Finally, compounds (I) in which R1 and/or R5 are a group of the formula RCONH--, ROCONH--, RNHCONH-- or RSO2 NH--, in which R is a C1 -C7 -alkyl, a phenyl or a benzyl, can be prepared by conventional reactions starting from compounds (I') in which R'1 and/or R'5 ???NH2.