la ##STR59## wherein each of R1, R2 and R3 is a hydrogen atom, a C1-6 alkyl group, a C1-6 alkoxy group or a nitro group;R4 is a C7-11 aromatic acyl group, a C2-7 aliphatic acyl group, a C2-7 alkoxycarbonyl group, a C6-10 aromatic sulfonyl group or a C1-6 aliphatic sulfonyl group; R5 is a C1-6 alkyl group, a C1-6 alkoxy group, a C6-10 aromatic group or a C6-10 aryloxy group; and (d) 0.01 to 10% by