One such process for preparing a compound of formula I wherein X represents oxygen, and R is other than hydrogen comprises reacting a compound of formula ##STR5## wherein R2, R3 and Y are as defined above in connection with formula I and W represents a halogen (e.g. chlorine or bromine) or an equivalent replaceable radical such as an organic sulphonyl radical, e.g. tosylate (-Otosyl), with a compo