The Impact of Solvent and the Receptor Structure on Chiral Recognition Using Model Acyclic Bisamides Decorated with Glucosamine Pendant Arms.

We investigated the influence of various factors (including solvent mixtures) on chiral recognition of chiral carboxylates, using the titration method under 1H NMR control. We found that strong binding carboxylates (geometrical matching) is not enough for the satisfactory differentiation of enantiomers. Moreover, solvent mixture studies indicate a significant influence of environment on the formation of diastereomeric complexes and variations among them. Our findings offer insights into the complementarity of chiral recognition processes.

Introduction

Solving the puzzle of how nature woran class="Chemical">ks remains as an unending source of challenges for researchers.[1] One such insufficiently understood problem lies in the subtlest types of selectivity known as chiral recognition.[2] Therefore, current research focuses on aspects such as asymmetric catalysis,[3] molecular recognition,[4] chiral separation,[5] interaction on surfaces,[6] and supramolecular assemblies of chiral molecules in solution.[7]

an class="Chemical">Chirne">al rean class="Chemical">cognition phenomena originate from differences in the Gibbs free energy (ΔGtotal) of diastereomeric complexes formed between chiral molecules.[8] ΔGtotal depends on the energy of intermolecular interactions (Einter), the energy from conformational changes (ΔEintra), and free solvation energy (ΔGsolv). The latter includes the free solvation energy of the complex (ΔGsolv), free receptor (ΔGsolv), and free guest (ΔGsolv) (Figure .).[9]

Figure 1

ΔGtotal equation.

Differenan class="Chemical">ces in interactions between solvent and anionic species and receptors lead to slight differences in ΔGsolv and depend on the medium. The solvent’s parameters define this distinctiveness. Relative polarity, dielectric constant (ε), and Gutmann numbers[10] are among the criteria describing divergences in solvation (see Table ). If host–guest interactions are electrostatic and the solvent weakly solvates both, the magnitude of the binding constant is inversely correlated to the dielectric constant of the solvent.[11] Gutmann’s Donor Number (DN) defines the donicity of a solvent, meaning its behavior as a Lewis base solvent, while Gutmann’s Acceptor Number (AN) reflects a solvent’s character as a Lewis acid. Hence, the interactions between a solvent and a charged anion and the H-donor cavity lie along the ion–dipole and dipole–dipole interactions. Analysis of differences in solvent features indicates that the medium could also be a meaningful factor in chiral recognition. To our knowledge, however, there have been no in-depth studies describing the influence of the medium on chiral recognition of anions.

Table 1

Properties of Solvents Used: Gutmann Donor Number (DN) (Kcal Mol–1), Gutmann Acceptor Number (AN) (Kcal Mol–1), Relative Polarity (ET), and Dielectric Constant (ε)

entry	solvent	DN[12]	AN[12]	ET[13]	ε[13]
1	CH3CN	14.1	18.9	0.460	37.5
2	DMSO	29.8	19.3	0.444	46.68
3	CHCl3	4.0	23.1	0.259	4.89
4	H2O	54.8	18.0	1.000	80.1

an class="Gene">Another factor is the proper design of the chiral receptor, crucial for binding the anion.[14] Many factors govern the chiral recognition phenomenon, making a prior prediction difficult. The appropriate arrangement of hydrogen bond donors and chiral fragments should ensure enantioselective interactions with chiral anions. Hence, effective chiral recognition of anions requires the synthesis and determination of binding affinities and enantioselectivities of prospective chiral receptors.[15]

Based on these an class="Chemical">considerations, we designed a series of chiral receptors (1a–1e) of various sizes and binding pocket geometries, consequently with different arrangements of hydrogen bonding donors (Figure ). To achieve this goal, we took the approach of using the covalent attachment of the chiral moiety to an anion binding backbone. Based on our experience[16] showing bisamides to be attractive building blocks for achiral receptors, we applied different simple aromatic platforms (benzene, pyridine, azulene, and pyrrole). As a chiral part, we chose a peracetylated glucosamine derivative,[17] which is a cheap and readily available source of chirality and can be easily functionalized by changing the protecting groups.

Figure 2

(A) Idea of a chiral receptor structure; (B) chiral receptors 1a–1e investigated in this study.

(n class="Gene">A) Idea of a chiral receptor structure; (B) chiral receptors 1a–1e investigated in this study.

Results and Discussion

First, we an class="Gene">synthesized a series of amide-based receptors using per-O-acetyl-D-glucosamine hydrochloride with acid dichlorides previously prepared from the corresponding dicarboxylic acid.[18] Due to the high nucleophilicity of the 1- and 3-position of the azulene moiety, we applied HBTU (2-(1-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate) to obtain 5,7-bisamide azulene receptor 1a (Scheme ).[19]

Scheme 1

Synthesis of Model Bisamides 1a–1e

We dan class="Gene">etermined the anion binding properties using the 1H NMR titration method. This technique keeps track of the binding process and shows differences in the formation of diastereomeric complexes.[20] Utilizing changes in chemical shifts, we obtained all global stability constants by nonlinear curve fitting to the 1:1 and 1:2 (receptor: anion) binding model[21] using the program HypNMR2008.[22]

Henan class="Chemical">ce, as to evne">aluate the influence of geometry on binding affinity and modes of anion binding by amide-based receptors, we conducted titration experiments with the series of receptors 1a–1e with achiral benzoate anion as tetrabutylammonium salt (TBA). Due to the solubility of receptors (all studied ligands are insoluble in aqueous media), we conducted titration experiments in CD3CN + 0.5% H2O. The respective binding constants toward benzoate anion (Ka), the geometrical parameters of the binding pocket, and the maximum chemical shift (Δδmax) of the protons in the binding cavity (green H in Figure ) are presented in Table .

Figure 3

Geometrical parameters presented in Table .

Table 2

Comparison of Geometrical Parameters of the Binding Pocket[16] with Binding Constants Ka [M–1]a for Complexes of Receptors 1a–1e with Benzoate in CD3CN + 0.5% H2Oa−c

receptor	1a	1b	1c	1d	1e
Ka	460	49	1400	>104c	>104c
Δδmax [ppm]	0.33	−	0.77	4.16	1.61
d [Å]b	4.0	4.6	5.0	5.4	5.6
α [o]b	117	117	123	139	145

Values determined by 1H NMR spectroscopy titration experiments at T = 303 K; estimated errors <10%; TBA salts were the sources of the anions.

Geometrical parameters determined for conformation syn–syn using X-ray for R = n-Bu.

Binding model 1:2 (host:guest), K1:2 is omitted, for more details see Supporting Information.

Geomn class="Gene">etrical parameters presented in Table .

Values dn class="Gene">etermined by 1H NMR spectroscopy titration experiments at T = 303 K; estimated errors <10%; TBA salts were the sources of the anions.

Geomn class="Gene">etrical parameters determined for conformation syn–syn using X-ray for R = n-Bu.

Binding model 1:2 (host:guest), an class="Gene">K1:2 is omitted, for more dan class="Gene">etails see Supporting Information.

In all an class="Chemical">cases, anion complexation caused a downfield shift of C–H and N–H protons located in the receptor binding pocket. The calculated affinity constants generally increased with the size of the binding cavity. Receptors 1d and 1e, based on a five-membered aromatic core, revealed the highest binding affinity toward benzoate anion (binding constants up to 10,000 M–1) and the 1:2 (host:guest) binding model. Dipicolinic acid derivative 1b binds the anion with the lowest constant Ka = 50 M–1 (Figure ). The repulsive interaction between anion and the electron free pair located on the nitrogen atom may attenuate the formation of hydrogen bonds. Growing changes in chemical shifts of green protons demonstrating weak C–H hydrogen bonds (for 1a, 1c, and 1e) and strong N–H bonds (for 1d) are in accordance with the calculated affinity constants.

Figure 4

Dependence of logKa with the size of the binding pocket.

Dependenn class="Chemical">ce of logKne">a with the size of the binding pocket.

Next, to estimate the rean class="Chemical">ceptors’ potentine">al for an class="Chemical">chiral recognition, we evaluated their binding properties with respect to two pairs of enantiomeric carboxylate derivatives: mandelic acid (Man) and N-Ac-phenylglycine (N-Ac-Phg) (see Figure ). Given the magnitude of the binding constants with benzoate, we conducted titration experiments in CD3CN + 0.5% H2O as a solvent mixture. We used chiral anions as TBA salts. We calculated the association constants (KR and KS) determined by separate titration experiments and then compared them. To evaluate the enantioselective properties, we applied thermodynamic selectivity (α), which is a ratio of the binding constants of two diastereomeric complexes (α = KR/KS).

Figure 5

Titration curves of receptor 1e with Man (R, pink; S, blue) in (A) CD3CN + 0.5% H2O and (B) CDCl3. (C) Receptor 1a with Man in CD3CN + 0.5% H2O.

Titration n class="Chemical">curves of rean class="Chemical">ceptor 1e with Man (R, pink; S, blue) in (A) CD3CN + 0.5% H2O and (B) CDCl3. (C) Receptor 1a with Man in CD3CN + 0.5% H2O.

Influence of Receptor’s Geometry on Chiral Recognition

By analogy to the aan class="Chemical">chirne">al an class="Chemical">benzoate anion, we observed that anions formed complexes with respect to receptors with the same stoichiometry (Table ). Receptors 1b–1d did not exhibit meaningful enantioselective properties toward the investigated chiral pairs of anions (see the Supporting Information, Figure S1). Ligand 1e recognized the model chiral pair of anion derivatives with α = 3.1 and α = ∼2, respectively, for Man and N-Ac-Phg. Receptor 1a bound the (R) enantiomer more strongly with α = 2.1, with no chiral recognition for Phg derivatives. Comparison analysis of titration curves revealed that the source of the differentiation of chiral mandelic anions for the 5,7-disubstituted azulene derivative 1a is the difference in binding of the anionic part with amide groups of the receptor (Figure c). For receptor 1e we observed perturbation in chemical shifts both for binding cavity protons and for sugar moieties (Figure a). Firstly, this may suggest that the sugar parts adopt a conformation preventing interaction with the side chain of the anions; secondly, it may indicate a close distance of chiral fragments from the binding pocket. Some explanation of origin enantioselective discrimination by 1a and 1e we could find in NOESY experiments, where we observe forming hydrogen bonding only by (R)-enantiomers of mandelate with azulene receptors (for more information see Supporting Information). The absence of significant chiral recognition for ligand 1d and its presence for 1e shows strong binding of the anion by the chiral receptor is insufficient for successfully recognizing enantiomers.

Table 3

Stability Constants Ka [M–1]a and Chiral Recognition α for Complexes of Hosts 1a–e with Chiral Anions in CD3CN + 0.5% H2O

		1a		1b		1c		1d		1e
entry	anion	Ka	α	Ka	α	Ka	α	Ka	αd	Ka	αd
1	R-Man	230	2.1	11	1.1	170	1.1	1400c	1.2	4400c	3.1
2	S-Man	110		10		160		1200c		1400c
3	L-N-Ac-Phg	130	1.1	18	1.1	350	0.9	6900c	0.9	>104b,c	b,c
4	D-N-Ac-Phg	140		20		330		6200c		5030c

Values determined by 1H NMR spectroscopy titration experiments at T = 303 K: estimated errors <10%; TBA salts were the sources of the anions.

Stability constants above the limit of the 1H NMR titration technique (Ka > 104); α was estimated.

Binding model 1:2 (host:guest), K1:2 is omitted, for more details see ESI

α given for KR1:1/KS1:1.

Values dn class="Gene">etermined by 1H NMR spectroscopy titration experiments at T = 303 K: estimated errors <10%; TBA salts were the sources of the anions.

Stability n class="Chemical">constne">ants above the limit of the 1H NMR titration technique (Ka > 104); α was estimated.

Binding model 1:2 (host:guest), an class="Gene">K1:2 is omitted, for more dan class="Gene">etails see ESI

α given for an class="Chemical">KR1:1/an class="Chemical">KS1:1.

Impact of the Guest on Chiral Recognition

Model an class="Chemical">chirne">al pne">airs of an class="Chemical">carboxylates (Man and N-Ac-Phg) exhibit different sizes, acidity, and ability to form hydrogen bonds on the substituent of the α carbon atom. All ligands revealed higher affinity constants toward N-Ac-Phg derivatives in comparison to mandalates. We observed enantioselectivity with amino acid derivatives only for receptor 1e. The results indicate that the guest’s structure and complex-forming properties influence the extent of chiral recognition.

Influence of Solvent on Chiral Recognition

Next, to estimate the influenan class="Chemical">ce of the solvent on chiral recognition properties, we conducted titration experiments with receptor 1e in the presence of model chiral anionic guests, mandelate derivatives, used as TBA salts in various solvent mixtures.

The addition of an class="Chemical">water into the solvent mixture an class="Chemical">caused a lowering of the binding constant of receptor 1e with the anions examined (Table , entry 1–6). The admixture of 5% water into CD3CN changed the affinity toward the (R) enantiomer. Experiments conducted in CD3CN + 5% H2O and DMSO-d6 + 0.5% H2O revealed similar binding affinities. Data fitting showed the considerable chiral recognition ability of receptor 1e in the CD3CN + 0.5% H2O mixture and in chloroform (α = 3.1 and 2.1, Table , entry 3 and 4, and 9 and 10, respectively). Comparison of the relative polarity of dimethylsulfoxide and acetonitrile (0.460 and 0.444, respectively, Table , entry 1 and 2) indicates the low impact of this parameter on the binding affinity. The higher value of DN points to stronger solvation of the host’s binding pocket and simultaneously weakened interaction of guest with the ligand (DNDMSO = 29.8 kcal mol–1, DNACN = 14.1 kcal mol–1). Similarly, the addition of water (DN = 54.8 kcal mol–1) into the solvent mixture causes an increase in environmental competitiveness.

Table 4

Stability Constants Ka [M–1]a and Chiral Recognition for Complexes of Host 1e with TBA Man in Various Solvent Mixtures

entry	solvent mixture	anion	Ka	αd
1	CD3CN	(R)-Man	>10 000b,c	b
2		(S)-Man	>10 000b,c
3	CD3CN + 0.5% H2O	(R)-Man	4400	3.1
4		(S)-Man	1400
5	CD3CN + 5% H2O	(R)-Man	11	0.7
6		(S)-Man	16
7	DMSO-d6 + 0.5% H2O	(R)-Man	11	1.0
8		(S)-Man	11
9	CDCl3	(R)-Man	2700	2.1
10		(S)-Man	1300

Values determined by 1H NMR spectroscopy titration experiments at T = 303 K; estimated errors <10%; anions added as TBA salts.

Stability constants above the limit of the 1H NMR titration technique (Ka > 104), α could not be determined.

Binding model 1:2 (host:guest), K1:2 is omitted, for more details see ESI.

α given for KR1:1/KS1:1.

Values dn class="Gene">etermined by 1H NMR spectroscopy titration experiments at T = 303 K; estimated errors <10%; anions added as TBA salts.

Stability n class="Chemical">constne">ants above the limit of the 1H NMR titration technique (Ka > 104), α could not be determined.

Binding model 1:2 (host:guest), an class="Gene">K1:2 is omitted, for more dan class="Gene">etails see ESI.

α given for an class="Chemical">KR1:1/an class="Chemical">KS1:1.

an class="Chemical">Comparative anne">alysis of the titration curves of receptor 1e in the CD3CN + 0.5% H2O mixture and in chloroform revealed differences in the formation of diastereomeric complexes, depending on the experimental mixture (Figure a,b). In CD3CN + 0.5% H2O, we observed disparities in chemical shifts for both the cavity’s protons and sugar moieties in relation to the respective enantiomer (Figure a). In contrast, similar comparison analysis for chloroform revealed that recognition of stereoisomers occurs by the interaction between the side chain of anion and sugar derivatives with no meaningful difference in chemical shifts of protons in the binding pocket (Figure b). These outcomes show that the chosen solvent contributes to the formation of different diastereomeric complexes and to chiral recognition.

Conclusions

We have reported here the n class="Gene">synthesis and anion carboxylate binding properties of a series of chiral receptors bearing glucosamine pendant arms with varied geometries of the binding site. In the course of these studies we discovered the following:

an class="Chemical">Compne">arison of geoman class="Gene">etric parameters showed that receptors based on a five-membered ring demonstrate the highest affinity toward the carboxylate anion, creating mixed complexes with 1:2 ligand:anion stoichiometry.

Strong binding of the an class="Chemical">anion by the an class="Chemical">chiral receptor is insufficient for successfully recognizing enantiomers; rather, proper conformation is needed to ensure enantioselective interactions.

The n class="Chemical">complexation medium has a significant impact on complex formation and on enantiomer differentiation.

Overall, the studies presented above have provided insights into the effean class="Chemical">cts of the geometry and size of the binding cavity on affinity for carboxylate anions and chiral recognition. We evaluated whether the choice of solvent mixture affected the formation of diastereomeric complexes, keeping track of the binding process using the 1H NMR titration technique, and discovered that it does indeed exert an impact. Overall, our findings offer insight into the complementarity of chiral recognition processes.

Experimental Section

an class="Gene">All precursors for synthesis were obtained from commercial suppliers and were used without further purification. All solvents were of reagent grade quality and were dried under standard conditions. Flash chromatography was carried out using silica gel 60 (63–100 mesh); typically, a 40-fold mass excess of gel was used. TLC analysis was carried out on precoated silica gel plates (60 F254). 1H and 13C NMR spectra were recorded with 400 and 600 MHz NMR instruments. HRMS measurements were performed with ESI ionization and a TOF analyzer.

General Procedure for the Preparation of Diamide Derivatives 1b–1e

The reaan class="Chemical">ction wne">as an class="Chemical">carried out under argon conditions. To the solution of dichloride acid (1.1 mmol) in dry dichloromethane (100 mL), triethylamine (4.4 mmol) was slowly added. After 5 min of stirring, per-O-Ac-glucosamine hydrochloride (2.2 mmol), obtained according to the literature procedure,[17] was added and the reaction mixture was stirred overnight. Afterward, the reaction mixture was washed with 0.1 M HCl (2 × 50ml), saturated NaHCO3 (2 × 50ml), and water (1 × 50ml). Then, the organic phase was separated and was dried over MgSO4, and then filtrated and evaporated under vacuum. The crude product was purified using silica gel column chromatography with mixtures of dichlomethane and metanol (200:1 > 30:1, v/v] as eluents. The product was crystallized from a dichloromethane:hexane (1:3 v/v) mixture, yielding as solid.

Receptor 1a

The reaan class="Chemical">ction wne">as an class="Chemical">carried out under argon conditions. To the solution of diacid 2a (216 mg, 1 mmol) (obtained according to the literature procedure)[19] in dry DMF (100 mL) , triethylamine (1.4 mL, 10 mmol) was slowly added followed with HBTU (1.5 g, 4 mmol). After 10 min of stirring, per-O-Ac-glucosamine hydrochloride (1.5 g, 4 mmol), obtained according to the literature procedure,[17] was added and the reaction mixture was stirred overnight. Afterward, the mixture was concentrated on a rotary evaporator to 1/3 of the starting volume and water (10 mL) was added and the precipitate was washed with water. The crude product was purified by column chromatography on silica gel with the dichloromethane:methanol (99:1, v/v) mixture as an eluent. The product was crystallized from the dichloromethane:hexane (1:3, v/v) mixture yielding 1a (0.5 g, 57%) as blue solid. Mp: 124–127 °C. 1H NMR (400 MHz, DMSO-d6): δ = 8.88 (d, J = 8.9 Hz, 2H), 8.75 (d, J = 1.7 Hz, 2H), 8.47 (s, 1H), 8.07 (t, J = 3.8 Hz, 1H), 7.73 (d, J = 3.8 Hz, 2H), 5.95 (d, J = 8.8 Hz, 2H), 5.41 (t, J = 9.9 Hz, 2H), 5.02 (t, J = 9.6 Hz, 2H), 4.30–4.20 (m, 4H), 4.08–4.01 (m, 4H), 2.05 (s, 6H), 2.04 (s, 6H), 2.01 (s, 6H), 1.92 (s, 6H). 13C{H} NMR (100 MHz, DMSO-d6): δ = 170.0, 169.7, 169.2, 168.9, 168.9, 138.7, 137.2, 136.4, 135.5, 127.4, 124.4, 91.8, 72.4, 71.7, 68.0, 61.5, 53.4, 45.8, 20.5, 20.4, 20.3. HRMS (ESI–TOF) m/z: [M + Na]+ calcd for C40H46N2O20Na, 897.2542; found, 897.2523. Anal. Calcd for C40H46N2O20: C, 54.92; H, 5.30; N, 3.20. Found: C, 54.81; H, 5.40; N, 3.25.

Receptor 1b

Rean class="Chemical">ceptor 1b was prepared according to the general procedure using commercially available 2,6-pyridinedicarbonyl dichloride (0.51 g, 2.5 mmol) yielding the product (1.35 g, 65%) as white powder. Mp: 196–197 °C. 1H NMR (400 MHz, Acetonitrile-d3): δ = 8.32 (d, J = 9.4 Hz, 2H), 8.21–8.16 (m, 2H), 8.11 (dd, J = 8.6, 6.7 Hz, 1H), 6.03 (d, J = 8.7 Hz, 2H), 5.62–5.54 (m, 2H), 5.19 (t, J = 9.8 Hz, 2H), 4.27 (dt, J = 13.5, 5.8 Hz, 4H), 4.12 (dd, J = 12.4, 2.2 Hz, 2H), 3.99 (ddd, J = 10.0, 4.7, 2.3 Hz, 2H), 2.04 (s, 6H), 2.04 (s, 6H), 1.99 (s, 6H), 1.87 (s, 6H). 13C{H} NMR (100 MHz DMSO-d6): δ = 172.1, 171.3, 170.5, 170.1, 164.6, 149.2, 140.8, 125.6, 93.4, 73.8, 73.2, 68.9, 62.7, 54.5, 21.0, 21.0, 20.9, 20.9. HRMS (ESI–TOF) m/z: [M + Na]+ calcd for C35H43N3O20Na, 847.2385; found, 847.2356. Anal. Calcd for C35H43N3O20: C, 50.91; H, 5.25; N, 5.09. Found: C, 50.89; H, 5.30; N, 5.13.

Receptor 1c

Rean class="Chemical">ceptor 1an class="Chemical">c was prepared according to the general procedure using commercially available isophthaloyl dichloride (0.5 g, 2.5 mmol) yielding the product (1.5 g, 75%) as white powder. Mp: 117–120 °C. 1H NMR (400 MHz, DMSO-d6): δ = 8.74 (d, J = 9.0 Hz, 2H), 8.17 (s, 1H), 7.87 (d, J = 6.7 Hz, 2H), 7.60 (t, J = 7.7 Hz, 1H), 5.91 (d, J = 8.7 Hz, 2H), 5.37 (t, J = 9.9 Hz, 2H), 4.99 (t, J = 9.6 Hz, 2H), 4.24 (m, J = 10.2 Hz, 4H), 4.01 (m, J = 11.3 Hz, 4H), 2.03 (s, 6H), 2.00 (s, 6H), 2.00 (s, 6H), 1.86 (s, 6H). 13C{H} NMR (100 MHz, DMSO-d6): δ = 170.0, 169.6, 169.2, 168.8, 165.8, 134.2, 129.8, 128.6, 126.2, 91.8, 72.4, 71.7, 68.0, 61.5, 52.7, 20.5, 20.4, 20.4, 20.2. HRMS (ESI–TOF) m/z: [M + Na]+ calcd for: C36H44N2O20Na, 847.2385; found, 847.2356. Anal. Calcd for C36H44N2O20·0.5 H2O: C, 51.86; H, 5.44; N, 3.36. Found: C, 51.57; H, 5.54, N, 3.36.

Receptor 1d

Rean class="Chemical">ceptor 1d was prepared according to the general procedure using acid dichloride obtained according to the literature procedure[18] (0.155 g, 1 mmol) yielding the product (320 g, 42%) as white powder. Mp: 231–234 °C. 1H NMR (400 MHz, DMSO-d6): δ = 11.82 (s, 1H), 8.40 (d, J = 8.6 Hz, 2H), 6.67 (s, 2H), 5.85 (d, J = 8.8 Hz, 2H), 5.30 (t, J = 10.1 Hz, 2H), 4.96 (t, J = 9.7 Hz, 2H), 4.28–4.09 (m, 4H), 4.03 (t, J = 11.1 Hz, 4H), 2.02 (s, 6H), 2.00 (s, 6H), 1.99 (s, 6H), 1.86 (s, 6H).13C{H} NMR (100 MHz, DMSO-d6): δ = 170.0, 169.5, 169.2, 168.8, 159.6, 128.4, 111.9, 91.8, 72.2, 71.5, 68.1, 61.5, 52.1, 20.5, 20.5, 20.4, 20.3. HRMS (ESI–TOF) m/z: [M + Na]+ calcd for C34H43N3O20Na, 836.2338; found, 836.2349. Anal. Calcd for C34H43N3O20·H2O: C, 49.10; H, 5.45; N, 5.05. Found: C, 49.13; H, 5.26; N, 5.02.

Receptor 1e

Rean class="Chemical">ceptor 1e was prepared according to the general procedure using acid dichloride obtained according to the literature procedure[16] (0.380 g, 1.5 mmol) yielding the product (750 g, 75%) as purple powder. Mp: 223–226 °C. 1H NMR (400 MHz, DMSO-d6): δ = 9.47 (d, J = 9.6 Hz, 2H), 8.45 (d, J = 8.7 Hz, 2H), 8.33 (s, 1H), 8.06 (t, J = 9.2 Hz, 1H), 7.72 (t, J = 9.7 Hz, 2H), 5.96 (d, J = 8.7 Hz, 2H), 5.44 (t, J = 9.9 Hz, 2H), 5.00 (t, J = 9.4 Hz, 2H), 4.30 (dd, J = 36.2, 9.4 Hz, 4H), 4.04 (d, J = 11.7 Hz, 4H), 2.04 (s, 12H), 2.00 (s, 6H), 1.87 (s, 6H). 13C{H} NMR (100 MHz, DMSO-d6): δ = 170.0, 169.7, 169.2, 168.9, 165.0, 141.4, 140.3, 138.8, 135.8, 128.7, 119.8, 92.0, 72.6, 71.6, 68.2, 61.5, 52.2, 20.6, 20.5, 20.4, 20.3. HRMS (ESI–TOF) m/z: [M + Na]+ calcd for C40H46N2O20Na, 897.2542; found, 897.2515. Anal. Calcd for C40H46N2O20: C, 54.92; H, 5.30; N, 3.20. Found: C, 54.86; H, 5.44; N, 3.15.