n optionally substituted hydrocarbon group) or ???NR12 R13 (R12 and R13 are the same or different and each is a hydrogen atom, an optionally substituted hydrocarbon group, an optionally substituted heterocyclic group, or an acyl group, or R12 and R13 are optionally bonded to form a ring); and R6 and R7 are the same or different and each is a hydrogen atom, an optionally substituted hydrocarbon gro