Compounds of formula (1) where R3 is a --C(O)R4 group in which R4 is lower alkoxy or aryloxy group, may be prepared by reaction of a compound of formula (1) in which R3 is a hydrogen atom with a reagent R4 OC(O)X (where R4 is as just described and X is a leaving group, such as a halogen atom, for example a chlorine atom) in a solvent, e.g. dichloromethane.