The present invention relates to compounds of formula I or salts thereof, wherein, R1 is selected from the group consisting of hydrogen and R4 R2 is selected from the group consisting of hydrogen, acetyl and R4 R3 is selected from the group consisting of alkyl, —O-alkyl, —NH-alkyl, aryl and heterocyclyl R4 represents a moiety (A) wherein, X is selected from the group consisting of a single bond, alkylene, alkenylene, alkynylene, arylene or a heterocyclylene moiety R5 and R6 are same or different and are independently selected from hydrogen, alkyl, aryl or heterocyclyl or R5 and R6 may form together with the nitrogen atom to which they are attached a heterocyclyl ring system.
