The present invention relates to compounds of formula (1) and salts, stereoisomeric forms, racemic mixtures, prodrugs, esters and metabolites thereof. R.sub.1 is a glycosyl moiety, hydroxyl-protected acetate derivatives thereof or amino derivatives thereof; R.sub.2, R.sub.3 and R.sub.4 are, each independently, selected from hydrogen, C.sub.1-6alkyl, C.sub.2-20alkenyl, C.sub.6-20arylalkyl, C.sub.3-7cycloalkyl, C.sub.3-7cycloalkylC.sub.1-6alkyl, C.sub.6-20aryl aralkyl, akylcarbonyloxy, arylcarbonyloxy, alkyloxy, alklylthio, alkylamino, alkyloxyalkyl, alkanoyl, alkylcarbonylalkyl, optionally substituted by one or more substituents independently selected from alkyl, aralkyl, aryl, cycloalkyl, alkyloxycarbonyl, carboxyl, aminocarbonyl, hydroxy, cyano, halogen or amino optionally mono- or disubstituted wherein the substituents are independently selected from alkyl, aryl, aralkyl, aryloxy, arylamino, aryloxyalkyl, arylaminoalkyl, aralkoxy, and alkoxy.