The present invention provides optionally substituted compounds of the formula I or salts thereof; ##STR00001## wherein R.sub.1 is O or S when double bonded to the ring or is OH, SH, or a protected equivalent, when single bonded to the ring, R.sub.2 is hydrogen or more preferably an C.sub.1-C.sub.10 organic group attached by a carbon atom, X is H, O, OO, S or SS R.sub.3 is absent where X.dbd.H, is hydrogen or is a hydroxyl or thiol protecting group, R.sub.4 is a hetero- or preferably homo-cyclic aryl group, optionally substituted with a further group R.sub.5 and groups T.sub.1 are each, independently, absent, hydrogen or an S--R.sub.6 group, where any/each R.sub.6 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.