The acylation is conveniently effected in an inert aprotic diluent such as an aliphatic or cyclic ether, e.g. diethyl ether, 1,2-dimethoxyethane or tetrahydrofuran, or a chlorinated aliphatic hydrocarbon, e.g. methylene chloride, using a base, e.g. triethylamine or pyridine, at temperatures between 0??? C. and 50??? C., preferably at room temperature.