Synthesis of the Stereoisomers of /?-Hydroxyhistidine and their Analytical Identification in Hydrolysates of Bacterial Peptides

Ser (= /3-hydroxy Ala) belongs to the proteinogenic amino acids while other /3-hydroxy amino acids prevail in bacterial peptidic metabolites, often acting as Fe3+ chelating ligands. Thus, ß-hydroxy Asp is frequently encountered in the Pseudomonas siderophores (pyoverdins) [1], /3-Hydroxy Leu and 3-hydroxy-3-methyl Pro were recently found in a depsipeptide from Streptomyces sp. [2], L-Erythro-ß-Hydroxy His is a charac­ teristic component of bleomycins from Streptomyces verticUlus [3]. L-Threo-ß-hydroxy His was found in exochelin MN from Mycobacterium neoaurum [4] and in pyoverdin PF244 from Pseudomonas fluorescens [5], In our studies of Pseudomonas siderophores we recently encountered twice amino acids that from spectral evidence could have been /3-hydroxy His [6 ]. Therefore, we needed authentic comparison material of all four stereoisomers. In the literature syntheses are de­ scribed for the S-erythro isomer [7] and one prepa­ ration without experimental details for the R-threo isomer in about 90% purity [7], So we decided to synthesize all four stereoisomers 1 4 for the for-