Lysine is proposed as an alternative biosynthetic precursor of the pyrrole-imidazole alkaloids frequently found in marine sponges. As a putative key intermediate, the natural product N-alpha-(4-bromopyrrolyl-2-carbonyl)-L-homoarginine (1) from the sponge Agelas wiedenmayeri was synthesized in the solid phase starting from the Fmoc/Pmc-protected L-homoarginine and in solution starting from readily available L-lysine methyl ester.