The total synthesis of alcaligin

Chemistry and Biology of Siderophores from Marine Microbes

Microbial siderophores are multidentate Fe(III) chelators used by microbes during siderophore-mediated assimilation. They possess high affinity and selectivity for Fe(III). Among them, marine siderophore-mediated microbial iron uptake allows marine microbes to proliferate and survive in the iron-deficient marine environments. Due to their unique iron(III)-chelating properties, delivery system, structural diversity, and therapeutic potential, marine microbial siderophores have great potential for further development of various drug conjugates for antibiotic-resistant bacteria therapy or as a target for inhibiting siderophore virulence factors to develop novel broad-spectrum antibiotics. This review covers siderophores derived from marine microbes.

The chemical biology and coordination chemistry of putrebactin, avaroferrin, bisucaberin, and alcaligin

Dihydroxamic acid macrocyclic siderophores comprise four members: putrebactin (putH₂), avaroferrin (avaH₂), bisucaberin (bisH₂), and alcaligin (alcH₂). This mini-review collates studies of the chemical biology and coordination chemistry of these macrocycles, with an emphasis on putH₂. These Fe(III)-binding macrocycles are produced by selected bacteria to acquire insoluble Fe(III) from the local environment. The macrocycles are optimally pre-configured for Fe(III) binding, as established from the X-ray crystal structure of dinuclear [Fe₂(alc)₃] at neutral pH. The dimeric macrocycles are biosynthetic products of two endo-hydroxamic acid ligands flanked by one amine group and one carboxylic acid group, which are assembled from 1,4-diaminobutane and/or 1,5-diaminopentane as initial substrates. The biosynthesis of alcH₂ includes an additional diamine C-hydroxylation step. Knowledge of putH₂ biosynthesis supported the use of precursor-directed biosynthesis to generate unsaturated putH₂ analogues by culturing Shewanella putrefaciens in medium supplemented with unsaturated diamine substrates. The X-ray crystal structures of putH₂, avaH₂ and alcH₂ show differences in the relative orientations of the amide and hydroxamic acid functional groups that could prescribe differences in solvation and other biological properties. Functional differences have been borne out in biological studies. Although evolved for Fe(III) acquisition, solution coordination complexes have been characterised between putH₂ and oxido-V(IV/V), Mo(VI), or Cr(V). Retrosynthetic analysis of 1:1 complexes of [Fe(put)]⁺, [Fe(ava)]⁺, and [Fe(bis)]⁺ that dominate at pH < 5 led to a forward metal-templated synthesis approach to generate the Fe(III)-loaded macrocycles, with apo-macrocycles furnished upon incubation with EDTA. This mini-review aims to capture the rich chemistry and chemical biology of these seemingly simple compounds.

Desferrithiocin: a search for clinically effective iron chelators

The successful search for orally active iron chelators to treat transfusional iron-overload diseases, e.g., thalassemia, is overviewed. The critical role of iron in nature as a redox engine is first described, as well as how primitive life forms and humans manage the metal. The problems that derive when iron homeostasis in humans is disrupted and the mechanism of the ensuing damage, uncontrolled Fenton chemistry, are discussed. The solution to the problem, chelator-mediated iron removal, is clear. Design options for the assembly of ligands that sequester and decorporate iron are reviewed, along with the shortcomings of the currently available therapeutics. The rationale for choosing desferrithiocin, a natural product iron chelator (a siderophore), as a platform for structure-activity relationship studies in the search for an orally active iron chelator is thoroughly developed. The study provides an excellent example of how to systematically reengineer a pharmacophore in order to overcome toxicological problems while maintaining iron clearing efficacy and has led to three ligands being evaluated in human clinical trials.

Unsaturated macrocyclic dihydroxamic acid siderophores produced by Shewanella putrefaciens using precursor-directed biosynthesis

To acquire iron essential for growth, the bacterium Shewanella putrefaciens produces the macrocyclic dihydroxamic acid putrebactin (pbH2; [M + H(+)](+), m/zcalc 373.2) as its native siderophore. The assembly of pbH2 requires endogenous 1,4-diaminobutane (DB), which is produced from the ornithine decarboxylase (ODC)-catalyzed decarboxylation of l-ornithine. In this work, levels of endogenous DB were attenuated in S. putrefaciens cultures by augmenting the medium with the ODC inhibitor 1,4-diamino-2-butanone (DBO). The presence in the medium of DBO together with alternative exogenous non-native diamine substrates, (15)N2-1,4-diaminobutane ((15)N2-DB) or 1,4-diamino-2(E)-butene (E-DBE), resulted in the respective biosynthesis of (15)N-labeled pbH2 ((15)N4-pbH2; [M + H(+)](+), m/zcalc 377.2, m/zobs 377.2) or the unsaturated pbH2 variant, named here: E,E-putrebactene (E,E-pbeH2; [M + H(+)](+), m/zcalc 369.2, m/zobs 369.2). In the latter system, remaining endogenous DB resulted in the parallel biosynthesis of the monounsaturated DB-E-DBE hybrid, E-putrebactene (E-pbxH2; [M + H(+)](+), m/zcalc 371.2, m/zobs 371.2). These are the first identified unsaturated macrocyclic dihydroxamic acid siderophores. LC-MS measurements showed 1:1 complexes formed between Fe(III) and pbH2 ([Fe(pb)](+); [M](+), m/zcalc 426.1, m/zobs 426.2), (15)N4-pbH2 ([Fe((15)N4-pb)](+); [M](+), m/zcalc 430.1, m/zobs 430.1), E,E-pbeH2 ([Fe(E,E-pbe)](+); [M](+), m/zcalc 422.1, m/zobs 422.0), or E-pbxH2 ([Fe(E-pbx)](+); [M](+), m/zcalc 424.1, m/zobs 424.2). The order of the gain in siderophore-mediated Fe(III) solubility, as defined by the difference in retention time between the free ligand and the Fe(III)-loaded complex, was pbH2 (ΔtR = 8.77 min) > E-pbxH2 (ΔtR = 6.95 min) > E,E-pbeH2 (ΔtR = 6.16 min), which suggests one possible reason why nature has selected for saturated rather than unsaturated siderophores as Fe(III) solubilization agents. The potential to conduct multiple types of ex situ chemical conversions across the double bond(s) of the unsaturated macrocycles provides a new route to increased molecular diversity in this class of siderophore.

Complexes formed in solution between vanadium(IV)/(V) and the cyclic dihydroxamic acid putrebactin or linear suberodihydroxamic acid

An aerobic solution prepared from V(IV) and the cyclic dihydroxamic acid putrebactin (pbH(2)) in 1:1 H(2)O/CH(3)OH at pH = 2 turned from blue to orange and gave a signal in the positive ion electrospray ionization mass spectrometry (ESI-MS) at m/z(obs) 437.0 attributed to the monooxoV(V) species [V(V)O(pb)](+) ([C(16)H(26)N(4)O(7)V](+), m/z(calc) 437.3). A solution prepared as above gave a signal in the (51)V NMR spectrum at δ(V )= -443.3 ppm (VOCl(3), δ(V) = 0 ppm) and was electron paramagnetic resonance silent, consistent with the presence of [V(V)O(pb)](+). The formation of [V(V)O(pb)](+) was invariant of [V(IV)]:[pbH(2)] and of pH values over pH = 2-7. In contrast, an aerobic solution prepared from V(IV) and the linear dihydroxamic acid suberodihydroxamic acid (sbhaH(4)) in 1:1 H(2)O/CH(3)OH at pH values of 2, 5, or 7 gave multiple signals in the positive and negative ion ESI-MS, which were assigned to monomeric or dimeric V(V)- or V(IV)-sbhaH(4) complexes or mixed-valence V(V)/(IV)-sbhaH(4) complexes. The complexity of the V-sbhaH(4) system has been attributed to dimerization (2[V(V)O(sbhaH(2))](+) ↔ [(V(V)O)(2)(sbhaH(2))(2)](2+)), deprotonation ([V(V)O(sbhaH(2))](+) - H(+) ↔ [V(V)O(sbhaH)](0)), and oxidation ([V(IV)O(sbhaH(2))](0) -e(-) ↔ [V(V)O(sbhaH(2))](+)) phenomena and could be described as the sum of two pH-dependent vectors, the first comprising the deprotonation of hydroxamate (low pH) to hydroximate (high pH) and the second comprising the oxidation of V(IV) (low pH) to V(V) (high pH). Macrocyclic pbH(2) was preorganized to form [V(V)O(pb)](+), which would provide an entropy-based increase in its thermodynamic stability compared to V(V)-sbhaH(4) complexes. The half-wave potentials from solutions of [V(IV)]:[pbH(2)] (1:1) or [V(IV)]:[sbhaH(4)] (1:2) at pH = 2 were E(1/2) -335 or -352 mV, respectively, which differed from the expected trend (E(1/2) [VO(pb)](+/0) < V(V/IV)-sbhaH(4)). The complex solution speciation of the V(V)/(IV)-sbhaH(4) system prevented the determination of half-wave potentials for single species. The characterization of [V(V)O(pb)](+) expands the small family of documented V-siderophore complexes relevant to understanding V transport and assimilation in the biosphere.

Vibriobactin antibodies: a vaccine strategy

A new target strategy in the development of bacterial vaccines, the induction of antibodies to microbial outer membrane ferrisiderophore complexes, is explored. A vibriobactin (VIB) analogue, with a thiol tether, 1-(2,3-dihydroxybenzoyl)-5,9-bis[[(4S,5R)-2-(2,3-dihydroxyphenyl)-4,5-dihydro-5-methyl-4-oxazolyl]carbonyl]-14-(3-mercaptopropanoyl)-1,5,9,14-tetraazatetradecane, was synthesized and linked to ovalbumin (OVA) and bovine serum albumin (BSA). The antigenicity of the VIB microbial iron chelator conjugates and their iron complexes was evaluated. When mice were immunized with the resulting OVA-VIB conjugate, a selective and unequivocal antigenic response to the VIB hapten was observed; IgG monoclonal antibodies specific to the vibriobactin fragment of the BSA and OVA conjugates were isolated. The results are consistent with the idea that the isolated adducts of siderophores covalently linked to their bacterial outer membrane receptors represent a credible target for vaccine development.

Temperature Effects on Stereocontrol in the Horner−Wadsworth−Emmons Condensation of α-Phosphono Lactones

The Horner-Wadsworth-Emmons condensation of some alpha-phosphono lactones has been examined for conditions that impact product stereochemistry. The temperature employed to quench the reaction was found to be a major factor. For example, after the diethyl phosphonate derivative of gamma-butyrolactone was treated with potassium hexamethyldisilazane, 18-crown-6, and propionaldehyde at -78 degrees C in THF, an aliquot transferred to a flask at approximately 30 degrees C gave almost exclusively the Z-olefin product, while one allowed to warm to room temperature over several hours greatly favored the E-olefin.

A General Synthesis of Substituted Formylpyrroles from Ketones and 4-Formyloxazole

A novel two-step synthesis of substituted 2-formylpyrroles is described. Aldol adducts of ketones and 4-formyloxazole undergo a dehydration/oxazole hydrolysis/cyclization cascade on sequential treatment with MsCl/Et3N and aqueous NaOH to yield 5-substituted and 4,5-disubstituted 2-formylpyrroles. The methodology was extended to an N-benzyl thiazolium salt.

Bordetella iron transport and virulence

Bordetella pertussis, Bordetella parapertussis, and Bordetella bronchiseptica are pathogens with a complex iron starvation stress response important for adaptation to nutrient limitation and flux in the mammalian host environment. The iron starvation stress response is globally regulated by the Fur repressor using ferrous iron as the co-repressor. Expression of iron transport system genes of Bordetella is coordinated by priority regulation mechanisms that involve iron source sensing. Iron source sensing is mediated by distinct transcriptional activators that are responsive to the cognate iron source acting as the inducer.

Alpha-phosphono lactone analogues of cytidine and cytosine arabinoside diphosphates: synthesis via ring closing metathesis

alpha-Phosphono lactone derivatives of the nucleosides cytidine and cytosine arabinoside have been prepared from the corresponding nucleoside aldehydes. The stereochemical outcome of allylation reactions with these aldehydes was found to be dependent upon both the choice of protecting groups for the 2'- and 3'-hydroxyl groups and, to some extent, the nature of the Lewis acid catalyst employed. Ultimately, conditions were found that favored either the 5'R or 5'S diastereomer from different cytidine aldehydes, and gave some stereoselectivity in additions to an aldehyde derivative of ara-C. The resulting homoallylic alcohols were used as substrates in attempted Knovenagel and Horner-Wadsworth-Emmons condensations, but elimination was found to predominate over lactone formation under the conditions employed. The desired alpha-phosphono lactones could be prepared through a reaction sequence that included ring-closing metathesis on acrylate esters of the homoallylic alcohols, followed by reduction of the resulting alpha,beta-unsaturated lactones and carbon-phosphorus bond formation on enolates generated from the saturated lactones.

Alpha-phosphono lactone analogues of farnesyl pyrophosphate: an asymmetric synthesis via ring-closing metathesis

An alpha-phosphono lactone derivative of farnesol has been prepared, in both racemic and nonracemic forms, to provide a new type of farnesyl pyrophosphate analogue. Attempted preparation of the racemic alpha-phosphono lactone through rearrangement of a vinyl phosphate derived from the parent lactone resulted in both rearrangement and lactone ring opening, revealing that the farnesyl lactone was not stable to the excess of strong base required for the rearrangement. A procedure for C-P bond formation based on generation of the lactone enolate, reaction with a P(III) reagent, and oxidation was successful in providing the racemic alpha-phosphono lactone, in part, because only 1 equiv of strong base was required. The same strategy for phosphonate synthesis then was applied to the nonracemic farnesyl lactone, prepared through a sequence including allylation of farnesal with a nonracemic borane reagent, reaction of the product alcohol with acryloyl chloride, and formation of an unsaturated lactone through ring-closing metathesis. A similar strategy gave the corresponding racemic alpha-phosphono lactam through a six-step sequence from farnesal.

Preparation of α-Phosphono Lactams via Electrophilic Phosphorus Reagents: An Application in the Synthesis of Lactam-Based Farnesyl Transferase Inhibitors

Conversion of N-alkyl lactams to the corresponding alpha-phosphono lactams has been investigated through procedures that involve formation of the lactam enolate and reaction with a phosphorus electrophile. With N-octylpyrrolidinone, the enolate could be trapped efficiently on oxygen by reaction with diethyl phosphorochloridate, and the resulting vinyl phosphate rearranges smoothly to the desired phosphonate upon treatment with additional LDA. Attempts to apply the same protocol to N-farnesyl lactams met with limited success. Studies with an isolated alpha-phosphono N-farnesyl lactam have shown that the farnesyl group is not stable to the excess of strong base required for rearrangement of a vinyl phosphate. However, a series of N-farnesyl lactams and imides was converted to the desired phosphonates through formation of the lactam enolate, reaction with diethyl phosphorochloridite, and subsequent oxidation of the phosphorus intermediate to the P(V) state.

Total synthesis of the siderophore danoxamine

The total synthesis of the linear trihydroxamate siderophore, Danoxamine, is described. Danoxamine is a siderophore component of the naturally occurring siderophore-drug conjugates Salmycin A-D. The synthesis of Danoxamine features a series of coupling reactions involving N-(5-benzyloxypentyl)-O-benzylhydroxylamine being linked by a succinoyl linker to N-(benzyloxy)-1,5-pentanediamine. Two more succinoyl linkers and another N-(benzyloxy)-1,5-pentanediamine were used in coupling reactions to afford the fully protected siderophore. The linear tetrabenzyl-protected trihydroxamate was deprotected to afford the natural product Danoxamine.

Synthesis and evaluation of hydroxylated polyamine analogues as antiproliferatives

The synthesis of four hydroxylated polyamine analogues, (2R, 10R)-N(1),N(11)-diethyl-2,10-dihydroxynorspermine, (2S,10S)-N(1), N(11)-diethyl-2,10-dihydroxynorspermine, (3S,12S)-N(1), N(14)-diethyl-3,12-dihydroxyhomospermine, and (3R,12R)-N(1), N(14)-diethyl-3,12-dihydroxyhomospermine, is described along with their impact on the growth and polyamine metabolism of L1210 murine leukemia cells. Four different synthetic approaches are set forth, two each for the hydroxylated norspermines and for the hydroxylated homospermines. The key step in the assembly of the norspermines was the coupling of either N-[(2R)-2,3-epoxypropyl]-N-ethyl p-toluenesulfonamide or N-[(2S)-2,3-epoxypropyl]-N-ethyl trifluoromethanesulfonamide to N,N'-dibenzyl-1,3-diaminopropane. The key step with homospermines employed alkylation of putrescine with (3S)-N-(benzyloxycarbonyl)-N-ethyl-3,4-epoxybutylamine or of N, N'-bis(mesitylenesulfonyl)-1,4-butanediamine with (2R)-2-benzyloxy-4-[N-(mesitylenesulfonyl)ethylamino]-O-tosyl-1-++ +butan ol. All of the hydroxylated analogues were active against L1210 cells with 96-h IC(50) values of </=2 microM, and they also effectively reduced putrescine and spermidine, although the effect on spermine pools ranged from moderate to insignificant. Interestingly, the impact of the hydroxylated analogues on ornithine decarboxylase (ODC) was significantly less than that of unhydroxylated parent drug (e.g., N(1),N(11)-diethylnorspermine [DENSPM]) at 1 microM; however, S-adenosylmethionine decarboxylase (AdoMetDC) depletion was nearly identical to what was observed in cells treated with parent drug. The most notable difference between the parent and hydroxylated analogues was seen with spermidine/spermine N(1)-acetyltransferase (SSAT) upregulation in the DENSPM series. The hydroxylated analogues, especially (R, R)-(HO)(2)DENSPM, were much less effective at upregulation than the parent DENSPM. Finally, a comparison of the toxicity of (R, R)-(HO)(2)DENSPM with that of DENSPM at subchronic doses revealed that the neurological effects seen with DENSPM were now absent.

A Preorganized Siderophore: Thermodynamic and Structural Characterization of Alcaligin and Bisucaberin, Microbial Macrocyclic Dihydroxamate Chelating Agents(1)

The iron coordination chemistry of two macrocyclic dihydroxamate siderophores, alcaligin (AG) and bisucaberin (BR), has been investigated thermodynamically and structurally. Alcaligin is a siderophore of freshwater bacteria as well as mammalian pathogens, including the bacterium that causes whooping cough in humans, while bisucaberin, a structural analogue of alcaligin, is produced by marine bacteria. Both alcaligin and bisucaberin form 1:1 ferric complexes (FeL(+)) in acidic conditions and 2:3 ferric complexes (Fe(2)L(3)) at and above neutral pH. The stability constants of these macrocyclic dihydroxamate siderophores differ significantly from that of rhodotorulic acid (RA), a linear dihydroxamate siderophore. Notably, K(FeL) of alcaligin is 32 times greater than that of rhodotorulic acid, while the subsequent stepwise formation constant for Fe(2)L(3) is 3 times less. The Fe(III) complexes of alcaligin are stereospecific; the absolute configuration of the Fe(2)L(3) complex (circular dichroism and X-ray structure) is Lambda. The structure of the Fe(2)L(3) alcaligin complex is a topological alternative to the triple-helicate structure of the rhodotorulic complex Fe(2)(RA)(3). The structures of the free ligand and the bisbidentate ligand in the FeL complex are essentially identical, indicating that alcaligin is highly preorganized for metal ion binding. This explains the difference in K(FeL) between alcaligin and rhodotorulic acid, as well as explaining the monobridged topology of the Fe(2)L(3) alcaligin complex. The protonation constants (log K(a1) and log K(a2)) are 9.42(5) and 8.61(1) for alcaligin and 9.49(2) and 8.76(3) for bisucaberin. The stepwise formation constants of the Fe(III) complexes (log K(ML) and log K(M)()2(L)()3) are 23.5(2) and 17.7(2) for alcaligin and 23.5(5) and 17.2(5) for bisucaberin. The overall formation constants (log beta(230)) of alcaligin and bisucaberin are 64.7(1) and 64.3(1). The solution chemistry of Fe(III) and alcaligin was further investigated at a lower ligand to metal ratio (1:1). At high pH, a novel 2:2 ferric bis-&mgr;-oxo-bridged complex of alcalagin forms (Fe(2)L(2)O(2)(2)(-)) with a log beta(22)(-)(4) of 16.7(2). This species exhibits behavior consistent with an iron bis-&mgr;-oxo complex, including antiferromagnetic coupling. Crystal data: Fe(2)(AG)(3).25H(2)O crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a =13.3374(4) Å, b = 16.1879(5) Å, c = 37.886(1) Å, V = 8179.7(4), Z = 4. For 5512 reflections with F(o)(2) > 3sigma(F(o)(2)) the final R (R(w)) = 0.053(0.068).

The origin of the differences in (R)- and (S)-desmethyldesferrithiocin. Iron-clearing properties

The iron clearance properties, toxicity, and pharmacokinetics of (R)- and (S)-desmethyldesferrithiocin (DMDFT) are described. The studies were performed in rodent and primate models. While both enantiomers were found to be effective iron chelators with minimal toxicity in the rodents, only (S)-DMDFT was able to induce the clearance of any iron in the primates. In addition, two out of nine of the monkeys given (R)-DMDFT died within 24 h of drug administration. The reason for the differences in iron clearance properties and the apparent toxicity of the (R)-enantiomer in the primates is likely related to the disparities in the pharmacokinetics of the two analogues. The pharmacokinetic data suggest enantioselectivity in renal clearance of the desferrithiocins and their iron complexes with (S)-DMDFT clearance 3.5 times greater than that of (R)-DMDFT, and FeIII [(S)-DMDFT]2 clearance 6.8 times greater than that of FeIII [R-DMDFT]2. In all primates studied FeIII [(R)-DMDFT]2 in the plasma exceeded 25 mg/L (50 microM) for several hours and remained above 10 mg/L (20 microM) at 8 h while levels of FeIII [(S)-DMDFT]2 never exceeded 50 microM and were at or below the limits of detection 8 h post-injection.