chirality(6)2009

‘Naked-Eye’Enantioselective Chemosensors for N-Protected Amino Acid Anions Bearing Thiourea UnitsGUANGYAN QING,1,2TAOLEI SUN,2ZHIHONG CHEN,1XI YANG,1XIAOJUN WU,1AND YONGBING HE1*1College of Chemistry and Molecular Sciences,Wuhan University,Wuhan430072,People’s Republic of China2Physikalisches Institut,Mu¨nster Universita¨t,Mu¨nster48149,GermanyABSTRACT Four linear thiourea anion receptors(1–3)derived from simple aminoacid have been synthesized and their bonding properties with various chiral N-protectedamino acid anions were examined by using UV–vis andfluorescence titration experi-ments.Receptors1a,2,and3exhibit excellent enantioselective recognition abilitiestowards N-Boc-protected alanine anion in the UV–vis spectra,obvious difference in thecolor of solution indicate that the enantiomers of N-Boc-alanine anion could be distin-guished by naked eye directly.Receptor1a is also found to carry out enantioselectivefluorescent recognition of the N-acetyl-glutamate.1H NMR experiments suggest thathydrogen-bonding interaction between the host and guest is the main factor in the rec-ognition process.Chirality21:363–373,2009.V C2008Wiley-Liss,Inc.KEY WORDS:enantioselective recognition;amino acid anion;UV–vis;fluorescence;thioureaINTRODUCTIONMolecular recognition,and in particular chiral recogni-tion,is a fundamental characteristic in the biochemical systems.The study of synthetic model systems could con-tribute to the understanding of these processes and,at the same time,offer new perspectives for the development of pharmaceuticals,enantioselective sensors,catalysts,and other molecular devices.1–3Amino acids and their deriva-tives are important components of chemical and biological systems,and their recognition,in particular,chiral recog-nition attracts considerable interest.4–7Various approaches involve the use of metal complexes,8,9imprinted poly-mers,10,11natural and modified cyclodextrins,12–14syn-thetic macrocycles(including calixarenes),15–17and differ-ent types of acyclic compounds18–20as host molecules. Often a chiral host is prepared by using a natural chiral compound as precursor or modifier.Amino acids and pep-tides have often been employed as chiral sources in the synthesis of chiral receptors because of their accessibility and biological relevance.21Neutral receptors typically con-tain an NÀÀH fragment(from amides,sulfonamides or pyr-roles)which acts as an H-bond donor.Especially,thiourea contains two rigidly positioned NÀÀH fragments suitable for the interaction with Y-shaped anions,like carboxy-lates.22Many literatures describe the thiourea containing receptors form stable complexes with various anions23–25; some of them also exhibit good enantioselective recogni-tion abilities towards various amino acid derivatives.7,15,26,27 Recently,many efforts involve the covalent linking of an optical signaling unit(a chromophore or afluorophore)to a specific receptor for the anion or chiral molecuales.28–32 Compared with other detection methods,such as NMR, HPLC,capillary electrophoresis or CD,colorimetric che-mosensor has a good useful value in thefield of analytical chemistry because it can be directly observed by naked eye and has the reliable calibration stability.29,33,34Fluores-cence techniques have often been used to study the inter-action between enantiomers and receptors because of their sensitivity,selectivity,and versatility.30,35On the ba-sis of their respective advantages,we attempt to design some receptors with a dual optical response to the enan-tiomers in the recognition interaction,which may offer a simple method to explore the recognition process for more information.In this article,four linear thiourea anion receptors(1–3)derived from amino acid have been synthesized easily, and their bonding properties with various chiral N-pro-tected amino acid anions have been examined by using UV–vis andfluorescence titration experiments in DMSO. Receptors1a,2,and3exhibited good enantioselective recognition abilities towards N-Boc-protected alanine anion and formed1:1complex with the guests in DMSO. While receptor1a also showed good enantioselectivefluo-rescent response to the N-acetyl-glutamate.1H NMR experiments suggested that hydrogen-bonding interaction between the host and guest were the main factor in the recognition process.*Correspondence to:Yongbing He,College of Chemistry and Molecular Sciences,Wuhan University,Wuhan430072,People’s Republic of China. E-mail:ybhe@Additional Supporting Information may be found in the online version of this article.Contract grant sponsor:National Natural Science Foundation;Contract grant number:20572080Received for publication30January2008;Accepted4April2008DOI:10.1002/chir.20593Published online20June2008in Wiley InterScience().CHIRALITY21:363–373(2009)V C2008Wiley-Liss,Inc.MATERIALS AND METHODSGeneralMelting point was determined with a Reichert7905 melting-piont apparatus(uncorrected).Optical rotations were taken on a Perkin-Elmer Model341polarimeter.IR spectra were obtained on a Nicolet670FTIR spectropho-tometer.1H NMR spectra were recorded in CDCl3or d6-DMSO on a Varian Mercury VX-300MHz spectrometer. 13C NMR spectra were recorded on a Varian Inova–600MHz spectrometer.Mass spectra were recorded on a Fin-nigan LCQ advantage mass spectrometer.Elemental anal-ysis was determined with a Carlo-Erba1106instrument. The UV–vis spectra were performed with a TU-1901spec-tro-photomer.Fluorescence spectra were obtained on a Shimadzu RF–5301spectrometer.Propane diamine was distilled before using.DMF was dried from CaH2and dis-tilled under reduced pressure.All other commercially available reagents were used without further purification. The anions were used as their tetrabutylammonium salts.UV–vis and Fluorescent Spectra MeasurementAll the host compounds1a,1b,2,and3were prepared as stock solution in DMSO for531024mol dm23.All anions used in this report were in the form of tetrabu-tylammonium salt.They were prepared to$0.175and 0.0175mol dm23of stock solution in DMSO.The work solutions were prepared by adding different volumes of anion solution to a series of test tubes,then,the same amount of stock solution of host compound was added into each of test tubes followed by dilution to3.5ml by DMSO.After being shaken for several minutes,the test solution could be measured immediately.Influorescence spectra measurement,the excited wavelengths were both 295nm,the excitation slit width was5nm and the emis-sion slit wavelength was10nm.The association constants were calculated by means of a nonlinear least-square curve fitting method with Origin7.0(Orignin-Lab Coroperation). To ensure the veracity of data,more than16series of con-tinuous data were collected in the optical titration experi-ments until the absorbance orfluorescence intensities approach equilibrium.NMR Shift ExperimentsNMR shift experiments were performed on a Mercury-VX300spectrometer at258C.Samples for analysis were prepared by mixing equimolar amounts of compounds1a, 2,and3with the N-Boc-L-or D-alanine anion(the concen-trations were2.031023mol dm23).Tetrabutylammonium SaltsThe tetrabutylammonium salts were prepared by adding 1equiv(for monocarboxylic acid)or2equiv(for dicarbox-ylic acid)tetrabutylammonium hydroxide in methanol to a solution of corresponding carboxylic acid(1equiv)in methanol.The mixture was stirred at room temperature for2h and evaporated to dryness under reduced pressure. The resulting syrup was dried at high vacuum for24h, checked by NMR and stored in the desiccators.SynthesisCompounds4and5were synthesized according to themethod reported in the literatures.17General procedure for the synthesis of compounds 1–pound5(1mmol)was added into dry DMF (10mL)and stirred.Then the p-nitrophenyl isothiocyanate(0.18g,1mmol)or p-methylphenyl isothiocyanate(0.15g,1mmol)was added into the mixture and stirred.Graduallythe mixture turned clear.After stirred for24h,a solutionof ethanol(10ml)and H2O(40ml)was added into the so-lution dropwise.The precipitate wasfiltered and the resi-due was washed with ethanol:H2O(4320ml,V:V51:4).The resulting solid was dried in vacuo to give powder.Then the powder was dissolved in200ml CHCl3,washedwith brine for three times,the organic layer was collectedand dried over anhydrous Na2SO4.Afterfiltration,the sol-vent was removed under reduced pressure,and the resi-due was purified by column chromatography on silica gel. tert-Butyl-(S)-1-(3-(3-(4-nitrophenyl)thioureido)pro-pylcarbamoyl)-2-(1H-indol-3-yl)ethylcarbamate[1a]. (eluant:CHCl3/CH3OH580:1(V/V)).Pure product wasobtained as a yellow powder(0.35g)in64.8%.m.p.104–1068C;[a]20D516.708(c0.010,DMSO);IR(KBr/cm21)m: 3401,2957,1653,1536,1508,1329,1302,1255,1164,1111, 851,745;1H NMR(CDCl3):d(ppm)1.42(s,9H,Bu t),1.64–1.71(m,2H,CCH2C),3.14–3.29(m,4H,Indole-CH2,NCH2C), 3.48(s,1H,NCH2C),3.71(s,1H,NCH2C),4.29–4.36(m,1H, NC*H CO), 5.11(s,1H,NH-Boc), 6.12(s,1H,CNHCS), 7.05(s,1H,Indole),7.10–7.16(m,1H,Indole),7.20(d,J5 7.5Hz,1H,Indole),7.38(d,J58.1Hz,1H,Indole),7.45(s, 1H,ArNHCS),7.57(d,J58.1Hz,1H,Indole),7.65(d,J5 9.6Hz,2H,ArH-N),8.20–8.23(m,3H,ArH-NO2,Indole-N H), 8.52(s,1H,CONHC);13C NMR(CDCl3):d(ppm)26.0,34.5, 39.3,53.5,64.7,78.5,107.3,109.2,116.1,117.3,119.4,119.9, 121.0,122.4,124.9,133.9,140.9,142.5,153.7,170.1,178.0; ESI-MS m/z(%):541.6(M111,100);Elemental analysis calcd.(%)for C26H32N6O5S:C,57.75;H, 5.97;N,15.55. Found:C,57.36;H,6.04;N,15.32.tert-Butyl-(S)-1-(3-(3-p-tolylthioureido)propylcar-bamoyl)-2-(1H-indol-3-yl)ethylcarbamate[1b].(eluant: CHCl3/C2H5OH5150:1(V/V)).Pure product was obtainedas a white powder(0.27g)in53.0%.m.p.72–748C;[a]20D5 112.718(c0.010,CHCl3);IR(KBr/cm21)m:3317,2973, 1696,1658,1536,1513,1385,1365,1250,1166,1119,870, 744;1H NMR(CDCl3):d(ppm)1.43(s,9H,Bu t),2.37(s,3H, Ar-CH3), 3.08–3.15(m,4H,Indole-CH2,NCH2C), 3.29–3.32(m,4H,NCH2C,CCH2C),4.38–4.41(m,1H,NC*H CO), 5.14(s,1H,NH-Boc),6.32(s,2H,CNHCS),7.05–7.15(m,6H, Indole-H,CH3Ar H),7.23–7.26(m,2H,CH3Ar H),7.34(d,J57.2Hz,2H,Indole-H),7.61(d,J56.6Hz,2H,Indole-N H),8.30(s,1H,CONHC);13C NMR(CDCl3):d(ppm)21.2,28.5,28.7,29.1,36.2,41.9,55.8,67.2,80.3,110.5,111.6,119.0,119.6,122.2,123.6,125.6,127.7,130.7,133.9,136.4,137.3,155.7,172.5,180.9;ESI-MS m/z(%):501.5(M121,100); Elemental analysis calcd.(%)for C27H35N5O3S:C,63.62;H, 6.93;N,13.75.Found:C,63.21;H,6.99;N,13.55.tert-Butyl(S)-1-(3-(3-(4-nitrophenyl)thioureido)-propylcarbamoyl)-2-phenylethylcarbamate[2].(eluant: CHCl3/CH3OH560:1(V/V)).Pure product was obtained364QING ET AL. Chirality DOI10.1002/chiras a yellow powder (0.30g)in 60%.m.p.146–1488C;[a ]20D 5127.928(c 0.015,CHCl 3);IR (KBr/cm 21)m :3306,3182,2986,1684,1656,1534,1385,1340,1248,1171,1110,1053,853,744,701;1H NMR (CDCl 3):d (ppm)1.40(s,9H,Bu t ),1.73–1.78(m,2H,CCH 2C),3.01(d,J 56.6Hz,2H,phenyl-CH 2),3.18(s,1H,NCH 2C),3.38(s,1H,NCH 2C),3.53(s,1H,NCH 2C),3.76(s,1H,NCH 2C),4.17–4.25(m,1H,NC*H CO),5.01(s,1H,NH-Boc), 6.16(s,1H,CNHCS),7.16(d,J 56.6Hz,2H,phenyl-H),7.30–7.35(m,3H,phenyl-H),7.51(s,1H,ArNHCS),7.65(d,J 59.0Hz,2H,ArH-N),8.21(d,J 58.7Hz,2H,ArH-NO 2),8.56(s,1H,CONHC);13C NMR (CDCl 3):d (ppm)18.5,28.4,28.8,38.5,41.6,56.5,58.4,67.2,80.9,121.8,124.9,127.3,128.9,129.3,136.5,143.4,145.4,156.1,163.2,173.0,180.7;ESI-MS m /z (%):500.1(M 121,100);Elemental analysis calcd.(%)for C 24H 31N 5O 5S:C,57.46;H,6.23;N,13.97.Found:C,57.23;H,6.35;N,13.82.(S )-tert -Butyl-2-(3-(3-(4-nitrophenyl)thioureido)pro-pylcarbamoyl)pyrrolidine-1-carboxylate [3].(eluant:CHCl 3/CH 3OH 540:1(V/V)).Pure product was obtained as a yellow powder (0.35g)in 77.5%.m.p.80–828C;[a ]20D 5125.648(c 0.010,DMSO);IR (KBr/cm 21)m :3328,2976,1660,1597,1539,1510,1391,1331,1258,1170,1112,851,720;1H NMR (CDCl 3):d (ppm)1.45(s,9H,Bu t ),1.81–1.90(m,4H,CCH 2C,Pro-ring CH 2),2.08–2.14(m,2H,Pro-ring CH 2), 3.26–3.31(m,1H,NCH 2C), 3.44–3.52(m,4H,Pro-ring CH 2N,NCH 2C), 3.96–4.00(m,1H,Pro-ring CH 2N),4.16–4.21(m,1H,NC*H CO),6.46(s,2H,CNHCS),7.63(s,1H,ArNHCS),7.74(d,J 58.1Hz,2H,ArH-N),8.17(d,J 58.1Hz,2H,ArH-NO 2),8.97(s,1H,CONHC);13C NMR (DMSO-d 6):d (ppm)23.8,24.6,28.2,28.7,36.7,42.1,47.3,60.6,79.3,121.0,125.1,142.4,147.0,154.0,173.1,180.7;ESI-MS m /z (%):450.3(M 121,100);Elemen-tal analysis calcd.(%)for C 20H 29N 5O 5S:C,53.20;H,6.47;N,15.51.Found:C,53.02;H,6.56;N,15.40.RESULTS AND DISCUSSIONDesign and Synthesis of the New Chiral ReceptorsThe synthesis of receptors 1–3is outlined in Scheme 1.N-Boc-protected amino acid methyl ester (4)were synthe-sized according to the literature with high yield,17and then reacted with excess amount of propane diamine to obtain the compounds 5.To avoid the partial racemization of 5in this reaction,the material 4and 1,3-diaminopropyl were soluble in large amount of methanol and continued to stir at room temperature.Especially at the end of reac-tion,the solvent and 1,3-diaminopropyl should be evapo-rated under high vacuum at 30–358C,because a little higher temperature may lead the racemization of com-pound.Then the condensation reactions between com-pounds 5and p -nitrophenylisothiocyanate or p -methylphe-nylisothiocyanate were performed in dry DMF at room temperature for 24h,giving the target materials (1–3).Receptors 1–3are common soluble in organic solvents such as CHCl 3,C 2H 5OH,CH 3CN,DMSO,and DMF;their structures were characterized by IR,MS,1H NMR,13C NMR spectra,and elemental analysis.Absorption Properties StudyUV–vis spectra were first used to investigate the enan-tioselective recognition abilities of these receptors.The UV–vis spectra were recorded from a solution of 1a ,2,or 3in the absence and presence of various chiral N-pro-tected amino acid anions,in each case the counter cation was tetrabutylammonium.DMSO was chose as the solvent considering the solubility of anions.First N -tert -butylcarbonate protected alanine anion (N -Boc-Ala anion)was chosen as the guest.Figure 1exhibits the UV–vis absorption spectra of receptor 1a upontheScheme 1.Synthesis of receptors 1–3.365ENANTIOSELECTIVE CHEMOSENSORS WITH THIOUREA UNITSChirality DOI 10.1002/chiraddition of N -Boc-L -Ala anion.In the absence of anion,1a has an absorption maximum at 360nm,which can be assigned to an intramolecular charge transfer (CT)absorp-tion band.With the addition of N -Boc-L -Ala anion to the so-lution of 1a in DMSO (5.0310–5mol dm 23),the charac-teristic absorption peak of the host at 360nm was decreased gradually with a red shift (about 10nm)and a new absorption peak at 482nm was produced.The new absorption suggested the formation of the complex between 1a and N -Boc-L -Ala anion.Gradually increasing the concentration of anion,the color of the solution of 1a changed from colorless to yellow,which could be observed by naked eye.As shown in the Figure 2,the similar phenomena were observed when N -Boc-D -Ala anion was added into the solu-tion of receptor 1a (5.0310–5mol dm 23)in DMSO.With the increasing concentration of anion,the new band at 482nm increased obviously,100equiv of anion could lead the absorbance intensity (at 482nm)approach to 0.65and the color of the solution changed from colorless to brown red.Although the intensity only approached to 0.18with the addition of 100equiv of N -Boc-L -Ala anion,the color of so-lution just changed from colorless to yellow.Obvious dif-ference in the absorbance spectra and the color of solution indicated that 1a had a good enantioselective recognition ability towards N -Boc-Ala anion.The satisfactory result (the correlation coefficient (R )is over 0.99)of nonlinear curve fitting (at 480nm)confirm that 1a and N -Boc-Ala anion form a 1:1complex (see the insets of Figs.1and 2)36,37;in addition,the association constants (K ass )are dif-ferent (K ass (L)5192.7dm 3mol 21;K ass (D)5519.7dm 3mol 21),which correspond to the D /L selectivity (K ass(D)/K ass(L))of 2.70for N -Boc-Ala anion.Receptor 1a was developed to give strong hydrogen bonding to anions,which could give rise to colorimetric changes upon anion recognition,as 4-nitro-phenyl moiety is part of amidothiourea anion receptor moiety,giving riseto an internal charge transfer (ICT)absorption band in the absorption spectra.Upon adding anions such as Boc-Ala anion to a solution of 1a ,a gradual shift to longer wave-length was observed,signifying a perturbation in the ICT character of the sensor,due to the recognition of anion at the amidothiourea moiety.This enhances the push–pull character of the ICT state,and consequently a red-shift is observed,upon hydrogen bonding of the anion to the re-ceptor.38–40To prove this explanation,receptor 1b was designed,in which the electron repulsive p -tolyl unit took instead of the p -nitro-phenyl unit.Similar experiments have been done,but there was nearly no change could be observed in the UV–vis spectra,which confirmed that charge-transfer interactions mainly happened between the thiourea units and electron-deficient p -nitro-phenyl moi-eties.When a protic solvent,such as methanol,was added to the solution of 1a and N-Boc-L -or D -Ala anion in DMSO,the color of solutions both changed to colorless.This phe-nomenon illustrated that addition of the protic solvent destroyed the complexation of host and guest,which dem-onstrated that the interaction between 1a and N-Boc-L -or D -Ala anion were largely relied on the hydrogen bonding.Similar thiourea-based anion receptors were reported by Fabbrizzi 24and Pfeffer 25recently,the effect of deprotona-tion versus hydrogen bond interaction has been described for fluoride and carboxylate anion for both urea and thiou-rea,the latter being more acidic and prone to deprotona-tion in DMSO.The tendency towards deprotonation increases with the acidity of the receptor and the stability of [HX 2]2self-complex.When fluoride or acetate was inves-tigated,small amount of anion is sufficient enough to induce large change in the UV–vis or 1H NMR spectra;it could be presumed that the deprotonation of thiourea con-tribute to the interaction.But in this work,only large amount of N -Boc-Ala anion could induce the change of ab-sorbance,small K ass also indicate that the interaction between the host and guest is not large,which indicate that Ala anion is not a strongly basic anion to lead thedeproto-Fig. 1.UV–vis absorption spectra of receptor 1a (5.031025mol dm 23)upon the addition of various amounts of N -Boc-L -Ala anion in DMSO.Equivalent of anion:0,1.5,4.0,6.5,10,15,25,30,45,60,80,95,and 115.The nonlinear fitting curve of change in absorbance at 482nm with respect to the amount of N-Boc-L -Ala anion is shown in the inset,the line is fittingcurve.Fig. 2.UV–vis absorption spectra of receptor 1a (5.031025mol dm 23)upon the addition of various amounts of N -Boc-D -Ala anion in DMSO.Equivalent of anion:0,1.2,2.5,3.5,5.0,6.5,7.5,9.5,12,15,16.5,20,22,25,30,35,37.5,42.5,50,57.5,65,75,85,100,and 115.The nonlin-ear fitting curve of change in absorbance at 482nm with respect to the amount of N -Boc-D -Ala anion is shown in the inset,the line is fitting curve.366QING ET AL.Chirality DOI 10.1002/chirnation of the thiourea.So we presume that deprotonation of thiourea is not the main factor for the complexation.The continuous variation methods were used to deter-mine the stoichiometric ratio of the receptor1a with N-Boc-Ala anion.The total concentration of host and anion guest was constant(5.0310–4mol dm23)in DMSO,with continuously variable molar fraction of guest([G]/ ([H]1[G])).The Job plots of1a interacted with N-Boc-L-or D-Ala anion(at482nm)were shown in the supporting information(SI)Figure S1.1.When the molar fraction of guest is0.50,the absorption reaches a maximum,which further demonstrates that receptors1a forms a1:1com-plex with guest.41,42For the complex of1:1stoichiometry,the association constant K ass can be calculated by using the following eq1 in Origin7.043,44:A¼A0þA limÀA02C0f C HþC Gþ1=K assÀ½ðC HþC Gþ1=K assÞ2À4C H C G 1=2gð1Þwhere A represents the absorption intensity,and C H and C G are the corresponding concentrations of host and anion guest.Then the association constants of1a with other chiral amino acid anions can be obtained using the same method.The association constants(K ass)and correlation coefficients(R)obtained by a non-linear least-squares anal-ysis of X versus C H and C G are listed in Tables1and2. From the data listed in the Table1,receptor1a is also found to have a good enantioselective recognition ability towards N-acetyl-alanine anion(N-Ac-Ala anion),the D/L selectivity(K ass(D)/K ass(L))is7.24.While the addition of N-benzoyl-L-or D-Ala anion could not cause any significant change to the UV–vis spectrum of1a,which indicates that acylamide of N-protected Ala anion could largely affect the complexation.When N-benzoyl-Ala anion is referred,the electron could partly transfer from acylamide to phenyl through the n-p conjugation,which will decrease the com-plexation ability of acylamide,while the basicity of anion also become weakened,both of them lead the rather weak interaction with host.To eliminate the effect of steric hin-drance,N-Boc-Phe anion,mandelate and phenylglycine were used as the guests,receptor1a all exhibited good response to these anions in the UV–vis spectra.Many literatures reported the chemosensors containing thiourea units had good recognition abilities towards dicar-boxylate,7,26,27,29we also used N-protected glutamate or aspartate as the guests to investigate recognition abilities of receptor1a,the association constants(K ass)between the host and guest are listed in the pared with the small K ass between receptor1a and monocarbox-ylates,the K ass are much larger when the1a interacted with dicarboxylates,which maybe due to the stronger hydrogen bonding could be formed through the more bonding sites.Receptor1a was found to have the best enantioselective recognition ability towards N-Boc-Ala anion in the UV–vis titration,which inspired us to change the structure of1a to investigate these phenomena further.Then receptor2 derived from L-phenylalanine was synthesized and also found to have a good chiral recognition ability towards N-Boc-Ala anion(see SI Figs.S2.1,S2.2).Job plots for the interaction between2and Ala anion indicated that a1:1 complex was formed,and the association constant of2TABLE1.Association constants(K ass)andenantioselectivities K ass(D)/K ass(L)of receptor1a with L/D-monocarboxylate in DMSO at258C Entry Guest a K ass(dm3mol21)b,c K(D)/K(L) 1N-Boc-L-Ala192.766.12N-Boc-D-Ala519.7613.4 2.703N-Ac-L-Ala79.968.64N-Ac-D-Ala578.3631.57.245N-Bz-L-Ala d6N-Bz-D-Ala d7N-Boc-L-Phe347.2624.18N-Boc-D-Phe83.368.80.249L-mandelate303.768.910D-mandelate619.5613.8 2.04 11L-phenylglycine(1.5260.07)310312D-phenylglycine(4.6260.24)3103 3.04a The anions were used their tetrabutylammonium salts.The amino groups of amino acid anions were protected by tert-butylcarbonate,acetyl or ben-zoyl respectively.b The data were calculated from UV-vis titration in DMSO.c All error values were obtained by the results of nonlinear curvefitting, the correlation coefficient(R)of nonlinear curvefitting is over0.99.d Reliable association constant could not be obtained due to the too small change in the UV-vis spectra.TABLE2.Association constants(K ass)andenantioselectivities K ass(D)/K ass(L)of receptor1awith L/D-dicarboxylate in DMSO at258CEntry Guest a K ass(dm3mol-1)b,c K(D)/K(L) 1N-Boc-L-Glu(6.5160.29)31032N-Boc-D-Glu(1.3060.08)3104 2.00 3N-Ac-L-Glu d4N-Ac-D-Glu d5N-Bz-L-Glu(3.4960.24)31036N-Bz-D-Glu694.2655.20.20 7N-Boc-L-Asp(3.8760.37)31038N-Boc-D-Asp(1.2060.05)3104 3.10 9N-Ac-L-Asp303.1622.510N-Ac-D-Asp432.7616.5 1.43 11N-Bz-L-Asp(1.9260.15)310412N-Bz-D-Asp(6.8860.58)31030.36 13L-malate(1.6460.09)310414D-malate(7.7460.38)31030.47 15L-tartrate(1.4760.14)310416D-tartrate(2.2260.17)3104 1.51a The anions were used their tetrabutylammonium salts.The amino groups of amino acid anions were protected by tert-butylcarbonate,acetyl or ben-zoyl respectively.b The data were calculated from UV-vis titration in DMSO.c All error values were obtained by the results of nonlinear curvefitting, the correlation coefficient(R)of nonlinear curvefitting is over0.99.d Reliable association constant could not be obtained due to the too small change in the UV-vis spectra.367ENANTIOSELECTIVE CHEMOSENSORS WITH THIOUREA UNITSChirality DOI10.1002/chirwith L -Ala anion was 92.5dm 3mol 21,while that was 277.2dm 3mol 21for its D -enantiomers.The colors of solution were also different when 100equiv of L -or D -Ala anion was added to the solution as shown in the Figure 3,which sug-gested that this kind of chiral recognition process could be observed by visual color change.Proline derivates are widely reported as chiral ligands used in the asymmetric catalysis,so we also synthesized receptor 3,in which the linear structure and most of bind-ing sites were remained.Similar titration experiments were done to investigate whether it had similar property to 1a or 2.As shown in the Figure 4,receptor 3was also found to have good enantioselective recognition ability towards N-Boc-Ala anion,large difference in the absorb-ance intensity changes and color of solution could confirm this conclusion (see SI Part 3).This phenomenon indi-cated that the linear structure derived from N-Boc-pro-tected amino acid may be the key factor for the good chi-ral recognition properties;further 1H NMR experiment will investigate the interaction in detail.The interaction between receptor 2or 3and various chiral anion guests were also investigated with UV–vis ti-tration;the results are listed in the Tables 3and 4.These receptors both have good enantioselective recognition abil-ities towards N-Ac-Ala anion and do not have any response to the N-Bz-Ala anion,which is similar to that of 1a .Othergood results can be obtained when receptors 2or 3inter-acts with N-Bz-glutamate (see SI S1.7).The absorbance intensities of 2or 3increased gradually with the addition of D -Glu anion,while there was nearly no change despite large amount of L -Glu anion was added,which indicated that receptors 2and 3could be also used as colorimetric chemosensors for the N-Bz-glutamate.On the basis of the good chiral recognition abilities of receptors 1a –3towards N -Boc-alanine anion,we tried to use them as the probes to detect the enantiomeric composi-tion of this anion.45,46The UV–vis absorbance intensities changes of 1a ,2,and 3with respect to the enantiomeric composition of N -Boc-alanine anion were studied.As shown in the Figure 5,the absorbance intensity of 1a increases lin-early with the increasing D -component of the alanine anion,similar linear increase could be also observed when 2or 3was used as the probe.Thus,these receptors can be used as colorimetric sensors to readily determine the enantio-meric composition of N -Boc-alanine anion.Fluorescence Properties StudyFluorescence techniques have often used to study the interaction between enantiomers and receptors because of their sensitivity,selectivity,and versatility.30To explore more details about the complexation,fluorometric method was also used to detect the fluorescence change when the guests were added.Fluorescence spectra were recorded from a solution of the receptor 1a in the presence of N -Boc-L -or D -Ala anion in DMSO.As shown in the Figure 6,the fluorescence emission of 1a (k ex 5295nm;k em 5347nm)increases gradually with the addition of D -Ala anion.The satisfactory result (the correlation coefficient is over 0.99)of nonlinear curve fitting confirm that 1a and D -Ala anion form 1:1com-plex 41,42(see the top right plots of Fig.6).Similar phenom-ena were found when 1a interacted with N -Boc-L -Ala anion (see Supporting Information Part 4Fig S4.1).The associa-tion constants (K ass )could also be calculated through the eq 1.The K ass of 1a with L -Ala anion is 200.4dm 3mol 21,while that is 685.4dm 3mol 21with its D -enantiomers,which are consistent with the results calculated from the UV–vis titration (see Table 1).Comparative experiments have been done to explore whether receptor 1b has the similar recognition abilities.There was little enhancement in the emission spectra of fluorescence,but 1b wasstillFig.3.Effects of N -Boc-L -or D -Ala anion (as Bu 4N 1salts)on color changes of 1a ,2,or 3(131024mol dm 23)in DMSO.From left to right:(a )1a ;(b )1a 1L -Ala anion;(c )1a 1D -Ala anion;(d )2;(e )21L -Ala anion;(f )21D -Ala anion;(g )3;(h )31L -Ala anion;(i )31D -Ala anion.N -Boc-L or D -Ala anion had been added with the amount of 100euqiv of the host.[Color figure can be viewed in the online issue,which is available at.]Fig.4.Absorbance intensity changes (at 482nm)of receptors 1a ,2,or 3(531025mol dm 23)with the addition of N -Boc-Ala anion in DMSO.The line is the fitting curve.368QING ET AL.Chirality DOI 10.1002/chir。