Synthesis of the unnatural amino acid AGDHE, a constituent of the cyclic depsipeptides callipeltins A and D

[GRAPHICS] The novel amino acid residue (2R,3R,4S)-4-amido-7-guanidino-2,3-dihydroxyheptanoic acid (AGDHE, 3), a constituent of the cyclic depsipeptides callipeltins A and D, and its (2S,3S,4S) diastereomer were synthesized from a protected L-ornithine derivative in 13 steps (15% overall yield), and its configurational assignment was reexamined by H-1 NMR.