Bis(amino amides) derived from natural amino acids as chiral receptors for N-protected dicarboxylic amino acids

Tetrahedron Letters 54 (2013) 72–79

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Bis(amino amides) derived from natural amino acids as chiral receptors for N-protected dicarboxylic amino acids Belén Altava a,⇑, M. Isabel Burguete a, Noèlia Carbó a, Santiago V. Luis a,⇑, Vicente Martí-Centelles a, Cristian Vicent b a b

Departamento de Química Inorgánica y Orgánica, Universitat Jaume I, Avda. Sos Baynat, s/n, 12071 Castellón, Spain Servei Centrals d0 Instrumentaciò Cinetífica, Universitat Jaume I, Avda. Sos Baynat, s/n, 12071 Castellón, Spain

a r t i c l e

i n f o

Article history: Received 14 September 2012 Revised 19 October 2012 Accepted 23 October 2012 Available online 1 November 2012 Keywords: Bis(amino amides) CSA Chiral receptors Dicarboxylic amino acids Enantiodiscrimination

a b s t r a c t A family of bis(amino amides) derived from natural amino acids has been synthesized and tested for the NMR enantiodiscrimination, as chiral receptors, of some N-protected dicarboxylic amino acids. The influence of the amino acid side chain is an important parameter to obtain good enantiodiscrimination. The binding between bis(amino amides) and N-protected dicarboxylic amino acids has been thoroughly studied by ESI-MS and NMR spectroscopic methods as well as by molecular modeling. Ó 2012 Elsevier Ltd. All rights reserved.

Introduction Selective receptors for chiral molecules have many technological, industrial, biomedical, and environmental applications.1 Thus, artificial chiral receptors for carboxylic acids are important as many relevant biomolecules contain this functionality. Dicarboxylates, implicated in different biomolecular processes, are particularly important targets in this regard.2 Some polyamine receptors are able to interact with dicarboxylic acids in both organic solvents and aqueous media.3 Although most dicarboxylic acids present in biological systems are chiral, only a few enantioselective receptors for chiral dicarboxylic acids have been described.4 Peptidomimetic structures have been designed for multiple purposes, including their use as chiral receptors, and we have been recently involved in the preparation and study of pseudopeptides with the general structure 4.5 In this context, some of those compounds have shown to be efficient chiral shift agents (CSAs) for the enantiodifferentiation of mandelic acid and some related guests, including relevant aryl propionic acids.6 Futhermore, in the literature several host compounds with bisamide functionalities have been developed and studied as CSAs,7 for amines,7a,7b derivatized amino acids,7c,7d and carboxylic acids.7e,7f,4q

⇑ Corresponding authors. Tel.: +34 6472 8237; fax: +34 6472 8214 (B.A.); tel.: +34 6472 8239; fax: +34 6472 8214 (S.V.L.). E-mail addresses: [email protected] (B. Altava), [email protected] (S.V. Luis). 0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2012.10.099

Taking this into account, we considered the synthesis of new bis(amino amides) derived from natural amino acids, with the general structure 4 as effective chiral solvating agents for N-protected dicarboxylic amino acids. An aliphatic spacer with five methylene groups was selected for our study after some molecular modeling in order to develop the appropriate ditopic receptor, and several starting amino acids were considered. Results and discussion Open-chain pseudopeptides 4 derived from L-phenylalanine, and L-proline, were prepared from the corresponding N-Cbz protected amino acid through the initial formation of their activated N-hydroxysuccinimide esters (2), coupling with 1,5-pentanediamine, and final N-deprotection, following reported procedures (Scheme 1).8 Overall yields, after the final deprotection step, were in the range of 80%. These chiral bis(amino amides) were fully characterized by 1H NMR, 13C NMR, IR, ESI-MS, and EA. L-tryptophan,

Chiral Shift Agents (CSAs) In preliminary experiments, 1H NMR spectroscopy was used to investigate their chiral recognition ability using the racemic mandelic acid 5.6 Table 1 shows the chemical shift non-equivalences (DDd) for the enantiomers of mandelic acid in the presence of 2 equiv of the different hosts in CDCl3/CD3OD (5%) at 30 °C. Higher chemical shift non-equivalences were obtained with the bis(amino

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B. Altava et al. / Tetrahedron Letters 54 (2013) 72–79

Cbz

O

H N

i)

OH

Cbz

R

O

H N

Cbz

2 O

N H

CαH C

O

R

O R

(a)

ii)

N

O

1 H N

O

N 3H

H N R

iii), iv) Cbz

O

H2N

N H

R

3a Phe 3b Trp 3c Pro

NH

O N 3 H

NH2

(b)

R

4a Phe 4b Trp 4c Pro

CαH C NH

Scheme 1. Synthesis of chiral bis(amino amide) ligands. Reagents and conditions: (i) DCC, N-hydroxysuccinimide, THF, rt, 24 h; (ii) H2NCH2(CH2)3CH2NH2, DME, rt 24 h; (iii) HBr/AcOH, rt 6 h; (iv) NaOHaq, rt. 6.5

CSA

DDd (ppm)b

1 2 3 4

4a 4a 4b 4c

0.081c 0.033 0.045 0.021

amides) containing aromatic side chains 4a and 4b (entries 1–3). This suggests the location of the aromatic ring of the mandelic acid close to the aromatic side chain of the hosts and the presence of intermolecular p–p interactions.6 The enantiomeric discriminating ability of these bis(amino amides) for the natural dicarboxylic amino acid derivatives Cbzaspartic acid 6 and Cbz-glutamic acid 7 (Chart 1) was then explored. When the bis(amino amide) derived from L-phenylalanine 4a was used as CSA, upon addition of 0.5 equiv of Cbz-aspartic acid in CDCl3/CD3OD (1.5%), the chemical shift values of the methine CaH and carbamide NH proton signals of (L)-and (D)-Cbz-aspartic acid exhibited large upfield and downfield shifts respectively, leading to significant 1H chemical shift non-equivalences (DDd). This is illustrated in Figure 1. In this case, Dd values for CaH were 0.789 and 0.720 ppm for (L)- and (D)-enantiomers respectively, and 0.507 and 0.386 ppm for carbamide NH protons. Table 2 shows the chemical shift non-equivalences (DDd) observed for the enantiomers of Cbz-aspartic and Cbz-glutamic acids

(a) R

6 N H

4 3 5

4a Phe 4b Trp 4c Pro

5.7

5.5

5.3

5.1

4.9

4.7

4.5

4.3

4.1

3.9

Table 2 1 H chemical non-equivalences (DDd, 500 MHz) of racemic dicarboxylic acids in the presence of 4a–c in CDCl3/CD3OD (1–10%) at 30 °C

All samples were prepared by mixing 1 equiv of racemic carboxylic acid (0.01 M) and 2 equiv of chiral host (0.01 M) in NMR tubes. b 1 H chemical shift non-equivalences of a-H of mandelic acid. c CDCl3/CD3OD (1.5%).

H2N

5.9

Figure 1. Partial 1H NMR spectra (CDCl3/CD3OD (1.5%), 500 MHz) of racemic CBzaspartic acid (a) before and (b) after the addition of 2 equiv of 4a (10 mM), showing the splitting of signals corresponding to the carbamide NH and CaH protons.

a

O

6.1

δ (ppm)

Table 1 1 H chemical non-equivalences (DDd, 500 MHz) for racemic mandelic acid in the presence of 4a–c in CDCl3/CD3OD (5%) at 30 °C Entrya

6.3

N H

(b)

O

Entrya

CSA

Dicarboxylic acid

DDd (ppm)b

DDd (ppm)c

1 2 3 4 5 6 7 8

4a 4a 4b 4c 4a 4a 4b 4c

Cbz-Asp Cbz-Asp Cbz-Asp Cbz-Asp Cbz-Glu Cbz-Glu Cbz-Glu Cbz-Glu

0.068d 0e 0e 0e 0.010d 0e 0f 0e

0.140d 0.150e 0.180e 0.050e 0.090d 0.075e — 0.170e

a All samples were prepared by mixing 1 equiv of racemic dicarboxylic acid and 2 equiv of chiral host (0.01 M in CDCl3/CD3OD (1–10%) in NMR tubes. b 1 H chemical shift non-equivalences of the a-H of dicarboxylic acids. c 1 H chemical shift non-equivalences of the carbamide NH proton of dicarboxylic acids. d NMR measurements in CDCl3/CD3OD (1.5%). e NMR measurements in CDCl3/CD3OD (5%). f NMR measurements in CDCl3/CD3OD (10%).

in the presence of 2 equiv of the different bis(amino amides) 4a–c. As a general trend, the CaH from the guests moves upfield (Fig. 1) suggesting a deprotonation of the carboxylic groups.6 Concomitantly, the amine NH proton signals of the receptors moves upfield (see Fig. 4), indicating its protonation, leading to the formation of the diastereomeric salts. As shown in Table 2, all assayed bis(amino amides) produce chemical shift non-equivalences for the carbamide NH proton

HO

H N

O

CO2H

O

2 NH2

CO2H CO2H

R 6

5 H N

O O

CO2H CO2H

7 Chart 1. (a) Labeling of atoms for the general structure of the bis(amino amides). (b) Guests studied in this work: mandelic acid 5, Cbz-aspartic acid (Cbz-AspOH) 6, and Cbzglutamic acid (Cbz-GluOH) 7.

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ee% (obseved)

(a)

100 90 80 70 60 50 40 30 20 10 0

y = 0,9808x + 0,3516 R² = 0,9982

0

20

40

60

80

100

ee % (prepared)

ee% (obseved)

(b) 100 90 80 70 60 50 40 30 20 10 0

Binding studies

y = 0,9776x - 0,234 R² = 0,9955

0

20

40

60

80

100

ee % (prepared) Figure 2. Correlation between prepared and observed ee values obtained by 500 MHz 1H NMR titrations of enantiomerically enriched mixtures of Cbz-AspOH using 4a as CSA in CDCl3/CD3OD (1.5%) at 303 K. (a) For CaH proton signals. (b) For carbamide NH proton signals.

Δ δ X H 0.25 0.2 0.15 0.1 0.05 0 0

0.2

0 .4

0.6

0 .8

XG

interactions and dissociation of the ion pairs. In this regard, DDd values were 0 or very small for the Ca-H protons when 4a–c were used as CSAs in CDCl3/CD3OD containing >5% of CD3OD (entries 3, 4). For 4b, a higher amount of CD3OD was needed, in general, to obtain clear solutions, explaining the lower DDd values observed in some cases (i.e. entry 7 in Table 2). As observed for mandelic acid, for Cbz-aspartic acid there were always higher DDd values for the carbamide NH when receptors 4a and 4b were used (DDd = 0.150 and 0.180 ppm), suggesting an important role of intermolecular interactions involving the aromatic ring from the amino acid side chains and the aromatic at the N-protecting group from the guest. The situation is, however, different for Cbz-glutamic acid, for which the best DDd values were obtained with the most basic receptor 4c (DDd = 0.170 ppm for the carbamide NH). The practical applicability of receptor 4a to determine the enantiomeric excess of different enantiomeric Cbz-AspOH mixtures by integration of the corresponding NMR signals in the presence of 2 equiv of 4a was also demonstrated. The results, based on the carbamide NH (Fig. 2a) and CaH (Fig. 2b) proton signals, are within ±3% of the actual enantiopurity of the samples, rendering an excellent linear response (R2 = 0.9982 and R2 = 0.9955 for the CaH and for the carbamide NH protons, respectively).

1

Figure 3. Job plots of 4a with L-Cbz-aspartic acid (stars) and with D-Cbz-aspartic acid (squares). The Dd stands for the chemical shift change of the CaH proton of Cbz-AspOH in the presence of host 4a. Total concentration is 10 mM, CDCl3/CD3OD (1.5%).

signals of the Cbz-aspartic and glutamic acids (0.050–0.180 ppm). The observed enantiodifferentiation was more efficient for both dicarboxylic acids than for the monocarboxylic acid 5, according to the design of the receptors. Compounds 4a and 4b were the best CSAs for the enantiodiscrimination of Cbz-aspartic acid, while receptor 4c exhibits best enantiodiscrimination for Cbz-glutamic acid (entry 8). The use of polar protic solvents decreases the chemical shift non-equivalences (DDd (compare entries 1–2 and 5–6), which can be associated to favoring a decrease in intermolecular

The stoichiometries of the complexes were analyzed by Job’s plot method. The determination of the stoichiometry of the species formed is important to analyze in more detail the structure of the complexes.9 As can be observed in Figure 3 for 4a, 1:1 receptor:guest stoichiometries were obtained for the complexes formed with both (L) and (D)-Cbz-aspartic acid. XG and XH are the molar fractions of the guest Cbz-AspOH and the host 4a, respectively. Similar results were obtained for other related systems. Complexation studies of N-protected dicarboxylic amino acids were also performed using 1H NMR titrations1,10 in CDCl3/CD3OD (5%) at a constant concentration of the receptor, by the addition of increasing amounts of the guest until saturation. Some interesting features can be pointed out. For 4a and Cbz-(D)-aspartic acid, the C2-H and the amine NH protons (Chart 1) for 4a moved significantly downfield (Fig. 4). As mentioned previously, this supports a proton transfer from the carboxylic group of the substrate to the amino group of the receptor and hence, the formation of an ionic pair. H4 and H5 protons in 4a also move upfield suggesting a hydrogen bond interaction of the amide group from 4a and Cbz(D)-aspartic acid. Finally, changes observed for the phenyl moieties of 4a (Fig. 4) point out to the participation of the aromatic rings in the complex formation. One important change upon diastereomeric complex formation is the different multiplicity pattern obtained for the Cbz methylene protons (see Fig. 5). This suggest that an important interaction for the enantiopreference is close to these positions, reducing the conformational freedom and making the two methylene protons of the Cbz group chemically non-equivalent (see Fig. 5a for the complexes formed between 4a and (D) and (L)-Cbz-AspOH. Similar results were observed when 4b was used as receptor in the titration experiments by the addition of increasing amounts of (D) and (L)-Cbz-aspartic acid (Fig. 5b), while no differences in the 1 H NMR between both enantiomers of Cbz-aspartic acid were observed in the titration experiments when 4c was used as receptor (Fig. 5c). This behavior of the methylene protons from (D) and (L)-Cbzaspartic acid 6 was not observed for the two enantiomers of Cbzglutamic acid 7 when receptors 4a and 4c were used (see Fig. S1). Taking into account the former results from NMR experiments, we performed molecular modeling calculations to better

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NH

H5

H2 NH

H2

H5

NH

H5

H2 NH

H2 7.35

7.30

7.25

7.20 7.15 δ ((ppm)

7.10

4.2

3.9

3.5

3.1

2.7

2.3

H5

1.9

1.5

H4

H4

H4

H4 1.1

δ (ppm)

Figure 4. Partial 1H NMR spectra (CDCl3, 300 MHz) of 4a in the presence of different amounts of (D)-Cbz-AspOH (from bottom to top 1:0, 3:1, 2:1, 1:1 receptor:guest ratios).

(a)

(b)

(c)

Figure 5. Partial 1H NMR spectra (CDCl3/CD3OD (5%), 300 MHz) for L-Cbz-aspartic acid (top) and D-Cbz-aspartic acid (bottom) in the presence of 1 equiv of (a) 4a (b) 4b and (c) 4c. All the signals correspond to the Cbz-methylene protons.

understand the nature of the complexes formed between 4a–4c and (D) and (L)-Cbz-AspOH. The calculations were carried out with the program Spartan’08,11 using the Monte Carlo conformational searches performed at the Merck molecular force field (MMFF). The most stable conformers obtained in the conformational search were then optimized with the semiempirical PM6 method using the GAUSSIAN 09 program12 in CHCl3 using the SCRF methodology. The stationary points have been characterized as true minima by the calculation of the normal vibration modes, being all the values positive. The conformers with the minimum energy obtained for the complexes between 4a–c and (L) and (D)-Cbz-AspOH (1:1 ratio,

after proton transfer from the aspartic acid to the amino group of the receptor) are shown in Fig. 6. The relative energies for the complexes of receptors 4a–c with both D and L guest enantiomers were also computed. For all the cases, the D-enantiomer was found to form more stable complexes than the L-enantiomer (Table 3). Moreover, several interactions are present in the optimized structures for the complexes of 4a–c with (L) and (D)-Cbz-AspOH (1:1 ratio). The first one is the electrostatic interactions due to ion-pair formation. An intermolecular hydrogen bond between one of the carboxylate groups and the two amide NH groups of the receptor is also present.

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Figure 6. Minimum energy structures for the complexes formed between receptors 4a–c and (L) or (D)-Cbz-AspOH. (a) 4a: D-Cbz-AspOH (b) 4a: L-Cbz-AspOH (c) 4b: D-Cbz-AspOH (d) 4b: L-Cbz-AspOH (e) 4c: D-Cbz-AspOH (f) 4c: L-Cbz-AspOH.

Table 3 Relative energies for the supramolecular complexes calculated at PM6 level of theory in CHCl3 Complex

DGD–L (kcal/mol)

4aZ-Asp 4bZ-Asp 4cZ-Asp

1.54 0.71 0.54

However, the most important differences between the diastereomeric complexes are associated to intermolecular interactions involving the Cbz moiety. One of them is a H-bond between one ammonium group of the receptors 4a and 4b and the carbamide CO group; this interaction is only present in the D-AspOH complexes (Fig. 6a and c). At the same time, intermolecular p–p interactions can be detected involving the aromatic rings of the receptors 4a and 4b and the aromatic ring of the Cbz-AspOH (i.e: p–p distances 2.849 and 2.817 Å for 4a:D-Cbz-AspOH and 4a:L-Cbz-AspOH respectively). These intermolecular interactions are not present in the diastereomeric complexes formed with 4c and this can explain the differences observed in 1H NMR experiments when the aromatic receptors 4a–4b and the non aromatic receptor 4c are used as CSA for the Cbz-AspOH. Moreover, the different environment observed in Fig. 6 for the protons of the NH carbamide group in diastereromeric complexes are in good agreement with the observed differences in chemical shifts (Table 2, entries 2–4).

Finally, the binding constants (Ka) were calculated by means of a nonlinear curve fitting method for the downfield shift of the C2-H proton signal observed upon addition of the guest (see Supplementary data for complexation curves).13 Association constants (Ka) and free energy changes (DGo) are shown in Table 4. Binding constants could not be obtained for 4b because some precipitation occurs in NMR tubes during titration experiments. In general, (D)Cbz-AspOH and (D)-Cbz-GluOH acids have association constants slightly higher than their (L)-enantiomers and data presented in Table 4 reveal that the enantiorecognition process is weak for the 1:1 equilibrium. It is interesting to note that 4a shows a small selective recognition of Cbz-aspartic acid over Cbz-glutamic acid (selectivity constant 2), while no selectivity at all is observed for 4c. In addition, the stronger association constants are observed for the most basic receptor 4c,5l which confirms that protonation of the receptor and the consequent ion pair formation is a critical event and determines the strength of such association process. As very low enantioselectivities were displayed by 4a and 4c in terms of association constants, the enantiodifferentiation process must be associated to the presence of clearly different structures for the two diastereomeric complexes. Finally, we also studied the supramolecular complex formed between 4a–4c with the (D)-Cbz-aspartic acid by ESI-MS experiments. The use of mass spectrometry in supramolecular chemistry has tremendously grown, as soft ionization techniques (such as ESI) usually render a good preservation of weak intermolecular interactions, allowing the detection of the corresponding receptor–substrate complexes.14 To study the binding difference between receptors 4a, 4b, and 4c a series of competition experiments were undertaken.15 Typically, to a equimolar mixture of 4a–4c in CH2Cl2:CH3OH (9:1) a deficit of (D)-Cbz-aspartic acid was added and the resulting mixture was directly electrosprayed using soft ionization conditions, with low cone voltages Uc = 15 V. As seen in Figure 7, peaks corresponding to the supramolecular [4a–4c + A + 2H]2+ species are clearly inferred. The corresponding doubly-charged [4a–4c + A + 2H]2+ supramolecular species (where A corresponds to (D)-Cbz-AspOH) were detected in the positive ESI scan mode, and clear differences in terms of signal abundances were observed. These can be used to extract relative binding affinities.14b,c The experimental ESI competition results suggest relative binding affinities following the 4c > 4b > 4a trend. This correlates well with the trends from solution binding constants. We also investigated the intrinsic gas-phase stability of these [4a–4c + A + 2H]2+ adducts by collision induced dissociation (CID) experiments (Figs. S3-S5). A number of dissociation channels were observed for the [4a–4c + A + 2H]2+ dications that are summarized in Eqs. (1) and (2).

Table 4 Asociation constants (Ka), Gibbs free energy changes (DGo), and enantioselectivity Ka(D)/Ka(L) (or DDGo calculated from DGo) for the complexation of 4a and 4c with (D)- or (L)-dicarboxylic acids in CDCl3/CD3OD (5%) at 30 °C

a b

Entry

Host

Guest

Ka (M1)

D G o (kcal/mol)

DDGo (kcal/mol)

1 2 3 4 5 6 7 8

4aa 4aa 4ab 4ab 4cb 4cb 4cb 4cb

(D)-CbzAspOH (L)-CbzAspOH (D)-CbzGluOH (L)-CbzGluOH (D)-CbzAspOH (L)-CbzAspOH (D)-CbzGluOH (L)-CbzGluOH

411 ± 37 378 ± 35 218 ± 19 227 ± 20 650 ± 51 572 ± 55 844 ± 50 750 ± 35

3.3 3.2 3.1 3.1 3.5 3.4 3.6 3.6

0.1

Concentration of host 0.016 M. Concentration of host 0.009 M.

0 0.1 0

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[4b+A+2H]2+ 100

371.7

[4a+A+2H]2+ 332.7

100

354.1

%

[4c+2H]2+

339.3

149.1

363.1 368.1

[4a+2H]2+

346.1

199.2 0

m/z

330 [4b+2H]2+ 238.2 [4c+A+2H] 2+ [4c+H]+

335

340

345

350

355

360

365

370

375

[4a+H]+ 397.3

297.3

%

[4b+H]+ 475.3 309.3 283.2

0

m/z 150

200

250

300

350

400

450

500

Figure 7. Positive ESI mass spectrum of an equimolar mixture of 4a–4c and 0.5 equiv of (D)-Cbz-AspOH recorded in CH2Cl2:CH3OH (9:1) and at Uc = 15 V. The circled area shows an inset of the m/z 330–380 region.

½4 þ A þ 2H2þ ! ½4 þ 2H2þ þ A

ð1Þ

½4 þ A þ 2H2þ ! ½4 þ Hþ þ ½A þ Hþ

ð2Þ

For [4a–4b + A + 2H]2+ dications, both dissociation channels depicted in Eqs. (1) and (2) are operative where the supramolecular [4a + A + 2H]2+ adduct is more prone to be dismantled at identical collision energies, thus suggesting an inherently lower gasphase stability than that of [4b + A + 2H]2+. CID spectra of [4c + A + 2H]2+ revealed the exclusive release of neutral A according to Eq. (1) while dissociation channel depicted in Eq. (2) was not operative. Higher collision energies to those observed for [4a–4b + A + 2H]2+ were required to promote dissociation of the [4c + A + 2H]2+ dication. Noticeably, dissociation of two neutral NH3 molecules from [4c + A + 2H]2+ was observed (most likely associated to the two amino groups of the 4c host) simultaneously with the liberation of the guest A. Dissociation of host:guest supramolecular adducts via covalent bond cleavages is less common than those of liberation of the guest. It has been observed for example in cucurbituril host:guest chemistry16 and it is indicative of a remarkable gas-phase stability.

basicity, according to previously reported data for similar compounds,5l which could indicate the key importance of proton transfer. In the case of receptors containing aromatic side-chains, p–p interactions seem to be involved in complex formation. A good agreement was observed between NMR results and those obtained by ESI-MS and calculations. Experimental section Materials and reagents All reagents were used as purchased from commercial suppliers without further purification. Chiral a-amino amides were synthesized as previously described7 while all the N-protected amino acids were commercially available. The NMR spectroscopic experiments were carried out on a 500 or 300 MHz for 1H and 13C NMR, respectively. The chemical shifts are reported in ppm using trimethylsilane (TMS) as a reference. FTIR spectra were acquired with a MIRacle single-reflection ATR diamond/ZnSe accessory. General procedure for the preparation of N-hydroxysuccinimide esters of amino acids7

Conclusions In conclusion, we have synthesized a new family of CSAs based on bis(amino amide) ligands derived from natural amino acids and demonstrated that they are good receptors for N-protected dicarboxylic amino acids. The formation of diastereomeric complexes is fast and quantitative, being possible the analysis in situ using NMR spectrometry. A multidisciplinary study using mass spectrometry, NMR, and molecular modeling, has allowed us to identify the structural factors responsible for the interactions. In solution, the substrate-induced chemical shifts of the NMR signals of receptors 4a–4c showed the formation of an ionic pair upon proton transfer from the carboxylic groups to the amino nitrogen of bis(amino amides). Different intermolecular interactions between the Cbz moiety of the guest and the receptors are observed, which could explain the strong chemical shift non-equivalence for the NH carbamide signal. In some instances, chemical shift nonequivalences are observed for the Ca-H of the guest, but only for the receptor 4a derived from Phe. It seems that the stronger complexes (Ka >500) are formed with receptor showing a higher

N-Cbz-L-aa (1 mmol) and N-hidroxysuccinimide (1 mmol) were dissolved in dry THF at 0 °C. Once a clear solution had been obtained, DCC (1 mmol) in anhydrous THF (40 mL) was added in several aliquots and the resulting solution was stirred at 0–5 °C for 3 h. The dicyclohexylurea formed was filtered off and the filtrate was concentrated to dryness. The crude product was recrystallized from 2-propanol to furnish the pure product. General procedure for the synthesis of Cbz-N-protected bis(amino amide) ligands7 The N-hydroxysuccinimide ester of N-Cbz-L-aa (1 mmol) was dissolved in anhydrous DME (30 mL) cooled in an ice bath. Penthylenediamine (0.5 mmol) dissolved in dry DME was added several times. The reaction mixture was stirred at room temp for 18 h and then was warmed for 6 h at 40–50 °C. The white solid was filtered off and washed with cold water and cold methanol. The obtained compounds showed high purity by NMR and were used in the next step without further purification steps.

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General procedure for the deprotection of N-protected bis(amino amide) ligands7 N,N0 -bis(N-Cbz-L-aa)-1,5-diaminopentane (1 mmol) was added to HBr/AcOH (33%) (20 mL) and the mixture was stirred at room temp. until CO2 evolution ceased. At this point, diethyl ether (50 mL) was added to the clear solution, which led to the deposition of a white precipitate. This was filtered off, washed with additional ether, and dissolved in distilled water, the resulting solution was extracted with chloroform (3  10 mL). Solid NaOH was then added up to a pH value of 12 and the resulting solution was saturated with NaCl and extracted with chloroform (3  10 mL). The organic phase was dried over MgSO4 and evaporated under vacuum to obtain a white solid. Synthesis of N,N0 -bis(N-L-phenylalanine)-1,5-diaminopentane This compound was obtained as described above starting from N,N0 -bis(N-Cbz-L-phenylalanine)-1,5-diaminopentane and HBr–AcOH solution. Yield = 82% (1.1 g); mp = 132–134 °C; ½a20 D = 82.4° (c = 0.01, CHCl3); ESI-MS m/z = 397.1 (100, M+H+), 419.1 (30, M+Na+); IR mmax = 3365, 3289, 1650, 1628, 1547, 1525 cm1; 1 H NMR (500 MHz, CDCl3/CD3OD (1.5%), 30 °C) d = 1.2–1.30 (m, 2H, (CH2)2CH2(CH2)2), 1.44–1.53 (m, 4H, CH2CH2CH2CH2CH2), 2.68 (dd, 2H, J = 13.52, 5.5 Hz, CH2Ph), 3.17–3.27 (m, 6H, CH2Ph, CH2CH2CH2CH2CH2), 3.56 (dd, 2H, J = 9.2, 4.2 Hz, CH⁄), 7.17–7.34 (m, 10H, Ar-H); 13C NMR (125 MHz, CDCl3, 30 °C) d = 26.3, 29.4, 38.8, 41.0, 56.4, 126.6, 128.5, 129.1, 137.8, 173.9; Elemental analysis calcd (%) for C23H32N4O2: C, 69.67; H, 8.13; N, 14.13. Found: C, 69.85; H, 8.21; N, 14.18 Synthesis of N,N0 -bis(N-L-triptophan)-1,5-diaminopentane This compound was obtained as described above starting from N,N0 -bis(N-Cbz-L-triptophan)-1,5-diaminopentane and HBr–AcOH solution. Yield: 93% (1.8 gr). Mp = 110–115 °C; ½a20 D = +19.9° (c = 0.01, MeOH); ESI-MS m/z = 475.3 (100, M+H+); IR mmax = 3288, 3112, 2929, 2861, 1643, 1534, 1456, 1439, 1023, 1004, 740 cm1; 1 H NMR (500 MHz, CDCl3/CD3OD (5%), 30 °C) d = 0.95–0.86 (m, 2H, (CH2)2CH2(CH2)2), 1.21–1.28 (m, 4H, CH2CH2CH2CH2CH2), 3.02–3.12 (m, 6H, CH2Ind, CH2CH2CH2CH2CH2), 3.20 (dd, 2H, J = 14.4, 5.6 Hz, CH2Ind), 3.60–3.68 (m, 2H, CH⁄), 7.01 (s, 2H, ArH), 7.06 (t, 2H, J = 7.5 Hz, Ar-H), 7.14 (t, 2H, J = 7.5 Hz, Ar-H), 7.36 (d, 2H, J = 8.1 Hz, Ar-H), 7.61 (d, 2H, J = 7.9 Hz, Ar-H); 13C NMR (125 MHz, CDCl3, 30 °C): 23.7, 29.3, 30.7, 38.8, 55.8, 110.8, 111.4, 111.6, 118.6, 118.7, 119.3, 119.5, 121.8, 122.0, 123.4, 123.6, 127.9, 136.4; Elemental analysis calcd (%) for C27H34N6O2. H2O: C, 65.85; H, 7.32; N, 17.07. Found: C, 65.33; H, 6.87; N, 16.61. Synthesis of N,N0 -bis(N-L-proline)-1,5-diaminopentane This compound was obtained as described above starting from N,N0 -bis(N-Cbz-L-proline)-1,5-diaminopentane and HBr–AcOH solution Yield = 80% (0.8 gr); mp = 95–100 °C; ½a20 D = 73.7° (c = 0.01, MeOH); ESI-MS m/z = 297.0 (100, M+H+); IR mmax = 3297, 2931, 2864, 1646, 1522 cm1; 1H NMR (500 MHz, CDCl3, 30 °C) d = 1.23–1.32 (m, 2H, (CH2)2CH2(CH2)2)), 1.40–1.50 (m, 4H, CH2CH2CH2CH2CH2), 1.58–1.68 (m, 4H, CH2), 1.77–1.89 (m, 2H, CH2), 2.00–2.10 (m, 2H, CH2), 2.78–2.85 (m, 2H, CH2), 2.89–2.97 (m, 2H, CH2), 3.15 (dd, 4H, J = 13.5, 6.8 Hz, CH2CH2CH2CH2CH2), 3.64 (dd, 2H, J = 9.1, 5.3 Hz, CH⁄), 7.53 (br, 2H, NH); 13C NMR (125 MHz, CDCl3, 30 °C) d = 24.2, 26.1, 29.3, 30.7, 38.6, 47.2, 60.6, 174.8; Elemental analysis calcd (%) for C15H28N4O2. H2O: C, 57.32; H, 9.55; N, 17.83 Found: 57.02; H, 10.01; N, 17.3

NMR chiral shift experiments The chiral shift experiments were performed at 30 °C. Samples for analysis were prepared by mixing CSA 4a–c with different carboxylic acids. NMR host–guest titration 1 H NMR titrations were performed by adding incremental amounts of (R)- or (S)-CBz-dicarboxylic amino acid to a NMR tube containing a solution of host 4a or 4c (10 mM, CDCl3/CD3OD (5%)) so that the total concentration of host was kept constant.

Determination of stoichiometry of the host–guest complex (Job plots) The CSA 4a and Cbz-AspOH were distributed among 10 NMR tubes, with various amounts of CSA and Cbz-AspOH, so that the total concentration of CSA and acid was 10 mM, and the molar ratio of the CSA to the acid in the 10 tubes was increased from 0.1 to 1. The 1H NMR spectrum of each sample was recorded on a 500 MHz spectrometer. Acknowledgments Financial support from Ministerio de Ciencia y Tecnología (CTQ2011-28903-C02-01), Fundación Caixa Castelló-Bancaixa (P1-1B-2009-58), and Generalitat Valenciana PROMETEO 2012/ 020 is acknowledged. N.C. thanks Bancaixa for a fellowship. The authors are grateful to the Serveis Centrals d’Instrumentació Científica (SCIC) of the Universitat Jaume I for the spectroscopic facilities. Supplementary data Supplementary data (partial 1H NMR for L-Cbz-GluOH and D-Cbz-GluOH in the presence of 4a, 4c. Complexation curves for 4a and 4c with (L) and (D)-Cbz-AspOH and (L) and (D)-Cbz-GluOH. CID spectra for mass selected [4a + A + 2H]2+. CID spectra for mass selected [4b + A + 2H]2+. CID spectra for mass selected [4c + A + 2H]2+. Cartesian coordinates for the minimized structures) associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.10.099. References and notes 1. (a)Comprehensive Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vögtle, F., Suslick, K. S., Eds.; Pergamon: Oxford, 1996; (b) Bianchi, A.; Bowman-James, K.; García-España, E. Supramolecular Chemistry of Anions; VCH: Weinheim, 1997; (c) Schneider, H.-J.; Yatsimirsky, A. Principles and Methods in Supramolecular Chemistry; Wiley: Chichester, 2000; (d) UccelloBarretta, G.; Balzano, F.; Salvadori, P. Curr. Pharm. Des. 2006, 12, 4023–4045; (e)Analytical Methods in Supramolecular Chemistry; Schalley, C., Ed.; Wiley-VCH: Weinheim, 2007; (f) Wenzel, T. J. Discrimination of chiral compounds using NMR spectroscopy; Wiley-Interscience: Hoboken, NJ, 2007; (g) Schneider, H. J. Angew. Chem., Int. Ed. 2009, 48, 3924–3977. 2. (a) Jane, D. E. Medicinal Chemistry into the Millennium In Campbell, M. M., Blagbrough, I. S., Eds.; Royal Society of Chemistry: Cambridge, 2001; pp 67–84; (b) Standaert, D. G.; Young, A. B. The Pharmacological Basis of Therapeutics In Hardman, J. G., Goodman Gilman, A., Limbird, L. E., Eds.; McGraw-Hill: New York, 1996; p 503. Chapter 22; (c) Fletcher, E. J.; Loge, D. An Introduction to Neurotransmission in Health and Disease In Riederer, P., Kopp, N., Pearson, J., Eds.; Oxford University Press: New York, 1990; p 79. Chapter 7; (d) Childers, W. E., Jr.; Baudy, R. B. J. Med. Chem. 2007, 50, 2557–2562; (e) Schkeryantz, J. M.; Kingston, A. E.; Johnson, M. P. J. Med. Chem. 2007, 50, 2563–2568; (f) Lo, A. S.-Y.; Liew, C.-T.; Ngai, S.-M.; Tsui, S. K.-W.; Fung, K.-P.; Lee, C.-Y.; Waye, M. M.-Y. J. Cell. Biochem. 2005, 94, 763–773; (g) Patrick, G. L. An Introduction to Medicinal Chemistry; Oxford University Press, Inc.: New York, 2005; (h) Barshop, B. A.

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