Abstract: The present invention provides optionally substituted compounds of the formula I or salts thereof; wherein R1 is O or S when double bonded to the ring or is OH, SH, or a protected equivalent, when single bonded to the ring, R2 is hydrogen or more preferably an C1-C10 organic group attached by a carbon atom, X is H, O, OO, S or SS R3 is absent where X?H, is hydrogen or is a hydroxyl or thiol protecting group, R4 is a hetero- or preferably homo-cyclic aryl group, optionally substituted with a further group R5 and groups T1 are each, independently, absent, hydrogen or an S—R6 group, where any/each R6 is independently an organic group of molecular weight up to around 500 amu. The invention further provides a method for the synthesis of such compounds and a method of treatment comprising administering such compounds to a mammalian subject.