Poor Enantioselectivity Research Articles

A Flavoprotein Monooxygenase that Catalyses a Baeyer–Villiger Reaction and Thioether Oxidation Using NADH as the Nicotinamide Cofactor

A gene from the marine bacterium Stenotrophomonas maltophilia encodes a 38.6 kDa FAD-containing flavoprotein (Uniprot B2FLR2) named S. maltophilia flavin-containing monooxygenase (SMFMO), which catalyses the oxidation of thioethers and also the regioselective Baeyer-Villiger oxidation of the model substrate bicyclo[3.2.0]hept-2-en-6-one. The enzyme was unusual in its ability to employ either NADH or NADPH as nicotinamide cofactor. The K(M) and k(cat) values for NADH were 23.7±9.1 μM and 0.029 s(-1) and 27.3±5.3 μM and 0.022 s(-1) for NADPH. However, k(cat) /K(M) value for the ketone substrate in the presence of 100 μM cofactor was 17 times greater for NADH than for NADPH. SMFMO catalysed the quantitative conversion of 5 mM ketone in the presence of substoichiometric concentrations of NADH with the formate dehydrogenase cofactor recycling system, to give the 2-oxa and 3-oxa lactone products of Baeyer-Villiger reaction in a ratio of 5:1, albeit with poor enantioselectivity. The conversion with NADPH was 15 %. SMFMO also catalysed the NADH-dependent transformation of prochiral aromatic thioethers, giving in the best case, 80 % ee for the transformation of p-chlorophenyl methyl sulfide to its R enantiomer. The structure of SMFMO reveals that the relaxation in cofactor specificity appears to be accomplished by the substitution of an arginine residue, responsible for recognition of the 2'-phosphate on the NADPH ribose in related NADPH-dependent FMOs, with a glutamine residue in SMFMO. SMFMO is thus representative of a separate class of single-component, flavoprotein monooxygenases that catalyse NADH-dependent oxidations from which possible sequences and strategies for developing NADH-dependent biocatalysts for asymmetric oxygenation reactions might be identified.

Easy Routes towards Chiral Lithium Binaphthylamido Catalysts for the Asymmetric Hydroamination of Amino‐1,3‐dienes and Aminoalkenes

AbstractThe preparation of chiral lithium salts of N,N′‐disubstituted binaphthyldiamines and their use as catalysts for asymmetric hydroamination/cyclisation of amino‐1,3‐dienes and aminoalkenes are reported. Several straightforward methods involving the combination of ligand with solutions of methyl‐ or [(trimethylsilyl)methyl]lithium (LiCH2TMS) by ex situ or in situ preparation have been investigated. The use of LiCH2TMS in an in situ procedure was revealed as theeasiest for carrying out reactions with reliable results by fine‐tuning the quantity of base. Screening of a variety of ligands led to the selection of binaphthyldiamines modified by benzyl, pyridyl or naphthyl groups for the cyclisation of conjugated aminodienes in pyrrolidine or piperidine with the highest stereo‐ and enantioselectivities described to date (up to 61 and 72 % ee, respectively). Primary and secondaryaminoalkenes are efficiently cyclised at room temperature, but with poor enantioselectivities.

ChemInform Abstract: Laboratory Evolution of Stereoselective Enzymes: A Prolific Source of Catalysts for Asymmetric Reactions

AbstractReview: 327 refs.

Highly Enantioselective Catalytic Benzoyloxylation of 3‐Aryloxindoles Using Chiral VAPOL Calcium Phosphate

Highly Enantioselective Catalytic Benzoyloxylation of 3‐Aryloxindoles Using Chiral VAPOL Calcium Phosphate

Evaluating the use of chiral anthracene templates to access pyroglutamic acids

An approach for the asymmetric synthesis of pyroglutamic acid derivatives is described based on an anthracene chiral auxiliary. The introduction of a furan ring as a masked carboxylic acid moiety proceeded with excellent levels of diastereo-selectivity, followed by conversion into a carboxylate ester. The ensuing retro-Diels–Alder procedure using flash vacuum pyrolysis (FVP) followed by reduction gave pyroglutamate esters in good yield but poor enantioselectivity, the latter of which was found to be dependant on the electronic nature of the N-protecting group.

Natural Diversity to Guide Focused Directed Evolution

Simultaneous multiple site-saturation mutagenesis was performed at four active-site positions of an esterase from Pseudomonas fluorescens to improve its ability to convert 3-phenylbutyric acid esters (3-PBA) in an enantioselective manner. Based on an appropriate codon choice derived from a structural alignment of 1751 sequences of alpha/beta-hydrolase fold enzymes, only those amino acids were considered for library creation that appeared frequently in structurally equivalent positions. Thus, the number of mutants to be screened could be substantially reduced while the number of functionally intact variants was increased. Whereas the wild-type esterase showed only marginal activity and poor enantioselectivity (E(true)=3.2) towards 3-PBA-ethyl ester, a significant number of hits with improved rates (up to 240-fold) and enantioselectivities (up to E(true)=80) were identified in these "smart" libraries.

Laboratory Evolution of Stereoselective Enzymes: A Prolific Source of Catalysts for Asymmetric Reactions

Asymmetric catalysis plays a key role in modern synthetic organic chemistry, with synthetic catalysts and enzymes being the two available options. During the latter part of the last century the use of enzymes in organic chemistry and biotechnology experienced a period of rapid growth. However, these biocatalysts have traditionally suffered from several limitations, including in many cases limited substrate scope, poor enantioselectivity, insufficient stability, and sometimes product inhibition. During the last 15 years, the genetic technique of directed evolution has been developed to such an extent that all of these long-standing problems can be addressed and solved. It is based on repeated cycles of gene mutagenesis, expression, and screening (or selection). This Review focuses on the directed evolution of enantioselective enzymes, which constitutes a fundamentally new approach to asymmetric catalysis. Emphasis is placed on the development of methods to make laboratory evolution faster and more efficient, thus providing chemists and biotechnologists with a rich and non-ending source of robust and selective catalysts for a variety of useful applications.

Synthesis and Catalytic Activity of Gold Chiral Nitrogen Acyclic Carbenes and Gold Hydrogen Bonded Heterocyclic Carbenes in Cyclopropanation of Vinyl Arenes and in Intramolecular Hydroalkoxylation of Allenes

Mononuclear and dinuclear chiral gold(I) carbene complexes with carbene ligands of the type HBHC (hydrogen bonded heterocyclic carbenes) and NAC (nitrogen acyclic carbenes) have been prepared by reaction of isocyanide gold(I) complexes and chiral amines or diamines. The reaction of [AuCl(CNPy-2)] (1) (Py = pyridyl) with the corresponding chiral primary amines afforded the chiral HBHC complexes (R)-[AuCl{C(NH(CHMePh))(NHPy-2)}] ((R)-2), and (S)-[AuCl{C(NH{CHMe(1-naphthyl)})(NHPy-2)}] ((S)-3), while the reaction of 2 equiv of 1 with diamines produced (S)-2,2'-bis[NH{C(AuCl)(NHPy-2)}](2)-binaphthyl ((S)-4), (1R,2R)-1,2-bis[NH{C(AuCl)(NHPy-2)}]-diphenylethane ((1R,2R)-5), and (1R,2R)-1,2-bis[NH{C(AuCl)(NHPy-2)}]-cyclohexane ((1R,2R)-6). On the other hand the addition of alkyl amines to (S)-2,2'-[NCAuCl](2)-binaphthyl ((S)-8) gave the chiral NAC complexes (S)-2,2'-bis[NH{C(AuCl)(NMe(2))}](2)-binaphthyl ((S)-9) and (S)-2,2'-bis[NH{C(AuCl)(N(i)Pr(2))}](2)-binaphthyl ((S)-10), while the addition to (S)-2,2'-[NCAuCl](2)-3,3'-Ph(2)-binaphthyl ((S)-12) yielded (S)-2,2'-bis[NH{C(AuCl)(NMe(2))}](2)-3,3'-Ph(2)-binaphthyl ((S)-13) and (S)-2,2'-bis[NH{C(AuCl)(NEt(2))}](2)-3,3'-Ph(2)-binaphthyl ((S)-14). All the complexes are active catalysts in the cyclopropanation of vinyl arenes and in the intramolecular hydroalkoxylation of allenes, providing good yields and modest or poor enantioselectivity. The results show that all these ligands are compatible with different functions and reaction conditions and are worth considering as alternative systems to NHCs or phosphines in gold catalyzed reactions.

Didymosphaeria igniaria: a new microorganism useful for the enantioselective reduction of aryl-aliphatic ketones

Didymosphaeria igniaria is a promising biocatalyst in asymmetric reductions of prochiral aromatic-aliphatic ketones such as acetonaphthones, acetophenones, and acetylpyridines. The organism converted the substrates mainly to (S)-alcohols. Excellent results in terms of conversion and enantioselectivity (100% yield, >99% ee) were obtained with acetonaphthones. In case of acetyl pyridines, the optical purity of the product depended on the position of the carbonyl group on the pyridine ring and followed the order 2-acetyl≫4-acetyl>3-acetyl-pyridine. Transformation of o-methoxy-acetophenone gave optically pure (S)-(-)-1-(2-methoxyphenyl)-ethanol in 95% yield. The transformation of para-methyl ketone gave (R)-alcohol (81% ee), whereas para-bromo ketone gave (S)-alcohol (98% ee). Monitoring of the biotransformation of these substrates over time led to the conclusion that for both substrates, non-selective carbonyl group reduction occurred in the first step, followed by selective oxidation of the (R)-isomer of p-bromo-phenylethanol and selective oxidation of the (S)-isomer of p-methyl-phenylethanol. D. igniaria exhibited poor enantioselectivity in the reduction of bicyclic aryl-aliphatic ketones such as 1- and 2-tetralones. Only (S)-5-methoxy-1-tetralol was obtained in optically pure (>99% ee) form.

Synthesis of function-oriented 2-phenyl-2H-chromene derivatives using L-pipecolinic acid and substituted guanidine organocatalysts.

Organocatalytic domino oxa-Michael/aldol reactions between salicylaldehyde with electron deficient olefins are presented. We screened guanidine, 1,1,3,3-tetramethylguanidine (TMG) and L-pipecolinic acid as organocatalysts for this transformation. 3-Substituted 2-phenyl-2H-chromene derivatives are synthesized with high yields and with poor enantioselectivity (5-17% ee) using L-pipecolinic acid while TMG works well with cinnamaldehyde without using co-catalyst. These 3-substituted-2-phenyl-2H-chromene derivatives are further derivatized to synthesize triazole and biotin-containing chromene derivatives, to facilitate purification of protein targets.

Enantioselective Hydrogenation in Water Over Chiral Modified Heterogeneous Catalyst Admixed With Organic Solvent

Hydrogenation of α,β-unsaturated acid over cinchonidine-modified Pd/C results in poor enantioselectivity in water, but the selectivity becomes much higher when Pd/C is admixed with a small amount of a hydrophobic water-insoluble organic solvent. With the admixed catalyst, the product is easily removed by separation of the aqueous phase under the hydrogen atmosphere and the remaining catalyst can be reused.

2,4-Dinitrophenol as an Effective Cocatalyst: Greatly Improving the Activities and Enantioselectivities of Primary Amine Organocatalysts for Asymmetric Aldol Reactions

Seven primary amine organocatalysts 1a-g were readily prepared from natural primary amino acids via two steps and then were used to catalyze the direct asymmetric aldol reaction, but they showed very poor enantioselectivities and activities. As an effective cocatalyst, 2,4-dinitrophenol (DNP) dramatically elevated the activities and enantioselectivities of these very inefficient primary amine organocatalysts. This remedial course to the very inefficient organocatalysts by selection and employment of the optimal cocatalyst was particularly cost-effective and environment-beneficial compared with de novo development of catalysts. The highest efficient organocatalytic system that was composed of 1f and DNP showed high enantioselectivities and good to high diastereoselectivities with a broad spectrum of seven ketones. The linear ketones and cyclopentanone got predominant syn products whereas cyclohexanone mainly gave anti products.

Helical Triskelion Monophosphites as Ligands in Asymmetric Catalysis

Members of a new family of chiral triskelion P ligands, namely helical C(3)-symmetric monophosphites P(OR)(3), have been prepared in two steps by monoacylation of (R)- or (S)-1,1'-binaphthyl-2,2'-diol (BINOL) or diphenol using a carboxylic acid chloride followed by PCl(3) phosphorylation. The most sterically hindered member of these monophosphites, derived from the compound accessible by monoacylation of BINOL using adamantane carboxylic acid chloride, has been characterized by X-ray crystallography and NMR spectroscopy as a single well-defined compound. It exists exclusively in the syn conformation, with a propeller-like (twisted) geometry resulting in helicity. Upon utilization of (R)- or (S)-BINOL in the two-step synthesis, the helicity proves to be P or M, respectively. When used as ligands in the Rh-catalyzed asymmetric hydrogenation of prochiral homoallylic alcohols, these bulky helical ligands lead to respectable enantioselectivities (79-98% ee). In contrast, the less sterically congested and more flexible BINOL-derived phenyl analogue exists in several conformeric forms, even in the crystal, and this leads to poor enantioselectivity in the model reactions (ee = 32%). For the purpose of structural comparison, the analogous monophosphites derived from diphenol were also prepared and characterized. These compounds, again in contrast to the BINOL-derived adamantyl derivatives, occur in several different conformeric states.

In search of enantioselective catalysts for the Henry reaction: are two metal centres better than one?

Catalysts for the asymmetric Henry reaction involving copper(II) complexes of the chiral Schiff bases N,N′-(1R,2R)-(−)-1,2-cyclohexylenebis(3-hydroxysalicylideneamine) (H21) and N,N′-(1R,2R)-(−)-1,2-cyclohexylenebis(3-ethoxysalicylideneamine) (H22), and H23, which is the reduced analogue of H21, have been studied. Whereas [Cu(1)] and [Cu(2)] give poor yields and enantioselectivity, [Cu(3)] produced moderate to high yields and enantioselectivities were optimal when reactions were carried out in toluene rather than a polar solvent. A significant finding is that both yield and enantioselectivity are enhanced when a second equivalent of Cu(OAc)2 is added to the catalyst. The single-crystal structures of [Cu(3)] and [Cu(1)(H2O)] are presented, and the host–guest interactions and molecular packing in the latter are compared with those in [Cu(2)(H2O)].

HP‐NMR Study of the Pd‐Catalyzed Methoxycarbonylation of Styrene Using Monodentate and Bidentate Phosphane‐Modified Systems

AbstractCatalytically active palladium systems bearing the monodentate phosphetane ligand 1 and diphosphane 2 were studied by multinuclear NMR and HP‐NMR spectroscopy under methoxycarbonylation conditions. The reaction of the complex [PdCl2(1)2] (1a) with pTsOH yields the dimeric complex [PdCl(μ‐Cl)(1)]2 (1b), which was characterized by multinuclear NMR spectroscopy and X‐ray crystallography. The isolated complex 1b was tested as a precursor in the methoxycarbonylation of styrene, providing poor activity and enantioselectivity. Evidences for the protonation of ligand 1 under catalytic conditions were obtained. When complex [Pd(OTs)(OH2)(R,R‐bdpp)][OTs] (2b) was used as the catalyst precursor evidences for both hydride and methoxycarbonylation mechanisms were achieved. Two isomeric forms of [Pd(π‐methylbenzyl)(R,R‐bdpp)], 2h and 2i, were characterized by multinuclear NMR spectroscopy and DFT calculations. The results obtained by DFT calculations showed that the two most stable isomers contain the ligand bdpp in the “chair” conformation and that both isomers have very similar energies. The hydrido‐carbonyl‐bridged complex [Pd2(μ‐H)(μ‐CO)(R,R‐bdpp)2][OTs] (2e) was also detected under catalytic conditions. Deuterium labeling experiments using [D4]MeOH and GC‐MS analysis of the unconverted substrate and products were performed and the results indicate that the styrene insertion step into the Pd–H bond of the catalyst is irreversible when the phosphetane containing system is used, whereas this step is reversible when the diphosphane precursor is used as the incorporation of deuterium was only observed in the latter case.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Enantioselective nucleophilic catalysis: the synthesis of aza-beta-lactams through [2+2] cycloadditions of ketenes with azo compounds.

Although aza-β-lactams have attracted interest due to their biological activity[1] and to their utility as intermediates in organic chemistry (e.g., for the generation of α-amino acids[2] and of hydantoins[3]),[4] only limited progress has been described with regard to the enantioselective synthesis of this family of heterocycles.[5] One attractive, convergent approach to the formation of aza-β-lactams is the [2+2] cycloaddition of a ketene with an azo compound [Eq. (1)].[6] To the best of our knowledge, no stereoselective variants of this process have yet been reported. (1) We have been exploring the use of chiral derivatives of PPY (PPY = 4-(pyrrolidino)pyridine; e.g., 1 and 2) as enantioselective catalysts for an array of transformations,[7] including couplings of ketenes with imines[8] and with aldehydes.[9,10] Although there are no reports of nucleophilic catalysis of [2+2] cycloadditions of ketenes with azo compounds, we were intrigued by the possibility that our planar-chiral pyridines might be effective in this role. In this investigation, we establish that PPY derivative 1 can achieve the first catalytic asymmetric synthesis of aza-β-lactams, via [2+2] cycloadditions of ketenes with azo compounds [Eq. (2)]. (2) As part of our initial study, we examined the cycloaddition of phenyl ethyl ketene with dimethyl azodicarboxylate (1.0 equiv). We were pleased to determine that planar-chiral PPY derivative 1 serves as an effective catalyst for the desired coupling, generating the aza-β-lactam in good ee and yield (Table 1, entry 1; in the absence of a catalyst, there is no reaction: entry 2). Under the same conditions, a related catalyst (2), as well as a variety of chiral phosphines and cinchona alkaloids, provide poor enantioselectivity or little of the cycloaddition product (e.g., entries 3–5).[11,12] The substituents of the azo compound have a significant impact on the ee and the yield, with the methoxycarbonyl group affording the best results (entry 1 vs. entries 6–9). If ClCH2CH2Cl, rather than CH2Cl2, is employed as the solvent, then formation of the aza-β-lactam proceeds less efficiently (entry 1 vs. entry 10). The reaction temperature of choice appears to be −20 °C (entry 1 vs. entries 11–12).[13] Table 1 Nucleophile-catalyzed enantioselective synthesis of aza-β-lactams: Effect of reaction parameters. The optimized conditions can be applied to the enantioselective synthesis of aza-β-lactams from a variety of ketenes (Table 2). If the alkyl group is small (Me or primary), the desired heterocycle is generally produced with good, not excellent, enantioselectivity (~85% ee; entries 1–7). Fortunately, the ee of the aza-β-lactams is readily enhanced by recrystallization (e.g., the product generated from phenyl ethyl ketene can be obtained in >99% ee after a single crystallization; see entry 2 of Table 2). In the case of ketenes that bear a secondary alkyl group, catalyst 1 typically furnishes the aza-β-lactam with very good enantioselectivity and yield (>90% ee; entries 8–13).[14] Table 2 Nucleophile-catalyzed enantioselective synthesis of aza-β-lactams (for the reaction conditions, see [Eq. (2)]). A plausible mechanism for this new nucleophile-catalyzed method for the synthesis of aza-β-lactams is illustrated in Figure 1. Interestingly, the configuration at the quaternary stereocenter is different from that produced in Staudinger reactions that are catalyzed by PPY derivative 1 [Eq. (3)],[8b] which are believed to proceed through a similar pathway.[15] Figure 1 Possible mechanism for the nucleophile-catalyzed synthesis of aza-β-lactams. (3) In conclusion, we have developed a new process, the nucleophile-catalyzed [2+2] cycloaddition of ketenes with azo compounds to generate aza-β-lactams. In addition, we have established that planar-chiral PPY derivative 1 achieves this convergent transformation with good enantioselectivity, thereby providing the first catalytic asymmetric synthesis of this useful family of heterocycles.

Fluorescence properties of ( R) - and ( S)-[1,1′-binaphthalene]-2,2′-diols solutions and their complexes with cyclodextrins in aqueous medium

Steady-state and time-resolved fluorescence techniques were used to study ( R)- and ( S)-[1,1′-binaphthalene]-2,2′-diol (1,1′-binaphthol or BINOL) dilute solutions of different polarity solvents, as well as their inclusion complexes with α- and βcyclodextrins (CDs) in water. BINOLs in dilute water solutions exhibited a surprisingly high fluorescence anisotropy that was explained as being due to the formation of fairly large order π–π stacking aggregates in aqueous polar media. Stoichiometries, formation constants and the changes of enthalpy and entropy upon inclusion were also obtained by measuring the variation of the fluorescence intensity with [CD] and temperature. Results agree with the formation of 1:1 stoichiometry complexes, but the association constants are rather low and very similar for both enantiomers. Molecular mechanic calculations in the presence of water were employed to study the formation of BINOL complexes with both α- and βCDs. For the most stable structures of any of the complexes only a small portion of the guests, in agreement with thermodynamics parameters and quenching experiments, penetrates inside the CD cavities. Driving forces for 1:1 inclusion processes may be dominated by non-bonded van der Waals host:guest interactions. The low guest:host binding constants and poor enantioselectivity of α- and βCDs for BINOLS may be a consequence of the BINOL aggregation in water.

A Study on Chiral Organocalcium Complexes: Attempts in Enantioselective Catalytic Hydrosilylation and Intramolecular Hydroamination of Alkenes

The chiral β -diketimine ligand [(S)-Ph(Me)CH-N=C(Me)]CH2 was prepared by condensation of acetylacetone with the commercially available chiral building block (S)-Ph(Me)CH-NH2. Reaction of bis(o-Me2N-α-Me3Si-benzyl)calcium with this β -diketimine led to double deprotonation. Reaction of bis(o-Me2N-α-Me3Si-benzyl)calcium with the commercially available chiral bis-oxazoline (S)-Ph-BOX gave diastereopure [(S)-Ph-BOX](o-Me2N-α-Me3Si-benzyl)calcium which in solution slowly decomposed with formation of o-Me2N-α-Me3Si-toluene. The corresponding amide complex [(S)-Ph-BOX]CaN(SiMe3)2·(THF)2 is stable and the crystal structure has been determined. In solution, this heteroleptic amide is in Schlenk equilibrium with the homoleptic species [(S)-Ph-BOX]2Ca and Ca[N(SiMe3)2]2·(THF)2. This Schlenk equilibrium can be steered to the heteroleptic side. Use of the enantiopure calcium amide catalyst for the hydrosilylation of styrene with PhSiH3 or in the intramolecular hydroamination of aminoalkenes gave good product yields, but only small ee-values were observed (5 - 10%). From stoichiometric reactions of the catalyst with the substrates it is concluded that the “true” catalytically active species is mainly present as a homoleptic calcium complex, which explains the poor enantioselectivities

Synthesis and Structural Variation of Iron, Rhodium, Palladium, and Silver Complexes of a Chiral N-Heterocyclic Carbene−Phenoxyimine Hybrid Ligand

A new class of chiral N-heterocyclic carbene (NHC) precursors has been prepared incorporating a phenoxyimine moiety. Reaction between 3-((1R,2R)-2-{[1-(3,5-di-tert-butyl-2-hydroxyphenyl)meth-(E)-ylidene]amino}cyclohexyl)-1-isopropyl-4-phenyl-3H-imidazol-1-ium bromide [C(H)NO(H))][Br] (2a) and Pd(OAc)2 gives the complex [Pd(κ3-CNO)Br] (3), which exhibits tridentate ligand coordination via double deprotonation of 2a. The NHC ligand transfer agents [Ag(κ1-CNO(H))X] (X = Br (5a), Cl (5b)) are prepared from 2a, chloride analogue 2b, and Ag2O. Reaction between 5b and [Rh(COD)Cl]2 gives the NHC complex [Rh(κ1-CNO(H))Cl] (6) and, on chloride abstraction, the bidentate NHC-imine-coordinated complex [Rh(κ2-CNO(H))][B{3,5-(CF3)2C6H3}4] (7). For complexes 5 and 6 coordination of the phenoxyimine and deprotonation of the phenoxy moiety appear to be modified by intramolecular hydrogen bonding between hydroxyl and imine nitrogen groups. Reaction between 5a and Fe{N(SiMe3)2} gives an unusual bimetallic complex, [Fe(κ2-C(AgBr)NO)2] (8), that exhibits a 1-D polymeric chain motif in the solid state. Preliminary catalytic properties of an acetonitrile adduct of 3, [Pd(κ3-CNO)MeCN][BF4] (4), and 7 are reported for hydroamination and phenylation of enones, respectively, that show good activity but poor enantioselectivity.

Asymmetric Cyclopropanation of Styrenes Catalyzed by Metal Complexes of D2‐Symmetrical Chiral Porphyrin: Superiority of Cobalt over Iron.

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