James S. McManis

Total synthesis and structure revision of petrobactin

Abstract The total synthesis and the revised structural assignment of petrobactin, a siderophore isolated from the marine bacterium Marinobacter hydrocarbonoclasticus, is reported. The key step in the synthesis involved condensation of N1-(2,3-dibenzoyloxybenzoyl)-N4-benzylspermidine with 1,3-di-(p-nitrophenyl)-2-tert-butyl citrate. Proton NMR spectra of the synthesized product compared with those reported for the natural product revealed that the compound did not contain 2,3-dihydroxybenzoyl moieties as published; instead, the splitting pattern suggested 3,4-dihydroxybenzoyl fragments. The 3,4-dihydroxybenzoyl analogue was accessed via a similar route; the proton and carbon-13 NMR spectra of this compound were consistent with those reported for natural petrobactin.

Design, Synthesis, and Testing of Non-Nephrotoxic Desazadesferrithiocin Polyether Analogues

A series of iron-clearing efficiencies (ICEs), ferrokinetics, and toxicity studies for ( S)-2-(2,4-dihydroxyphenyl)-4,5-dihydro-4-methyl-4-thiazolecarboxylic acid (deferitrin, 1), ( S)-4,5-dihydro-2-[2-hydroxy-4-(3,6,9-trioxadecyloxy)phenyl]-4-methyl-4-thiazolecarboxylic acid ( 2), and (S)-4,5-dihydro-2-[2-hydroxy-3-(3,6,9-trioxadecyloxy)phenyl]-4-methyl-4-thiazolecarboxylic acid ( 3) are reported. The ICEs in rodents are shown to be dose-dependent and saturable for ligands 2 and 3 and superior to 1. Both polyether analogues in subcutaneous (sc) versus oral (po) administration in rodents and primates demonstrated excellent bioavailability. Finally, in a series of toxicity studies of ligands 1- 3, the dosing regimen was shown to have a profound effect in animals treated with ligand 1. When ligand 1 was given at doses of 237 micromol/kg/day twice a day (b.i.d.), there was serious proximal tubule damage versus 474 micromol/kg/day once daily (s.i.d.). With 2 and 3, in iron-overloaded and/or non-iron-loaded rodents, kidney histopathologies remained normal.

The total synthesis of bisucaberin

Abstract The first total synthesis of 6,17-dihydroxy-1,6,12,17-tetraazacyclodocosane-2,5,13,16-tetrone (bisucaberin) is presented. The synthetic scheme employed in this study illustrates the utility of O-benzyl-N-( tert -butoxycarbonyl)-N-(4-cyanobutyl)hydroxylamine 3 , as an intermediate in the synthesis of both bisucaberin and desferrioxamine B. The bisucaberin macrocyclic lactam precursor, a linear ω-amino acid 10 , is constructed from the intermediates 3 and O-benzyl-N-(4-cyanobutyl)hydroxylamine 4 , utilizing a series of sequential acylations and nitrile reductions. Cyclization of 10 generates the 22 membered ring, 6,17-dibenzylbisucaberin 11 . Deprotection of the hydroxamates in the last step affords the natural product, bisucaberin 1 .

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.

Total synthesis of vibriobactin

The first synthesis of the natural product N-[3-(2,3-dihydroxy-benzamido) propyl]-1,3-bis[2-(2,3-dihydroxyphenyl)-trans-5-methyl-2-oxazoline-4-carboxamido] propane (vibriobactin) is described. The synthesis illustrates the problems and solutions involved in the asymmetric functionalization of norspermidine, a triamine which consists of a symmetrical methylene backbone. Finally, the synthesis provides an independent and definitive proof of Neilands suggested structure of vibriobactin.

The total synthesis of desferrioxamines E and G

Abstract The total syntheses of the hexacoordinate amino acid, 32-amino-5,16,27-trihydroxy-4,12,15, 23, 26-pentaoxo-5,11,16,22,27-pentaazadotriacontanoic acid (desferrioxamine G) and the corresponding macrocyclic lactam 1,12,23-trihydroxy-1,6,12,17,23,28-hexaazacyclotritriacontane-2, 5,13,16,24,27-hexone (desferrioxamine E, nocardamine) are reported. The synthetic route utilized here is predicated on the efficient formation and selective transformations of O-benzyl-N-(tert-butoxy- carbonyl)-N-(4-cyanobutyl)hydroxylamine 4 , a key reagent in our previous syntheses of bisucaberin and desferrioxamine B. The O-benzyl protected trihydroxamate nitrile acid 9 , which is constructed from 4 by a series of selective deprotections and regiospecific acylations, is hydrogenated under mild conditions (Pd, dilute HCl) to give desferrioxamine G directly. Reduction of the nitrile group of 9 leads to amino acid 10 , which is cyclized to generate the 33 membered ring, 1,12,23-tribenzylnocardamine 11 . Unmasking the hydroxamates in the final step affords the natural product, nocardamine. Synthetic methodology is now in place for accessing all of the natural product hydroxamate siderophores isolated from Streptomyces pilosis.

Substituent effects on desferrithiocin and desferrithiocin analogue iron-clearing and toxicity profiles.

Desferrithiocin (DFT, 1) is a very efficient iron chelator when given orally. However, it is severely nephrotoxic. Structure-activity studies with 1 demonstrated that removal of the aromatic nitrogen to provide desazadesferrithiocin (DADFT, 2) and introduction of either a hydroxyl group or a polyether fragment onto the aromatic ring resulted in orally active iron chelators that were much less toxic than 1. The purpose of the current study was to determine if a comparable reduction in renal toxicity could be achieved by performing the same structural manipulations on 1 itself. Accordingly, three DFT analogues were synthesized. The iron-clearing efficiency and ferrokinetics were evaluated in rats and primates; toxicity assessments were carried out in rodents. The resulting DFT ligands demonstrated a reduction in toxicity that was equivalent to that of the DADFT analogues and presented with excellent iron-clearing properties.

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.

Total synthesis of rhizoferrin, an iron chelator

Abstract The absolute configuration of rhizoferrin, a novel siderophore isolated from the fungus Rhizopus microsporus var. rhizopodiformis, was determined via its total synthesis. The principal steps in the synthesis of this naturally occurring iron chelator involve a simple coupling of key intermediate (R)-1,2-dimethyl citrate with N1,N4-dibenzyl-1,4-diaminobutane followed by ester hydrolysis and debenzylation. This is the first example of a total synthesis of a citrate-based siderophore from a chiral citric acid fragment, the structure of which was confirmed by single crystal X-ray diffraction.

(R)- and (S)-N-(Benzyloxycarbonyl)-3,4-epoxybutylamine. New 4-amino-2-hydroxybutyl synthons for the synthesis of hypusine reagents and (R)-6- and (S)-7-hydroxyspermidine

Abstract The synthesis and application of the chiral reagents (R)- and (S)-N-(benzyloxycarbonyl)-3,4-epoxybutylamine is described for the first time. These 4-amino-2-hydroxybutyl synthons are successfully employed in the assembly of two hydroxylated triamines, (R)-6- and (S)-7-hydroxyspermidine, and a previously described hypusine reagent, (2S,9R)-11-[(benzyloxycarbonyl)amino]-7-(benzyloxycarbonyl)-2-[(9-fluorenylmethoxycarbonyl)amino]-9-(tetrahydropyran-2-yloxy)-7-azaundecanoic acid, useful for solution- and solid-phase peptide synthesis.