r 3 denotes u2014 o u2014 co u2014 c 1 c 4 alkyl are obtainable via conventional ester syntheses either under acid or basic reaction conditions starting from compounds of formula i wherein r 2 denotes hydroxy compounds of formula i u2032 wherein r 3 denotes u2014 n c 1 c 4 alykl 2 are obtainable from the compounds of formula i wherein r 2 denotes nh 2 or u2014 nhc 1 c 4 alkyl via conventional methods by treatment with c 1 c 4 alkyl halides triflates mesylates or p toluenesulfonates under basic reaction conditions compounds of formula i u2032 wherein r 3 denotes u2014 nh u2014 co u2014 c 1 c 4 alkyl are obtainable from the compounds of formula i wherein r 2 denotes nh 2 via conventional methods by treatment with c 1 c 4 alkyl carboxylic acid esters anhydrides or acyl halydes under basic reaction conditions compounds of formula i u2032 wherein r 3 denotes u2014 nh u2014 co u2014 oc 1 c 4 alykl are obtainable from the compounds of formula i wherein r 2 denotes nh 2 via conventional methods by treatment with c 1 c 4 alkyl chloroformates under basic reaction conditions compounds of formula i u2032 wherein r 3 denotes u2014 nh u2014 co u2014 nh u2014 c 1 c 4 alkyl are obtainable via conventional methods by treatment with u2014 c 1 c 4 alkyl isocyanates compounds of formula i u2032 wherein.