pyrimidinyl ring substituted at the 6-position by halo, hydroxy, 2-cyanophenoxy, 2,6-difluorophenoxy, 2-nitrophenoxy or 2-thiocarboxamidophenoxy and R2 is any group which can be transesterified to form a methyl ester, which comprises treating a compound of general formula (II): wherein R1 and R2 have the meanings given above, with a formylating agent and subsequently treating the formylated product with a methylating agent. 1