Chiral recognition of amino-acid esters by a glucose-derived macrocyclic receptor

Article abstract of DOI:10.1039/d1cc00878a

We report a glucose-based crown ether capable of chiral recognition of a wide range of amino-acid methyl esters in aqueous environments. The enantioselectivities towards amino-acids with extended hydrophobic side chains displayed by the glucose-derived ma

Full text of DOI:10.1039/d1cc00878a

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Chiral recognition of amino-acid esters by a
glucose-derived macrocyclic receptor†
Cite this: Chem. Commun., 2021,
57, 3476
Pit Dominique,
Martin Schnurr and Bartosz Lewandowski
*
Received 16th February 2021,
Accepted 2nd March 2021
DOI: 10.1039/d1cc00878a
rsc.li/chemcomm
We report a glucose-based crown ether capable of chiral recognition previously been utilised for the purposes of asymmetric catalysis
of a wide range of amino-acid methyl esters in aqueous environments. (Fig. 1a)34–36 as well as for binding of neutral (Fig. 1b)37 and cationic
The enantioselectivities towards amino-acids with extended hydro- guests (incl. chiral cations, Fig. 1c).38–43
phobic side chains displayed by the glucose-derived macrocycle are
Herein we present a versatile glucose-derived crown-ether-
among the highest observed for small molecule synthetic receptors type receptor capable of enantioselective recognition of a range
to date.
of amino-acid methyl ester hydrochlorides in water.
We designed a macrocyclic glucose derivative with a tetraethy-
Chirality is a key feature of many bioactive molecules and their lene glycol fragment attached in positions 1 and 4 of the pyranose
absolute configuration determines their chemical and biologi- ring (Fig. 1d) with the aim of enantioselective binding of chiral
cal properties.1 Efficient methods for chiral recognition and ammonium cations derived from amino-acids. The crown-ether-
separation of enantiomers are therefore highly sought after.2–5 type macrocyclic ring was intended as the primary binding site for
Amino-acids are one of the most widespread and important the cationic ammonium group of the guest as crown ethers of
classes of biomolecules applied in e.g. asymmetric synthesis and analogous ring-sizes are known to bind ammonium cations
catalysis,6–8 chemical biology,9 drug development10–12 and materials efficiently.44,45 The carbohydrate scaffold was embedded directly
science.13,14 Their enantioselective binding is crucial for diagnostic, into the macrocycle to provide the chiral environment and thus
therapeutic and technological purposes.15–18 Chiral recognition favour binding of one enantiomer of the guest over the other.
of amino-acids is usually achieved using complex supramole-
The macrocyclic receptor was prepared from phenyl-b-D-gluco-
cular receptors (e.g. resorcin- and calixarenes,19,20 cyclodex- pyranoside following a 7 step synthetic procedure (Scheme 1).46
trins,21 porphyrins22 or cyclophanes23) which often function
efficiently in non-aqueous environments.22–29 Since amino-
acids are broadly applied for biological and medicinal purposes,
methods for their chiral recognition in aqueous media are
particularly desirable.30 Yet, small molecule receptors capable
of enantiomeric discrimination of amino-acids in water are
rare.31
Carbohydrates are particularly attractive candidates for the
construction of novel chiral receptors as they are water soluble,
biocompatible and available in bulk quantities as enantiomerically
pure compounds.32 Their chemical reactivity is well explored and
they possess multiple functionalisation sites which can be utilised to
endow carbohydrate derivatives with additional properties.33 Macro-
cyclic, crown-ether type receptors based on saccharide scaffolds have
¨
Laboratory of Organic Chemistry, ETH Zurich, Vladimir-Prelog-Weg 1-5,
¨
Fig. 1 Examples of carbohydrate-based crown ethers applied in: (a) asym-
metric phase transfer catalysis;35 (b) binding of aspirin and paracetamol;37
(c) enantioselective binding of organic ammonium cations;39 (d) chiral recogni-
tion of amino-acid methyl ester salts (this work).
Zurich 8093, Switzerland. E-mail: bartosz.lewandowski@org.chem.ethz.ch
† Electronic supplementary information (ESI) available: Synthesis and character-
isation, NMR and ITC titration data and curves, conformational analysis data. See
DOI: 10.1039/d1cc00878a
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Scheme 1 Synthesis of receptor 8: (a) benzaldehyde dimethyl acetal, 10-camphorsulfonic acid, CH3CN/DMF, r.t., 24 h, 80%; (b) benzyl bromide, NaH,
DMF, r.t., 24 h, 95%; (c) NaBH3CN, 1N HCl in dioxane, THF, r.t., 2 h, 55%; (d) bis(2-chloroethyl) ether, TBAHS, 50%aq KOH, THF, 50 1C, 12 h, 90%; (e) KOH,
diethylene glycol, 80 1C, 8 h, 80%; (f) FeCl3, NH4PF6, CH3CN, 40 1C, 48 h, 60%; (g) H2, Pd/C, CH3OH, r.t., 48 h, 99%.
Selective protection of the 4- and 6-OH groups as benzaldehyde
acetal (2), followed by alkylation of the remaining hydroxy groups
with benzyl bromide, yielded the fully-protected derivative 3. Reduc-
tive cleavage of the acetal provided the 2,3,6-tri-O-benzylated com-
pound 4. The introduction of tetraethylene glycol unit at position
4 of the pyranose ring and subsequent macrocyclisation were
performed following a slightly modified protocol from Miethchen
and coworkers.34 Reaction with bis(2-chloroethyl) ether and
subsequent treatment of the product by potassium hydroxide
in diethylene glycol furnished the macrocycle precursor 6. The
macrocyclisation was achieved under high dilution conditions at
40 1C using NH4PF6 as a template and a catalytic amount of
FeCl3 as Lewis acid to promote the trans-glycosylation reaction.
The product macrocycle 7 was obtained in a very good 60% yield
exclusively as the a-anomer. Finally, the benzyl functionalities
were removed using H2 and Pd/C to yield the target receptor 8.
Fig. 2 (a) Proposed conformation of receptor 8 based on the NMR
In order to gain insight into the three-dimensional structure
analysis, with marked NOEs between the glucopyranose and tetraethylene
of the receptor we performed structural analysis of compound 8
by NOESY NMR experiment in D2O. The analysis revealed
strong through-space interactions of both H-1 and H-4 of the
sugar backbone to their adjacent CH2 protons in the tetraethy-
lene glycol chain (Fig. 2a and Fig. S2, ESI†).46 Interactions
between H-6 of the sugar and the ethylene glycol chain, albeit
glycol protons (top); fragment of the NOESY spectrum indicating the NOE
between H-1 of the glucopyranose and the CH2 protons of the adjacent
ethylene substituent in the crown-ether fragment (bottom); (b) lowest
energy conformation of receptor 8 obtained in the OPLSe3 Force Field
calculations (top – side view, bottom – top view).
weaker, were also detected. Notably no interactions between and tryptophan – to verify whether the size and character of
H-2 nor H-3 and the tetraethylene glycol protons were observed. the side chain (hydrophobic vs. hydrophilic) has an influence on
These findings indicate that the tetraethylene fragment of the the binding affinity and selectivity. Additionally proline as the
crown-ether-like ring is arranged in a nearly perpendicular only example of a proteinogenic amino-acid bearing a cyclic
manner with respect to the glucopyranose ring (Fig. 2a). In this secondary amine was studied. Firstly, we performed titration
1
arrangement part of the hexose ring and the primary alcohol experiments in D2O monitored by H NMR spectroscopy. In a
functionality at C-6 shield the top face of the crown ether typical experiment a solution of a given enantiomerically pure
structure. To further validate the proposed conformation we guest was added in portions into the solution of 8. The changes
performed a conformational search (10 000 steps), with water as of chemical shifts of several receptor protons upon each addition
a solvent model, using the OPLSe3 Force Field and an energy were followed and then fitted into the equation for single-site
window of 5 kcal mol .
À1 46 The calculations revealed an ensemble non-competitive binding.46 The results of the titration experi-
of similar conformers within an energy window of 2 kcal molÀ1 ments are summarised in Table 1.
(Fig. S3, ESI†)46 with the lowest energy conformer corresponding
A general preference for binding of the L-enantiomers of
very well to the structure derived from the NMR spectroscopic guests by receptor 8 was observed. In case of free amino-acids
analysis (Fig. 2b).
both the selectivity as well as binding affinity was rather low
Next, we evaluated the host–guest interactions between recep- (entries 1–3, Table 1). The binding affinities and selectivities
tor 8 and a series of free unprotected amino-acids and their were considerably improved when amino-acid methyl esters
methyl ester hydrochloride salts (L- and D-enantiomers). Eight were used as guests. The lowest selectivity was determined
representative examples of amino-acids were selected – alanine, for secondary-amine-containing proline and for asparagine
threonine, valine, cysteine, phenylalanine, lysine, asparagine (approx. 1.5 : 1, entries 10 and 11, Table 1). Moderate selectivity
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Table 1 Binding affinities of receptor 8 to amino-acids and their methyl
ester hydrochlorides determined by titration experiments monitored
by NMR
KaDb
Selectivity
b
Entry
Guest
KaL
1
2
3
4
5
6
7
8
H-Ala-OH
H-Val-OH
H-Phe-OH
18 Æ 4
14 Æ 2
19 Æ 4
21 Æ 4
69 Æ 9
61 Æ 9
46 Æ 5
59 Æ 7
45 Æ 5
45 Æ 8
63 Æ 6
44 Æ 5
49 Æ 6
1.3 : 1
1.6 : 1
1.8 : 1
1.9 : 1
2.7 : 1
5.1 : 1
2.6 : 1
6.2 : 1
5.6 : 1
1.7 : 1
1.6 : 1
3.4 : 1
30 Æ 5
37 Æ 5
H-Ala-OMea
H-Thr-OMe
H-Val-OMe
H-Cys-OMe
H-Phe-OMe
Ne-Boc-Lys-OMe
H-Asn-OMe
H-Pro-OMe
H-Trp-OMe
133 Æ 14
166 Æ 22
232 Æ 43
159 Æ 15
275 Æ 30
250 Æ 38
108 Æ 19
70 Æ 5
9
10
11
12
166 Æ 28
a
b
Hydrochloride salts of methyl esters were used as guests. Kas are
reported in MÀ1
.
was observed for alanine bearing the least sterically demanding
side chain (2 : 1, entry 4, Table 1). Similar values were also
found for amino-acids with more polar side chains, cysteine
and threonine (entries 5 and 7, Table 1).
The highest enantioselectivities towards the L-enantiomer
were observed for amino-acids with extended, hydrophobic side
chains (valine, phenylalanine and tryptophan, 5 : 1, 6 : 1 and
3.5 : 1, respectively, entries 6, 8 and 12, Table 1) and for Ne-Boc-
protected lysine (5.6 : 1, entry 9, Table 1). These values are
among the highest enantioselectivities of amino-acid binding
in aqueous media reported to date for a small molecule
receptor.31,47
1
Fig. 3 (a) H NMR (D2O) stacked plot: top – receptor 8; middle – H-L-Ala-
OMe xHCl + receptor 8 (1 : 1); bottom – H-L-Ala-OMe xHCl – the dotted lines
indicate shifts of the protons upon complex formation; (b) Job plot of the
complex between H-L-Ala-OMe xHCl and receptor 8; (c) graphical representa-
tion of the proposed binding mode between receptor 8 and the L-enantiomer
of amino-acid guest (obtained by docking of the lowest energy conformers of
the separate host and guest molecules using MacroModel).
In order to gain insight into the binding mode between the
The results of the NMR experiments were further corrobo- amino-acid methyl ester salts and receptor 8 we studied the
rated by Isothermal Titration Calorimetry (ITC) studies.46 In NMR data from the titration experiments in further detail.
general a very good agreement between the Kd values, obtained Upon addition of the guests to the host solution notable
for the amino-acid complexes with receptor 8 was observed downfield changes in chemical shifts were observed for the
using the two techniques. The ITC measurements also revealed H-1, H-2, H-3 and H-5 protons of the glucopyranose ring in 8
the other thermodynamic parameters of the host–guest inter- (Fig. 3a). Smaller shifts were observed for H-4 and H-6 protons.
actions. The binding of amino-acids by receptor 8 is predomi- Interestingly in the case of tryptophan upfield, instead of
nantly entropy-driven which suggests that the hydrophobic downfield, shifts of the host receptor protons occurred
effect plays an important role in the binding process. For the (Fig. S7, ESI†).46 Additionally, for each amino-acid guest, the
strongest binders (H-L-Val-OMe, H-L-Phe-OMe and H-L-(Boc) signals of the a- and the side-chain protons were shifted upfield
Lys-OMe)
a significant favourable enthalpic contribution upon formation of the complex with receptor 8 (Fig. 3a). The
(DH = À0.7 to À1.2 kcal molÀ1) was also observed. This methyl ester signal of the guest was affected by the complex
indicates that these particular amino-acids form stronger formation to a much smaller extent.
non-covalent interactions (e.g. van der Waals forces or CH–p
interactions) with the macrocyclic receptor compared to the measurements indicated a 1 : 1 stoichiometry of the complexes
other studied compounds. formed between the amino-acids and receptor 8. This stoichio-
Both the fitting of the NMR titration data as well as the ITC
The capacity for enantioselective binding of amino-acid metry was further supported by a Job plot method analysis
esters by receptor 8 was further confirmed in experiments with (Fig. 3b).46
mixtures of L- and D-enantiomers of the guests (alanine and
All these observations imply that the amino-acid methyl
valine were used as representative examples). Mixtures of L- and ester salts primarily bind to the crown ether fragment of the
D-H-AA-OMe, each with different enantiomeric excess, were receptor from the bottom face (as the top face is shielded by the
dissolved in D2O together with receptor 8. The resulting solu- glucopyranose ring and the –CH2OH moiety). The binding
1
tions were analysed by H NMR and the observed differences occurs through a combination of Coulombic and hydrogen-
in chemical shifts of the guest protons between each mixture bonding interactions between the ammonium group of the
(Fig. S4 and S5, ESI†) were then correlated to the enantiomeric guest and the oxygen atoms in the macrocyclic ring. The side-
excess of the studied guest (Fig. S6, ESI†).46
chain of the amino-acid guest can then additionally engage in
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