Formula I wherein, R'is selected from H, C106 alkyl, C2-7 alkenyl, C6-10 aryl or heteroaryl,-C (O) C 6 alkyl, -C(O)C2-7 alkenyl, -C(O)C6-10 aryl or heteroaryl ; R2 is a hydrogen; or R1 and R2 may be taken together to form a second bond between the attached oxygen and carbon atoms to form a carbonyl ; R3 is selected from H or CH3 ; R4 is selected from -COOH, -COOR5, -CH2OC(O)R6 and -CH2OR7 R5 and R