CHIRALITY (2014)

Preparation and Enantioselectivity Binding Studies of a New Chiral Cobalt(II)porphyrin-Tröger’s Base Conjugate AMENEH TATAR,1 MARTIN VALÍK,1 JANA NOVOTNÁ,1,2 MARTIN HAVLÍK,1 BOHUMIL DOLENSKÝ,1* VLADIMÍR KRÁL,1 2 AND MARIE URBANOVÁ 1 Department of Analytical Chemistry, Institute of Chemical Technology Prague, Praha, Czech Republic 2 Department of Physics and Measurements, Institute of Chemical Technology Prague, Praha, Czech Republic

ABSTRACT A new bis[cobalt(II)porphyrin]-Tröger’s base conjugate was studied as a potential receptor for methyl esters of several amino acids. The conjugate was prepared as racemate, and then resolved via preparative high-performance liquid chromatography (HPLC) on a chiral column. The high affinity to lysine, histidine, and proline methyl esters was found by complexation studies followed by UV-Vis spectroscopy. The studies of pure enantiomers, followed by UV-Vis and electronic circular dichroism spectroscopy, revealed the highest enantioselectivity for lysine methyl ester. Chirality 00:000–000, 2014. © 2014 Wiley Periodicals, Inc. KEY WORDS: Tröger’s base conjugate; enantioselectivity; chiral receptors INTRODUCTION

The design and preparation of chiral receptors is one of the important issues of the host-guest supramolecular chemistry. One of the perspective concepts is based on derivatives of Tröger’s base (TB), which are attractive for the chiral concave shape of their molecule.1–4 Those receptors act as bidentate, since contain two binding units due to C2 symmetry of the TB skeleton. As a different family of efficient bidentate receptors are regularly reported, bis(metalloporphyrin) derivatives, which utilize metalloporphyrins for their high binding abilities and high absorbance in the visible region.5–16 Thus, it is expected that metalloporphyrin-Tröger’s base conjugates would be effective enantioselective receptors, since they join the best properties from both structural motives. The first reported metalloporphyrin-Tröger’s base conjugate 1 (Fig. 1), having metalloporphyrin units as a constituent of the rigid TB skeleton, has very promising properties.17–19 The proximity of metalloporphyrin units within TB 1 leads to the highest affinity for 1,2-diaminoethane (K = 1.9 · 108 M 1), and decreasing affinity for higher α,ω-diaminoalkanes.17 The optically pure enantiomers of TB 1 exhibited enantioselective binding of amino acids (AA) derivatives, histidine, and lysine esters.18,19 The (+)-1 enantiomer prefers to bind L-His-OCH3, L-His-OCH2Ph, and L-Lys-OCH2Ph, than their D-forms by factors of (a ratio of binding constants) 13.3, 9.2, and 2.8, respectively. The efforts to prepare new metalloporphyrin-TB derivatives and to study their binding ability are continuing.20,21 Here we present preparation and binding studies of a new cobalt(II) porphyrin-TB conjugate 2 (Fig. 1). In contrast to TB 1, the metalloporphyrin units of TB 2 are flexibly attached to the TB skeleton, thus their conformational freedom is much higher. Our results demonstrate an influence of the flexibility on the binding affinity and selectivity, and uncover a potential role of the TB compartment. MATERIALS AND METHODS The nuclear magnetic resonance (NMR) spectra were recorded on 1 13 Mercury Plus 300 (Varian, Palo Alto, CA). The H and C NMR spectra 19 are referenced to tetramethylsilane; the F NMR spectra are referenced to trichlorofluoromethane. The chemical shifts (δ) of signals are given in ppm; their relative integral intensity, multiplicity, and absolute values of © 2014 Wiley Periodicals, Inc.

homonuclear (if not stated otherwise) coupling constants (J in Hz) follow in parentheses, respectively. The electronic circular dichroism (ECD) spectra of (+)-2 and ( )-2 were measured using spectrometer J-810 (Jasco, Japan) in the spectral regions of 200 to 350 nm (cuvette 0.5 cm) and 350 to 550 nm (cuvette 0.1 cm). The 0.1 cm cuvette was used for complexation studies in the spectral range 220 to 550 nm. The final spectra were accumulated 10 times at scanning speed 100 nm/min, data pitch 0.1 nm, bandwidth 1 nm, standard sensitivity 100 mdeg, and resolution 0.5 nm. The spectrum of pure solvent was used for the baseline correction. Savitzky-Golay method was used for smoothing of final spectra. The UV-vis spectra were measured using Cary 400 (Varian) spectrophotometer in the spectra regions of 200–900 nm (cuvette 1 cm). Compounds were measured in pure dichloromethane. The spectrum of pure solvent was used for the baseline correction.

Preparation of Dihydroxy TB 4 and TB 5 a) BBr3 (1.41 ml, 15.2 mmol) was slowly added to the solution of dimethoxy TB 3 (0.85 g, 3.0 mmol) in dry dichloromethane (30 ml) at room temperature. The reaction mixture was stirred for 2 days. The mixture was diluted with water (50 ml), and white solid dihydroxy TB 4 1 (0.61 g, 80 %) was filtered off and dried in vacuum. H NMR (DMSO-d6): 9.71 (2H, br s), 7.24 (2H, d, 7.2), 6.78 (2H, d, 8.2), 6.53 (2H, s), 4.84 (4H, 13 m), 4.26 (2H, d, 16.5). C NMR (DMSO-d6): 156.30, 131.08, 125.41, 115.78, 112.84, 66.61, 56.62. Elemental analysis calcd for C15H14N2O2: 70.85 %C, 5.55 %H, 11.02 %N, found: 70.59 %C, 5.71 %H, 10.99 %N. b) A solution of 3 (1.06 g, 3.55 mmol) in mixture of a concentration of HCl (10 ml) and acetic acid (10 ml) was heated at 110°C for 40 h under nitrogen atmosphere. The reaction mixture was cooled to room temperature, diluted with water (20 ml), and alkalized with ammonia solution to pH ~12. The formed precipitate was filtered off, washed with water (2 × 10 ml) and methanol (2 × 5 ml), and dried in vacuum. The starting TB 3 (0.38 g, 36%) and TB 5 (0.30 g, 30%) were obtained from the solid residue by column chromatography on silica (dichloromethane/acetone 1 from 1:1 to 0:1). H NMR (DMSO-d6): 9.02 (1H, s), 6.98 (1H, d, 8.8), 6.89 (1H, d, 8.8), 6.71 (1H, dd, 8.8, 2.7), 6.55 (1H, dd, 8.8, 2.7), 6.50 (1H, Contract grant sponsor: BIOMEDREG; Contract grant number: CZ.1.05/2.1.00/ 01.0030. Contract grant sponsor: PROPMEDCHEM; Contract grant number: CZ.1.07/ 2.3.00/30.0060. *Correspondence to: Bohumil Dolenský, Department of Analytical Chemistry, Institute of Chemical Technology Prague, Technická 5, 166 28, Praha 6, Czech Republic. E-mail: [email protected] Received for publication 25 November 2013; Accepted 11 March 2014 DOI: 10.1002/chir.22327 Published online in Wiley Online Library (wileyonlinelibrary.com).

TATAR ET AL.

due to overlaps and too low intensity coursed by very high multiplicity owing to the number of fluorine atoms. 1 Compound 9: H NMR (CDCl3): 8.98 (8H, d, 6.1), 8.92 (16H, m), 7.36 (4H, d, 8.8), 7.22 (4H, dd, 8.8, 2.7), 6.96 (4H, d, 2.7), 4.89 (4H, d, 16.8), 19 4.42 (4H, s), 4.36 (4H, d, 16.9), 2.90 (2H, s), 2.91 (4H, s). F NMR (CDCl3): 136.9 (16F, m), 137.6 (8F, m), 151.7 (6F, t, 21), -151.7 + (2F, t, 21), 154.1 (8F, m), 161.8 (16F, m). HRMS (OrbiTrap, ESI ): + for C162H55F56N16O2 [M+H+1] calcd 3352.3726, found 3352.3811; for 2+ C162H55F56N16O2 [M+2H+1] calcd 1676.6899, found 1676.6916.

Preparation of Porphyrin TB 2 Compound 7 (100 mg, 0.046 mmol), anhydrous CoCl2 (400 mg, 3.08 mmol), triethylamine (0.4 ml, 2.8 mmol) were dissolved in dioxane (40 ml). The mixture was treated at 70°C for 2 d. The volatile part was evaporated and the residue was purified by the column chromatography on silica (acetone/petroleum ether 1:4) to give pure porphyrin TB 2 + + (92 mg, 88%). HRMS (OrbiTrap, ESI ): for C103H29F38N10O2Co2 [M+H+1] 2+ calcd 2277.0482, found 2277.0490; for C103H29F38N10O2Co2 [M+H+1] calcd 1138.5238, found 1138.5240.

Enantioseparation

Fig. 1. Rigid and flexible metalloporphyrin-Tröger’s base conjugates.

d, 2.7), 6.29 (1H, d, 2.7), 4.50 (1H, d, 16.5), 4.47 (1H, d, 16.5), 4.12 (2H, s), 3.95 13 (1H, d, 16.5), 3.91 (1H, d, 16.5), 3.63 (3H, s). C APT NMR (DMSO-d6): 153.29, 141.12, 139.51, 129.00, 128.70, 125.65 (CH), 125.56 (CH), 114.47 (CH), 113.68 (CH), 112.26 (CH), 110.69 (CH), 66.70 (CH2), 58.40 (CH2), 58.30 (CH2), 55.10 (CH3). Elemental analysis calcd for C16H16N2O2: 71.62 % C, 6.01 %H, 10.44 %N, found: 71.53 %C, 6.21 %H, 10.21 %N.

Preparation of Free Base Porphyrin TB Derivatives Sodium hydride (34 mg, 1.26 mmol) and dihydroxy TB 4 (120 mg, 0.46 mmol) was dissolved in DMF (5 ml) and stirred at room temperature for 4 h. The obtained suspension was added to the solution of porphyrin 6 (1.85 g, 1.90 mmol) in DMF (2 ml), and stirred at 60°C for 4 d. The volatile part was evaporated in vacuum and the residue was separated by column chromatography on silica to provide the starting porphyrin 6 (1.30 g, 70%) by dichloromethane with petroleum ether 9:1, compound 7 (191 mg, 18%), and compound 9 (35 mg, 5%) by dichloromethane with diethyl ether 95:5, and compound 8 (40 mg, 7%) by dichloromethane with diethyl ether 8:2. 1 Compound 7: H NMR (CDCl3): 8.95 (16H, m), 7.38 (2H, d, 8.8), 7.23 (2H, dd, 8.8, 2.7), 6.96 (2H, d, 2.7), 4.91 (2H, d, 17.0), 4.47 (2H, s), 4.37 13 (2H, d, 17.0), 2.91 (4H, br s). C NMR (CDCl3): 146.55 (br d, JCF = 262), 137.58 (br d, JCF = 255), 126.80, 115.86, 115.56, 114.19, 104.31, 103.58, 13 58.78, 58.49, other C signals were not observed due to overlaps and too low intensity coursed by very high multiplicity owing to the number 19 of fluorine atoms. F NMR (CDCl3): 137.0 (12F, m), 137.7 (4F, m), 151.8 (6F, t, 21), 154.3 (4F, m), 161.9 (12F, t, 23). HRMS (OrbiTrap, + + ESI ): for C103H33F38N10O2 [M+H+1] calcd 2164.2209, found 2164.2212. 1 Intermediate 8: H NMR (CDCl3): 8.98 (2H, d, 4.8), 8.93 (6H, m), 7.31 (1H, d, 8.8), 7.19 (1H, dd, 8.8, 2.7), 7.12 (1H, d, 8.7), 6.92 (1H, d, 2.7), 6.73 (1H, dd, 8.7, 2.7), 6.46 (1H, d, 2.7), 4.82 (1H, d, 16.9), 4.73 (1H, d, 16.9), 4.39 (2H, s), 4.26 (1H, d, 16.9), 4.18 (1H, d, 16.9), 2.87 (2H, s), the signal 19 of phenolic OH is too broad to be detected. F NMR (CDCl3): -136.9 (6F, m), 137.6 (2F, m), 151.7 (3F, t, 21), 154.1 (2F, m), 161.8 13 (6F, m). C NMR (CDCl3): 153.55, 152.64, 146.55 (d, JCF = 248), 144.02, 142.30 (dm, JCF = 259), 141.53 (dm, JCF = 251), 140.08, 137.48 (d, JCF = 256), 135.24 (tm, JCF = 12), 131.17 (br s), 129.44, 128.65, 126.63, 126.20, 116.27 (t, JCF = 19), 115.68 (m), 115.66, 115.25, 114.18, 13 112.91, 104.38, 103.58, 66.99, 58.70, other C signals were not observed Chirality DOI 10.1002/chir

Enantioseparations of (±)-2 was performed via HPLC utilizing a chiral column Reprosil Chiral-NR (8 μm, Dr. Maisch, Germany; Whelk O1 equivalent) at ambient temperature, using HPLC grade solvents, and a UV detector at 254 nm. The analytical separation was performed on the analytical size of the column (250 × 4.6 mm) using the isocratic elution (CH2Cl2:MeOH 5:95, 1.0 ml/min) with injection volume 20 μl. The capacity factors were calculated from retention time and the column dead time k = (tR t0)/t0 = t/t0, the selectivity factor was calculated using α = k2/k1, and the resolution factor using RS = 2(t’2 - t’1)/1.7(w1 + w2), where w is the peak width at the half peak height. The preparative separation (5 mg per round) was performed on the preparative column (250 × 20 mm) using the same isocratic elution (14.0 ml/min) with injection volume 0.5 ml.

Complexation Studies of TB 2 With AA-OCH3 The stock solution of TB 2 [(±)-2, (+)-2, or ( )-2] was prepared by (6.2 mg) dissolving in 3 ml of dichloromethane, and by dissolving of 100 μl (150 μl in the case of (±)-2) of that solution up to 25 ml (100 ml in the case of (±)-2) with dichloromethane (or toluene). The concentrations of the stock solution of (±)-2, (+)-2, and ( )-2 were 1.36, 3.63, and 1 3.63 μmol.l , respectively. The stock solution of AA-OCH3 was prepared by dissolving of an accurate amount of hydrochloride of AA-OCH3 (approx. 20 mg of DL-, D-, or L-AA-OCH3) in 1 ml of methanol containing 0.5% of triethylamine, and by dissolving of 30 μl of that solution up to 1 ml with the stock solution of TB 2. Then the stock solution of AA-OCH3 was added to 2.0 ml (1.5 ml in the case of DL-AA-OCH3) of the stock solution of TB 2 stepwise. After each addition, the UV-vis spectrum (and ECD spectrum in the case of D-His-OCH3, L-His-OCH3, D-Lys-OCH3, and L-Lys-OCH3) of the resulting mixture was measured. Following the standard procedure,22 the absorbance at 404 nm was evaluated as the function of AA-OCH3 concentration to give the corresponding binding constant.

RESULTS AND DISCUSSION

The studied receptor 2 was prepared in four steps (Scheme 1). The initially prepared dimethoxy TB derivative 323 was demethylated to the dihydroxy TB derivative 4 by BBr3 at room temperature for 2 d in 80% yield.24,25 Although it was successful on the 1,3,4,7,9,10-hexamethyl analog,26 demethylation of 3 by the mixture of acetic acid and concentrated aqueous hydrochloric acid at reflux for 40 h gave only partial demethylation; the half demethylated intermediate 5 was isolated in 30% yield. In the third step, the dihydroxy TB 4 was converted in situ into disodium salt by treatment with NaH. The salt was treated with perfluorophenylporphyrin 6 to give the expected bisporphyrin TB derivative 7 in 18% yield. Compound 7 was

ENANTIOSELECTIVITY OF BISPORPHYRIN-TRÖGER’S BASE CONJUGATE

Scheme 1. Preparation of porphyrin TB 2. The reaction conditions: i) BBr3, CH2Cl2, r.t. 2 days, 80% of 4; ii) conc. CH3COOH, conc. aq. HCl, reflux, 2 days, 30% of 5; iii) NaH, 6, DMF, 60°C, 4 days, 18% of 7, 7% of 8, and 5% of 9; iv) CoCl2, TEA, dioxane, 70°C, 2 d, 88% of 2.

followed by several byproducts; two were isolated. We isolated compound having NMR characteristics corresponding to monoporphyrin TB derivative 8; unfortunately, we were unable to measure the molecular ion peak in highresolution mass spectrometry (MS). Next, we isolated trisporphyrin TB derivative 9, which was confirmed by high-resolution MS; the 5,15-substitution on the middle porphyrin was estimated based on the 1H NMR spectrum. The observed doublet signal at 8.98 ppm of integral intensity 8H can be assigned to hydrogen atoms of the pyrrole units closed to the substituted phenyls, i.e., four hydrogen atoms of side porphyrins and four hydrogen atoms of the middle porphyrin, which are expected to be very similar in chemical shift, and thus produce a single doublet of intensity 8H. Such signal is not expected in the case of the isomer of 5,10-substitution. Finally, TB 7 was metalated by treatment with CoCl2 in dioxane to produce TB 2 in 88% yield. Unfortunately, due to the paramagnetic property of cobalt(II), it was impossible to record a reasonable NMR spectrum of this compound; thus, the purity was checked by thin-layer chromatography (TLC) and HPLC, and characterized by high-resolution MS, and also by characterization of its enantiomers (vide infra). To resolve racemic TB 2 we employed HPLC column Reprosil Chiral NR; an equivalent of Whelk O1, which is efficient for resolving various TB derivatives.27,28 We reached separation selectivity 1.16, and resolution 0.83. We separated a sufficient amount of each enantiomer in excellent HPLC purity (Fig. 2) by several runs on the semipreparative column.

Fig. 2. Chromatograms of HPLC analysis (Reprosil Chiral-NR, CH2Cl2: MeOH 5:95, detection at 254 nm) of racemic TB 2 and its isolated enantiomers.

The purity of the obtained enantiomers was checked also by comparison of UV-vis spectra at the same concentrations (Fig. 3). The spectra uncovered small impurities, since the spectra did not match tightly. On the other hand, the ECD spectra of the enantiomers are nearly mirror images of each other (Fig. 3). Based on the Cotton effect corresponding to the Soret absorption band (404 nm), the less retarded enantiomer (tR = 9.4 min) can be assigned to ( )-2 (negative couplet), Chirality DOI 10.1002/chir

TATAR ET AL.

Fig. 4. The example of the complexation study followed by UV-vis.

Fig. 3. ECD and UV-vis spectra of ( )-2 (blue) and (+)-2 (red); c = 20 μgml in CH2Cl2.

1

since the more retarded enantiomer (tR = 10.8 min) can be assigned to (+)-2 (positive couplet).29 That is in accord with the ECD spectra of TB 1 enantiomers.18 Before a study of enantioselectivity of TB 2, we carried out a screening for binding ability utilizing racemic compounds. Complexation of the racemic TB 2 with a racemic amino acid methyl ester (AA-OCH3) was followed by UV-Vis spectra (e.g., Fig. 4). The obtained data were processed by standard procedure22 to estimate binding constants (Table 1). The binding constants varied from 103 to 106 M-1 in dichloromethane, and usually significantly lower in toluene (up to 6 · 104 M-1). In all cases the binding 1:1 was identified, with the exception of His-OCH3 and Lys-OCH3, for which formation of 1:2 complexes with binding constants approx. 103 M-2 were traced. The high binding constants were found for ProOCH3 and Cys-OCH3, the compounds having highly nucleophilic amine groups (as suggested from the high pKa value of the parent amino acids; Table 1). In spite of lower amino group nucleophility, the higher constants were found for His-OCH3, obviously due to presence of two amino groups. The highest binding constant was found for LysOCH3, which contains two rather nucleophilic amino groups. The strong binding of diamino compounds supports the idea that TB 2 is a bidentate ligand. Chirality DOI 10.1002/chir

To support the bidentate binding mechanism, we compared the binding constants of the bis(cobalt(II)porphyrin) TB 2 with the ones of the parent cobalt(II)porphyrin 10. The binding constant of 10 with a bidentate ligand, CysOCH3, His-OCH3, and Lys-OCH3, decreased considerably, while the binding constants to Pro-OCH3 remained and became the highest (Table 1). These observations are in accord with the expected bidentate binding mechanism. For the most strongly bound AA-OCH3 (Lys-OCH3, HisOCH3, and Pro-OCH3) we determined the binding constants of each enantiomer of TB 2 to each enantiomer of AAOCH3. In accord with the results on racemic compounds, the addition of AA-OCH3 shifted the Soret band from 404 to 406 nm with the appearance of a shoulder at 426, 429, and 425 nm for lysine, histidine, and proline, respectively. The most intense shoulder was observed in the case of histidine enantiomers (Fig. 4). The binding constants, summarized in the Table 2, were estimated analogously to the studies on racemic compounds (vide supra). Although the enantioselectivity was very low, we found that (+)-2 has higher affinity for D-AA-OCH3 than for L-AA-OCH3. That is opposite to the enantioselectivity of TB 1, where (+)-1 has higher affinity for L-AA-OCH3; however, it must be emphasized that the absolute configuration was determined neither for TB 1 nor TB 2 enantiomers. The selectivity factor, the ratio of the binding constant of a preferred pair (e.g., ( )2 · L-AA-OCH3) to the binding constant of the opposite enantiomers (i.e., ( )-2 · D-AA-OCH3), is for His-OCH3 only 1.1, while the selectivity factor of TB 1 is 13.3.18 Surprisingly, the enantioselectivity for Pro-OCH3 is the same as for His-OCH3, which indicates that the bidentate binding, as suggested for His-OCH3 (vide supra), is not the only factor to achieve a high enantioselectivity, in general. To obtain more insight on the binding, we also performed a complexation study followed by ECD spectroscopy in the case of His-OCH3 (Fig. 5) and Lys-OCH3 (Fig. 6). Upon

ENANTIOSELECTIVITY OF BISPORPHYRIN-TRÖGER’S BASE CONJUGATE

TABLE 1. The binding constants for racemic compounds

a

K [103 dm3.mol-1] AA-OCH3

Parent aminoacids pKa of α-NH2

2 in CH2Cl2

2 in toluene

10 in CH2Cl2

pI

0.5

0.3

3

6.00

2

5.07

7

7

5.97

9.58

30

60

7.59

9.09 (6.04 for NH)

2

5.98

9.58

30

9.74

9.16 (10.67 for NH2)

3

5.48

9.09

100

6.30

10.47

0.3

5.60

8.96

5

5.89

9.34

2

5.66

9.04 (10.10 for OH)

8

5.96

9.52

Ala-

Cys-

c

70 ± 30

Gly-

6

His-

b

300 ± 100

Leu-

c

2

Lys1100 ± 300

b

Phe3

60

c

9.71

10.28 (8.14 for SH)

Pro100 ± 50

Thr1

Trp4

Tyr3

20

c

c

c

Val5

0.3

a

The uncertainty of binding constants are reasonable only for the four most affinity compounds. -2 Also K1:2 approx. 1000 M was traced. c Almost no change to evaluate K. b

addition of AA-OCH3, we did not observe an ECD variation in the UV region (the bands of the TB compartment), but in the Vis region (the bands of the metalloporphyrin units). As expected, the enantiomeric pairs have nearly mirror ECD spectra.

The addition of both His-OCH3 enantiomers decreased the intensity of the ECD band in the Soret band of both TB 2 enantiomers. The decreasing of the ECD band at 410 nm was accompanied by the appearance of a new weak ECD band at 433 nm. Chirality DOI 10.1002/chir

TATAR ET AL.

TABLE 2. The binding constants K of enantiomers in CH2Cl2 K [103 dm3.mol-1] (-)-2

(+)-2

Selectivity factor

8.9 ± 0.8 7.9 ± 1.2 41.7 ± 2.2 20.9 ± 2.7 9.8 ± 0.9 8.9 ± 0.9

7.9 ± 0.7 8.9 ± 0.9 21.7 ± 3.6 42.3 ± 4.1 8.9 ± 0.8 9.8 ± 0.4

1.1 1.1 1.9 2.0 1.1 1.1

AA-OCH3 L- His-OCH3 D- His-OCH3 L- Lys-OCH3 D- Lys-OCH3 L- Pro-OCH3 D- Pro-OCH3

1

Fig. 6. The ECD spectra of TB 2 enantiomers at concentration 8.3 mg.l (red) in the presence of 1.1 eq (gray), 3.2 eq (green), and 9.3 eq (blue) of a Lys-OCH3 enantiomer.

general knowledge,29 the reversing of the Cotton effect can be the outcome of a mirror reorientation of the porphyrin moieties, e.g., from a left-handed to a right-handed. Since this phenomenon was observed for Lys-OCH3 only, we assume it is the consequence of a selective action of the TB compartment. Unfortunately, our attempts to uncover an origin of the behavior via molecular modeling were unsuccessful; we emphasize that the 2 · AA-OCH3 complexes are rather large and flexible, and solvation can play an important role. 1

Fig. 5. The ECD spectra of TB 2 enantiomers at concentration 8.3 mg.l (red) in the presence of 1.1 eq (gray), 3.2 eq (green), and 9.3 eq (blue) of a His-OCH3 enantiomer.

CONCLUSION

In contrast, the changes are different and very interesting in the case of the Lys-OCH3 enantiomers (Fig. 6). The changes of the ECD spectra for the preferred pairs (having the highest binding constants, ( )-2 · L-Lys-OCH3 and (+)-2 · D-Lys-OCH3) are similar to the changes induced by His-OCH3; although a raising of the weak ECD band at 433 nm was not observed, the ECD band at 410 nm decreased similarly to His-OCH3. However, in the case of the less preferred pairs, ( )-2 · DLys-OCH3 and (+)-2 · L-Lys-OCH3, both ECD bands of the couplet (398 nm and 412 nm) reversed their signs and amplified intensities; the weak band at 425 nm is broadly overlapped with the 412 nm band (Fig. 6). Based on the

We described the preparation and resolving of a new bis [cobalt(II)porphyrin]-Tröger’s base conjugate 2, wherein the porphyrin units are covalently bound via single bonds to the TB scaffold. Contrary to the known zinc(II)porphyrin-TB derivative 1,18 we observed higher affinity to the lysine ester than the histidine ester. That is understood as the consequence of a higher distance between the porphyrin cores within TB 2 than TB 1. Although the affinities of TB 2 and TB 1 are similar, the enantioselectivity of TB 2 is lower. That is understood as the consequence of the higher conformation freedom of porphyrin units within TB 2. Interestingly, the higher flexibility of TB 2 enables reorientation of its porphyrin units upon an interaction with a guest, and thus, the sign of the Cotton effect in ECD spectra can reflect the structure

Chirality DOI 10.1002/chir

ENANTIOSELECTIVITY OF BISPORPHYRIN-TRÖGER’S BASE CONJUGATE

and chirality of the guest, as observed for Lys-OCH3 enantiomers. That also indicates an importance of the TB compartment for the binding selectivity of TB 2, as well as the irreplaceability of ECD studies. LITERATURE CITED 1. Dolenský B, Elguero J, Král V, Pardo C, Valík M. Current Tröger’s base chemistry. Adv Heterocycl Chem 2007;93:1–56. 2. Sergeyev S. Recent developments in synthetic chemistry, chiral separations, and applications of Tröger’s base analogues. Helv Chim Acta 2009;92:415–444. 3. Dolenský B, Havlík M, Král V. Oligo Tröger’s base—new molecular scaffolds. Chem Soc Rev 2012;41:3839–3858. 4. Rúnarsson ÖV, Artacho J, Wärnmark K. The 125th Anniversary of the Tröger’s base molecule: synthesis and applications of Tröger’s base analogues. Eur J Org Chem 2012;36:7015–7041. 5. Hayashi T, Aya T, Nonoguchi M, Mizutani T, Hisaeda Y, Kitagawa S, Ogoshi H. Chiral recognition and chiral sensing using zinc porphyrin dimers. Tetrahedron 2002;58:2803–2811. 6. Guo YM, Oike H, Saeki N, Aida T. One-pot optical resolution of oligopeptide helices through artificial peptide bundling. Angew Chem Int Ed 2004;43:4915–4918. 7. Johnston MR, Lyons DM. Synthesis and complexation studies of a convex bis-porphyrin tweezer—a molecular capsule precursor. Supramol Chem 2005;17:503–511. 8. Ema T, Ouchi N, Doi T, Korenaga T, Sakai T. Highly sensitive chiral shift reagent bearing two zinc porphyrins. Org Lett 2005;7:3985–3988. 9. Flamigni L, Talarico AM, Ventura B, Rein R, Solladi N. A versatile bisporphyrin tweezer host for the assembly of noncovalent photoactive architectures: A photophysical characterization of the tweezers and their association with porphyrins and other guests. Chem Eur J 2006;12:701–712. 10. Feng DJ, Wang GT, Wu J, Wang RX, Li ZT. Hydrogen bonding-driven elastic bis(zinc)porphyrin receptors for neutral and cationic electrondeficient guests with a sandwich-styled complexing pattern. Tetrahedron Lett 2007;48:6181–6185. 11. Berova N, Pescitelli G, Petrovic AG, Proni G. Probing molecular chirality by CD-sensitive dimeric metalloporphyrin hosts. Chem Commun 2009;5958–5980. 12. Zhou Z, Cao C, Yin Z, Liu Q. Bis(zinc porphyrin) bridged by benzo orthocarbonates as a conformational switch under regulation of DABCO + and a Cu ion. Org Lett 2009;11:1781–1784. 13. Lee CH, Yoon H, Jang WD. Biindole-bridged porphyrin dimer as allosteric molecular tweezers. Chem Eur J 2009;15:9972–9976. 14. Fathalla M, Jayawickramarajah J. Configurational isomers of a stilbenelinked bis(porphyrin) tweezer: synthesis and fullerene-binding studies. Eur J Org Chem 2009;6095–6099. 15. Kim D, Lee S, Gao G, Kang HS, Ko J. A molecular-clip-based approach to cofacial zinc–porphyrin complexes. J Organometallic Chem 2010;695:111–119.

16. Van W, Kundrat RO, Ngo TH, Lhoták P, Dehaen W, Maes W. An oxacalix [2]arene[2]pyrimidine-bis(Zn-porphyrin) tweezer as a selective receptor towards fullerene C70. Tetrahedron Lett 2010;51:2423–2426. 17. Crossley MJ, Hambley TW, Mackay LG, Try AC, Walton R. Porphyrin analogues of Tröger’s base: large chiral cavities with a bimetallic binding site. J Chem Soc Chem Commun 1995;1077–1079. 18. Crossley MJ, Mackay LG, Try AC. Enantioselective recognition of histidine and lysine esters by porphyrin chiral clefts and detection of amino acid conformations in the bound state. J Chem Soc Chem Commun 1995;1925–1927. 19. Allen PR, Reek JNH, Try AC, Crossley MJ. Resolution of a porphyrin analogue of Tröger’s base by making use of ligand binding affinity differences of the enantiomers. Tetrahedron: Asymmetry 1997;8:1161–1164. 20. Brotherhood PR, Luck IJ, Blake IM, Jensen P, Turner P, Crossley MJ. Regioselective reactivity of an asymmetric tetravalent di[dihydroxotin (IV)] bis-porphyrin host driven by hydrogen-bond templation. Chem Eur J 2008;14:10967–10977 21. Tatar A, Dolenský B, Dvořáková H, Král V. Selective formation of either Tröger’s base or spiro Tröger’s base derivatives from [2aminoporphyrinato(2-)]nickel by choice of reaction conditions. Tetrahedron Lett 2012;53:6015–6017. 22. Hirose K. A practical guide for the determination of binding constants. J Incl Phenom Macrocycl Chem 2001;39:193–209. 23. Bag BG, Maitra U. A convenient method for the synthesis of Tröger’s Base analogues. Synth Commun 1995;25:1849–1856. 24. Muthyala RS, Carlson KE, Katzenellenbogen JA. Exploration of the bicyclo[3.3.1]nonane system as a template for the development of new ligands for the estrogen receptor. Bioorg Med Chem Lett 2003;13:4485–4488. 25. Kiehne U, Lützen A. Synthesis of 2,8-disubstituted analogues of Tröger’s Base. Synthesis 2004;10:1687–1695. 26. Smith LI, Schubert WM. Polyalkylbenzenes. The reaction between polymethyl-p-methoxyanilines and formaldehyde. J Am Chem Soc 1948;70:2656–2661. 27. Sergeyev S, Stas S, Remacle A, Velde CMLV, Dolenský B, Havlík M, Král V, Čejka J. Enantioseparations of non-benzenoid and oligo-Tröger’s bases by HPLC on Whelk O1 column. Tetrahedron: Asymmetry 2009;20:1918–1923. 28. Sergeyev S, Didier D, Boitsov V, Teshome A, Asselberghs I, Clays K, Velde CMLV, Plaquet A, Champagne B. Symmetrical and nonsymmetrical chromophores with Tröger’s bases skeleton: chiroptical, linear, and quadratic nonlinear optical properties—A joint theoretical and experimental study. Chem Eur J 2010;16:8181–8190. 29. Kobayashi N. Optically active porphyrin systems analyzed by circular dichroism. In: Kadish KM, Smith KM, Guilard R, editors. Handbook of porphyrin science — with applications to chemistry, physics, materials science, engineering, biology and medicine, vol. 7: Physicochemical characterization. Hackensack, NJ: World Scientific Publishing; 2010;147–245.

Chirality DOI 10.1002/chir