The present invention relates to compounds of the general formula (I) and
salts, prodrugs, and stereoisomers thereof, wherein Y independently
represents S, O, NR.sup.2, SO, SO.sub.2; A independently represents a
fife- or six-membered aromatic carbocycle or heterocycle and wherein
R.sup.1 to R.sup.20 in formula (I) represent independently of each other
a variety of different substituents comprising alkyl, aryl, aralkyl,
alkylaryl, heteroaryl groups and monofunctional moieties.