re hydrogen, with a compound of generalformula (VI) ##STR10##in which A is a group of formula --COR.sub.1 or --CHRR.sub.2 wherein R has the meaning defined above, R.sub.1 is halogen or C.sub.1 -C.sub.4 alkoxy and R.sub.2 is a leaving group such as halogen or sulfonyloxy, in at least unitary molar ratio, ata temperature above 50.degree.