Discrimination of enantiomers of dipeptide derivatives with two chiral centers by tetraaza macrocyclic chiral solvating agents using ¹H NMR spectroscopy

Abstract

¹H NMR spectroscopy is often used to discriminate enantiomers of chiral analytes and determine their enantiomeric excess (ee) by various chiral auxiliaries. In reported research, these studies were mainly focused on chiral discrimination of chiral analytes with only one chiral center. However, many chiral compounds possessing two or more chiral centers are often found in natural products, chiral drugs, products of asymmetric synthesis and biological systems. Therefore, it is necessary to investigate their chiral discrimination by effective chiral auxiliaries using ¹H NMR spectroscopy. In this paper, a new class of tetraaza macrocyclic chiral solvating agents (TAMCSAs) with two amide (CONH), two amino (NH) and two phenolic hydroxyl (PhOH) groups has been designed and synthesized for chiral discrimination towards dipeptide derivatives with two chiral centers. These dipeptide derivatives are important chiral species because some of them are used as clinical drugs and special dietary supplements for treatment of human diseases, such as L-alanyl-L-glutamine and aspartame. The results show that these TAMCSAs have excellent chiral discriminating properties and offer multiple detection possibilities pertaining to ¹H NMR signals of diagnostic split protons. The nonequivalent chemical shifts (up to 0.486 ppm) of various types of protons of these dipeptide derivatives were evaluated with the assistance of well-resolved ¹H NMR signals in most cases. In addition, enantiomeric excesses (ee) of the dipeptide derivatives with different optical compositions have been calculated based on integration of well-separated proton signals. At the same time, the possible chiral discriminating behaviors have been discussed by means of Job plots, ESI mass spectra and a proposed theoretical model of (±)-G1 with TAMCSA 1c. Additionally, the association constants of enantiomers of (±)-G5 with TAMCSA 1a were calculated by employing the nonlinear curve-fitting method.

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Introduction

In the field of chiral molecular recognition, NMR spectroscopy has been extensively utilized as one of the most effective techniques towards discriminating enantiomers of various chiral analytes by virtue of chiral auxiliaries,¹ such as chiral derivatizing agents (CDAs),² chiral lanthanide shift reagents (CLSR),³ chiral shift reagents (CSRs)⁴ and chiral solvating agents (CSAs).⁵ Compared with chiral chromatography and X-ray crystallography, the more prominent advantages of NMR spectroscopy are its attributes as a fast, accurate and convenient method.⁶ In reported papers, NMR spectroscopy was mainly focused on enantiodifferentiation of chiral analytes with only one chiral center by chiral auxiliaries.⁷ However, studies on enantiomeric discrimination of chiral compounds with two or more chiral centers were rarely reported in the past.⁸ Nevertheless, these chiral compounds often exist in natural products, chiral drugs, products of asymmetric synthesis and biological systems.⁹ Therefore, it is necessary to intensively develop more effective chiral auxiliaries for their chiral discrimination by ¹H NMR spectroscopy.¹⁰ Herein, dipeptide derivatives were selected as one of the most representative species with two stereo configurations. Generally, dipeptides or their derivatives contain two identical or different chiral amino acid residues with one or two stereocenters. Thus, these dipeptides or their derivatives are likely composed of one, two, three and even four stereoisomers (D,D; L,L; D,L; L,D) under the same amino acid sequence, due to the reasons noted above. Of course, they are also achiral compounds. However, different stereoisomers have different biological and pharmacological activities, and even result in the opposite physiological effect.¹¹ Therefore, analysis and determination of the optical compositions of these dipeptides or their derivatives, especially used as clinical drugs and special dietary supplements (such as L-alanyl-L-glutamine¹² and aspartame¹³), are of important significance for the treatment of human diseases. Furthermore, this work addresses a challenging and fundamental problem, due to the very similar chemical structures and more complicated stereochemistries involved. In this paper, we report methodology towards achieving highly effective chiral discrimination of dipeptide derivatives with two chiral centers, in the presence of a new class of tetraaza macrocyclic chiral solvating agents (TAMCSAs) 1a–1c, by employing ¹H NMR spectroscopy.

Results and discussion

Design and synthesis of TAMCSAs 1a–1c for chiral discrimination

TAMCSAs 1a–1c have been designed and synthesized from D-(−)-α-phenylglycine and (1S,2S)-(+)-1,2-diaminocyclohexane for chiral discrimination of dipeptide derivatives with two chiral centers by ¹H NMR spectroscopy.¹⁴ The detailed procedure of synthesis of TAMCSAs 1a–1c is available in the ESI.† Their chemical structures are shown in Fig. 1.

Fig. 1 Structures of TAMCSAs 1a–1c.

As shown in Fig. 1, the most outstanding features of TAMCSAs 1a–1c are the presence of two amide, two amino and two phenolic hydroxyl groups as potential intermolecular interaction sites. Other notable features include an overall C₂-symmetry and a 12-membered cavity as part of the framework.

The newly generated chiral centers of TAMCSAs 1a–1c in the reductive coupling reaction are assigned to possess S, S absolute configurations based on their X-ray crystallographic analysis. Their X-ray crystal structures are shown in Fig. 2.

Fig. 2 X-ray crystal structures of TAMCSAs 1a·2CH₃COCH₃ (a), 1b (b) and 1c·CDCl₃ (c).

To investigate the chiral discriminating ability of TAMCSAs 1a–1c towards dipeptide derivatives with two chiral centers by ¹H NMR spectroscopy, the following stereoisomers of dipeptide derivatives as guests were derived from the corresponding D- and L-amino acids (Fig. 3).

Fig. 3 Structures of dipeptide derivatives with two chiral centers G1-1 – G8-2.

Their chemical structures are characterized by ¹H NMR, ¹³C NMR, IR and HRMS methods. However, some protons cannot be clearly assigned on this NMR evidence alone, due to close chemical shift values, almost identical coupling constants and similar peak shapes. Therefore, ¹H–¹H COSY and ¹H–¹³C HSQC spectra were measured and the assignments of the disputed protons were properly determined on the basis of their correlated signals.

As a typical example, the assignments of some related protons of G4-1 were clarified by ¹H–¹H COSY and ¹H–¹³C HSQC. In the ¹H NMR spectrum of G4-1, signals of the protons of PhCH₂ and BnCH groups of phenylalanine residue of G4-1 are easily determined according to their chemical shift values, coupling constants and split peak shapes. However, the protons with chemical shift values at 4.93 and 5.40 ppm cannot be properly assigned owning to their same coupling constants and peak shapes (d, J = 6.96 Hz). Nevertheless, based on the correlation of their ¹H–¹H COSY and ¹H–¹³C HSQC features, they can be very clearly assigned to be PhCH (5.40 ppm) of the phenylglycine residue and NH proton (4.93 ppm) of the TsNH group of N-Ts-phenylalanine residue. All relevant ¹H–¹H COSY and ¹H–¹³C HSQC spectra are available in the ESI.†

To evaluate chiral discriminating properties of TAMCSAs 1a–1c, first, the ¹H NMR spectrum of (±)-G1 (G1-1 (○) and G1-2 (●)) was measured in the presence of TAMCSA 1b. The result shows that the ¹H NMR signals of the corresponding protons for the PhCH₂, PhCH₂O, TsNH, CONH and ArH groups of (±)-G1 were split and their nonequivalent chemical shifts (ΔΔδ) were found to be 0.013, 0.020, 0.012, 0.013, 0.151, 0.081 and 0.020 ppm (Fig. 4).

Fig. 4 Overlaid ¹H NMR spectra (400 MHz) of (±)-G1 in the presence of TAMCSA 1b (TAMCSA 1b/(±)-G1 = 1 : 1, 10 mM) in CDCl₃, free (±)-G1 and free TAMCSA 1b from top to bottom.

Among them, ΔΔδ of the NH proton for the TsNH group was demonstrated to provide the maximum value (up to 0.151 ppm). Two protons belonging to PhCH₂ or PhCH₂O groups, with a subtle difference in stereochemistry, are also differentiated. The aforementioned results suggest that strong hydrogen bonding interactions may exist in diastereomeric complexes of TAMCSA 1b with (±)-G1. Consequently, under the same conditions, ¹H NMR spectra of G1-1 and G1-2 were also measured, respectively. Assignment of the two enantiomers of (±)-G1 was easily determined by comparing their chemical shift values, coupling constants and peak shapes. These preliminary results suggest that TAMCSA 1b has excellent binding properties towards chiral discrimination of (±)-G1.

To obtain better baseline resolution for chiral discrimination, the following chiral discriminating conditions were optimized. Firstly, ¹H NMR spectra of (±)-G1 with TAMCSA 1b were performed in different deuterated solvents such as CDCl₃, CDCl₃ containing 10% C₆D₆, CD₃COCD₃, CD₃OD and DMSO-d₆, respectively. The changes of ΔΔδ of the selected protons without any interfering signals are summarized in Table 1.

Table 1 Nonequivalent chemical shifts (ΔΔδ, ppm) of protons of PhCH₂O, TsNH, CONH and ArH of (±)-G1 in the presence of TAMCSA 1b in different deuterated solvents by ¹H NMR spectroscopy (400 MHz) at room temperature^a

Solvent	PhCH2O	TsNH	CONH	ArH
a Concentrations of (±)-G1 and TAMCSA 1b are 10 mM (1 : 1).
CDCl3	0.012/0.013	0.151	0.081	0.020
CDCl3/C6D6 (10%)	0.003/0.004	0.060	0.035	0.007
CDCl3/CD3COCD3 (10%)	0.000/0.000	0.038	0.018	0.000
CDCl3/CD3OD (10%)	0.000/0.000	—	—	0.000
CDCl3/DMSO-d6 (10%)	0.000/0.000	—	—	0.000

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As shown in Table 1, upon enhancement of solvent polarity, the intermolecular interactions between enantiomers of (±)-G1 and TAMCSA 1b were weakened and even disappeared. The results suggest that CDCl₃ is more suitable for chiral discrimination by ¹H NMR spectroscopy.

Secondly, samples of (±)-G1 with different concentrations were prepared in the presence of TAMCSA 1b in CDCl₃, and their ¹H NMR spectra were measured. Overlaid ¹H NMR spectra featuring the NH proton of the CONH group are shown in Fig. 5.

Fig. 5 Overlaid ¹H NMR spectra of NH proton of CONH group of (±)-G1 (G1-1 (○), G1-2 (●)) with different concentrations in the presence of TAMCSA 1b. The mole ratio of TAMCSA 1b and (±)-G1 is 1 : 1, unchangeable.

As the concentration gradually increase, the ΔΔδ values of the NH proton of the CONH group exhibit a gradually increasing trend from 0.021 (2 mM) to 0.166 ppm (40 mM). Based on general requirements for concentration in ¹H NMR spectroscopy, and common solubilities of hosts and guests, a concentration of 10 mM was used for chiral discrimination. It should be clarified that both a double peak at 6.17 ppm and another double peak between 6.40 and 6.58 ppm at 40 mM derive from ¹H NMR signals of NH protons of the TsNH group of a pair of enantiomers of (±)-G1.

Lastly, upon increasing the molar ratio of TAMCSA 1b, the ΔΔδ values of the NH protons of the CONH group of (±)-G1 were showing a gradually increasing trend. The largest change of ΔΔδ was found to be 0.038 ppm upon varying the molar ratios of TAMCSA 1b over (±)-G1 from 1 : 1.5 to 1 : 1 (Fig. 6). Based on these results, the 1 : 1 molar ratio of (±)-G1 with TAMCSAs 1b was used for chiral discrimination.

Fig. 6 Overlaid ¹H NMR spectra of NH proton of CONH group of (±)-G1 (G1-1 (○), G1-2 (●)) with various molar ratios in the presence of TAMCSA 1b. The concentration of (±)-G1 is 10 mM in CDCl₃, unchangeable.

Upon completion of the aforementioned optimization study, and in order to obtain further evidence to demonstrate the chiral discriminating ability of TAMCSAs 1a–1c, ¹H NMR spectra were measured for all (±)-G1–G8 samples prepared in the presence of TAMCSAs 1a–1c, with the exception of (±)-G1 that was prepared only in the presence of TAMCSAs 1b. The ¹H NMR spectra show that signals of various types of key protons belonging to (±)-G1–G8 exhibit splitting. Furthermore, TAMCSAs 1a–1c provide multiple detection capabilities for chiral discrimination in most cases. Among them, ΔΔδ value of the NH proton (TsNH group) of (±)-G8 is found to exhibit a maximum (0.486 ppm) in the presence of TAMCSA 1a. In particular, ¹H NMR signals due to eight types of protons of (±)-G5 can be differentiated in the presence of TAMCSAs 1a–1c. ¹H NMR spectra of the corresponding protons of (±)-G5 in the presence of TAMCSA 1a are shown in different colors (Fig. 7).

Fig. 7 Overlaid ¹H NMR spectra (400 MHz) of (±)-G5 in the presence of TAMCSA 1a (TAMCSA 1a/(±)-G5 = 1 : 1, 10 mM), free (±)-G5 and free TAMCSA 1a from top to bottom.

As shown in Fig. 7, the ΔΔδ values of protons belonging to CH₃CH, PhCH₃, PhCH₂CH, CH₃CH, PhCH₂O, TsNH, CONH and ArH groups of the (±)-G5 enantiomers are found to be 0.024, 0.025, 0.054, 0.054, 0.006, 0.293, 0.136 and 0.025 ppm in sequence. In particular, two aromatic hydrogens (ArH, pink) of the benzene ring (Ts group) were discriminated as well. This indicates that π–π stacking interactions may operate in the diastereoisomeric complexes of (±)-G5 with TAMCSA 1a.

To further test the intermolecular non-covalent interactions, the association constants of complexes of G5-1 and G5-2 with TAMCSA 1a were calculated by the nonlinear least-squares methods, respectively.¹⁵ The detailed results are summarized in Table 2.

Table 2 Association constants of G5-1 and G5-2 with TAMCSA 1a

TAMCSA	Guest	K a ^a (M^−1)	−ΔG (KJ mol^−1)
a K a values were calculated by the nonlinear curve-fitting method.
1a	G5-1	(1.05 ± 0.45) × 10^3	17.04 ± 1.14
1a	G5-2	(1.06 ± 0.45) × 10^3	17.07 ± 1.14

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Following measurements of the enantiomeric discrimination patterns noted above, ¹H NMR spectra of each enantiomer of G1–G8 with TAMCSAs 1a–1c were obtained under the same conditions. The assignments of enantiomers of (±)-G1–G8 were determined by comparing their chemical shift values, coupling constants and peak shapes. As an example, the partial ¹H NMR spectra and nonequivalent chemical shifts of the NH proton belonging to the TsNH group of (±)-G1–G8 are demonstrated in Table 3, with the exception of (±)-G3 in the presence of TAMCSA 1a and (±)-G6 in the presence of TAMCSAs 1b and 1c, since the ¹H NMR signal of the corresponding NH proton (TsNH group) exhibit interference by signals from other protons or are not discriminated (Table 3). Nonequivalent chemical shifts of other discriminated protons of (±)-G1–G8 are summarized in Table 4. Their partial ¹H NMR spectra are available in the ESI.†

Table 3 Nonequivalent chemical shifts (ΔΔδ, ppm) and partial ¹H NMR spectra of NH proton of TsNH group of (±)-G1–G8 in the presence of TAMCSAs 1a–1c (except (±)-G3 with TAMCSA 1a and (±)-G6 with TAMCSAs 1b and 1c) by ¹H NMR spectroscopy at room temperature, respectively (400 MHz)^a

TAMCSA/guest/ΔΔδ	Spectra^b^,^d	Spectra^c^,^d	TAMCSA/guest/ΔΔδ	Spectra^b^,^d	Spectra^c^,^d
a ΔΔδ = |Δδ1 − Δδ2|, Δδ1 = |δ1 − δfree|, Δδ2 = |δ2 − δfree|, the numbers “1 and 2” present two enantiomers of (±)-G1-8, respectively. b Partial ^1H NMR spectra of NH protons of Ts group of (±)-G1-8 (10 × 10^−3 M) in the presence of TAMCSAs 1a–1c, respectively, H : G = 1 : 1. c Overlaid partial ^1H NMR spectra of NH protons of Ts group of a single enantiomer of (±)-G1-8 (10 × 10^−3 M) under the same conditions noted above, H : G = 1 : 1. d The following signs stand for the corresponding stereoisomers: G1-1 (○), G1-2 (●); G2-1 (□), G2-2 (■); G3-1 (▽), G3-2 (▼); G4-1 (◊), G4-2 (◆); G5-1 (△), G5-2 (▲); G6-1 (☆), G6-2 (★); G7-1 (♤), G7-2 (♠); G8-1 (♡), G8-2 (♥).
1a/(±)-G1/0.154			1a/(±)-G5/0.293
1b/(±)-G1/0.151			1b/(±)-G5/0.133
1c/(±)-G1/0.160			1c/(±)-G5/0.132
1a/(±)-G2/0.142			1a/(±)-G6/0.048
1b/(±)-G2/0.143			1a/(±)-G7/0.141
1c/(±)-G2/0.140			1b/(±)-G7/0.104
1b/(±)-G3/0.060			1c/(±)-G7/0.105
1c/(±)-G3/0.059			1a/(±)-G8/0.486
1a/(±)-G4/0.199			1b/(±)-G8/0.425
1b/(±)-G4/0.243			1c/(±)-G8/0.432
1c/(±)-G4/0.242

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Table 4 Nonequivalent chemical shifts (ΔΔδ, ppm) of corresponding protons of (±)-G1–G8 (except NH protons of TsNH group) in the presence of TAMCSAs 1a–1c obtained by ¹H NMR spectroscopy at room temperature, (400 MHz)^a

TAMCSA/guest	Proton	ΔΔδ (ppm)	TAMCSA/guest	Proton	ΔΔδ (ppm)
a Nonequivalent chemical shifts (ΔΔδ, ppm) of (±)-G1–G8 (10 × 10^−3 M) in the presence of TAMCSAs 1a–1c, respectively, H : G = 1 : 1.
1a/(±)-G1	PhCH2	0.021		ArH	0.025
		0.032	1b/(±)-G5	CH 3	0.014
	PhCH2O	0.011		PhCH3	0.015
		0.012		PhCH2	0.029
	CONH	0.066		CH3CH	0.048
	ArH	0.021		PhCH2O	0.007
1b/(±)-G1	PhCH2	0.013			0.009
		0.020		CONH	0.075
	PhCH2O	0.012		ArH	0.019
		0.013	1c/(±)-G5	CH 3	0.014
	CONH	0.081		PhCH3	0.014
	ArH	0.020		PhCH2	0.029
1c/(±)-G1	PhCH2	0.014		CH3CH	0.049
		0.022		PhCH2O	0.008
	PhCH2O	0.013			0.010
		0.013		CONH	0.072
	CONH	0.089		ArH	0.017
	ArH	0.021	1a/(±)-G6	(CH3)2CH	0.011
1a/(±)-G2	PhCH2	0.013			0.009
		0.013		PhCH2	0.025
	PhCH2O	0.004			0.014
		0.004		(CH3)2CHCH	0.057
	CONH	0.034		PhCH2O	0.010
	ArH	0.008		CONH	0.089
1b/(±)-G2	PhCH2	0.008		ArH	0.021
		0.007	1b/(±)-G6	(CH3)2CH	0.007
	CONH	0.035			0.005
	ArH	0.007		PhCH2	0.015
1c/(±)-G2	PhCH2	0.007		(CH3)2CHCH	0.048
		0.008		PhCH2O	0.010
	CONH	0.073		CONH	0.081
	ArH	0.006		ArH	0.017
1a/(±)-G3	CH 3	0.009	1c/(±)-G6	(CH3)2CH	0.007
	PhCH2	0.009			0.005
		0.014		PhCH2	0.014
	PhCH2O	0.004		(CH3)2CHCH	0.043
	PhCH	0.008		PhCH2O	0.008
	ArH	0.006		CONH	0.074
1b/(±)-G3	CH 3	0.011		ArH	0.015
	PhCH2	0.015	1a/(±)-G7	PhCH3	0.022
		0.017		PhCH2	0.017
	PhCH2O	0.007			0.035
	PhCH	0.003		PhCH2O	0.008
	ArH	0.006			0.008
1c/(±)-G3	CH 3	0.011		ArH	0.005
	PhCH2	0.015	1b/(±)-G7	PhCH3	0.012
		0.016		PhCH2O	0.007
	PhCH2O	0.006	1c/(±)-G7	PhCH3	0.012
	PhCH	0.004		PhCH2O	0.007
	ArH	0.007	1a/(±)-G8	CH 3	0.100
1a/(±)-G4	PhCH2O	0.052		PhCH	0.060
		0.071		PhCH2O	0.011
	PhCH	0.047		CONH	0.250
1b/(±)-G4	PhCH2O	0.005		ArH	0.055
		0.029	1b/(±)-G8	CH 3	0.077
	PhCH	0.031		PhCH	0.058
1c/(±)-G4	PhCH2O	0.006		PhCH2O	0.009
		0.029		CONH	0.247
	PhCH	0.031		ArH	0.052
1a/(±)-G5	CH 3	0.024	1c/(±)-G8	CH 3	0.078
	PhCH3	0.025		PhCH	0.056
	PhCH2	0.054		PhCH2O	0.009
	CH3CH	0.054		CONH	0.253
	PhCH2O	0.006		ArH	0.052
	CONH	0.136

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As shown in Table 4, an unprecedented number of different types of protons from the same chiral analyte can be discriminated by TAMCSAs 1a–1c using ¹H NMR spectroscopy. These results show that TAMCSAs 1a–1c are highly effective chiral solvating agents with respect to the dipeptide derivatives (±)-G1–G8.

Now that TAMCSAs 1a–1c have been established to demonstrate excellent chiral discriminating abilities, their practical applicability was tested in determining enantiomeric excess (ee) of dipeptide derivatives with different optical compositions. Samples containing G1-1 with 85%, 65%, 45%, 25%, 0%, −25%, −45%, −65% and −85% ee were prepared in the presence of TAMCSA 1c and then their ¹H NMR spectra were recorded. The enantiomeric excesses (ee) of all samples were accurately obtained based on the integration of ¹H NMR signals of the NH protons (TsNH group) of enantiomers G1-1 and G1-2 (Fig. 8).

Fig. 8 Determination of enantiomeric excesses of G1, ee% = [(G1-1)–(G1-2)] %, G1-1 and G1-2 stand for two enantiomers of (±)-G1. (a) The overlaid ¹H NMR spectra of NH proton of TsNH group of G1-1 (left) and G1-2 (right) in the presence of equal amounts TAMCSA 1c (5 mM) in CDCl₃ at room temperature. (b) Linear correlation between the theoretical (Y) and observed (X) ee% values.

Additionally, in order to further investigate chiral discriminating behavior, Job plots of (±)-G1 were performed in the presence of TAMCSA 1c. The Job plots of (±)-G1 exhibited a maximum value (X*Δδ = 0.042 ppm) at a molar fraction of X = 0.5, which suggests that a diastereoisomer complex with 1 : 1 stoichiometry was established between (±)-G1 and TAMCSA 1c (Fig. 9).

Fig. 9 Job plots for complex of (±)-G1 and TAMCSA 1c. Δδ stands for chemical shift change of NH proton of TsNH group of G1-1 and G1-2 in the presence of TAMCSA 1c. X stands for the molar fraction of (±)-G1, (X = [(±)-G1]/[(±)-G1] + [1c]).

To verify the results noted above, the stoichiometric interaction of (±)-G1 with TAMCSA 1c was evaluated by electrospray ionization (ESI) mass spectroscopy. The partial ESI mass spectrum of TAMCSA 1c and (±)-G1 exhibit formation of a 1 : 1 complex [1c + G1 + H]⁺ (m/z 1289.3025). Partial and expanded ESI mass spectra derived from the same spectrum are shown in Fig. 10. The full ESI mass spectrum of (±)-G1 and TAMCSA 1c is available in the ESI.†

Fig. 10 Partial and expanded ESI mass spectra of (a) [G1 + Na]⁺: calcd for C₃₁H₃₀N₂NaO₅S [M + Na]⁺ 565.1773, found 565.1765; (b) [1c + H]⁺: calcd for C₃₆H₃₇Br₂N₄O₄ [M + H]⁺ 747.1176, found 747.1177; (c) [1c + G1 + H]⁺: calcd for C₆₇H₆₇Br₂N₆O₉S [M + H]⁺ 1289.3051, found 1289.3025.

Finally, to better understand the chiral discriminating behavior between (±)-G1 and TAMCSA 1c, theoretical calculations were carried out using an AM1 model by Gaussian 03 program.¹⁶ The proposed model suggests that a pair of diastereoisomer complexes of (±)-G1 with TAMCSA 1c are formed with 1 : 1 stoichiometry. The proposed model exhibits the distances of hydrogen bonds (S=O⋯HNCO and NHCO⋯HNSO₂) of G1-1 and G1-2 with TAMCSA 1c as 1.91 and 2.02, 2.05 and 1.97 Å, respectively. The models separated from the following proposed binding model are available in the ESI.† The proposed binding model is shown in Fig. 11.

Fig. 11 Proposed model for the intermolecular interactions of G1-1 and G1-2 with TAMCSA 1c.

Conclusions

In summary, we have developed a new class of tetraaza macrocyclic chiral solvating agents (TAMCSAs) 1a–1c with two amide, two amino and two phenolic hydroxyl groups, featuring C₂-symmetry and a 12-membered cavity as a framework for chiral discrimination. All enantiomers of the dipeptide derivatives with two chiral centers (±)-G1–G8 were successfully discriminated by the corresponding ¹H NMR signals of various types of protons in the presence of TAMCSAs 1a–1c. Enantiomeric excesses (ee) of G1 with different optical compositions were calculated based on the integration of ¹H NMR signals of NH protons of the TsNH group of enantiomers G1-1 and G1-2 in the presence of TAMCSA 1c. The results have proved that the unique structural features of TAMCSAs 1a–1c are enormously helpful for promoting chiral discrimination. The chiral discriminating behavior has been evaluated by means of Job plots, ESI mass spectra and a theoretically proposed model of (±)-G1 with TAMCSA 1c. In addition, the association constants of enantiomers of (±)-G5 with TAMCSA 1a were calculated by the nonlinear curve-fitting method. In conclusion, TAMCSAs 1a–1c have exhibited strong binding properties, better baseline resolution and multiple detection windows for chiral discrimination. These results demonstrate that TAMCSAs 1a–1c are especially suitable for chiral discrimination of these dipeptide derivatives by ¹H NMR spectroscopy.

Acknowledgements

This work was supported by The Beijing Municipal Commission of Education. Work in the Missouri lab (P. S.) is supported by the National Institute of General Medical Sciences of the National Institutes of Health under Award Number R15GM117508.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis and characterization of TAMCSAs 1a–1c and enantiomers of dipeptide derivatives with two chiral centers G1–8, experimental detail and original NMR spectra of chiral discrimination. CCDC 1007044, 1007045 and 1007047. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6qo00521g

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