Synthetic Approaches to <i>N</i><sup>δ</sup>-Methylated <scp>l</scp>-Arginine, <i>N</i><sup>ω</sup>-Hydroxy-<scp>l</scp>-arginine, <scp>l</scp>-Citrulline, and <i>N</i><sup>δ</sup>-Cyano-<scp>l</scp>-ornithine
led to threeend compounds in excellent yields. Simultaneous protection of the α-amino acid moiety by formation of boroxazolidinones, particularly by employing 9-borabicyclo[3.3.1]nonane (9-BBN-H), proved to be a convenient option to perform side chain modifications and led to all of the desired end compounds. Additionally, enantiomeric excess (ee, %) of crucial synthetic intermediates and end compounds