yl or alkenyl, hydroxyl, C1 -C6 -alkoxy, carboxyl,or amino or R1 and R2 are together and are an amino or substituted amino derivative of a benzo-group, and each of R3 and R4 is H, unsubstituted or substituted C1 -C6 -alkyl or alkenyl, hydroxyl, C1 -C6 -alkoxy, carboxyl or amino.