Purification, general procedures amino acids

Colistin (COL) is a multicomponent antibiotic (polymyxins E) that is produced by strains of inverse Bacillus polymyxa. It consists of a mixture of several closely related decapeptides with a general structure composed of a cyclic heptapeptide moiety and a side chain acetylated at the N-terminus by a fatty acid. Up to 13 different components have been identified. The two main components of colistin are polymyxins El and E2 they include the same amino acids but a different fatty acid (216). A selective and sensitive HPLC method was developed for the determination of COL residues in milk and four bovine tissues (muscle, liver, kidney, and fat). The sample pretreatment consists of protein precipitation with trichloracetic acid (TCA), solid-phase purification on Cl 8 SPE cartridges, and precolumn derivatization of colistin with o-phthalaldehyde and 2-mercaptoethanol in borate buffer (pH 10.5). The last step was performed automatically, and the resulting reaction mixture was injected into a switching HPLC system including a precolumn and the reversed-phase analytical column. Fluorescence detection was used. The structural study of El and E2 derivatives was carried out by HPLC coupled with an electrospray MS. Recoveries from the preseparation procedure were higher than 60%. 

Alternative procedures for the preparation of N-carboxyanhydrides include reaction of the N -protected amino acids with PBrj. Generally, methods involving the use of phosphoro halides are hampered by the need for very long reaction times and extensive product purification, and often give only very poor yields. 

A salient feature of the oxazolidinone auxiliaries is the fact that they are easily synthesized from inexpensive, commercially available starting materials. The L-amino acids valine and phenylalanine provide access to oxazolidi-nones 114 and 115, respectively, while oxazolidinone 116 is conveniently derived from norephedrine. Moreover, their derivatives are typically crystalline, allowing for ease of purification and handling. The general procedure for the preparation of these chiral oxazolidinones is illustrated with the synthesis of the N-propionyl oxazolidinone 127 derived from phenylalanine (130, Scheme 3.20) [86]. 

---