Compounds of formula (1) where R3 is a --C(O)R4 group in which R4 is an alkyl group, may be prepared by reacting a compound of formula (1) where R3 is a hydrogen atom with (1) an alkylanhydride, such as propionic anhydride in a solvent or mixture of solvents such as dichloromethane and pyridine; or (2) with an acid chloride, where necessary in the presence of a base, such as an organic amine, e.g.