| abstract |
The present invention relates to a compound of formula [Ig] or a salt thereof:n n n n n n n n Wherein R 1 is hydrogen or an amino protecting group,n n n R 2 is hydrogen or hydroxy,n n n R 3 and R 4 are independently N-methyl-methoxycarbonylamino, N-ethyl-methoxycarbonylamino, N-propyl-methoxycarbonylamino or 3-ethylureido,n n n R 3 and R 4 are both substituted methoxyxacarbonylamino at the meta position of the benzene ring. |