Enterobactin model compound

There are two catecholate siderophores which may be chosen as model compounds for synthesis the cyclic enterobactin and the linear parabactin precursor N. N8-bis(2,3-dihydroxybenzoyl)spermidine. Both of these natural products are capable of the rapid removal of iron from transferrin, the human iron transport protein 89-90). The synthesis of these and other catecholate ligands routinely requires protection of the phenolic oxygens (for example, by methyl, benzyl or acetyl groups). Very few preparations of catechol-containing siderophores have appeared in which the unprotected 2,3-dihydroxybenzoyl group is used in the synthesis 91,92). 

Table 2 UV/vis and CD spectra in solution of various geometrical and optical isomers of chromium (III) siderophores and of model compounds rhodoturulic acid (A), men (B), coprogen (C), enterobactin (D), catechol (E), and ferrioxamine B (F). UV/vis values are for Xmax (nm) and e (M cm ) CD values are for X ax (nm) and 0 (M cm )...

Fig. 11. Possible protonation schemes of tris catecholate metal complexes. The compound shown is [M3+(MECAMS)]3-. In scheme 1 the metal complex undergoes a series of two overlapping one-proton steps to generate a mixed salicylate-catecholate coordination about the metal ion. Further protonation results in Ihe precipitation of a tris salicylate complex (e.g. enterobactin, MECAM). This differs from scheme 2, in which a single two-proton step dissociated one arm of the ligand to form a bis(catecholate) chelate (e.g. TRIMCAM). Scheme 3 incorporates features of scheme 1 and 2. In this model the metal again undergoes a series of two overlapping one-proton reactions. However, unlike the case of scheme 1, the second proton displaces a catecholate arm, which results in a bis-(catecholato) metal complex. This scheme is discussed for Ga and In complexes of enterobactin analogs in Ref. 141)...

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