High-yield synthesis of the enterobactin trilactone and evaluation of derivative siderophore analogs

A novel one-step synthesis of the macrocyclic triserine trilactone scaffold of the siderophore enterobactin, which eliminates the beta-lactonization step of N-tritylserine, is presented. The cyclization reaction is based on a stannoxane template and leads to an overall yield of similar to 50%. This enables the practical functionalization of the trilactone by attaching chelating groups other than catecholamides. The conformational stability of the trilactone ring has been examined by high-resolution X-ray diffraction studies of the N-trityl intermediate: crystals grown from methylene chloride:methanol are orthorhombic, space group P2(1)2(1)2(1) with unit cell dimensions a = 9.2495(5) Angstrom, b = 11.3584(1) Angstrom, c = 48.945(1) Angstrom, V = 5142.1(2) Angstrom(3) and Z = 4. A hydroxypyridinonate analog of enterobactin, N,N',N''-tris[(3-hydroxy-1-methyl-2-oxo-(1H-pyridinyl)carbonyl]-4-cyclotriseryl trilactone (hopobactin), has been prepared by attachment of three 3-hydroxy-1-methyl-2(1H)-pyridinonate (3,2-HOPO) moieties to the triserine trilactone. This ligand represents the first enterobactin analog that retains the trilactone scaffold, but employs chelates other than catecholamides. Crystals of the chiral ferric complex grown from DMF:diethyl ether are monoclinic, space group P2(1), with unit cell dimensions a = 13.0366(9) Angstrom, b = 22.632(2) Angstrom, c = 27.130(2) Angstrom, b = 100.926(1)degrees, V = 7860(1) Angstrom(3), and Z = 8. The Delta configuration of enterobactin metal complexes is also enforced in those of hopobactin and persists in aqueous or methanolic solution, as demonstrated by circular dichroism. The ferric hopobactin complex is the first reported chiral complex of hydroxypyridinonate ligands. The solution coordination chemistry of this new ligand and its iron(III) and iron(II) complexes have been studied by means of H-1 NMR, potentiometric, spectrophotometric, and voltammetric methods. The average protonation constant of the hopobactin free ligand (log K-av = 6.1) is typical of other 3-hydroxy-1-methyl-2-oxo-1H-pyridin-4-carboxamide ligands. The stability constants of the iron(III) complex formed with hopobactin (log beta(110) = 26.4) and with the tris(2-aminoethyl)amine-based analog, TRENHOPO, (log beta(110) = 26.7) are of the same order of magnitude, unlike the catecholamide-based species, where enterobactin (log beta(110) = 49) is 6 orders of magnitude more stable than TRENCAM (log beta(110) = 43.6). The stability enhancement reflects the specific predisposition by the triserine scaffold of the catecholamide binding units. in spite of a significantly lower affinity of 3,2-hydroxypyridinonates for iron(III) compared with the more basic catecholates, hopobactin is an extraordinarily powerful chelating agent under acidic conditions: No measurable dissociation is observed even in 1.0 M HCl. In contrast to enterobactin and its synthetic derivatives, the hopobactin ferric complex undergoes no sequential protonation above pH 1. The affinity of hopobactin and TRENHOPO for iron(III) relative to iron(II) results in strongly negative reduction potentials, -782 mV vs 0.01 M Ag+/Ag in CH3CN or -342 mV vs NHE in water and -875 mV vs 0.01 M Ag+/Ag in CH3CN or -435 mV vs NHE in water, respectively.