Scheme 6.

Synthetic scheme for generating (S)-2-amino-4-(3-methylguanidino)butanoic acid (4124W), as reported by Ueda et al. [80] using the methods of Ogawa et al. [32] and Corbin et al. [111]. Reagents and Conditions: (i) (1) 2,4-diamino-n-butyric acid dihydrochloride (20), copper acetate, 1:1 NH₄OH/H₂O; (2) N,S-dimethylthiopseudouronium iodide (19) RT, 24 h; (3) Dowex 50WX4 (NH₄⁺ form), 4124W (21) was crystallized as the hydrochloride salt.