Compounds of formula (1) where R3 is a --C(O)NH2 group may be prepared by reacting a compound of formula (1), where R3 is a hydrogen atom, with an isocyanate, such as chlorosulphonyl isocyanate ClSO2 NCO, or trichloroacetyl isocyanate Cl3 C(O)NCO (followed by reaction with ammonia) or sodium cyanate NaOCN, in a solvent such as a cyclic ether, e.g. tetrahydrofuran or a halogenated solvent, e.g. dic