Preparation of a New Chiral Pyridino-Crown Ether-Based Stationary

Sep 15, 2000 - Department of Organic Chemistry, Technical University of Budapest, H-1521 ... Hungarian Academy of Sciences, H-1521 Budapest, Hungary,...
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Ind. Eng. Chem. Res. 2000, 39, 3576-3581

Preparation of a New Chiral Pyridino-Crown Ether-Based Stationary Phase for Enantioseparation of Racemic Primary Organic Ammonium Salts Gyo1 rgy Horva´ th,† Pe´ ter Huszthy,*,‡ Szilvia Szarvas,§ Gyula Szo´ ka´ n,§ J. Ty Redd,| Jerald S. Bradshaw,⊥ and Reed M. Izatt⊥ Department of Organic Chemistry, Technical University of Budapest, H-1521 Budapest, Hungary, Research Group for Alkaloid Chemistry, Hungarian Academy of Sciences, H-1521 Budapest, Hungary, Department of Organic Chemistry, Eo¨ tvo¨ s Lorand University, H-1518 Budapest, Hungary, Department of Chemistry, Southern Utah University, Cedar City, Utah 84720, and Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602

Three novel enantiomerically pure chiral pyridino-18-crown-6 ligands [(S,S)-7, (S,S)-8 and (R,R)9] containing a linker with a terminal carboxyl function were prepared. One of them [(R,R)-9] containing two tert-butyl groups at the stereogenic centers was covalently attached to silica gel by an amide bond using 3-aminopropyltrimethoxysilane. The resulting chiral stationary phase [(R,R)-11] separated the enantiomers of racemic R-(1-naphthyl)ethylammonium perchlorate and 1-phenylethylammonium perchlorate by high-performance liquid chromatography. Introduction Enantiomeric recognition is an essential process in living organisms involving the differentiation of one enantiomer of the guest from the other by a chiral host. Examples of enantiomeric discrimination can be found in many natural processes such as enzyme-substrate interactions, immunological responses, the mechanism of drug action, and the storage and retrieval of genetic information. Imitating biochemical phenomena using synthetic compounds has demonstrated that biological behavior can be engineered into simple molecules. Crown ethers, for example, have demonstrated excellent enantiodiscrimination for the enantiomers of chiral organic ammonium guests.1 Since Cram and co-workers reported their impressive enantiomeric recognition work using a binaphthylderived chiral crown,2 many different chiral macrocyclic ligands have been prepared for enantiomeric recognition studies. Some of these ligands include those containing amino acid units,3 sugar molecules,4,5 diaza-crown ether units,6 and chiral crown ethers containing the pyridine subcyclic unit.1,7-16 Our interest in enantiomeric recognition has focused, to a large extent, on the interaction of chiral organic ammonium salts with chiral crown ethers containing pyridine and other heterocyclic ring derivatives as part of the macroring. In certain cases, these chiral crown ethers have demonstrated appreciable enantiomeric recognition for the enantiomers of chiral organic ammonium salts.1 Chiral host-guest interactions in the latter case have been characterized by calorimetry,7,17,20 high-resolution NMR spectroscopy,7,8-11,16,17,20 mass spectrometry,21-23 X-ray crystallography,7,24-26 solvent extraction,27 mo* Corresponding author. Phone: 36 1 463 2111. Fax: 36 1 463 3297. E-mail: [email protected]. † Technical University of Budapest. ‡ Hungarian Academy of Sciences. § Eo ¨ tvo¨s Lorand University. | Southern Utah University. ⊥ Brigham Young University.

lecular mechanics calculations,9-11,28 circular dichroism spectroscopy,29,30 and potentiometry.31 The studies of the interactions of chiral pyridino-18crown-6 ligands with chiral organic ammonium salts have established a tripodlike hydrogen bonding involving the pyridine nitrogen and two alternate oxygen atoms of the ligand and the three protons of the ammonium salt, π-π stacking between the pyridine ring of the macrocycle and the aromatic moiety of the ammonium ion, and steric repulsion between the bulky groups on the stereogenic centers of the ligand and the substituents of the ammonium salt, respectively. Our extensive studies on enantiomeric recognition have also shown that the bulkiness of the substituents at the stereogenic centers paralleled very well the enantioselectivity17,32 and, when using them as a chiral selector on chiral stationary phases (CSPs), the effectiveness in separation of the enantiomers of chiral organic ammonium salts.13,15 Chiral liquid chromatography (CLC) relying on enantiomeric recognition by a CSP has proved to be one of the most important techniques for the determination of enantiomeric composition and enantioseparation of enantiomerically impure and racemic mixtures of drugs, pesticides, and other chemicals of importance.33-35 In the past decades there has been a great interest in designing new CSPs capable of resolving many classes of compounds.33-38 In the late seventies, Cram and co-workers reported the attachment of substituted bis(binaphthyl)-22-crown-6 ligands to both silica gel39 and polymer resin.40 Using these CSPs, they separated several racemic organic ammonium salts, mainly protonated amino acid esters. A few years ago, we attached dimethyl- and diphenyl-substituted chiral pyridino-18crown-6 ligands to silica gel and demonstrated that these CSPs separated racemic R-(1-naphthyl)ethylammonium (NapEtNH3+) perchlorate13 (see Figure 1). Recently, we attached a chiral di-tert-butylpyridino-18crown-6 ligand to silica gel, and this CSP separated the enantiomers of selected racemic organic ammonium perchlorates.15

10.1021/ie000272a CCC: $19.00 © 2000 American Chemical Society Published on Web 09/15/2000

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Figure 1. Organic ammonium cations.

Figure 3. Pyridono- and pyridino-18-crown-6 ligands.

Scheme 1. Preparation of Chiral Crown Ethers Figure 2. Amide-containing 18-crown-6 ligand (R,R)-10 and CSP (R,R)-11.

In continuation of our studies on developing novel CSPs capable of resolving racemic organic ammonium salts by CLC, we report here the preparation of a new type of chiral pyridino-18-crown-6 ligand containing an amide bond and a trimethoxysilyl group [(R,R)-10] and its attachment to HPLC quality silica gel to obtain CSP (R,R)-11 (Figure 2). The novel CSP was tested for the enantiomeric separation of racemic NapEtNH3+ and R-phenylethylammonium (PhEtNH3+) perchlorates (Figure 1) by high-performance liquid chromatography (HPLC). Results and Discussion The preparation of dimethyl-substituted optically pure pyridono-18-crown-6 ligand (S,S)-1 (see Figure 3) has been reported.41 The diisobutyl- and di-tert-butylsubstituted analogues of (S,S)-1, that is, (S,S)-2 and (R,R)-3, were prepared in a similar manner as described for (S,S)-1, and their preparation is shown in Scheme 1. First the appropriate 4-tetrahydropyranyloxypyridino-18-crown-6 derivatives were prepared by a Williamson type ether synthesis from disubstituted tetraethylene glycols (S,S)-1216 and (R,R)-1315 using 4-tetrahydropyranyloxy-2,6-pyridinedimethanol ditosylate41 under strong basic conditions. The THP protecting group was removed using a mixture of ethanol, water, and acetic acid to obtain pyridono-crown ether ligands (S,S)-2 and (R,R)-3. The pyridono ligands (S,S)-1, (S,S)-2, and (R,R)-3 were alkylated with benzyl 2-chloroacetate using K2CO3 as a base in DMF to give pyridino-crown compounds (S,S)-4, (S,S)-5, and (R,R)-6. As discussed in more detail earlier,41 no N-alkylation occurred during this reaction. The esteric benzyl protecting group of ligands (S,S)-4, (S,S)-5, and (R,R)-6 was cleaved by catalytic

hydrogenation to give acids (S,S)-7, (S,S)-8, and (R,R)-9 (Scheme 1). Acylation of 3-(trimethoxysilyl)propylamine by (R,R)-9 was carried out using DCC in dichloromethane, a method generally applied in peptide chemistry.42 Crude (R,R)-10 was then treated with HPLC quality silica gel in refluxing toluene to give CSP (R,R)-11 (Scheme 2). The CSP contained about 0.25 mmol of chiral crown compound for each gram of adsorbent. An HPLC column made of (R,R)-11 was tested for the enantioseparation of racemic NapEtNH3+ and PhEtNH3+ perchlorates. Under isocratic conditions, optimum analysis times and effective resolutions were achieved with an optimization of the eluent composition in both cases (see Figures 4 and 5 and Table 1). The elution order of the enantiomers was determined by injection of standard authentic (R)-enantiomers. It was shown in both cases that the (R)-enantiomer eluted with a shorter retention time than that of its antipode. This

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Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 Table 1. Chromatographic Data of the Racemic Ammonium Perchlorates Separated on CSP (R,R)-11a salt

k1′(R)

k2′(S)

R

N1

N2

Rs

NapEtNH3+ PhEtNH3+

1.89 4.61

2.88 5.92

1.52 1.28

331 617

558 685

1.54 1.34

a k′ ) capacity factor, R ) separation factor, N ) number of theoretical plates, Rs ) peak resolution.

Experimental Section

Figure 4. Chromatographic pattern for racemic NapEtNH3+: support, (R,R)-11; eluent, CH2Cl2-CH3OH ) 90:10 (v/v); flow rate, 1 mL/min; detection, 280 nm.

Figure 5. Chromatographic pattern for racemic PhEtNH3+: support, (R,R)-11; eluent, CH3CN-CH2Cl2 ) 70:30 (v/v); flow rate, 1.2 mL/min; detection, 254 nm.

Scheme 2. Preparation of CSP (R,R)-11

behavior is in full agreement with our observation using CSPs containing similar pyridino-crown ethers as chiral selectors attached to ordinary silica gel12,13,15 or Merrifield resin41 at atmospheric pressure. This demonstrates the generally observed higher stability of heterochiral complexes [i.e. (R,R)-crown ether-(S)-ammonium salt or (S,S)-crown ether-(R)-ammonium salt] compared to that of homochiral complexes [i.e. (S,S)-crown ether(S)-ammonium salt or (R,R)-crown ether-(R)-ammonium salt] in the case of interactions of chiral pyridino18-crown-6 ligands with NapEtNH3+ and PhEtNH3+ perchlorates.17

Infrared spectra were obtained on a Zeiss Specord IR 70 spectrometer. Optical rotations were taken on a Perkin-Elmer 241 polarimeter that was calibrated using both enantiomers of menthol. 1H (500 MHz) and 13C (125 MHz) NMR spectra were taken on a Bruker DRX500 Advance spectrometer in CDCl3 unless otherwise indicated. Elemental analyses were performed in the Microanalytical Laboratory of the Department of Organic Chemistry, Eo¨tvo¨s Lorand University, Budapest, Hungary. Melting points were taken on a Boetius micromelting point apparatus and were uncorrected. Starting materials were used as purchased from Aldrich Chemical Co. unless otherwise noted. Silica gel 60 F254 (Merck) and aluminum oxide 60 F254 neutral type E (Merck) plates were used for TLC. Aluminum oxide (neutral, activated, Brockman I) and silica gel 60 (70200 mesh, Merck) were used for column chromatography. Solvents were dried and purified according to the well-established methods.43 Evaporations were carried out under reduced pressure unless otherwise stated. General Procedure for the Preparation of Pyridono-Crown Ethers (S,S)-2 and (R,R)-3. To a stirred suspension of NaH (4.2 g, 138 mmol, 80% dispersion in mineral oil) in pure and dry THF (20 mL) under argon, at 0 °C, was added dropwise a solution of disubstituted tetraethylene glycol (8.0 g, 26.1 mmol) [(S,S)-12 or (R,R)-13] in THF (50 mL). The mixture was stirred at 0 °C for 10 min and at room temperature (rt) for 30 min and was refluxed for 4 h. The mixture was then cooled to -60 °C, and a solution of 4-tetrahydropyranyloxy-2,6-pyridinedimethanol ditosylate (15 g, 26.1 mmol) in THF (100 mL) was added. The mixture was stirred at -60 °C for 30 min and at room temperature for 1 week. The solvent was removed, and the residue was dissolved in a mixture of ether (200 mL) and ice cold water (100 mL). Phases were shaken and separated, and the aqueous phase was extracted with ether (3 × 100 mL). The combined organic phase was dried over anhydrous Na2SO4 and filtered, and the solvent was removed to give a crude product which was dissolved in a mixture of ethanol (50 mL), water (1 mL), and acetic acid (1 mL) and stirred at room temperature overnight. Solvents were evaporated and traces of acetic acid were removed by repeated evaporation of toluene from the mixture. The resulting oil was purified by column chromatography on alumina to give the pyridono-18-crown-6 ligands as waxy solids. (4S,14S)-(+)-4,14-Diisobutyl-3,6,9,12,15-pentaoxa21-azabicyclo[15.3.1]heneicosa-17,20-diene-19(21H)one [(S,S)-2]. Ligand (S,S)-2 was obtained as a waxy solid: 4.56 g (41%). [R]D25 ) +1.21 (c 1.66, CH2Cl2). IR (film): ν 3070, 3040, 2975, 2930, 2860, 1640, 1600, 1550, 1470, 1360, 1100, 880, 740 cm-1. 1H NMR: δ 0.93 (d, 12 H, J ) 7 Hz), 1.24-1.29 (m, 2 H), 1.51-1.57 (m, 2H), 1.68-1.73 (m, 2 H), 3.46-3.50 (m, 2 H), 3.61-3.73 (m, 12 H). AB spin system of benzylic CH2 in the macroring: δA 4.55, δB 4.41 (JAB ) 12 Hz, 4 H), 6.26 (s, 2 H), 11.43 (NH). 13C NMR: δ 22.76, 24.78, 39.79, 67.82,

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70.11, 70.86, 73.99, 77.61, 114.77, 147.33, 180.54. Anal. Calcd for C23H39NO6‚0.5H2O: C, 63.56; H, 9.28; N, 3.22. Found: C, 63.47; H, 9.34; N, 3.29. (4R,14R)-(-)-4,14-Di-tert-butyl-3,6,9,12,15-pentaoxa-21-azabicyclo[15.3.1]heneicosa-17,20-diene19(21H)-one [(R,R)-3]. Ligand (R,R)-3 was obatined as a waxy solid: 7.0 g (63%). [R]D25 ) -38.5 (c 0.91, CH2Cl2). IR (film): ν 3072, 3040, 2976, 2960, 2928, 2860, 1640, 1590, 1530, 1460, 1370, 1250, 1110, 880, 550 cm-1. 1H NMR: δ 0.95 (s, 18 H), 3.27 (d, 2H, J ) 9 Hz), 3.613.73 (m, 12 H). AB spin system of benzylic CH2 in the macroring: δA 4.73, δB 4.58 (JAB ) 12 Hz, 4 H), 6.36 (s, 2 H), 9.84 (NH). 13C NMR: δ 26.59, 34.70, 70.64, 71.29, 71.34, 73.23, 88.73, 114.06, 148.26, 180.86. Anal. Calcd for C23H39NO6‚H2O: C, 62.47; H, 9.31; N, 3.16. Found: C, 62.68; H, 9.01; N, 3.06. General Procedure for the Preparation of Macrocyclic Benzyl Esters (S,S)-4, (S,S)-5, and (R,R)6. A solution of the pyridono-18-crown-6 ligand (7.0 mmol, 2.4 g of (S,S)-1, 3.0 g of (S,S)-2, or 3.0 g of (R,R)3) and benzyl 2-chloroacetate (3.88 g, 21.0 mmol) in pure and dry DMF (50 mL) was stirred under argon. To this solution was added K2CO3 (2.9 g, 21.0 mmol), and the mixture was stirred at room temperature for 48 h. The solvent was evaporated, and the residue was dissolved in a mixture of ether (50 mL) and water (50 mL). The phases were shaken well and separated. The aqueous phase was shaken with ether (1 × 50 mL). The combined organic phase was dried (Na2SO4) and filtered, and the solvent was removed. The residue was purified by column chromatography on alumina. Benzyl 2-[(4S,14S)-4,14-Dimethyl-3,6,9,12,15-pentaoxa-21-azabicyclo[15.3.1]heneicosa-1(21),17,19trien-19-yloxy]acetate [(S,S)-4]. Ligand (S,S)-4 was obtained as an oil: 2.95 g (86%). [R]D25 ) -12.33 (c 1.05, CH2Cl2). IR (film) ν 2980, 2920, 2870, 1750, 1600, 1580, 1500, 1460, 1390, 1360, 1280, 1200, 1180, 1110, 1080, 950, 880, 760, 710, 680 cm-1. 1H NMR: δ 1.10 (d, 6 H, J ) 7 Hz), 3.32-3.65 (m, 12 H), 3.68-3.72 (m, 2 H), 4.68 (s, 2 H). AB spin system of benzylic CH2 in the macroring: δA 4.83, δB 4.76 (JAB ) 15 Hz, 4 H), 5.23 (s, 2H), 6.88 (s, 2H), 7.39-7.35 (m, 5 H). 13C NMR: δ 16.50, 68.82, 70.01, 70.51, 70.63, 72.84, 73.20, 74.34, 106.25, 128.68, 128.85, 128.88, 135.12, 160.89, 165.14, 168.03. Benzyl 2-[(4S,14S)-4,14-Diisobutyl-3,6,9,12,15pentaoxa-21-azabicyclo[15.3.1]heneicosa-1(21),17,19-trien-19-yloxy]acetate [(S,S)-5]. Ligand (S,S)-5 was obtained as an oil: 2.41 g (60%). [R]D25 ) -8.20 (c 1.05, CH2Cl2). IR (film) ν 2952, 2870, 1764, 1664, 1600, 1528, 1456, 1416, 1352, 1304, 1276, 1228, 1196, 1160, 1120, 992, 752, 696 cm-1. 1H NMR: δ 0.90 (d, 6 H, J ) 7 Hz), 0.93 (d, 6 H, J ) 7 Hz), 1.19-1.23 (m, 2 H), 1.481.53 (m, 2H), 1.75-1.81 (m, 2H), 3.49-3.60 (m, 12H), 3.69-3.72 (m, 2 H), 4.72 (s, 2 H). AB spin system of benzylic CH2 in the macroring: δA 4.79, δB 4.75 (JAB ) 14 Hz, 4 H). AB spin system of benzylic CH2 of the benzyl ester function: δA 5.26, δB 5.24 (JAB ) 19 Hz, 2 H), 6.82 (s, 2H), 7.39-7.35 (m, 5 H). 13C NMR δ 22.55, 23.56, 24.86, 41.23, 64.81, 67.41, 70.80, 71.08, 72.38, 75.39, 76.36, 106.67, 128.69, 128.85, 128.87, 135.14, 160.88, 165.17, 168.01. Benzyl 2-[(4R,14R)-4,14-Di-tert-butyl-3,6,9,12,15pentaoxa-21-azabicyclo[15.3.1] heneicosa-1(21),17,19-trien-19-yloxy]acetate [(R,R)-6]. Ligand (R,R)-6 was obtained as an oil: 2.73 g (68%). [R]D25 ) +16.10 (c 1.04, CH2Cl2). IR (film): ν 2952, 2928, 2872, 2368, 1764,

1672, 1516, 1456, 1416, 1396, 1360, 1300, 1256, 1196, 1180, 1112, 912, 864, 796, 656 cm-1. 1H NMR: δ 0.96 (s, 18 H), 3.23-3.25 (m, 2 H), 3.47-3.54 (m, 10 H), 3.663.68 (m, 2H), 4.72 (s, 2 H). AB spin system of benzylic CH2 in the macroring: δA 4.84, δB 4.79 (JAB ) 14 Hz, 4 H), 5.25 (s, 2H), 6.85 (s, 2H), 7.40-7.35 (m, 5 H). 13C NMR: δ 26.59, 34.71, 64.78, 67.40, 70.67, 71.10, 72.62, 74.21, 85.43, 106.50, 128.67, 128.83, 128.85, 135.11, 160.91, 165.07, 168.08. General Procedure for the Preparation of Macrocyclic Acids (S,S)-7, (S,S)-8, and (R,R)-9. A solution of the benzyl ester (3.5 mmol, 1.7 g of (S,S)-4, 2.0 g of (S,S)-5, or 2.0 g of (R,R)-6) in MeOH (30 mL) was hydrogenated in the presence of Pd/C catalyst (340 mg, Merck palladium/charcoal; activated, 10% Pd) until the theoretical volume of hydrogen was consumed. Conversion of compounds was checked by alumina TLC using 1:4 MeOH-toluene as an eluent. After the reaction was completed, the catalyst was filtered and washed with MeOH (2 × 5 mL). The filtrate and washings were combined and evaporated to give pure products. 2-[(4S,14S)-4,14-Dimethyl-3,6,9,12,15-pentaoxa21-azabicyclo[15.3.1]heneicosa-1(21),17,19-trien19-yloxy]acetic Acid [(S,S)-7]. Ligand (S,S)-7 was isolated as a waxy solid: 1.32 g (95%). Rf ) 0.3 (silica gel TLC, 20% MeOH in toluene). [R]D25 ) +36.10 (c 0.95, 10% MeOH in water). IR (film): ν 2980, 2872, 2376, 2344, 1632, 1584, 1456, 1400, 1368, 1304, 1116, 1056, 972, 856, 728 cm-1. 1H NMR: δ 1.10 (d, 6 H, J ) 6 Hz), 2.99 (OH), 3.53-3.81 (m, 14 H). AB spin system of benzylic CH2 in the macroring: δA 4.38, δB 4.27 (JAB ) 12 Hz, 4 H). AB spin system of CH2 adjacent to the carboxyl group: δA 4.46, δB 4.43 (JAB ) 15 Hz, 2 H), 6.51 (s, 2 H). 13C NMR: δ 16.05, 68.60, 69.93, 70.11, 70.24, 72.65, 75.48, 108.72, 158.40, 167.32, 173.32. Anal. Calcd for C19H29NO8: C, 57.13; H, 7.32; N, 3.51. Found: C, 57.28; H, 7.44; N, 3.48. 2-[(4S,14S)-4,14-Diisobutyl-3,6,9,12,15-pentaoxa21-azabicyclo[15.3.1]heneicosa-1(21),17,19-trien19-yloxy]acetic Acid [(S,S)-8]. Ligand (S,S)-8 was isolated as a waxy solid: 1.19 g (70%). Rf ) 0.5 (silica gel TLC, 20% MeOH in toluene). [R]D25 ) +9.56 (c 2.76, MeOH). IR (film): ν 3456, 2872, 1635, 1600, 1456, 1368, 1192, 1112, 912, 872, 780, 736, 652 cm-1. 1H NMR: δ 0.92 (d, 6 H, J ) 7 Hz), 0.93 (d, 2 H, J ) 7 Hz), 1.251.29 (m, 2 H), 1.52-1.57 (m, 2H), 1.68-1.75 (m, 2 H), 3.44-3.62 (m, 12 H), 3.71 (d, 2 H, J ) 6 Hz), 4.70-4.80 (m, 6 H), 5.36 (OH), 7.04 (s, 2H). 13C NMR: δ 22.71, 23.07, 24.82, 40.23, 50.29, 67.07, 69.32, 70.25, 70.92, 74.08, 109.25, 156.72, 169.00, 170.28. Anal. Calcd for C25H41NO8: C, 62.09; H, 8.55; N, 2.90. Found: C, 62.21; H, 8.71; N, 2.85. 2-[(4R,14R)-4,14-Di-tert-butyl-3,6,9,12,15-pentaoxa21-azabicyclo[15.3.1]heneicosa-1(21),17,19-trien19-yloxy]acetic Acid [(R,R)-9]. Ligand (R,R)-9 was isolated as white crystals: 1.24 g (73%). Rf ) 0.45 (silica gel TLC, 20% MeOH in toluene). Mp: 132-134 °C. [R]D25 ) -51.72 (c 0.41, MeOH). IR (film): ν 3432, 2960, 2940, 1636, 1560, 1480, 1428, 1400, 1376, 1168, 1128, 912, 872, 768, 528 cm-1. 1H NMR: δ 0.95 (s, 18 H), 3.28 (d, 2 H, J ) 8 Hz), 3.49-3.65 (m, 10 H), 3.66 (d, 2 H, J ) 9 Hz), 4.79 (s, 2 H). AB spin system of benzylic CH2 in the macroring: δA 4.94, δB 4.88 (JAB ) 15 Hz, 4 H), 7.08 (s, 2H), 7.40 (OH). 13C NMR: δ 26.64, 34.77, 67.21, 70.54, 71.22, 71.97, 72.64, 87.27, 108.52, 157.67, 169.28, 170.42. Anal. Calcd for C25H41NO8: C, 62.09; H, 8.55; N, 2.90. Found: C, 61.85; H, 8.32; N, 2.84.

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Preparation of 2-[(4R,14R)-4,14-Di-tert-butyl3,6,9,12,15-pentaoxa-21-azabicyclo[15.3.1]heneicosa1(21),17,19-trien-19-yloxy]-N-(3-trimethoxysilylpropyl)acetamide [(R,R)-10]. A solution of (R,R)-9 (600 mg, 1.2 mmol) in pure and dry CH2Cl2 (5 mL) was stirred at room temperature, and DCC (270 mg, 1.3 mmol) was added. The mixture was stirred at room temperature for 10 min, and a solution of 3-aminopropyltrimethoxysilane (90 mg, 0.5 mmol) in CH2Cl2 (3 mL) was added. The mixture was stirred at room temperature for 16 h. The precipitate was filtered and washed with CH2Cl2 (2 × 5 mL). The filtrate and washings were combined and evaporated, and the crude product [(R,R)10] was used for the preparation of CSP (R,R)-11 without further purification: 500 mg (80%). IR (film): ν 3328, 2928, 2872, 1628, 1596, 1448, 1420, 1312, 1248, 1088, 832, 776, 640. 1H NMR: δ 0.68 (t, 2H, J ) 8 Hz), 0.97 (s, 18H), 1.53-1.59 (m, 2H), 3.24-3.26 (m, 2H), 3.37 (q, 2H), 3.45-3.62 (m, 19H), 3.65-3.70 (m, 2H), 4.55 (s, 2H). AB spin system of the benzylic CH2 in the macroring: δA ) 4.87, δB ) 4.80 (4H, JAB ) 14 Hz), 6.72 (s, 1H), 6.89 (s, 2H). Preparation of CSP (R,R)-11. A solution of crude (R,R)-10 (500 mg) in pure and dry toluene (20 mL) was stirred with HPLC quality silica gel (2.0 g, SHANDON HYPERSIL, mean practical size 5 µm) at reflux temperature for 33 h. The modified adsorbent was filtered and washed with solvents in the following order: toluene (3 × 10 mL), CH2Cl2 (2 × 15 mL), 2-propanol (2 × 15 mL), MeOH (2 × 15 mL), CH2Cl2 (2 × 15 mL) and n-hexane (2 × 15 mL). The solid CSP was dried in air to give 2.15 g of CSP (R,R)-11. Elemental analysis: C, 7.77; H, 1.45; N, 0.69. Blank silica gel treated in the same way as (R,R)-11 gave the following elemental analysis: C, 0.3; H, 0.0; N, 0.0. This result shows that each gram of CSP (R,R)-11 contained 0.25 mmol (by N%), 0.20 mmol (by C%), and 0.26 mmol (by H%) of crown ether. Benzyl 2-Chloroacetate. Benzyl 2-chloroacetate (26) was prepared as reported.44 Bp: 86-88 °C/0.4 mmHg. IR (film): ν 3032, 2984, 2960, 1760, 1560, 1496, 1456, 1412, 1376, 1312, 1228, 1168, 1052, 972, 940, 856, 772, 748, 696, 672. 1H NMR: δ 4.05 (s, 2H), 5.22 (s, 2H), 7.32-7.45 (m, 5H). High-PerformanceLiquidChromatography(HPLC) Experiments. Separations were performed on a Knauer system consisting of two Model 64 pumps with an analytical pumphead, a Model 50B programmer, an injection valve with a 20 µL sample loop, and a variable wavelength UV monitor with an analytical flow cell (Knauer GmbH, Germany). The chromatographic data were collected and processed with CHROMAPEX software (Data-Apex Ltd., Czech Republic). The column packing material was (R,R)-11. The 150 mm × 4.6 mm column was packed with the chiral sorbent (particle size 5 µm, MeOH-acetone 1:1 (v/v) slurry packing at 350 bar by a Haskel-pump). Mixtures of CH2Cl2, MeOH, and MeCN were used as mobile phase systems (see Figures 4 and 5). Column effluents were monitored by UV at 254 and 280 nm. The chromatograph was operated isocratically between 0.6 and 2.0 mL/min. Samples were dissolved in a small amount of an effective solvent (MeOH, CH2Cl2, or MeCN) and then diluted with the eluents. The starting concentration was usually 1 mg/mL, and the solutions were diluted 5-10 times according to sensitivity requirements.

The identified enantiomers of the chiral primary alkylammonium perchlorates were characterized chromatographically by retention time (tR) and capacity factor (k′). Enantiomer separations were characterized by selectivity (R) and resolution (Rs) calculated from the chromatographic patterns (see Table 1 and Figures 4 and 5). Acknowledgment Financial support by the Hungarian Scientific Research Fund (OTKA T-25071) and the Office of Naval Research (USA) is gratefully acknowledged. Supporting Information Available: 1H (500 MHz) and 13C (125 MHz) NMR spectra for (S,S)-5 and (S,S)-6 and 1H NMR (80 MHz) spectrum for (R,R)-4. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Zhang, X. X.; Bradshaw, J. S.; Izatt, R. M. Enantiomeric Recognition of Amine Compounds by Chiral Macrocyclic Receptors. Chem. Rev. 1997, 97, 3313. (2) (a) Kyba, E. P.; Koga, K.; Sousa, L. R.; Siegel, M. G.; Cram, D. J. Chiral Recognition in Molecular Complexing. J. Am. Chem. Soc. 1973, 95, 2692. (b) Cram, D. J. The Design of Molecular Hosts, Guest, and Their Complexes. Science 1988, 240, 760. (3) Erickson, S. D.; Simon, J. A.; Still, W. C. Practical Synthesis of a Highly Enantioselective Receptor For Peptides. J. Org. Chem. 1993, 58, 1305. (4) Stoddart, J. F. Synthetic Chiral Receptor Molecules From Natural Products. In Progress in Macrocyclic Chemistry; Izatt, R. M., Christensen, J. J., Eds.; Wiley-Interscience: New York, 1981; Vol. 2. (5) Bako´, P.; To¨ke, L. Novel Monoaza- and Diazacrown Ethers Incorporating Sugar Units and Their Extraction Ability Towards Picrate Salts. J. Inclusion Phenom. 1995, 23, 195. (6) Chadwick, D. J.; Cliffe, I. A.; Sutherland, I. O. Chiral Diaza18-Crown-6 Derivatives. J. Chem Soc., Chem. Commun. 1981, 992. (7) Davidson, R. B.; Bradshaw, J. S.; Jones, B. A.; Dalley, N. K.; Christensen, J. J.; Izatt, R. M.; Morin, F. G.; Grant, D. M. Enantiomeric Recognition of Organic Ammonium Salts by Chiral Crown Ethers Based on the Pyridino-18-Crown-6 Structure. J. Org. Chem. 1984, 49, 353. (8) Bradshaw, J. S.; Thompson, P. K.; Izatt, R. M.; Morin, F. G.; Grant, D. M. The Preparation of New Chiral DiphenylSubstituted Macrocyclic Polyether-Diester Compounds and Their Enantiomeric Recognition of Chiral Organic Ammonium Salts. J. Heterocycl. Chem. 1984, 21, 897. (9) Bradshaw, J. S.; Huszthy, P.; McDaniel, C. W.; Zhu, C. Y.; Dalley, N. K.; Izatt, R. M.; Lifson, S. Enantiomeric Recognition of Organic Ammonium Salts by Chiral Dialkyl-, Dialkenyl- and Tetramethyl-Substituted Pyridino-18-Crown-6 and TetramethylSubstituted Bis-Pyridino-18-Crown-6 Ligands: Comparison of the Temperature Dependent 1H NMR and the Empirical Force Field Techniques. J. Org. Chem. 1990, 55, 3129. (10) Huszthy, P.; Bradshaw, J. S.; Zhu, C. Y.; Izatt, R. M.; Lifson, S. Recognition by Symmetrically Substituted Chiral Diphenyl and Di-t-butyl and Asymmetrically Substituted Chiral Dimethylpyridino-18-Crown-6 Ligands of the Enantiomers of Various Organic Ammonium Perchlorates. J. Org. Chem. 1991, 56, 3330. (11) Huszthy, P.; Oue, M.; Bradshaw, J. S.; Zhu, C. Y.; Wang, T. M.; Dalley, N. K.; Curtis, J. C.; Izatt, R. M. New Symmetrical Chiral Dibenzyl- and Diphenyl-Substituted Diamido-, Dithionoamido-, Diaza- and Azapryidino-18-Crown-6 Ligands. J. Org. Chem. 1992, 57, 5383. (12) Bradshaw, J. S.; Huszthy, P.; Wang, T.-M.; Zhu, C.-Y.; Nazarenko, A. Y.; Izatt, R. M. Enantiomeric Recognition and Separation of Chiral Organic Ammonium Salts by Chiral Pyridino18-crown-6 Ligands. Supramol. Chem. 1993, 1, 267-275. (13) Huszthy, P.; Bradshaw, J. S.; Bordunov, A. V.; Izatt, R. M. Enantiomeric Separation of Chiral R-(1-Naphthyl)ethylammonium Perchlorates by Silica Gel-Bound Chiral Pyridino-18Crown-6 Ligands. ACHsModels Chem. 1994, 131, 445.

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Received for review February 28, 2000 Accepted July 26, 2000 IE000272A