At least a part of the isopropylideneglycerol of formula (IV) and the basic catalyst are first mixed under an inert atmosphere, for example under a nitrogen atmosphere, and the mixture is heated to a temperature of between 50??? and 190??? C., and preferably in the region of 150??? C. The mixing is preferably performed in the absence of solvent, but solvents such as dimethylformamide or methylpyrr