Compounds of formula (I): formula (I) and salts, solvates, chemically protected forms, and prodrugs thereof, are disclosed wherein R.sup.2 is selected from: an optionally substituted napthyl group; an optionally substituted thiophenyl or furanyl group; and a phenyl group substituted by: one or more chloro or fluoro groups; an ethyl or propyl group; a 4-t-butyl group; a 2-methyl group; or two methyl groups in the 2- and 6-positions. ##STR00001##