(b) a 1,6-naphthyridine derivative of the formula ##STR5## in which R1, R2 and R3 have the above-given meanings, R4 is a methyl radical and R5 is a benzyloxy-carbonyl radical, is split hydrogenolytically in known manner to give the corresponding carboxylic acid and this, in turn, is reacted, via the intermediate stage of an acid halide, with an appropriately substituted alcohol or amine to give