HIGH-YIELD SYNTHESIS OF THE ENTEROBACTIN TRILACTONE AND EVALUATION OFDERIVATIVE SIDEROPHORE ANALOGS

A novel one-step synthesis of the macrocyclic triserine trilactone sca ffold of the siderophore enterobactin, which eliminates the beta-lacto nization step of N-tritylserine, is presented. The cyclization reactio n is based on a stannoxane template and leads to an overall yield of s imilar to 50%. This enables the practical functionalization of the tri lactone by attaching chelating groups other than catecholamides. The c onformational stability of the trilactone ring has been examined by hi gh-resolution X-ray diffraction studies of the N-trityl intermediate: crystals grown from methylene chloride:methanol are orthorhombic, spac e 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) Angst rom(3) and Z = 4. A hydroxypyridinonate analog of enterobactin, thyl-2 -oxo-(1H-pyridinyl)carbonyl]-4-cyclotriseryl trilactone (hopobactin), has been prepared by attachment of three 3-hydroxy-1-methyl-2(1H)-pyri dinonate (3,2-HOPO) moieties to the triserine trilactone. This ligand represents the first enterobactin analog that retains the trilactone s caffold, but employs chelates other than catecholamides. Crystals of t he 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 entero bactin metal complexes is also enforced in those of hopobactin and per sists in aqueous or methanolic solution, as demonstrated by circular d ichroism. The ferric hopobactin complex is the first reported chiral c omplex of hydroxypyridinonate ligands. The solution coordination chemi stry 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 hopo bactin free ligand (log K-av = 6.1) is typical of other 3-hydroxy-1-me thyl-2-oxo-1H-pyridin-4-carboxamide ligands. The stability constants o f 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 catechola mide-based species, where enterobactin (log beta(110) = 49) is 6 order s of magnitude more stable than TRENCAM (log beta(110) = 43.6). The st ability enhancement reflects the specific predisposition by the triser ine scaffold of the catecholamide binding units. in spite of a signifi cantly lower affinity of 3,2-hydroxypyridinonates for iron(III) compar ed with the more basic catecholates, hopobactin is an extraordinarily powerful chelating agent under acidic conditions: No measurable dissoc iation 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 TREN HOPO for iron(III) relative to iron(II) results in strongly negative r eduction potentials, -782 mV vs 0.01 M Ag+/Ag in CH3CN or -342 mV vs N HE in water and -875 mV vs 0.01 M Ag+/Ag in CH3CN or -435 mV vs NHE in water, respectively.