ydrogen atom, an alkyl group, a substituted or unsubstituted aromatic ring, or a substituted or unsubstituted heterocyclic ring; each of R5 and R6 is a hydrogen atom, a halogen atom, an alkyl group, an aralkyl group, an alkoxy group, a substituted or unsubstituted aromatic ring, or a substituted or unsubstituted heterocyclic ring.