Besides guanidinium derivative 8b (ca. 12%), the experiment with the COMU reagent also produced oligomeric ester-type by-products that resulted from the self-condensation of 1 (characteristic 1 H NMR signals of benzylic CH2 at δ 5.5-5.3 ppm). These by-products impeded the separation of pure amide 9 during a conventional work-up procedure that involved a simple washing of the solid reaction mixture with water and filtration.