It is preferred that R2 is hydrogen and R3 is hydrogen, an alkyl group which is unsubstituted or substituted by at least an electron donating or electron withdrawing group or ZY. In this preferred embodiment, it is more preferred that R3 is hydrogen or an alkyl group such as methyl, which is unsubstituted or substituted by an electron donating group, NR4OR5 or ONR4R7, wherein R4, R5 and R7 are ind