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.