Thiourea Derivative of 2-[(1 R)-1-Aminoethyl]phenol: A Flexible Pocket-like Chiral Solvating Agent (CSA) for the Enantiodifferentiation of Amino Acid Derivatives by NMR Spectroscopy

Article abstract of DOI:10.1021/acs.joc.0c00027

Thiourea derivatives of 2-[(1R)-1-aminoethyl]phenol, (1S,2R)-1-amino-2,3-dihydro-1H-inden-2-ol, (1R,2R)-(1S,2R)-1-amino-2,3-dihydro-1H-inden-2-ol, and (R)-1-phenylethanamine have been compared as chiral solvating agents (CSAs) for the enantiodiscrimination of derivatized amino acids using nuclear magnetic resonance (NMR) spectroscopy. Thiourea derivative, prepared by reacting 2-[(1R)-1-aminoethyl]phenol with benzoyl isothiocyanate, constitutes an effective CSA for the enantiodiscrimination of N-3,5-dinitrobenzoyl (DNB) derivatives of amino acids with free or derivatized carboxyl functions. A base additive 1,4-diazabicyclo[2.2.2]octane(DABCO)/N,N-dimethylpyridin-4-amine (DMAP)/NBu₄OH) is required both to solubilize amino acid derivatives with free carboxyl groups in CDCl₃ and to mediate their interaction with the chiral auxiliary to attain efficient differentiation of the NMR signals of enantiomeric substrates. For ternary systems CSA/substrate/DABCO, the chiral discrimination mechanism has been ascertained through the NMR determination of complexation stoichiometry, association constants, and stereochemical features of the diastereomeric solvates.

Full text of DOI:10.1021/acs.joc.0c00027

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Article
Thiourea derivative of 2-[(1R)-1-aminoethyl]phenol: a
flexible pocket like chiral solvating agent (CSA) for the
enantiodifferentiation of amino acid derivatives by NMR spectroscopy
Alessandra Recchimurzo, Cosimo Micheletti, Gloria Uccello-Barretta, and Federica Balzano
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.0c00027 • Publication Date (Web): 19 Mar 2020
Downloaded from pubs.acs.org on March 19, 2020
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Thiourea derivative of 2-[(1R)-1-aminoethyl]phenol: a flexible pocket like chiral
solvating agent (CSA) for the enantiodifferentiation of amino acid derivatives by
NMR spectroscopy
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Alessandra Recchimurzo, Cosimo Micheletti, Gloria Uccello-Barretta, Federica Balzano*
Department of Chemistry and Industrial Chemistry, University of Pisa, via Moruzzi 13, 56124 Pisa, Italy.
*federica.balzano@unipi.it
TOC
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ABSTRACT
Thiourea derivatives of 2-[(1R)-1-aminoethyl]phenol, (1S,2R)-1-amino-2,3-dihydro-1H-inden-2-ol, (1R,2R)-
(1S,2R)-1-amino-2,3-dihydro-1H-inden-2-ol, and (R)1-phenylethanamine have been compared as chiral
solvating agents (CSAs) for the enantiodiscrimination of derivatized amino acids by using Nuclear Magnetic
Resonance (NMR) spectroscopy. Thiourea derivative, prepared by reacting 2-[(1R)-1-aminoethyl]phenol with
benzoyl isothiocyanate, constitutes an effective CSA for the enantiodiscrimination of N-3,5-dinitrobenzoyl
derivatives of amino acids with free or derivatized carboxyl functions. A base additive
(DABCO/DMAP/NBu4OH) is required both to solubilize amino acid derivatives with free carboxyl groups in
CDCl3 and to mediate their interaction with the chiral auxiliary, in order to attain efficient differentiation of
the NMR signals of enantiomeric substrates. For ternary systems CSA/substrate/DABCO chiral discrimination
mechanism has been ascertained through the NMR determination of complexation stoichiometry,
association constants and stereochemical features of the diastereomeric solvates.
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INTRODUCTION
In the continuous search of new efficient and direct methods of chiral analysis, NMR spectroscopy offers
several opportunities based on the use of chiral auxiliaries able to transfer enantiomers in a diastereomeric
environment, thus generating differentiation of their observable NMR parameters. Some chiral auxiliaries,
which are named chiral derivatizing agents (CDAs),1-3 are employed for the chemical derivatization of the two
enantiomers, via formation of covalent linkages. Chiral solvating agents (CSAs, diamagnetic)3-5 and chiral
lanthanide shifts reagents (CLSRs, paramagnetic)3,6 are, instead, simply mixed to the enantiomeric substrates
and, based on non-covalent intermolecular interactions in solution, diastereomeric solvates or complexes
are formed directly into the NMR tube. The use of diamagnetic CSAs has emerged as particularly convenient
on the practical point of view: one or two equivalents of the suitable CSA are added directly into the NMR
tube and corresponding signals of the two enantiomers can be easily differentiated and identified in the NMR
spectrum without significant line broadening effects, as in the case of paramagnetic CLSRs. The prominent
role of CSAs is clearly witnessed by the flourishing literature dedicated to this class of chiral auxiliaries for
NMR spectroscopy, spanning from the milestone Pirkle’s alcohol firstly proposed in 19777 to highly
preorganized complex structures.3-5 In particular, thiourea8 and bisthiourea9-13 CSAs have been proposed for
the NMR analyses of chiral anionic substrates, such as α-hydroxy and α-aminocarboxylates. Preformed
carboxylates can be analysed as in the case of tetrabutylammonium salts8,12,13 or the use of strong bases such
as 1,4-diazabicyclo[2.2.2]octane (DABCO) or N,N-dimethylpyridin-4-amine (DMAP)9-11 has been suggested in
order to mediate the interaction between the carboxylic acid and the CSAs.
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In consideration of their potentialities as efficient and versatile CSAs for NMR spectroscopy, we focused on
thiourea derivatives 1-TU, 2-TU, 3-TU, and 4-TU of commercially available amino alcohols 1-3 and amine 4 (Figure
1), the amino groups of which were selectively and quantitatively derivatized by the reaction with benzoyl
isothiocyanate. Compound 1 is endowed with an acidic phenolic hydroxyl, which was not present in 4. Compounds
2 and 3 have rigid structures with cis and trans amino and hydroxy groups, respectively.
Figure 1. CSAs structures.
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Efficiency of CSAs has been compared in multinuclear NMR enantiodiscrimination experiments of several
kinds of amino acid derivatives (Figure 2), including N-derivatives of α-amino acids with free carboxyl groups,
and -acceptor (5-9) or -donor (11) aromatic moieties. Compound 10 is endowed with a fluorinated probe
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for the enantiodifferentiation by F NMR. Derivatization as in the case of compounds 12-16 allowed to
evaluate the contribution of the carboxyl group to enantiodifferentiation. A base (DABCO, DMAP, NBu4OH)
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is required to solubilize the amino acid derivatives 5-11 in CDCl3.
Figure 2. Chemical structures of derivatives 5-17 (DNB=3,5-dinitrobenzoyl, TFA=trifluoroacetyl, DMB=3,5-
dimethoxybenzoyl).
With the aim of gaining insight into the origin of enantiodiscrimination processes, NMR investigations have
been carried out on the stereochemical and thermodynamic features of the diastereomeric solvates formed
by a selected CSA and the enantiomeric substrates.
RESULTS AND DISCUSSION
Chiral auxiliaries 1-TU, 2-TU, 3-TU, and 4-TU were obtained in a quantitative yield by reacting commercially
available amino alcohols 1-3 and amine 4 (Figure 1) with benzoyl isothiocyanate. CSAs were characterized by
use of 2D NMR techniques (see Experimental Section).
1H NMR enantiodiscrimination
Enantiodiscrimination efficiencies both of thiourea derivatives and their amino alcohol precursors were
evaluated in the NMR spectra by measuring the nonequivalence (ΔΔδ=|δR-δS|, where δR=δRmixture-δfree
and δS=δSmixture-δfree), i.e. the magnitude of the splitting of corresponding resonances of the enantiomeric
substrate in its mixture containing CSAs. Underivatized amino acids were not considered because of their low
solubility in CDCl3 or DMSO-d6, also in the presence of the base. Their derivatives 5-16 were analysed, among
which compounds 5-11 were solubilized in CDCl3 by adding one equivalent of base (DABCO/DMAP/NBu4OH).
Compounds 12-17 are completely soluble in CDCl3 and employed without base.
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Amino alcohols 1-3 showed very poor solubility in CDCl3 and enantiodiscrimination experiments could be
carried out only in DMSO-d6 or mixtures CDCl3/DMSO-d6 containing the minimum amount of DMSO-d6
needed to solubilize the CSA. In such conditions, however, any doublings of NMR signals of amino acid
derivatives were not detected. Amine 4 is soluble in CDCl3, but slow-exchange processes between its
protonated and unprotonated forms were detected in the NMR spectra in the presence of amino acids with
free carboxyl groups, pure or premixed with DABCO.
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Thiourea derivatives 1-TU, 2-TU, 3-TU, and 4-TU all are soluble in CDCl3. Adding one equivalent of 1-TU to
the equimolar mixture 5/DABCO in CDCl3 produced the same amount of enantiomers differentiation as in the
case of addition of one equivalent of DABCO to the equimolar mixture 1-TU/5. At 60 mM very high
nonequivalences of 0.147 ppm and 0.090 ppm were measured for the ortho and para protons of 3,5-
dinitrobenzoyl moiety, respectively (Table 1). Lower, but still relevant, doublings of 0.019 ppm and 0.031
ppm were measured at the NH and CH protons, respectively (Table 1). Even higher nonequivalences were
measured in the mixture containing 60 mM 1-TU and 30 mM 5/DABCO (Table 1). On changing both CSA
concentration and CSA to substrate molar ratio, appreciable differentiations of enantiotopic nuclei were
obtained till to 5 mM equimolar amounts of CSA and 5/DABCO (Figure 3, Table 1).
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Table 1. H NMR (600 MHz, CDCl3, 25 °C) nonequivalences (ppm) for 5 in the presence of one equivalent of DABCO in
5/DABCO/1-TU mixtures
[5]
5/1-TU
60 mM
1:1
30 mM
15 mM
5 mM
1:1
1:2
1:1
1:2
1:3
1:4
1:1
1:2
1:3
1:4
oDNB
pDNB
NH
0.147
0.090
0.019
0.031
0.129
0.079
0.010
0.034
0.203
0.094
0.156
0.096
0.045
0.039
0.193
0.224
0.027 0.044 0.077
0.016 0.029 0.048
0.014 0.037 0.052
0.097
0.124
0.032
0.049
0.058
0.024
0.024
0.112
0.057
0.047
0.135
0.066
0.053
0.061
0.066
0.012
CH
-
0.005 0.010
Different base additives were probed in the mixture 5/1-TU, among which DMAP was comparable to DABCO
for the ortho and para protons of 3,5-dinitrobenzoyl moiety, lower nonequivalences were detected for the
methine proton and better result for the NH group (Table 2). The effect of 1-TU on the tetrabutylammonium
salt of 5 was quite similar to 5/DABCO and 5/DMAP mixtures, to indicate a scarce dependence on the nature
of the base additive. However, on considering the spectral features of the three bases, DABCO was selected
since its 12 isochronous protons originated a unique resonance centered at 2.78 ppm.
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Figure 3. 1H NMR (600 MHz, CDCl3, 25 °C) spectral regions corresponding to the aliphatic methine protons of (R,S)-5 in
1-TU/DABCO/5 (1:1:1) mixtures: [5]=60 mM (a), 30 mM (b), and 15 mM (c), in 1-TU/DABCO/5 (2:1:1) mixture ([5]=15
mM) (d), and in DABCO/5 (1:1) mixture ([5]=30 mM) (e).
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Table 2. Effect of one equivalent of base on H NMR
(600 MHz, CDCl3, 25 °C) nonequivalences (ppm) for 5
(30 mM) in the presence of one equivalent of 1-TU
Base
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proton
oDNB
pDNB
NH
DABCO
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0.079
0.010
0.034
DMAP
0.140
0.081
0.040
0.009
NBu4
0.088
0.052
0.028
0.022
CH
In order to ascertain the role of the base additive on the enantiodiscrimination processes, beyond its
solubilizing efficacy, the equimolar mixture 1-TU/5 (without base additive) was analysed in CDCl3 containing
the minimum amount of DMSO-d6 (6% v/v) needed for the solubilization of the substrate: no significant
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doublings of 5 resonances were detected. By contrast, the addition of DABCO in the analogous solvent
mixture of 1-TU/5 caused doublings, which were significant even though lower than they were in sole CDCl3
(Supporting Information, Figure S1). Therefore, the base additive plays a fundamental role in the
enantiodiscrimination processes.
In consideration of above said results, the experimental conditions of 30 mM CSA and 15 mM equimolar
substrate/DABCO mixture were selected for the enantiodiscrimination experiments of substrates 5-11 (Table
3), where nonequivalences even higher than those obtained at the 60 mM equimolar conditions were
measured (Table 1); therefore lower amounts of CSA and substrate are required. Among 5-11, magnitude of
enantiomers differentiation in the NMR spectra was quite similar for the 3,5-dinitrobenzoyl derivatives of
phenylglycine 5, leucine 6 and alanine 7, whereas lower values were measured in the cases of valine 8 and
phenylalanine 9.
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The importance of 3,5-dinitrobenzoyl moiety of the amino acid derivatives was demonstrated by the
comparison with compounds 10 and 11, respectively containing trifluoroacetyl and 3,5-dimethoxybenzoyl as
derivatizing groups, for which no significant doublings of protons (or fluorine in the case of 10) resonances
were observed (Table 3).
Carboxyl groups derivatization in the form of methyl esters (12-14) allowed to improve solubility in CDCl3 and
DABCO was not required, but in the same experimental conditions, nonequivalences were nearly half of the
analogous derivatives with underivatized carboxyl groups (Table 3).
Table 3. 1H NMR (600 MHz, CDCl3, 25 °C)
nonequivalences (ppm) of 5-16 (15 mM) in the
presence of 1-TU (30 mM) and DABCO (15 mM) for 5-
11
substrate
oDNB
0.156
0.132
0.134
0.084
0.034
pDNB
0.096
0.082
0.095
0.055
0.016
NH
CH
0039
0.004
0.037
0.008
-
5
0.045
0.025
6
7
0.072
0.005
nda
8
9
10
11
12
13
14
15
16
nda
0.006
-
nda
0.005b
0.079
0.029
0.050
0.069
0.138
0.003c
0.050
0.023
0.038
0.035
0.078
-
0.087
0.038
nda
0.014
-
0.011
0.033
0.011
-
and=not determined. boDMB. cpDMB.
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The presence of an additional NH group, as for derivatives 15 and 16 of valine and phenylglycine, led to obtain
nonequivalences analogous to those measured for the corresponding derivatives (8 and 5) with free carboxyl
groups (Table 3).
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The presence of the carboxyl function, derivatized or underivatized, is essential since in the case of the 3,5-
dinitrobenzoyl derivative of 1-phenylethanamine, 17 (Figure 2), no differentiation was produced by 1-TU.
Therefore, it seems that derivatizing amino acids only at their amino groups by introducing a 3,5-
dinitrobenzoyl moiety (5-9) represents the easiest and more effective way for the optimization of
enantiomers differentiation by 1-TU, provided that a base additive is employed.
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Selected substrates (5, 6, 12, and 16) were mixed with equimolar amounts (30 mM) of 2-TU, 3-TU, and of the
thiourea derivative of 1-phenylethanamine, 4-TU, having the same main skeleton of 1-TU, but devoid of the
phenolic hydroxyl. In all the cases enantiodifferentiation was less efficient than it was in the presence of 1-
TU (Supporting Information, Table S1). As an example, 2-TU caused very poor differentiation of the ortho
protons of 3,5-dinitrobenzoyl moiety and of the methine proton at the chiral centre of 5: 0.004 ppm and
0.006 ppm, respectively, to be compared with 0.129 ppm and 0.034 ppm measured in the presence of 1-TU.
An enantiodifferentiation of 0.028 ppm was observed for the para proton of the same moiety, almost a third
of that measured in the presence of 1-TU. The corresponding trans derivative, namely 3-TU, did not produce
any signals splitting of 5, 6, 12 and 16, even though NH proton of 5 underwent a relevant line broadening and
a low-frequencies complexation shift (Δδ=-0.087 ppm, greater than those measured for the same proton in
the presence of 2-TU and 1-TU, Supporting Information Table S2), which both suggest the binding ability of
the chiral auxiliary. In the mixture 4-TU/5/DABCO, protons of 3,5-dinitrobenzoyl group were not
differentiated at all and very low doublings of 0.005 ppm were obtained for the NH and CH protons. 4-TU
showed lower enantiodiscrimination efficiency even towards the leucine derivative 6. For the substrates 12
and 16 with derivatized carboxyl functions, nonequivalences between 0.001 ppm and 0.004 ppm were
measured, i.e. remarkably lower in comparison with the values obtained in the presence of 1-TU (0.011-0.127
ppm) in the same experimental conditions (Supporting Information, Table S1). Therefore, the role of the
phenolic hydroxy group is clearly demonstrated.
13C{1H} NMR enantiodiscrimination
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Among the NMR active nuclei, H is traditionally privileged in the detection of chiral discrimination
phenomena by NMR: its high sensitivity arising both from the very high natural abundance and high
gyromagnetic ratio, as well as quite low recovery times of magnetization allow to obtain quantitative results
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in reduced experimental times. As a counterpart of the above said advantages, H-1H scalar couplings and
couplings with other nuclei with high natural abundance may produce complex multiplets, thus reducing the
accuracy of the quantitative determinations. For this reason several kinds of NMR experimental tools have
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two enantiomers are singlets, as in the case of “pure shift” experiments,14,15 which have become increasingly
popular in chiral discrimination investigations. The observation of other high sensitivity heteronuclei, such as
19F or 31P, has been frequently described.16 Detection of 13C nuclei, which are present in the molecular
skeleton of all the organic molecules, would be very attractive, but their low sensitivity has so far hampered
this possibility. However, recent improvements in high field NMR instruments have made accessible the
observation of 13C nuclei at moderate concentrations and with reasonable experimental times.4,16-20
Advantages of observing 13C nuclei are that quaternary carbons in addition to protonated ones can be
detected and carbon resonances of proton decoupled spectra have very narrow linewidths, which enable to
perform accurate quantification of the two enantiomers even for small nonequivalences. One of the major
concerns, which are raised about the observation of quaternary carbons, deals with the fact that they
produce signals with reduced intensity in comparison with protonated carbons. However, in
enantiodiscrimination experiments we compare the integrated area of corresponding signals of the two
enantiomers and diamagnetic CSAs usually affect linewidths of the two enantiomers to the same extent.
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Therefore, we carried out C{1H} NMR enantiodiscrimination experiments of selected substrates (5, 6, 12,
and 16) (Figure 4 and Supporting Information Table S3).
Figure 4. 13C{1H} NMR (150 MHz, CDCl3, 25 °C) spectral regions corresponding to carboxy (C-d) and amide (C-b) carbonyl
carbons, C-NO2 (C-c) and aliphatic methine (CH) of 5, 6, 12, 16 (30 mM) in the presence of one equivalent of 1-TU and
one equivalent of DABCO for 5 and 6. *Resonance of CSA.
In the equimolar mixture 1-TU/5/DABCO (30 mM) very high differentiations of 0.313 ppm and 0.241 ppm
were obtained for the quaternary carbonyl carbons of carboxy (C-d) and amide (C-b) functions, respectively
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(Figure 4 and Supporting Information Table S3). Differentiation of quaternary carbons directly bound to the
nitro groups (C-c) was similarly high and equal to 0.219 ppm. Among protonated carbons, the methine carbon
at the chiral centre of 5 was differentiated by 0.236 ppm (Figure 4 and Supporting Information Table S3). As
shown in Table S3 (Supporting Information) and represented in Figure 4, correspondingly high
nonequivalences were measured for the analogous derivative of leucine 6 with the free carboxyl function,
and for the two derivatives 12 and 16.
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Interaction mechanism 1-TU/(S)-5 and 1-TU/(R)-5
The complexation stoichiometries of the diastereomeric complexes 1-TU/(S)-5 and 1-TU/(R)-5 were defined
by using the Job’s method:21,22 the chemical shifts of selected protons were measured in CDCl3 solutions of
CSA/5 mixtures at variable molar ratios and constant total concentration (10 mM). By graphing the
normalized complexation shifts of one component as a function of the molar fraction of the other one, bell-
curves were obtained with a well-defined maximum at 0.5 molar fraction corresponding to 1 to 1
complexation stoichiometry (Figure 5).
Figure 5. Stoichiometry determination based on ortho () and para () protons of DNB group for (S)-5/1-TU/DABCO
complex.
The non-linear fitting of dilution data23 (40 mM ÷ 0.5 mM, Figure S2 in Supporting Information) based on
Equation 1 gave the association constants (K) of the two diastereomeric complexes 1-TU/(S)-5 and 1-TU/(R)-5
(Table 4).
C= 1/K {[(δobs - δf)( δb - δf)] / (δb - δobs)2}
Eq.(1)
where C is the molar concentration of CSA or substrate, f and b are the chemical shifts of the free and
bound species, respectively, and obs is the chemical shift measured for the selected proton in the equimolar
mixture CSA/5.
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Table 4. Association constants (K, M-1)
obtained by using dilution method for (S)-
5/DABCO/1-TU and (R)-5/DABCO/1-TU
K (M-1)
(S)-5/DABCO/1-TU
(R)-5/DABCO/1-TU
65.8 ± 5
30.2 ± 0.6
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To ascertain the nature of the interactions which contribute to the stabilization of the two diastereomeric
complexes, their stereochemistry was investigated by means of 1D and 2D ROE measurements, starting from
the analysis of the conformation of pure 1-TU.
As shown in Figure S3a (Supporting Information), proton NH(3) did not give ROE at the frequency of NH(2)
and the magnitudes of inter-ROEs H4-H5 and H4-NH(3) (Figure S3b, Supporting Information) were
comparable, therefore NH(3) is almost coplanar to the benzoyl moiety and transoid with respect to NH(2).
On this basis a possible cisoid arrangement of NH(3) and carbonyl function (which would lead the amide
proton NH(3) far away from the benzoyl aromatic ring) can be also ruled out in favour of their transoid
arrangement. Very low intensity NH(3)-CH3 ROE was detected (Supporting Information, Figure S3a,c), which
supported the cisoid arrangement of NH(3) and thiocarbonyl group; as a matter of fact, their transoid relative
positions would bring H3 close to the CH-CH3 moiety. Finally, H7 proton of the phenolic moiety must be in the
proximity of the CH-CH3 fragment far away from NH(2), as supported by the ROE patterns produced by
perturbation at H7 frequency (Supporting Information, Figure S3d).
Therefore, phenolic OH, NH(2) and carbonyl moiety all are in proximity, probably due to the formation of an
extended pool of acceptor/donor hydrogen bond interactions. In such a way a flexible pocket like
conformation (Figure 6) is stabilized, inside which not only extended hydrogen bond interactions can be
established, responsible for the stabilization of diastereomeric complexes with enantiomeric substrates, but
also relevant anisotropic effects can be exerted by the two aromatic moieties of the CSA.
Figure 6. Representation of 3D structure of 1-TU according to ROE data.
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Comparison of intermolecular dipolar interactions detected in the equimolar mixtures 1-TU/(S)-5/DABCO
and 1-TU/(R)-5/DABCO allowed to ascertain the nature of the interactions responsible for the stabilization of
the two diastereomeric solvates. ROEs patterns originated by the methyl and methine protons of 1-TU were
particularly informative: in the mixture containing (S)-5, the methyl proton of CSA originated only
intramolecular dipolar interactions (Supporting Information, Figure S4a), whereas its methine proton
(Supporting Information, Figure S4b) produced ROEs both at the amide proton of (S)-5 and at its phenyl
moiety. The reverse was found for the mixture containing (R)-5, with the methine proton of the CSA
producing only intramolecular dipolar interactions (Supporting Information, Figure S5b) and its methyl
protons showing intermolecular dipolar interactions with the 3,5-dinitrobenzoyl moiety of (R)-5 (Supporting
Information, Figure S5a). Accordingly, a reversal of magnitudes of complexation shifts was found for the
methine and methyl protons of the CSA in the two mixtures (Table 5). The presence of (S)-5 produced a
complexation shift of -0.19 ppm at the methine proton of the CSA and a minor effect at its methyl protons (-
0.09 ppm); in the mixture containing (R)-5, the methyl protons of 1-TU underwent a greater shift (-0.11 ppm)
in comparison with the methine proton (-0.06 ppm).
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Table 5. Complexation shifts (δ=δmixture-δfree
,
ppm) of methyl and methine protons of the two
enantiomers of 5 (30 mM) in presence of one
equivalent of DABCO and of 1-TU
Δδ
1-TU
(R)-5/DABCO/1-TU (S)-5/DABCO/1-TU
CH
-0.06
-0.11
0.06
0.03
-0.19
-0.09
0.09
0.03
CH3
NH(2)
NH(3)
Protons of the phenolic moiety of 1-TU produced through space dipolar interactions with the 3,5-
dinitrobenzoyl groups of both enantiomeric substrates (Supporting Information, Figure S6).
Interestingly, different 15N complexation shifts of NH(2) and NH(3) groups of 1-TU were detected in the 1H-
15N HSQC map of the mixture 1-TU/5/DABCO (Supporting Information, Figure S7), -0.8 ppm and +0.5 ppm
respectively, suggesting that NH(2) is more effectively involved in the stabilization of the diastereomeric
1
solvates. The major involvement of NH(2) is also witnessed by the fact that NH(2) underwent greater H
complexation shifts in comparison with NH(3) in both mixtures (Table 5) and thiocarbonyl group of the CSA
13
underwent remarkably higher C complexation shifts in comparison with the carbonyl group (-0.289 ppm
and - 0.075 ppm, respectively in the mixture containing racemic phenylglycine derivative).
Finally, DABCO produced multiple intense ROEs at the frequencies of protons of the CSA and of the
enantiomeric substrates (Supporting Information, Figure S8). Therefore it can be concluded that in both
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diastereomeric solvates DABCO acts as a bridge between the carboxyl function of the two enantiomers and
the pool of hydrogen bond donor/acceptor groups of the CSA and the electron-rich phenolic aromatic moiety
of the CSA is involved in π-π interactions with the 3,5-dinitrobenzoyl moiety of both enantiomers. In this way
(S)-5 and (R)-5 face opposite sides of the CSA pointing at its CH and CH3 groups, respectively (Figure 7).
Probably, approaching the same less hindered surface of the CSA does not allow hydrogen bond and 
interactions to be simultaneously guaranteed.
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Figure 7. Representation of the interaction model between 1-TU and (S)-5 (left) or (R)-5 (right) in the presence of DABCO
(green sphere) according to NMR data with black skeleton for 5.
Active role of DABCO in the stabilization of diastereomeric solvates has been also confirmed by comparison
of its diffusion coefficient (D), measured by Diffusion-Ordered Spectroscopy (DOSY),24 as pure compound, in
the binary mixtures (R)-5/DABCO and (S)-5/DABCO and in the ternary mixtures 1-TU/(R)-5/DABCO and 1-
TU/(S)-5/DABCO.
Diffusion coefficients describe the translational diffusion of the molecules in solution and can be correlated
to the molecular sizes by means of the Stokes-Einstein Equation (Equation (2)), which strictly holds for
spherical molecules:
D = kT / (6πηrH)
Eq.(2)
where k is the Boltzmann constant, T the absolute temperature, rH the hydrodynamic radius, and η the
solution viscosity. For quite diluted solutions viscosity is not affected significantly by the presence of solute
and it can be approximated to the viscosity of the solvent.
In the case of complexation equilibria, measured diffusion coefficient (Dobs) in fast exchanging conditions
represents the weighted average of its values in the bound (Db) and free (Df) states (Equation (3))
Dobs = χb Db + χf Df
Eq.(3)
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where χb and χf are the molar fractions of bound and free species, respectively.
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Any complexation phenomena, therefore, brings about an increase of the apparent molecular sizes which in
turn causes a decrease of the diffusion coefficient, the magnitude of which depends on the value of the
bound molar fraction.
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In our case, the diffusion coefficient of pure DABCO was 12.3 x 10-10 m2s-1 (30 mM in CDCl3), which remarkably
lowered to 7.3 x 10-10 m2s-1 in the presence of equimolar amount of 5: this value was very similar to that one
measured for 5 (6.9 x 10-10 m2s-1), to indicate the formation of a tight ionic pair. DABCO diffusion motion was
furtherly affected in the ternary mixture 1-TU/5/DABCO, where its diffusion coefficient was even more
lowered to the value of 6.4 x 10-10 m2s-1, which demonstrated that DABCO simultaneously interacted with the
enantiomeric substrates and the CSA, acting as a bridge between them.
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CONCLUSION
The quantitative derivatization reaction of 2-[(1R)-1-aminoethyl]phenol with one equivalent of benzoyl
isothiocyanate affords an efficient CSA for the enantiodiscrimination of amino acid derivatives. Provided a
3,5-dinitrobenzoyl group is present at the amino group of the amino acids, any further derivatization at the
carboxyl function is not needed in order to obtain efficient enantiodifferentiation by the CSA: only an achiral
base additive is required in order to attain solubilization in CDCl3. Very high nonequivalences are measured
in the presence of 1-TU till to about 0.2 ppm in the 1H NMR spectra and even more in the 13C{1H} NMR spectra.
15N nuclei of enantiomeric substrates can be differentiated too. Enantiodifferentiations remain considerable
also in the mixtures containing quite low CSA concentrations (5 mM) with obvious advantages on the
economic point of view. The role of DABCO is not restricted to its solubilizing effects on the amino acid
derivatives, but rather the base promotes the stabilization of the diastereomeric solvates since it acts as a
bridge between the CSA and the enantiomeric substrates for the enhancement of hydrogen bond
donor/acceptor propensities of the polar groups of the two counterparts. Phenolic hydroxy group of the CSA
plays a fundamental role in two respects: it is endowed with enhanced hydrogen bond donor propensity and
makes electron-rich the aromatic ring it is bound to, thus simultaneously favoring π-π interaction with the
electron-poor 3,5-dinitrobenzoyl moiety of the amino acid derivatives. This last interaction is so much
relevant that, in order to preserve it, the two enantiomers approach the two different faces of the CSA, that
one containing the methine hydrogen at the chiral center in the case of (S)-enantiomer and its methyl group
in the case of (R)-enantiomer, from which chiral discrimination originates.
EXPERIMENTAL SECTION
Materials. All commercially available substrates, reagents and solvents were purchased from Aldrich and
used without further purification. Tetrahydrofuran (THF) was distilled from sodium. Derivative 10 and
derivatives 5-9, 11-17 were prepared as described in reference 25 and 26, respectively.
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General methods. H and C{1H} NMR measurements were carried out in CDCl3 solution on spectrometer
1
13
15
operating at 600 MHz, 150 MHz and 60.7 MHz for H, C and N nuclei, respectively. The samples were
analyzed in CDCl3 solution; 1H and 13C chemical shifts are referred to tetramethylsilane (TMS) as secondary
reference standard; 15N chemical shifts are referred to nitromethane as external standard; the temperature
was controlled (25 °C). For all the 2D NMR spectra the spectral width used was the minimum required in both
dimensions. The gCOSY (gradient COrrelation SpectroscopY) and TOCSY (TOtal Correlation SpectroscopY)
maps were recorded by using a relaxation delay of 1 s, 256 increments of 4 transients, each with 2K points.
For TOCSY maps a mixing time of 80 ms was set. The 2D-ROESY (Rotating-frame Overhauser Enhancement
SpectroscopY) maps were recorded by using a relaxation time of 5 s and a mixing time of 0.5 s; 256 increments
of 16 transients of 2K points each were collected. The 1D-ROESY spectra were recorded using a selective
inversion pulse, transients ranging from 256 to 1024, a relaxation delay of 5 s and a mixing time of 0.5 s. The
gHSQC (gradient Heteronuclear Single Quantum Coherence) spectra were recorded, with a relaxation time
of 1.2 s, 128-256 increments with 32 transients, each of 2K points. The gHMBC (gradient Heteronuclear
Multiple Bond Correlation) experiments were optimized for a long-range coupling constant of 8 Hz. DOSY
(Diffusion-Ordered SpectroscopY) experiments were carried out using a stimulated echo sequence with self-
compensating gradient schemes and 64 K data points. Typically, g was varied in 20 steps (2–32 transients
each) and Δ and δ were optimized in order to obtain an approximately 90–95% decrease in the resonance
intensity at the largest gradient amplitude. The baselines of all arrayed spectra were corrected prior to
processing the data. After data acquisition, each FID was apodized with 1.0 Hz line broadening and Fourier
transformed. The data were processed with the DOSY macro (involving the determination of the resonance
heights of all the signals above a pre-established threshold and the fitting of the decay curve for each
resonance to a Gaussian function) to obtain pseudo two-dimensional spectra with NMR chemical shifts along
one axis and calculated diffusion coefficients along the other.
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1H NMR and 13C{1H} NMR characterization data, reported below, are referred to numbered protons/carbons
of chemical structures reported in Figures S9 and S10 (Supporting Information).
Synthesis of chiral auxiliaries 1-TU, 2-TU, 3-TU, and 4-TU
To a suspension of 1-4 (2 mmol) in CH2Cl2 (20 mL) was added, under a nitrogen atmosphere, benzoyl
isothiocyanate (1.1 equiv.). The reaction mixture was stirred at room temperature for 24 h. The reaction was
monitored by recording 1H-NMR and the solvent removed by evaporation under vacuum to afford chemically
pure products in nearly quantitative yield.
1-TU. Amber amorphous solid (598 mg, 99.5% yield). 1H NMR (600 MHz, CDCl3, 25 °C) δ: 1.71 (Me, d, J=6.9
Hz, 3H); 5.81 (H-1, dq, J = 8.1 Hz, J = 6.9 Hz, 1H); 6.77 (H-11, s, 1H); 6.90 (H-10, d, J= 7.9 Hz, 1H); 6.94 (H-8, t,
J =7.9 Hz, 1H); 7.19 (H-9, t, J = 7.9 Hz, 1H); 7.30 (H-7, d, J = 7.9 Hz, 1H); 7.50 (H-5, t, J = 7.8 Hz, 2H); 7.61 (H-6,
t, J = 7.8 Hz, 1H); 7.79 (H-4, d, J= 7.8 Hz, 2H); 8.92 (H-3, br s, 1H); 11.27 (H-2, d, J = 8.1 Hz, 1H). 13C{1H} NMR
(150 MHz, CDCl3, 25 °C) δ: 20.1 (C-Me); 51.3 (C-1); 117.5 (C-10); 121.1 (C-8); 127.4 (C-4, C-15); 127.5 (C-7);
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129.2 (C-5, C-9); 131.6 (C-12); 133.7 (C-6); 153.8 (C-16); 166.9 (C-13); 178.4 (C-14). Anal. Calcd for
C16H16N2SO2: C, 63.98; H, 5.37; N, 9.33. Found: C, 63.90; H, 5.38; N, 9.35.
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2-TU. White solid (621 mg, 99.4% yield). 1H NMR (600 MHz, CDCl3, 25 °C) δ: 2.28 (H-8, s, 1H); 3.04 (H-9, dd, J
= 16.6 Hz, J = 2.2 Hz, 1H); 3.27 (H-9’, dd, J = 16.6 Hz, J = 5.3 Hz, 1H); 4.90 (H-7, dt, J = 5.3 Hz, J = 2.2 Hz, 1H);
5.93 (H-1, dd, J = 7.8 Hz, J = 5.3 Hz, 1H); 7.26 (H-12, m, 1H); 7.29 (H-11, m, 1H; H-10, d, J = 6.8 Hz, 1H); 7.46
(H-13, d, J = 7.3 Hz, 1H); 7.50 (H-5, t, J = 7.4 Hz, 2H); 7.62 (H-6, t, J = 7.4 Hz, 1H); 7.84 (H-4, d, J= 7.4 Hz, 2H);
9.14 (H-3, s, 1H); 11.20 (H-2, d, J = 7.8 Hz, 1H). 13C{1H} NMR (150 MHz, CDCl3, 25 °C) δ: 39.8 (C-9); 63.8 (C-1);
73.5 (C-7); 124.9 (C-13); 125.5 (C-10); 127.4 (C-12); 127.5 (C-4); 128.7 (C-11); 129.1 (C-5); 131.7 (C-14); 133.6
(C-6); 139.2 (C-18); 139.9 (C-17); 166.6 (C-15); 180.6 (C-16). Anal. Calcd for C17H16N2SO2: C, 65.36; H, 5.16; N,
8.97. Found: C, 65.29; H, 5.15; N, 8.95.
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3-TU. Brownish solid (623 mg, 99.6% yield). 1H NMR (600 MHz, CDCl3, 25 °C) δ: 3.03 (H-9, dd, J = 16.2 Hz, J =
6.8 Hz, 1H); 3.43 (H-9’, dd, J = 16.2 Hz, J = 7.8 Hz, 1H); 4.05 (H-8, s, 1H); 4.71 (H-7, ddd, J = 7.8 Hz, J = 6.8 Hz; J
= 5.7 Hz, 1H); 5.73 (H-1, t, J = 5.7 Hz, 1H); 7.26 (H-10, d, J = 7.1 Hz, 1H); 7.29 (H-12, t, J = 7.5 Hz, 1H); 7.31 (H-
11, dd, J = 7.5 Hz, J = 7.1 Hz, 1H); 7.35 (H-13, d, J = 7.5 Hz, 1H); 7.53 (H-5, t, J = 7.8 Hz, 2H); 7.64 (H-6, t, J = 7.8
Hz, 1H); 7.85 (H-4, d, J= 7.8 Hz, 2H); 9.14 (H-3, s, 1H); 11.11 (H-2, d, J = 5.7 Hz, 1H). 13C{1H} NMR (150 MHz,
CDCl3, 25 °C) δ: 39.3 (C-9); 68.9 (C-1); 81.0 (C-7); 123.8 (C-13); 125.3 (C-10); 127.5 (C-4); 127.6 (C-12); 129.1
(C-11); 129.2 (C-5); 131.5 (C-14); 133.8 (C-6); 138.3 (C-18); 140.8 (C-17); 167.0 (C-15); 181.0 (C-16). Anal. Calcd
for C17H16N2SO2: C, 65.36; H, 5.16; N, 8.97. Found: C, 65.43; H, 5.16; N, 8.95.
4-TU. Yellow gum (565 mg, 99.3% yield). 1H NMR (600 MHz, CDCl3, 25 °C) δ: 1.66 (Me, d, J=7.1 Hz, 3H); 5.62
(H-1, dq, J = 7.4 Hz, J = 7.1 Hz, 1H); 7.29 (H-9, t, J = 7.4 Hz, 1H); 7.37 (H-8, t, J = 7.4 Hz, 2H); 7.40 (H-7, d, J = 7.4
Hz, 2H); 7.51 (H-5, t, J = 7.5 Hz, 2H); 7.62 (H-6, t, J= 7.8 Hz, 1H); 7.82 (H-4, d, J = 7.8 Hz, 2H); 8.96 (H-3, s, 1H);
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11.11 (H-2, d, J = 7.4 Hz, 1H). C{1H} NMR (150 MHz, CDCl3, 25 °C) δ: 21.6 (C-Me); 55.2 (C-1); 126.3 (C-7);
127.4 (C-4); 127.7 (C-9); 128.8 (C-8); 129.1 (C-5); 131.8 (C-10); 133.6 (C-6); 141.6 (C-13); 166.8 (C-11); 178.8
(C-12). Anal. Calcd for C16H16N2SO: C, 67.58; H, 5.67; N, 9.85. Found: C, 67.63; H, 5.66; N, 9.87.
Synthesis of N-3,5-dinitrobenzoyl amino acids 5-9 and of N-3,5-dimethoxybenzoylalanine (11)
A solution of the appropriate amino acid (2 mmol), propylene oxide (6 mmol) and N-3,5-dinitrobenzoyl
chloride (or N-3,5-dimethoxybenzoyl chloride) (2 mmol) in anhydrous THF (30 mL) was stirred, under nitrogen
atmosphere, overnight at room temperature. The residue, obtained by solvent evaporation under reduced
pressure, was dissolved in ethyl acetate and treated with a solution of HCl (10%), a saturated solution of NaCl
and dried over anhydrous Na2SO4. The crude product was suspended in petroleum ether/ethanol (5:1, 10
mL) at 0 °C, under stirring for 10-15 minutes. The product was filtered and dried under vacuum. 1H NMR (600
MHz, 25 °C, 30 mM) spectra of 5-9, and 11, reported below, were recorded in CDCl3 solution in the presence
of one equivalent of DABCO.
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5. White crystalline solid; 78% yield (539 mg). 1H NMR δ (ppm): 5.47 (H-1, d, J = 6.0 Hz, 1H); 7.24 (H-4, t, J =
7.6 Hz, 1H); 7.31 (H-3, t, J = 7.6 Hz, 2H); 7.49 (H-2, d, J = 7.6 Hz, 2H); 8.35 (H-5, d, J = 6.0 Hz, 1H); 9.01 (H-6, d,
J = 2.1, 2H); 9.11 (H-7, t, J = 2.1, 1H).
6. Mustard yellow crystalline solid; 75% yield (488 mg). 1H NMR δ (ppm): 0.95 (H-4, d, J = 6.5 Hz, 3H); 0.98 (H-
4’, d, J = 6.5 Hz, 3H); 1.81 (H-3, H-2, and H-2’, m, 3H); 4.63 (H-1, dt, J = 7.8 Hz, J = 6.5 Hz, 1H); 7.98 (H-5, d, J =
7.8 Hz ,1H); 8.98 (H-6, d, J = 2.0 Hz, 2H); 9.09 (H-7, t, J = 2.0 Hz, 1H).
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7. Pale brown amorphous solid; 72% yield (408 mg). 1H NMR δ (ppm): 1.54 (H-2, d, J = 6.7 Hz, 3H); 4.50 (H-1,
quint, J = 6.7 Hz, 1H); 7.80 (H-3, d; J = 6.7 Hz, 1H); 9.00 (H-4, d, J = 1.9 Hz, 2H); 9.18 (H-5, t, J = 1.9 Hz, 1H).
1
8. White amorphous solid; 78% yield (486 mg). H NMR δ (ppm): 0.99 (H-3, d, J = 6.8 Hz, 3H); 1.12 (H-3’, d, J
= 6.8 Hz, 3H); 2.34 (H-2, m, 1H); 4.59 (H-1, dd, J = 7.9 Hz, J = 4.3 Hz, 1H); 7.54 (H-4, d, J = 7.9 Hz, 1H); 9.00
(H-5, d, J= 2.0 Hz, 2H); 9.10 (H-6, t, J = 2.0, 1H).
9. White crystalline solid; 77% yield (553 mg). 1H NMR δ (ppm): 3.27 (H-2, dd, J = 13.5 Hz, J = 5.3 Hz, 1H); 3.41
(H-2’, dd, J = 13.5 Hz, J = 5.3 Hz, 1H); 4.79 (H-1, m, 1H); 7.16-7.24 (H-3, H-4, and H-5, m, 5H); 7.70 (H-6, d, J =
6.7 Hz, 1H); 8.87 (H-7, d, J = 2.0, 2H); 9.27 (H-8, t, J = 2.0, 1H).
11. Pale brown crystalline solid; 70% yield (355 mg). 1H NMR δ (ppm): 1.51 (H-2, d, J = 7.2 Hz, 3H); 3.81 (H-6,
s, 6H); 4.77 (H-1, quint, J = 7.2 Hz, 1H), 6.66 (H-5, t, J = 2.3 Hz, 1H), 6.68 (H-4, d, J = 2.3, 2H), 6.91 (H-3, d, J =
7.2 Hz, 1H).
Synthesis of N-trifluoroacetylvaline (10)
To a solution of valine (11 mmol), triethylamine (11 mmol) in MeOH (10 mL), ethyl trifluoroacetate (14.3
mmol) were added and the solution was stirred for 24 hours. After solvent evaporation, the solid was purified
by treating with water and HCl. The organic phase was extracted in ethyl acetate, washed with a saturated
solution of NaCl and dried with anhydrous Mg2SO4. By removing the solvent by evaporation under reduced
pressure, racemate 10 was obtained as white crystalline solid (1.990 g, 85% yield). 1H NMR (600 MHz, CDCl3,
25 °C) in the presence of one equivalent of DABCO, δ (ppm): 0.94 (H-3, d, J = 6.7 Hz, 3H); 0.95 (H-3’, d, J = 6.7
Hz, 3H); 2.27 (H-2, m, 1H), 4.29 (H-1, dd, J = 8.7 Hz, J = 4.5 Hz, 1H), 7.27 (H-4, d, J = 8.7 Hz, 1H).
Synthesis of N-3,5-dinitrobenzoyl derivatives of amino acid methyl esters 12-14
A solution of 5, 7 or 8 (3 mmol) in anhydrous MeOH (30 mL) saturated with HCl gas was refluxed for 1 h. The
crude product, obtained by solvent evaporation, was dissolved in CH2Cl2, and washed with a saturated
NaHCO3 solution, H2O, and dried over Na2SO4. 12-14 were obtained chemically pure.
12. White crystalline solid; 78% yield (842 mg). 1H NMR (600 MHz, CDCl3, 25 °C), δ (ppm): 3.81 (H-8, s, 3H);
5.78 (H-1, d, J = 6.8, 1H); 7.35-7.46 (H-2, H-3, and H-4, m, 5H), 7.50 (H-5, d, J = 6.8 Hz, 1H); 8.98 (H-6, d, J =
1.9 Hz, 2H); 9.16 (H-7, t, J = 1.9 Hz, 1H).
13. Pale brown amorphous solid; 80% yield (781 mg). 1H NMR (600 MHz, CDCl3, 25 °C), δ (ppm): 1.03 (H-3, d,
J = 6.5 Hz, 3H); 1.04 (H-3’, d, J = 6.5, 3H); 2.30 (H-2, m, 1H); 3.81 (H-7, s, 3H); 4.81 (H-1, dd, J = 8.5 Hz, J = 4.8
Hz, 1H); 6.84 (H-4, d, J = 8.5 Hz, 1H); 8.96 (H-5, d, J = 2.2 Hz, 2H); 9.19 (H-6, t, J = 2.2 Hz, 1H).
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14. White crystalline solid; 80% yield (713 mg). 1H NMR (600 MHz, CDCl3, 25 °C), δ (ppm): 1.56 (H-2, d, J = 7.3
Hz, 3H); 3.81 (H-6, s, 3H); 4.81 (H-1, m, 1H); 7.26 (H-3, d, J = 6.1 Hz, 1H); 8.93 (H-4, d, J = 2.0 Hz, 2H); 9.14
(H-5, t, J = 2.0 Hz, 1H).
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Synthesis of N-3,5-dinitrobenzoyl derivatives of amino acid alkylamides 15 and 16
To a mixture of 8 or 5 (16 mmol) and 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinine (16 mmol) in anhydrous
THF (120 mL), stirred under nitrogen atmosphere at room temperature for 3 h, was added the appropriate
amine (8.03 mmol), and stirred at room temperature for further 15 h. After solvent evaporation, the crude
products, 15 and 16, were purified by recrystallization from THF/hexane.
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15. White crystalline solid; 45% yield (1.527 g). 1H NMR (600 MHz, CDCl3, 25 °C), δ (ppm): 0.86 (H-15, t, J =
6.8 Hz, 3H); 0.96 (H-3, d, J = 6.8 Hz, 3H); 1.02 (H-3’, d, J = 6.8 Hz, 3H); 1.20-1.35 (H-10—H14, m, 10H); 1.53
(H-9, m, 2H); 2.14 (H-2, m, 1H); 3.27 (H-8, m, 1H); 3.34 (H-8’, m, 1H); 4.35 (H-1, t, J = 8.0 Hz, 1H); 6.18 (H-7, t,
J = 5.6 Hz, 1H); 8.20 (H-4, d, J = 8.0 Hz, 1H); 9.07 (H-5, d, J = 2.2 Hz, 2H); 9.15 (H-6, t, J = 2.2 Hz, 1H).
16. Pale brown crystalline solid; 30% yield (1.100 g). 1H NMR (600 MHz, CDCl3, 25 °C), δ (ppm): 0.86 (H-16, t,
J = 7.1 Hz, 3H); 1.15-1.30 (H-11—H15, m, 10H); 1.45 (H-10, m, 2H); 3.27 (H-9, m, 2H); 5.70 (H-1, d, J = 6.6 Hz,
1H); 5.83 (H-8, t, J = 5.8 Hz, 1H); 7.31 (H-4, t, J = 7.5 Hz, 1H); 7.35 (H-3, t, J = 7.5 Hz, 2H); 7.47 (H-2, d, J = 7.5
Hz, 2H); 8.50 (H5, d, J = 6.6 Hz, 1H); 8.97 (H-6, d, J = 2.1 Hz, 2H); 9.11 (H-7, t, J = 2.1 Hz, 1H).
Synthesis of N-(3,5-dinitrobenzoyl)-1-phenylethanamine (17)
A solution of 3,5-dinitrobenzoyl chloride (8.3 mmol) in THF was added dropwise at 0 °C to a solution of 1-
phenylethanamine (8.3 mmol) and triethylamine (9.0 mmol) in anhydrous THF (50 mL). The mixture was
stirred at room temperature for 16 h. The reaction was quenched by adding H2O. The solvent was removed
under reduced pressure, the residue was dissolved in CH2Cl2; the organic layer was washed with HCl (10%),
Na2CO3 (10%), H2O, and dried over anhydrous Na2SO4. The evaporation of the solvent under reduced pressure
afforded (2.250 g, 86% yield) as white crystalline solid, 17. 1H NMR (600 MHz, CDCl3, 25 °C) : 1.68 (H-2,
d, J=6.9 Hz, 3H); 5.36 (H-1, m, 1H); 6.51 (H-6, d, J=7.3 Hz, 1H); 7.30-7.43 (H-3, H-4, and H-5, m, 5H);
8.92 (H-7, d, J=2.0 Hz, 2H); 9.15 (H-8, t, J=2.0 Hz, 1H).
Supporting Information. Role of the base on enantiodiscrimination (Figure S1). Nonequivalences in presence
of 1-TU, 2-TU, 4-TU (Table S1). Complexation shifts (Table S2). 13C nonequivalences in the presence of 1-TU
1
(Table S3). Non-linear fittings of dilution data (Figure S2). 1D ROESY spectra (Figures S3-S6 and S8). H-15N
HSQC maps (Figure S7). CSAs structures with protons and carbons numbering (Figure S9). Structures of
substrates 5-17 with protons numbering (Figure S10). 1H NMR (600 MHz, CDCl3, 25 °C) and 13C{1H} NMR (150
MHz, CDCl3, 25 °C) spectra of CSAs (Figures S11-S18). 1H NMR (600 MHz, CDCl3, 25 °C) spectra of 5-17 (Figures
S19-S31).
ACKNOWLEDGMENTS
The work was supported by University of Pisa (PRA2018_23 “Functional Materials”).
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