There are provided compounds of formula (I) ##STR00001## wherein R.sub.1 represents O, S or NH; R.sub.2 represents --C(.dbd.O)-aryl or --C(.dbd.O)-heteroaryl; R.sub.3 represents hydrogen, methyl (which may be in either the .alpha. or .beta. configuration) or methylene; R.sub.4 and R.sub.5 are the same or different and each represents hydrogen or halogen; and represents a single or a double bond; and salts and solvates thereof; process for preparing them, compositions containing them and their use in therapy.