The invention provides compounds of Formula I and derivatives thereof, pharmaceutical compositions comprising a compound of Formula I, and methods of treatment utilizing such compounds and compositions: ##STR00001## wherein: R.sub.1 is chosen from optionally substituted aryl, optionally substituted aralkyl, optionally substituted heteroaryl, and optionally substituted heteroaralkyl; L is a linker; R.sub.2 is chosen from optionally substituted aryl, optionally substituted aralkyl, optionally substituted heteroaryl, and optionally substituted heteroaralkyl; and ##STR00002## or single stereoisomers, mixtures of stereoisomers, or the pharmaceutically acceptable salts, amides, or esters thereof.