DOI: 10.1002/ejoc.201800321 Communication

Amino Acid Chirality | Very Important Paper |

Discrimination of the Chirality of α-Amino Acids in ZnII

Complexes of DPA-Appended Binaphthyl ImineMukesh Eknath Shirbhate,[a] Raju Nandhakumar,*[b] Youngmee Kim,[a] Sung-Jin Kim,[a]

Seong Kyu Kim,*[c] and Kwan Mook Kim*[a]

Abstract: Binaphthol aldehydes with appended dipicolylaminemoieties, (R)-1, and (R)-2, were synthesized. The reaction of (R)-1 with an amino acid together with the ZnII ion produces amonomeric complex that discriminates the chirality of the

Introduction

Amino acids and their derivatives are involved in a wide varietyof biological processes and used as key intermediates in thesynthesis of pharmaceuticals.[1] Over 20 % of the drugs intro-duced into the market in the last decade include tailor-madeamino acid moieties in their chemical structures, and the effi-cacy of the drugs are significantly influenced by the chiralitydetermined by the amino acids.[2] Therefore, the recognition ofthe chirality of amino acids and the preparation of their opti-cally pure enantiomers have been important goals.[3,4] Chin etal. reported the chiral cobalt complex that successfully derace-mized the coordinated amino acids, where the strong metalbinding of the amino acids increased the acidity of thehydrogen on the asymmetric carbon.[5] Our group, and alsoChin's group, have developed a lot of binol aldehydes that bindamino acids through an imine bond and exhibit deracemizationactivity.[6,7]

In the last decade, Soloshonok et al. have extensively devel-oped NiII complexes of amino acids in the form of Schiff basesthat lead to deracemization and stereoselective alkylation ofthe coordinated amino acids under a suitable chiral environ-ment.[8–10] The complexes developed by Soloshonok et al.proved that activation of amino acids by both imine formationand metal binding is an efficient approach for versatile chiral

[a] Department of Chemistry and Nanosciences, Ewha Womans University,11-1 Daehyund-Dong, Seodaemun-Ku, Seoul 120-750, KoreaE-mail: kkmook@ewha.ac.krhttp://home.ewha.ac.kr/~kkmook/

[b] Department of Chemistry, Karunya University,Coimbatore 641 114, Tamil Nadu, IndiaE-mail: nandhakumar@karunya.eduhttp://sites.google.com/site/rajunandha/Home

[c] Department of Chemistry, Sungkyunkwan University,Suwon-Si, Gyeong Gi-Do, KoreaE-mail: skkim@skkku.eduhttp://chem.skku.edu/sub02/sub01_08.phpSupporting information and ORCID(s) from the author(s) for this article areavailable on the WWW under https://doi.org/10.1002/ejoc.201800321.

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bound amino acid by 1H NMR spectroscopy. The same reactionof (R)-2, which is different from (R)-1 by only one nitrogen atom,generates a dimeric complex that is highly enantiospecific forthe L-form of amino acids such as Phe and Trp.

recognition and stereoselective derivatization of the aminoacids.

Zn is an essential trace element, and the ZnII ion is the sec-ond most abundant transition metal ion in organisms after iron,appearing in a variety of metalloenzymes.[11] ZnII complexes areinert to redox reactions and suitable for NMR spectroscopicstudies owing to their diamagnetic properties.[12] The ligandsof ZnII complexes are relatively more labile than those of CoIII

and NiII complexes and can be recovered in milder conditions.Therefore, it is beneficial to utilize ZnII complexes in enantio-meric discrimination of amino acids. In this work, we have de-signed and synthesized novel binaphthol aldehydes (R)-1 and(R)-2 with appended dipicolylamine moieties. The aldehydefunctional group will react with an amino acid to form a Schiffbase and bind the ZnII ion together with the amine bases. Weexpected the discrimination of the chirality of the bound aminoacid due to the reinforced rigidity and asymmetric environmentof the metal complex.

Results and DiscussionAs shown in Scheme 1, (R)-1 was synthesized by a four-stepprotocol starting from (R)-3, which was obtained by the reduc-tion of the corresponding naphthol acid followed by the protec-tion with 2,2-dimethoxypropane as was reported in the litera-ture.[13] The reaction of m-xylylene dibromide with (R)-3 in thepresence of potassium carbonate gave benzyl bromide com-pound, (R)-4a. The substitution reaction of (R)-4a with dipicolyl-amine followed by an acid deprotection and manganese diox-ide oxidation afforded the desired compound (R)-1. Compound

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Scheme 1. Reagents and conditions for the synthesis of (R)-1 and (R)-2.

(R)-2 was synthesized using 2,6-dibromomethylpyridine insteadof m-xylylene dibromide by applying the same procedures.

As a representative experiment, (R)-1, Zn(ClO4)2, and thepotassium salt of racemic phenylalanine were reacted in meth-anol, and the reaction mixture was extracted with CH2Cl2 andwashed with water. TLC for the reaction mixture with an eluentof ethyl acetate/hexane indicated the formation of twoproducts, diastereomers. The two diastereomers, (R)-1-Zn–D-Phe and (R)-1-Zn–L-Phe, could be successfully separated usingcolumn chromatography. The each diastereomer could alsohave been obtained separately by the same procedures withoptically pure D- and L-phenylalanine.

Figures 1a and 1b show the 1H NMR spectra of the diastereo-mers, (R)-1-Zn–D-Phe and (R)-1-Zn–L-Phe, respectively, in the re-gion of the –CH2– protons. There exist five –CH2– groups ineach complex, and all of those exhibit AB type multiplet signals,which are normally observed in both organic and inorganiccompounds with rigid structures.[14] The difference between thetwo signals in one –CH2– group numbered as (5) reaches to1.6 ppm. The large splitting of AB type signals implies the highrigidity and asymmetry environment of the complex generatedby the coordination of ZnII ion to (R)-1 and the amino acid.

The optimum single crystal for a structural study was ob-tained with the ZnII complex of (R)-1 and L-serine. Figure 2shows the structure which was crystallized in the monoclinicP21 space group in a dichloromethane solution mixed with di-methylformamide.[15]

The coordination geometry around the ZnII ion is highlydistorted octahedral and is constructed by three N atoms fromthe dipyridylamine moiety, the imine N and carboxylate Oatoms from L-serine, and hydroxyl O from the binaphthol group.The Zn–N(2) bond length is 2.343(10) Å, where N(2) is the terti-ary N atom; this is longer than the other coordination bondswhose distances range from 2.015(8) Å for Zn–O(2) to 2.229(8) Åfor Zn–O(4). The large distortion of the coordination sphere

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Figure 1. The 1H NMR spectra showing the five AB type multiplets of benzylprotons in the complexes of (R)-1-Zn–L-Phe (a) and (R)-1-Zn–D-Phe (b). Eachsignal marked with a star (*) is due to the asymmetric hydrogen of thephenylalanine in each complex. AB type multiplet pairs were assigned by theanalysis of the 2D 1H NMR spectrum. The pairs numbered (4) and (5) aresignals due to the methylene groups of the pyridyl group.

from regular octahedron reflects on the crowded state of thebound ligand.

The 1H NMR spectra obtained with the complexes of (R)-1and ZnII with amino acids such as phenylalanine, tryptophan,serine, alanine, and valine all show a similar structure, thus allthose complexes are considered to have the same structuraltype (Supporting Information).

Well-resolved signals discriminating between the two dia-stereomers of the complex of Phe appear in the region 6.3–6.7 ppm as shown in Figure 3a. The integration ratio for thetwo signals obtained with the racemic form is nearly 1:1. Thusthe combination of (R)-1 and ZnII can be used as an efficientchiral shift reagent for the amino acid.[16] Figure 3b shows thesignals in the 1H NMR spectra for the other representative com-plexes of amino acids such as Ala, Val, Ser, and Trp in the race-mic forms. The differences of the signals between each dia-stereomeric couple for Phe, Val, and Trp are 0.18–0.20, while

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Figure 2. The crystal structure of (R)-1-Zn–L-Ser with atomic labelling schemes (a) and its chemical drawing (b). All hydrogen atoms were omitted for clarityin (a).

those for Ser and Ala are 0.11–0.12. The former are smaller thanthe latter, which is closely associated with the size of the alkylgroups in the amino acids, the cause for the steric hindrance.

Figure 3. Partial 1H NMR spectrum (in CDCl3) for (R)-1-Zn–rac-Phe, (R)-1-Zn–L-Phe, (R)-1-Zn–D-Phe (a) and (R)-1-Zn–rac-Ala, (R)-1-Zn–rac-Val, (R)-1-Zn–rac-Ser and (R)-1-Zn–rac-Trp (b). The samples were prepared by the extraction ofthe mixture of ZnII and (R)-1, and the corresponding amino acids were re-acted in methanol.

Phenylalanine did not exhibit significant stereoselectivity be-tween the L- and D-forms in the complexation with (R)-1 andZnII, whereas the L-form of valine, an amino acid with a bulkygroup, is included in the complex more than the D-form in anapproximate ratio of 2:1 (Figure 3b). Stereoselectivities of theamino acids tested increase in the order of Phe ≈ Ala > Ser >Trp > Val.

To estimate the structural difference between (R)-1-Zn–L-Alaand (R)-1-Zn–D-Ala, B3LYP/6-311G* level calculations were car-ried out, and the resultant optimized structures are shown inFigure 4. The methyl group of L-Ala in the complex appears tohave steric interaction with the nearby pyridine ring, while thatof the D-Ala complex with the neighboring imine proton. The

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consequent distortions of the coordination geometries of bothdiastereomers are slightly different from each other. The calcu-lated free energy of the D-isomer is higher than that of theL-isomer by 1.01 kcal/mol. Thus, the L-isomer is expected to bemore stable. Actually, a slight dominance of L-isomers is ob-served in the 1H NMR spectra of the ZnII complexes of Ser, Val,and Trp, as shown in Figure 3b.

Figure 4. Structures of (R)-1-Zn–L-Ala (left) and (R)-1-Zn–D-Ala (right) opti-mized at the B3LYP/6-311G* level calculations.

The reactions of (R)-2 with ZnII and amino acids gave remark-ably different results from those of (R)-1. Figure 5 shows 1HNMR spectra in CDCl3 for the crude products obtained from thereaction of (R)-2, ZnII and Phe in methanol. Broad signals wereshown from the reaction with D-Phe, thus the structure of theproduct cannot be rightly characterized (Figure 5c). On theother hand, well resolved 1H NMR signals were obtained fromthe reaction with L-Phe (Figure 5b), which were very similar tothose obtained with rac-Phe (Figure 5a). Figure 5 demonstratesthat (R)-2 and ZnII react exclusively with L-Phe even in the pres-ence of D-Phe. The enantiomerically pure L-Phe could have beenobtained from the hydrolysis of [(R)-2-Zn–L-Phe]2 in MeOH/HCl,thus this process may be developed for the isolation of enantio-mers of similar amino acids.

The study on the crystal grown in the solution of (R)-2, Zn,and L-Phe revealed the dimeric structure, [(R)-2-Zn–L-Phe]2, asshown in Figure 6.[17–19] The ZnII ion is coordinated by four N

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Figure 5. 1H NMR spectrum (in CDCl3) of (R)-2-Zn–rac-Phe (a), (R)-2-Zn–L-Phe (b), and (R)-2-Zn–D-Phe (c). The samples were prepared by the extraction of themixture of ZnII and (R)-2, and the corresponding amino acids were reacted in methanol.

atoms of the imine, the tertiary amine, and two pyridine groups,the carboxylate oxygen, and the hydroxyl oxygen. The bondlengths of Zn–N(2) and Zn–N(3) are 2.227(5) and 2.260(11) Å,respectively, where N(2) is the pyridyl N atom and N(3) is thetertiary amine N atom, and those of Zn–N(1) and Zn–N(4) are2.062(9) and 2.193(5) Å, where N(1) is the imine N atom andN(4) is the pyridyl N atom. The bond lengths of Zn–O(1) andZn–O(3) are 2.088(8) and 2.023(6) Å, where O(1) is the carboxyl-ate O atom and O(3) is the hydroxyl O atom. The coordinatedgeometry around ZnII ion is a less distorted octahedral com-pared to that of (R)-1-Zn–L-Ser.

The exclusive formation of dimeric ZnII complex of (R)-2 withL-Phe even in the presence of D-Phe can be explained by thesteric hindrance around the benzyl group of Phe. If any L-Pheis replaced by a D-Phe in the complex [(R)-2-Zn–L-Phe]2, thebenzyl ring of D-Phe approaches the binaphthyl ring side togenerate severe steric repulsion.

As the number of atoms in the dimeric complexes is toolarge to perform the density function theory calculation, theenergy difference between [(R)-2-Zn–L-Phe]2 and the assumedstructure, {(R)-2-Zn}2–L-Phe–D-Phe, could not be reliably deter-mined by a model calculation. When the calculation was carriedout at the semiempirical level using PM6 parametrized Hamilto-nian, the optimized structure of {(R)-2-Zn}2–L-Phe–D-Phe waslargely distorted from the initially starting state which was es-tablished based on the crystal coordinates of [(R)-2-Zn–L-Phe]2

due to a large repulsion between the benzyl ring and bi-naphthyl ring (Figure 7).

The reaction products of (R)-2 and ZnII with another bulkyamino acid, Trp, has shown the same features of 1H NMR spec-tra as like as those with phenylalanine. Thus, it can be safelysaid that the dimeric complex was formed with only L-Trp even

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Figure 6. Molecular structure of [(R)-2-Zn–L-Phe]2 with atomic labellingschemes. All hydrogen atoms were omitted for clarity. Symmetry operation:i (y, x, –z).

in the presence of D-form. In the reaction with Ala, Met, andSer, however, reactions of both L- and D-forms resulted in the

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Figure 7. Calculated molecular structure for the assumed complex [{(R)-2-Zn}2–L-Phe–D-Phe]. One L-Phe was replaced by D-Phe in the crystal structureshown in Figure 6.

products showing well-resolved 1H NMR spectra of similar struc-tures, indicating the formation of the same dimers. Analysis ofthe 1H NMR spectra obtained in the reaction of those aminoacids in racemic forms indicate the presence of the three dia-stereomeric species containing (L-, L-), (D-, D-), and (L-, D-) aminoacid pairs in the complex.

Conclusion

We synthesized binaphthol aldehydes with appended dipicolyl-amine moieties, (R)-1 and (R)-2, and studied their reactions withamino acids together with the ZnII ion. The crystal study on (R)-1-Zn–L-Ser revealed a monomeric structure. 1H NMR spectra ofthe complexes with D- and L-amino acids are well discriminatedfrom each other, and thus (R)-1 might be conveniently usedtogether with the ZnII ion as a chiral shift reagent for aminoacids. As for aldehyde (R)-2, a dimeric ZnII complex was pro-duced, which is highly enantiospecific for the L-form of aminoacids such as Phe and Trp. This system can be compared withSoloshonok's NiII complexes in the sense that the chiral Schiffbase of an amino acid binds a metal ion. It is worthy of furtherdevelopment, because the ZnII ion is a diamagnetic and envi-ronmentally safe transition metal.

AcknowledgmentsThis work was supported by a National Research Foundation ofKorea (NRF) grant funded by the Korea government (MSIP) (no.NRF-2017R1A2B2009672).

Keywords: Chirality · Amino acids · Zinc ·Enantioselectivity

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[14] T. M. Fletcher, J. A. Walmsley, J. Inorg. Biochem. 1997, 68, 239–249.[15] Crystallographic data for (R)-1-Zn–L-Ser {C44H36N4O5Zn·[0.5(CH3)2-

NCHO]·(0.5CH2Cl2)·(0.5H2O)}: A light pink plate, approximate dimensions0.010 mm × 0.080 mm × 0.200 mm, was used for the X-ray crystallo-graphic analysis. The frames were integrated with the Bruker SAINT soft-ware package using a narrow-frame algorithm. The integration of thedata using a monoclinic unit cell yielded a total of 57551 reflections toa maximum θ angle of 25.00°, of which 8589 were independent (Rint =0.1627). The final cell constants for the monoclinic space group P21 area = 12.8655(15) Å, b = 8.9081(11) Å, c = 21.613(3) Å, � = 95.7534(16)°,V = 2464.5(5) Å3. Data were corrected for absorption effects using themultiscan method SADABS. The final anisotropic full-matrix least-squaresrefinement on F2 with 572 variables converged at R1 = 8.47 % for theobserved data. The goodness-of-fit is 0.987.

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[17] Crystallographic data for [(R)-2-Zn–L-Phe]2 (C98H78N10O8Zn2): A pinkblock of approximate dimensions 0.10 × 0.30 × 0.30 mm was used forthe X-ray crystallographic analysis. The frames were integrated with theBruker SAINT software package using a narrow-frame algorithm. The in-

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tegration of the data using a monoclinic unit cell yielded a total of 73241reflections to a maximum θ angle of 28.375°, of which 11261 were inde-pendent (Rint = 0.0932). The final cell constants for the tetragonal spacegroup P43212 are a = b = 14.6370(10) Å, c = 42.771(3) Å, V =9163.4(14) Å3. Data were corrected for absorption effects using the mul-tiscan method (SADABS). The final anisotropic full-matrix least-squaresrefinement on F2 with 473 variables converged at R1 = 11.38 % for theobserved data. The goodness-of-fit is 1.038. CCDC 1549322 [for (R)-1-Zn–L-Ser] and 1556304 {for [(R)-2-Zn–L-Phe]2} contain the supplementary

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crystallographic data for this paper. These data can be obtained free ofcharge from The Cambridge Crystallographic Data Centre.

[18] Structure solution program of direct method: A. Altomare, G. Cascarano,C. Giacovazzo, A. Guagliardi, M. C. Burla, G. Polidori, M. Camalli, J. Appl.Crystallogr. 1994, 27, 435–436.

[19] Structure solution program for refinement: G. M. Sheldrick, Acta Crystal-logr., Sect. C 2015, 71, 3–8.

Received: February 23, 2018