1 -Hydroxy-2,5-diketopiperazine

Defensive Compounds. The pygidial gland secretion of Hygrobia hermanni contains unusual 2-hydroxy acids such as 2-hydroxyhexanoic 31 acid and 2-hy-droxy-4-(methylthio)butanoic acid 32. The compounds may form lactides 33, which are the oxygen-analogues of diketopiperazins. Traces of benzoic acid and p-hydroxybenzaldehyde could be identified [104]. 

Three diketopiperazines 165 were isolated from the cell-free culture supernatant of the Antarctic psychrophilic bacterium P. haloplanktis TAC125 (05MI34). Xylogranatinin (9-hydroxy-3-methoxy-6H-pyrido[l,2-flJpyrazin-6-dione) was isolated from the fruit of a Chinese mangrove... 

The first synthesis of a siderophore was the preparation of ferrioxamine B over 20 years ago in order to confirm the chemical structure of this natural product67). Synthesis of the other hydroxamate containing siderophores has as a central problem preparation of the constituent to-N-hydroxy amino acid in an optically pure form. The most important such subunit in hydroxamate siderophores is Ns-hydroxy ornithine. This is a chiral building block of the diketopiperazine-containing siderophores (rhodo-torulic acid 68), dimerum acid 69), coprogen 70) and coprogen B 69>), the cyclic hexa-peptides of the ferrichrome family27), the fusarinines 71 -73) and the antibiotic ferri-chrome derivatives albomycines Sl5 S2 and e 61-62). 

As was shown by Jacobs and Craig at the Rockefeller Institute and by Stoll and coworkers at Sandoz, the peptide moiety consists of the diketopiperazine from the amino acids L-proline and L-phenylalanine, combined with an a-hydroxy-L-alanine residue with its oxygen bound to the 6-membered ring thus forming a cyclol structure. 

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