Functionalized Chiral Bambusurils: Synthesis and Host-Guest Interactions with Chiral Carboxylates

Article abstract of DOI:10.1002/cplu.202000261

Bambusurils are a class of macrocyclic anion receptors that exhibit notable anion recognition properties, able to bind various inorganic anions as well the carboxylates or sulfonates. Recently, we reported enantioselective recognition of chiral carboxylates using non-functionalized chiral bambusuril derivatives. Herein, we report the synthesis and host-guest properties of two new representatives of chiral bambusuril macrocycles bearing ester functional groups, differing by the substituents attached to their portals. Their supramolecular properties in terms of carboxylate binding were studied by means of NMR in DMSO-d₆. The reported bambusurils bind selected chiral carboxylates with enantioselectivity factors up to 3.1. The results indicated that the selectivity towards different carboxylates is governed by the steric constraint of the substituents surrounding bambusuril portals. No clear trend in the binding affinities and their enantioselectivities was found.

Full text of DOI:10.1002/cplu.202000261

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Functionalized Chiral Bambusurils: Synthesis and Host-
Guest Interactions with Chiral Carboxylates
Jan Sokolov, Adam Štefek, and Vladimír Šindelář*[a]
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Bambusurils are a class of macrocyclic anion receptors that
exhibit notable anion recognition properties, able to bind
various inorganic anions as well the carboxylates or sulfonates.
Recently, we reported enantioselective recognition of chiral
carboxylates using non-functionalized chiral bambusuril deriva-
tives. Herein, we report the synthesis and host-guest properties
of two new representatives of chiral bambusuril macrocycles
bearing ester functional groups, differing by the substituents
attached to their portals. Their supramolecular properties in
terms of carboxylate binding were studied by means of NMR in
DMSO-d6. The reported bambusurils bind selected chiral
carboxylates with enantioselectivity factors up to 3.1. The
results indicated that the selectivity towards different carbox-
ylates is governed by the steric constraint of the substituents
surrounding bambusuril portals. No clear trend in the binding
affinities and their enantioselectivities was found.
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Introduction
binding properties at higher pH due to deprotonation. On the
other hand, neutral receptors can retain their binding ability
also at higher pH. In addition, unlike charged receptors, neutral
receptors do not possess counter-ions that could interfere with
the guest binding and reduce their affinity and selectivity.
Traditional neutral anion receptors interact with chiral anions
through hydrogen bonding of amide,[13,14] urea[15–20] or
thiourea[21–24] moieties. In addition, there are also examples of
chiral hosts that utilize halogen bonding[25–28] and metal
coordination[29,30] for enantioselective recognition of anionic
guests.
Many important biological molecules and active pharmaceutical
ingredients are chiral.[1,2] With respect to drug research, chiral
compounds are often found as single enantiomers, since the
opposing enantiomers may not contribute to the treatment or
may exhibit undesirable biological activity. This encouraged the
development of enantioselective synthetic methodologies as
well the analytical methods for determination of enantiomeric
excess.[3] Supramolecular chemistry features a variety of systems
with high potential for utilization in asymmetric catalysis or
chiral sensing. Many representatives of chiral coordination
compounds possess catalytic properties,[4] and chiral host
molecules such as cyclodextrins or crown ethers can be used in
preparing chiral stationary phases for the chromatographic
separation of enantiomers.[5,6]
There are many relevant chiral compounds that contain
carboxylic or sulfonic groups. These groups are readily deproto-
nated, yielding anionic forms of these compounds. This opens
the opportunity to exploit anion interactions in preparing
optically pure compounds. In this respect, there exists an
interest to prepare chiral receptors that are able to discriminate
enantiomers of anions. Such receptors can be divided into
several classes based on major interactions responsible for the
anion binding. Major representatives of chiral anion receptors
are positively charged. They consist of one or more ammonium
and/or guanidinium moieties[7–12] and are usually found proto-
nated at physiological conditions, exploiting charge-assisted
hydrogen bonding. However, they are prone to lose their
Bambus[6]urils[31–34] are a family of macrocyclic compounds
pioneered by our group. They are composed of six repeating
2,4-dialkylglycoluril units connected by a row of methylene
bridges. Alternating arrangement and the presence of ethyl-
eneurea motives in their building blocks make bambusurils
closely related to hemicucurbituril macrocycles.[35–38] Bambusur-
ils, similar as hemicucurbiturils, can be classified as CÀ H donors
as they bind anionic guests via twelve methine CÀ H groups
inside their cavity. These macrocycles can encapsulate not only
a large variety of inorganic anions but also carboxylates and
sulfonates.[39,40] Previously, several chiral hemicucurbituril deriva-
tives were reported and their binding to chiral carboxylates
were investigated.[41–43]
In our previous research, we developed the synthesis of first
chiral bambusurils and studied their affinity and enantioselec-
tivity towards different chiral carboxylate anions.[44] We showed
that these bambusurils were able to bind enantiomers of chiral
carboxylates with selectivity up to 3.2. However, the absence of
functional groups prevented further application of the macro-
cycles (e.g. for chiral stationary phases of chromatography
column). In this work, we present the synthesis of functionalized
chiral bambusuril derivatives bearing carboxylic groups, likely
to broaden the range of potential application of chiral
bambusurils.
[a] J. Sokolov, A. Štefek, Prof. V. Šindelář
Department of Chemistry and RECETOX
Faculty of Science, Masaryk University
Kamenice 5, 625 00 Brno (Czech Republic)
E-mail: sindelar@chemi.muni.cz
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cplu.202000261
This article is part of a Special Collection on “Chemistry in the Czech Re-
public”.
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Results and Discussion
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Synthesis of glycoluril building blocks
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We prepared chiral glycolurils using unsymmetrically substi-
tuted chiral urea (Scheme 1). The condensation of 3 with 11[45]
produced two diastereomeric glycolurils 4a and 4b. Diaster-
eomer 4a readily precipitated during the reaction and was
isolated by simple filtration. On the other hand, 4b remained
dissolved in the mother liquor. Crude 4b was obtained by
evaporation of the solvent and extraction into acetone. Ester
4b was subsequently hydrolyzed to yield carboxylic acid 4bH,
which was purified by precipitation from water in the presence
of a mineral acid. However, low yields of 4bH (5%) discouraged
us from further preparing the bambusuril using this compound.
We also decided to synthesize enantiomers of glycolurils
(�)-6 (Scheme 2) which are analogues of 4a and 4b lacking α-
methyl groups. In this case, the resulting glycoluril was
prepared as a racemate (�)-6 by the reaction of achiral urea 5
and vicinal diol 11. Prior to the macrocyclization reaction with
formaldehyde, the racemate (�)-6 has to be separated into
enantiomerically pure glycolurils to ensure that the subsequent
macrocyclization produces chiral bambusuril as a single isomer.
To perform the resolution, glycoluril (�)-6 had to be modified.
Firstly, the ester group was hydrolyzed to give carboxylic acid,
producing glycoluril (�)-7. Compound (�)-7 was subsequently
treated with Ac2O, yielding (�)-8 having enhanced solubility in
organic solvents. Enantiomers of (�)-8 were then resolved by
the crystallization with enantiomerically pure 1-phenylethyl-
amine 12 from dioxane. After the resolution, acetyl groups were
removed by treatment with KOH yielding enantiomerically pure
(1S,5R)-7 and (1R,5S)-7 suitable for bambusuril synthesis.
Prior to the further use of glycolurils 7, we assessed their
optical purity. Instead of using enantioselective HPLC analysis
we were able to utilize NMR spectroscopy for this purpose. We
found out that tetrabutylammonium salt of (�)-7 interacts with
the previously reported bambusuril BU-3[44] in DMSO-d6 (Fig-
ure 1). The interaction was identified through splitting of the
singlet of the methyl group of (�)-7 into two signals in the
presence of excess of BU-3. Each signal belongs to different
diastereomeric complexes of BU-3 with enantiomers of 7.
Therefore, it was possible to use 1H NMR to distinguish between
enantiomers of 7 and to determine its enantiomeric composi-
tion.
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Scheme 1. Synthetic procedures towards chiral macrocycle BU-1. Conditions:
°
°
(a) Boc2O, TEA, DCM, 25 C, 6 h, 98%. (b) 1. MeLi, n-BuLi, THF, À 78 C, 0.5 h, 2.
°
°
CO2, from À 78 to 0 C, 52%. (c) SOCl2, MeOH, 60 C, 4 h, 95%. (d) 10, TEA,
°
°
DCM, 25 C, 16 h, 82%. (e) 11, aq. HCl, 80 C, 2 h, 39% (4a). (f) 1. CH2O, H2SO4,
°
°
dioxane, 80 C, 1.5 h, 2. MeOH, H2O, 80–25 C, 16 h, 51%. (g) 1. KOH, MeOH,
65 C, 4 h, 2. aq. HCl, 5%.
°
As all three glycolurils 4a, (1S,5R)-7, and (1R,5S)-7 were
successfully prepared, their absolute configuration was not
known. Therefore, we converted the glycolurils to correspond-
ing amides 13, 14a and 14b by the reaction with (S)-1-(2-
naphthyl)ethylamine (Figure 2). These amides were subse-
quently crystallized to produce crystals suitable for X-ray
diffractometry.[46] Analysis of crystal structures allowed determi-
nation of absolute configuration of the corresponding glycolur-
ils.
Scheme 2. Synthetic procedures towards chiral macrocycles (1R,5S)-BU-2
°
and (1R,5S)-BU-2. Conditions: (a) 10, TEA, DCM, 25 C, 16 h, 92%. (b) 11, aq.
°
°
HCl, 80 C, 2 h, 88%. (c) 1. KOH, MeOH, 65 C, 4 h, 2. aq. HCl, 90%. (d) Ac2O,
°
H2SO4, 25 C, 16 h, 93%. (S-e) 1. (S)-12, dioxane, 2. aq. HCl. (R-e) 1. (R)-12,
°
dioxane, 2. aq. HCl (f) 1. KOH, MeOH, 25 C, 10 min, 2. aq. HCl 97%. (g) 1.
CH2O, H2SO4, dioxane, 80 C, 1.5 h, 2. H2O, NH3, 3. H3PO4. (h) 1. SOCl2, MeOH,
60 C, 2 h, 2. AgSbF6, MeOH, DCM, 39% over (g)+(h).
°
°
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Macrocyclization of 7 produced crude bambusuril BU-
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COOH, bearing carboxylic groups (Scheme 2). Therefore, bam-
busuril BU-COOH was esterified in the presence of SOCl2
providing the bambusuril BU-2 as its complex with HCl that
precipitates from the solution. Chloride anions were removed
by treatment of the complex with AgSbF6.[49] Chlorides were
precipitated as an insoluble AgCl and replaced with SbF6À inside
À
the BU-2 cavity. The SbF6 anion is poorly bound by the
bambusuril and was removed by simple washing with methanol
and water to yield the anion-free bambusuril BU-2.
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1
°
Figure 1. Partial H NMR spectra (DMSO-d6, 300 MHz, 30 C) of (�)-7 in the
absence of any host (A), (�)-7 in the presence of BU-3 (B), (1R,5S)-7 in the
presence of BU-3 (C), (1S,5R)-7 in the presence of BU-3 (D). Diastereomeric
complexes (1R,5S)-7·BU-3 and (1S,5R)-7·BU-3 can be distinguished following
the methyl signal of 7 (highlighted with red color). Compound 7 is present
as TBA salt at 2.2 mM concentration in all cases, concentration of BU-3 was
4.4 mM. Signal at 2.5 ppm is DMSO.
Host-guest properties
Previously, we published binding affinities of chiral bambusuril
BU-3 (structure shown in Figure 1) with ibuprofen G1, N-acetyl
phenylalanine G2, N-acetyl leucine G3, mandelic acid G4 and α-
methoxyphenyl acetic acid G5 (Figure 3).[44] In this study we
further included the amino acid proline G6 and have inves-
tigated these molecules as guests for the newly prepared
bambusurils. Tetrabutylammonium was used as a counter
cation for all the guests.
Supramolecular interactions of bambusurils BU-1 and BU-2
with chiral carboxylate guests were studied by means of 1H
NMR in DMSO-d6 (Figure 4). Bambusurils were used in the form
of methyl esters, as free carboxylic groups would interfere with
the binding of carboxylate guests. The complexation was slow
Figure 2. Molecular formulas and X-ray structures of chiral amides 13 (CCDC
2005908), 14a (CCDC 2005909), and 14b (CCDC 2005910).
Synthesis of bambusurils
Glycolurils 4a, (1S,5R)-7 and (1R,5S)-7 were subjected to macro-
cyclization reactions, in which corresponding glycolurils reacted
with paraformaldehyde in dioxane. Without the presence of
template, the reaction would yield unwanted four-membered
bambusuril as a main product.[47] Therefore, the reaction was
performed in the presence of sulfuric acid, which provides
HSO4À anion as a template that facilitates the formation of a six-
membered macrocycle and also acts as a catalyst. Correspond-
ing bambusurils were isolated from their reaction mixtures as
complexes with HSO4À (Scheme 1).
Our aim was to prepare all bambusurils as methyl esters,
suitable derivatives for future supramolecular study in DMSO.
Bambusurils bearing carboxylic function were undesired as
these groups would compete for binding with the carboxylate
guests. Bambusuril BU-1 was available in its ester form directly
Figure 3. Guests used in this supramolecular study. Counter-ion is tetrabuty-
lammonium in all cases.
À
after the synthesis as complex with HSO4 . It was previously
1
À
°
Figure 4. H NMR spectra (DMSO-d6, 300 MHz, 30 C) of BU-1 (0.69 mM) in
described that, in aqueous media, HSO4 is deprotonated to
the absence of any guest (A), in the presence of 0.5 equivalent of TBA salt of
R-G4 (B) and in the presence of 0.5 equiv. of TBA salt of S-G4 (C). *Signal of
the complexed host. Assignment of BU-1 signals (a, b, and c) is shown in
Scheme 1.
SO42À that is poorly complexed by bambusurils due to its strong
hydration.[48] Thus, anion-free BU-1 was obtained after washing
the complex in the methanol/water mixture in 51% yield.
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on the NMR time scale for all investigated host-guest systems.
Therefore, signals of the free host and the complex could be
distinguished. Integration of the corresponding signals was
used to determine the concentrations of both species from
which the association constants were calculated for host-guest
complexes of the corresponding bambusurils and chiral anions.
We also performed Job plot (Figure S68) to confirm that BU-1
and BU-2 and the investigated guests form complexes of 1:1
stoichiometry.
The results of the binding studies are summarized in
Table 1. The structure of bambusuril BU-1 is closely related to
that of BU-3. BU-1 differs from BU-3 by the presence of the
additional methoxycarbonyl groups, however, with the oppo-
site absolute configurations on chiral centers of both macro-
of enantiomers. The better enantioselectivity of BU-1 is
probably due to the presence of extra methyl groups on its
benzyl substituents which makes its structure more rigid
compared to BU-2. Higher flexibility of BU-2 could be also
reason for relatively narrow range of binding affinities from
1.5×103 MÀ 1 to 3.5×103 MÀ 1 (except for the BU-2·S-G3 com-
plex). Higher enantioselectivity of BU-1 could be also due to the
presence of additional chiral centers on its portals which are in
proximity of bound chiral guests. On the other hand, the range
of association constants of BU-1 complexes lays between 2.6×
103 MÀ 1 (for the complexes with S-G1 and R-G1) and >1×
104 MÀ 1 (for complexes with S-G3, S-G5, S-G6 and R-G6).
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cycles. For this reason, it was assumed that BU-1 would Conclusion
preferentially bind enantiomers of guests that are disfavored by
the BU-3. Indeed, this was observed for G5. Enantioselectivity of
bambusuril BU-1 for S-G5 over R-G5 is equal or greater than 3.1
as the association constant of the BU-1·S-G5 complex is too
high to be determined by direct measurement. Bambusuril BU-
3 also showed the highest enantioselectivity (3.2) towards G5
among other guests but the macrocycle prefers to bind of R-G5
over S-G5. On the other hand, in the case of guests G3 and G4
both BU-1 and BU-3 preferentially bind the same enantiomers.
Bambusurils BU-3 and BU-1 preferentially bound S-G3 over R-
G3 by a factor of 2.2 and >1.7, respectively. Also, S-G4 was
bound preferentially over R-G4 by a factor of 1.9 and 1.2.
However, no significant differences were observed for guests
G1, G2 and G6. These results indicate that simple modification
of the bambusuril structure has a great impact on the inclusion
of chiral carboxylates making it difficult to predict the
substituent effect on the guest binding.
In conclusion we reported the synthesis of two enantiomerically
pure bambusuril derivatives bearing ester groups, which are
suitable for subsequent modifications. We also demonstrated
that previously reported bambusuril BU-3 can be used as chiral
shift agent for determination of enantiomeric excess of
glycoluril 7, which is essential for the synthesis of bambusuril
BU-2. Supramolecular studies proved enantioselective binding
of chiral carboxylate anions. The comparison of bambusuril BU-
1 with BU-3 shows that introduction of ester function impacts
binding preferences. Comparison of BU-1 and BU-2 indicate
that selectivity of these macrocycles towards structurally differ-
ent carboxylate guests is influenced by steric constraints of the
substituents on the glycoluril moiety.
Experimental Section
When comparing BU-1 with BU-2 it is clear that BU-1
exhibits higher enantioselectivity for the corresponding couples
Methyl 4-[(1R)-1-aminoethyl]benzoate hydrochloride 2
Compound 1[50] (1.05 g, 4.0 mmol) was dissolved in MeOH (20 mL)
and SOCl2 (0.6 mL). The solution was heated to 60 C for 2 h. The
°
reaction mixture was then concentrated on rotary vacuum evapo-
rator to approximately 1 mL and diluted with diethyl ether (20 mL).
The resulting precipitate was collected by filtration, washed with
diethyl ether (10 mL) and dried on air to provide compound 2 as a
white powder (0.82 g, 3.8 mmol, 95%).
Table 1. Association constants and corresponding enantioselectivity values
of bambusuril complexes determined by 1H NMR measurements in DMSO-
°
d6 at 30 C.
K [MÀ 1]×103
Host
BU-1
[1R,5S]-BU-2
BU-3
Characterization spectra match the reported data.[51]
S-G1
R-G1
S-G2
R-G2
S-G3
R-G3
S-G4
R-G4
S-G5
R-G5
S-G6
R-G6
2.6�0.2
2.6�0.3
3.6�0.4
3.0�0.2
>10
5.8�0.5
9.5�0.4
4.9�0.6
>10
2.0�0.5
3.0�0.5
1.6�0.3
1.7�0.4
6.1�0.7
3.1�1.1
2.6�0.3
1.6�0.2
2.7�0.5
2.5�0.5
2.9�0.5
2.7�0.5
6.7�1.0
7.9�1.0
7.9�0.6
5.9�0.8
9.1�0.9
4.2�0.5
1.6�0.4
1.4�0.3
2.4�0.4
7.6�1.1
9.5�1.1
>10
N-[(1R)-1-(4-Methoxycarbonylphenyl)ethyl]-N’-methylurea 3
Compound 2 (4.51 g, 20.9 mmol) and N-methylcarbamoylimidazole
10[52] (4.11 g, 32.9 mmol) were diluted with DCM (100 mL), triethyl
amine (5.8 mL, 41.6 mmol) was added and the resulting mixture
was stirred at room temperature overnight. DCM was then removed
on rotary vacuum evaporator providing crude oily material. The
crude material was sonicated with water (50 mL), which resulted in
crystallization of the product. Crystals were collected by filtration,
washed with water (2×10 mL) and dried under vacuum yielding
urea 3 (4.04 g, 17.1 mmol, 82%).
3.2�0.6
>10
>10
Host
KS/KR
G1
G2
G3
G4
G5
G6
1.0/1
1.2/1
>1.7/1
1.9/1
>3.1/1
–
1/1.5
1/1.1
2.0/1
1.6/1
1.1/1
1.1/1
1/1.1
1.3/1
2.2/1
1.2/1
1/3.2
<1/1.2
1H NMR (500 MHz, DMSO-d6) δ 7.90 (d, J=8.2 Hz, 2H), 7.42 (d, J=
8.2 Hz, 2H), 6.43 (d, J=7.9 Hz, 1H), 5.67 (q, J=4.6 Hz, 1H), 4.79 (m,
1H), 3.84 (s, 3H), 2.53 (d, J=4.7 Hz, 3H), 1.31 (d, J=7.0 Hz, 3H).
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13C NMR (126 MHz, DMSO-d6) δ 166.10, 157.78, 151.68, 129.12,
127.75, 126.06, 51.95, 48.54, 26.26, 22.91.
(5 mL) and dried under yielding glycoluril 4bH (75 mg, 0.25 mmol,
5%).
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4
5
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7
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9
HR-MS (APCI+): m/z [C12H14N2O+H]+ observed: 237.1234, calcu-
lated: 237.1235.
1H NMR (500 MHz, DMSO-d6) δ 12.82 (s, 1H), 7.86 (d, J=8.3 Hz, 2H),
7.54 (s, 1H), 7.46 (d, J=8.3 Hz, 2H), 7.35 (s, 1H), 5.40 (d, J=8.3, 1H),
5.17 (d, J=8.3, 1H), 4.82 (q, J=7.3 Hz, 1H), 2.65 (s, 3H), 1.57 (d, J=
7.3 Hz, 3H).
(1S,5R)-2-[(1R)-1-(4-Methoxycarbonylphenyl)
ethyl]-4-methylglycoluril 4a
13C NMR (126 MHz, DMSO-d6) δ 167.15, 161.00, 157.27, 148.70,
129.14, 129.11, 126.63, 67.87, 64.62, 51.21, 27.76, 17.91.
Urea 3 (1.03 g, 4.4 mmol) and 11 (1.09 g, 9.2 mmol) were stirred in
HR-MS (ESI): m/z [C14H16N4O4À H]À observed: 303.1097, calculated:
°
aq. HCl (50 mL H2O and 0.5 mL 35% HCl) at 80 C for 2 h. The
303.1099.
resulting suspension was cooled in an ice bath, solids were
collected by filtration and washed with water (2×10 mL). The
collected solid was dried under vacuum yielding glycoluril 4a as a
single stereoisomer (0.55 g, 1.7 mmol, 39%).
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Bambusuril BU-1
Glycoluril 4a (360 mg, 1.13 mmol) and paraformaldehyde (43 mg,
1.43 mmol) were dissolved in the mixture of dioxane (5 mL) and
1H NMR (500 MHz, DMSO-d6) δ 7.93 (d, J=8.3 Hz, 2H), 7.53 (s, 1H),
7.49 (s, 1H), 7.44 (d, J=8.2 Hz, 2H), 5.10 (d, J=8.3, 1H), 5.03 (m, 2H),
3.85 (s, 3H), 2.65 (s, 3H), 1.55 (d, J=7.3 Hz, 3H).
°
H2SO4 (0.3 mL). The solution was stirred at 80 C for 90 min. Mixture
was then cooled to room temperature, solids were collected by
filtration and washed with diethyl ether (10 mL). Crude product was
13C NMR (126 MHz, DMSO-d6) δ 166.00, 161.15, 157.05, 146.68,
129.26, 128.48, 127.28, 67.60, 64.34, 52.06, 50.39, 27.62, 18.30.
°
diluted with methanol (5 mL) and water (50 mL), heated to 80 C for
2 h and then cooled down to room temperature and stirred
overnight. Precipitated solids were collected by filtration and dried
under vacuum. The solid material was diluted with DCM (15 mL),
the insoluble particles were filtered off, DCM was removed on
rotary vacuum evaporator yielding anion-free bambusuril BU-1
(190 mg, 0.095 mmol, 51%) as a white solid.
HR-MS (APCI+): m/z [C16H18N4O4 +H]+ observed: 319.1402, calcu-
lated: 319.1401.
23
°
½a�D =À 15.2 (MeOH, c=0.50 g/100 mL)
(1S,5R)-2-[(1R)-1-(4-Carboxyphenyl)ethyl]-4-methylglycoluril
4aH
1H NMR (500 MHz, DMSO-d6) δ 7.83 (d, J=8.1 Hz, 12H), 7.37 (d, J=
8.1 Hz, 12H), 5.35 (d, J=6.7 Hz, 6H), 5.27 (d, J=6.7 Hz, 6H), 5.15 (m,
6H), 4.85 (s, 6H), 4.19 (s, 6H), 3.85 (s, 18H), 3.02 (s, 18H), 1.62 (d, J=
6.8 Hz, 18H).
Glycoluril 4a (207 mg, 0.60 mmol) and powdered KOH (72 mg,
1.28 mmol) were dissolved in MeOH (4 mL). The solution was
°
heated to 60 C for 4 h. MeOH was removed on rotary vacuum
13C NMR (126 MHz, DMSO-d6) δ 165.91, 158.38, 157.74, 147.71,
129.20, 128.14, 126.39, 69.68, 69.00, 54.27, 52.05, 48.57, 47.60, 30.69,
18.80.
evaporator and the resulting solid was dissolved in water (5 mL).
35% HCl (0.5 mL) was added dropwise to the solution to precipitate
white solid. Solid was collected by filtration, washed with water and
dried under vacuum to produce compound 4aH (148 mg,
0.45 mmol, 75%).
HR-MS (ESI): m/z [C96H108 N24O24 +Cl]À observed: 2016.7669, calcu-
lated: 2016.7691, m/z [C96H108 N24O24 +SbF6]À observed: 2215.6927,
calculated: 2215.6911
1H NMR (500 MHz, DMSO-d6) δ 12.87 (s, 1H), 7.90 (d, J=8.4 Hz, 2H),
7.53 (s, 1H), 7.49 (s, 1H), 7.41 (d, J=8.2 Hz, 2H), 5.10 (d, J=8.2, 1H),
5.06–5.00 (m, 2H), 2.65 (s, 3H), 1.54 (d, J=7.3 Hz, 3H).
23
°
½a�D =21.4 (MeOH/DCM 1:1, c=0.50 g/100 mL)
13C NMR (126 MHz, DMSO-d6) δ 167.02, 161.14, 157.05, 146.12,
129.63, 129.39, 127.08, 67.59, 64.29, 50.36, 27.62, 18.35.
N-(4-Methoxycarbonylbenzyl)-N’-methylurea 5
Methyl-(4-aminomethyl)benzoate
hydrochloride[53]
(15.0 g,
HR-MS (ESI): m/z [C14H16N4O4 +H]+ observed: 305.1244, calculated:
305.1244.
74.3 mmol) and 10 (11.3 g, 90.3 mmol) were diluted with DCM
(300 mL), triethyl amine (48 mL) was added and the reaction
mixture was stirred at room temperature overnight. DCM was then
removed on rotary vacuum evaporator providing crude oily
material. The crude material was sonicated with water (100 mL),
which resulted in crystallization of the product. Crystals were
collected by filtration, washed with water (2×20 mL) and dried
under vacuum yielding compound 5 (15.2 g, 68.4 mmol, 92%)
(1R,5S)-2-[(1R)-1-(4-Carboxyphenyl)ethyl]-4-methylglycoluril
4bH
Urea 3 (1.08 g, 4.6 mmol) and 11 (0.75 g, 6.4 mmol) were stirred in
°
aq. HCl (50 mL H2O and 0.5 mL 35% HCl) at 80 C for 2 h. The
resulting suspension was cooled in an ice bath, solids were
collected by filtration and washed with water (2×10 mL). The
collected solid was dried under vacuum yielding glycoluril 4a as a
single stereoisomer (0.31 g, 3.9 mmol, 21%). The filtrate was
evaporated to dryness, resulting solid was treated with acetone
(20 mL) and stirred for 1 h. Insoluble solid was removed by filtration
and the acetone was evaporated. Resulting solid was dissolved in
MeOH (20 mL) and treated with powdered NaOH (0.19 g,
1H NMR (500 MHz, DMSO-d6) δ 7.90 (d, J=8.5 Hz, 2H), 7.37 (d, J=
8.5 Hz, 2H), 6.47 (t, J=6.0 Hz, 1H), 5.86 (d, J=4.6 Hz, 1H), 4.27 (d,
J=6.1 Hz, 2H), 3.84 (s, 3H), 2.57 (d, J=4.7 Hz, 3H).
13C NMR (126 MHz, DMSO-d6) δ 166.13, 158.60, 146.94, 129.09,
127.83, 127.05, 51.96, 43.50, 42.69, 26.43.
HR-MS (APCI+): m/z [C12H16N2O3 +H]+ observed: 223.1076, calcu-
lated: 223.1077.
°
4.8 mmol). The solution was heated to 60 C for 4 h. MeOH was
then evaporated and solids were dissolved in water (10 mL). 35%
HCl (0.5 mL) was added to precipitate the product. The precipitate
was collected by filtration, washed with water (5 mL) and acetone
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(�)-2-(4-Methoxycarbonylbenzyl)-4-methylglycoluril (�)-6
Urea 5 (15.2 g, 68.4 mmol) and 11[45] (15.0 g, 127 mmol) were stirred
Enantiomerically pure (1S,5R)-7and (1R,5S)-7
1
2
3
4
5
6
7
8
9
Glycoluril (�)-8 (17.0 g, 46.7 mmol) was dissolved in dioxane
(600 mL). Upon stirring S-12 (2.83 g, 3.01 mL, 23.4 mmol) was
added. The addition of amine resulted in crystallization of salt of
enantiomerically enriched (1R,5S)-8 as within 1 h. Solid was
collected by filtration. The mother liquor was treated with R-12
(2.83 g, 3.01 mL, 23.4 mmol) to crystallize (1S,5R)-8. Crystals were
collected by filtration. To isolate glycoluril 8 from the salt as free
carboxylic, the salt was placed into separatory funnel, treated with
DCM (100 mL), water (95 mL) and concentrated hydrochloric acid
(5 mL) and shaken until dissolution. The organic layer was
separated, aqueous layer was once more extracted with DCM
(50 mL), combined extracts were dried over anhydrous MgSO4,
filtered and evaporated providing enriched enantiomer of as a
white solid. Yield of the (1R,5S)-8 after first crystallization was
5.50 g, yield of (1S,5R)-8 was 4.54 g.
°
in aq. HCl (175 mL H2O with 1.75 mL 35% HCl) at 80 C for 1 h.
More 11 (1.50 g, 12.7 mmol) was then added and the stirring of the
°
mixture at 80 C for 1 h continued for one more hour. The resulting
suspension was cooled in an ice bath, solids were collected by
filtration and washed with water. The collected solid was dried
under vacuum yielding glycoluril (�)-6 (18.25 g, 60.0 mmol, 88%).
1H NMR (500 MHz, DMSO-d6) δ 7.91 (d, J=8.3 Hz, 2H), 7.58 (s, 1H),
7.53 (s, 1H), 7.39 (d, J=8.3 Hz, 2H), 5.18 (d, J=8.1 Hz, 1H), 5.09 (d,
J=8.1, 1H), 4.60 (d, J=16.1 Hz, 1H), 4.14 (d, J=16.1 Hz, 1H), 3.84 (s,
3H), 2.70 (s, 3H).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
13C NMR (126 MHz, DMSO-d6) δ 166.07, 160.93, 157.52, 143.32,
129.33, 128.50, 127.82, 67.38, 65.33, 52.06, 43.88, 27.76.
HR-MS (APCI+): m/z [C14H16N4O4 +H]+ observed: 305.1243, calcu-
lated: 305.1244.
The crystallization of each enriched enantiomer was twice repeated.
For the following crystallizations only 200 mL of dioxane and 1
equivalent of 12 were used. Final yield of (1R,5S)-8 was 4.51 g, yield
of the (1S,5R)-8 4.04 g.
(�)-2-(4-Carboxybenzyl)-4-methylglycoluril (�)-7
Enantiomerically pure glycoluril (1R,5S)-8 (4.51 g. 12.0 mmol) was
dissolved in MeOH (40 mL), powdered KOH (2.70 g, 48.0 mmol) was
added. Potassium salt of glycoluril (1S,5R)-7 crystallized within
10 min. Crystals were isolated by filtration, dissolved in water
(15 mL) and the glycoluril (1S,5R)-7 was precipitated with concen-
trated HCl (2 mL). Precipitate was collected by filtration, washed
with water and dried under vacuum providing glycoluril (1S,5R)-7
as a white solid (3.38 g, 11.6 mmol, 97%).
Glycoluril (�)-6 (5.5 g, 18.1 mmol) and powdered KOH (2.0 g,
35 mmol) were dissolved in MeOH (45 mL). The solution was heated
°
to 60 C for 3 h. MeOH was removed on rotary vacuum evaporator
and the resulting solid was dissolved in water (25 mL). 35% HCl
(6 mL) was added dropwise to the solution to produce an
amorphous solid. The suspension was sonicated to make the solid
crystallize, crystals were collected by filtration, washed with water
and dried under vacuum to produce compound (�)-7 (4.70 g,
16.2 mmol, 90%).
23
°
½a�D =52.0 ((1R,5S)-7, MeOH, c=0.50 g/100 mL)
1H NMR (500 MHz, DMSO-d6) δ 12.86 (s, 1H), 7.91 (d, J=8.3 Hz, 2H),
7.57 (s, 1H), 7.53 (s, 1H), 7.36 (d, J=8.3 Hz, 2H), 5.17 (d, J=8.1 Hz,
1H), 5.08 (d, J=8.1 Hz, 1H), 4.60 (d, J=16.0 Hz, 1H), 4.12 (d, J=
16.1 Hz, 1H), 3.31 (s, 3H).
23
°
½a�D =À 48.0 ((1S,5R)-7, MeOH, c=0.50 g/100 mL)
Bambusuril BU-2
13C NMR (126 MHz, DMSO) δ 167.07, 160.90, 157.46, 142.74, 129.63,
129.45, 127.63, 67.33, 65.24, 43.82, 27.74.
Glycoluril (1R,5S)-7 (0.60 g, 2.1 mmol) and paraformaldehyde
(72.0 mg, 2.4 mmol) were dissolved in the mixture of dioxane
(10 mL) and H2SO4 (0.3 mL). The solution was stirred at 80 C for
HR-MS (APCI+): m/z [C13H14N4O4 +H]+ observed: 291.1089, calcu-
lated: 291.1088.
°
90 min. Mixture was then cooled to room temperature, solids were
collected by filtration, washed with diethyl ether and dissolved in a
mixture of water (9 mL) and 25% ammonia (1 mL) and stirred for
15 min. Phosphoric acid (1.5 mL) was then added to the solution to
precipitate the macrocycle. Crude macrocycle (570 mg) bearing
COOH groups was isolated. Solids were collected by filtration,
washed with water (3×10 mL) and dried under vacuum. Dry solid
was then dissolved in the mixture of MeOH (10 mL) and SOCl2
(�)-2,4-Diacetyl-6-(4-carboxybenzyl)-8-methylglycoluril (�)-8
Glycoluril (�)-7 (14.2 g, 48.9 mmol) was suspended in acetic
anhydride (36 mL). Upon stirring concentrated H2SO4 (36 drops)
was added. The reaction mixture was stirred at room temperature
overnight providing clear yellowish solution. The solution was
concentrated on rotary vacuum evaporator, diluted with water
(50 mL) and extracted with DCM (2×50 mL). Combined extracts
were dried over anhydrous MgSO4, filtered and evaporated. Acetic
acid in the crude product was removed by co-evaporation with
acetonitrile (2×50 mL) on rotary vacuum evaporator. Compound
(�)-8 was isolated as a white solid (17.0 g, 45.4 mmol, 93%).
1H NMR (500 MHz, DMSO-d6) δ 12.83 (s, 1H), 7.88 (d, J=8.3 Hz, 2H),
7.30 (d, J=8.1 Hz, 2H), 5.79 (d, J=8.5 Hz, 1H), 5.76 (d, J=8.6 Hz,
1H), 4.55 (d, J=16.3 Hz, 1H), 4.47 (d, J=16.3 Hz, 1H), 2.87 (s, 3H),
2.46 (s, 3H), 2.26 (s, 3H).
°
(0.4 mL) and the solution was heated to 55 C for 90 min. Pure
product precipitated during the reaction. After cooling to room
temperature, the precipitate was collected by filtration and washed
with MeOH (2×5 mL), yielding BU-2·HCl complex (295 mg,
0.15 mmol, 44%). The solid was dissolved in the mixture of DCM
(5 mL) and MeOH (5 mL). The solution was treated by AgSbF6
(86 mg, 0.25 mmol) in MeOH (5 mL) and stirred for 15 min.
Precipitated solid removed by filtration through filter paper,
solution was concentrated to approximately 2 mL. Solution was
diluted with demineralized water (10 mL) to precipitate the macro-
cycle. Solid was collected by filtration and dried under vacuum
yielding anion-free bambusuril BU-2 (253 mg, 0.13 mmol, 39%) as a
white solid.
13C NMR (126 MHz, DMSO-d6) δ 171.29, 171.02, 167.12, 158.48,
151.57, 143.44, 129.38, 129.27, 126.80, 66.62, 64.92, 46.39, 31.05,
24.38, 23.93.
1H NMR (500 MHz, DMSO-d6) δ 7.84 (d, J=8.1 Hz, 12H), 7.30 (d, J=
8.1 Hz, 12H), 5.22 (d, J=8.0 Hz, 6H), 5.13 (d, J=7.9 Hz, 6H), 4.83 (s,
6H), 4.71 (d, J=16.9 Hz, 6H), 4.58 (d, J=17.0 Hz, 6H), 4.10 (s, 6H),
3.85 (s, 18H), 3.03 (s, 18H).
HR-MS (APCI+): m/z [C17H18N4O6 +H]+ observed: 375.1298, calcu-
lated: 375.1299.
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13C NMR (126 MHz, DMSO-d6) δ 165.87, 158.93, 158.14, 143.68,
129.21, 128.41, 126.82, 69.64, 68.93, 52.02, 47.84, 47.54, 46.91, 31.03.
127.39, 126.05, 125.53, 124.97, 124.02, 67.29, 65.16, 48.54, 43.78,
27.73, 22.00.
1
2
3
4
5
6
7
8
9
HR-MS (ESI): m/z [C90H96N24O24 +Cl]À observed: 1932.6728, calcu-
lated: 1932.6752, m/z [C90H96N24O24 +SbF6]À observed: 2131.6022,
calculated: 2131.5972.
HR-MS (APCI+): m/z [C25H25N5O3 +H]+ observed: 444.2027, calcu-
lated: 444.2030.
23
°
½a�D =À 17.8 ((1R,5S)-BU-2, MeOH/DCM 1:1, c=0.50 g/100 mL)
14b
1H NMR (500 MHz, DMSO-d6) δ 8.85 (d, J=8.0 Hz, 1H), 7.87 (m, 6H),
7.61–7.53 (m, 3H), 7.52–7.43 (m, 2H), 7.34 (d, J=8.2 Hz, 2H), 5.33 (m,
1H), 5.16 (d, J=8.1 Hz, 1H), 5.05 (d, J=8.1 Hz, 1H), 4.60 (d, J=
15.8 Hz, 1H), 4.08 (t, J=15.8 Hz, 1H), 2.70 (s, 3H), 1.57 (d, J=7.1 Hz,
3H).
Glycoluril amide 13
Glycoluril 4aH (50 mg, 0.16 mmol), N-hydroxysuccinimide (NHS)
(26 mg, 0.22 mmol) and DCC (46 mg, 0.22 mmol) were dissolved in
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
°
DMF (4 mL) and stirred at 25 C for 16 h. Solution was then treated
with (S)-1-(2-naphthyl)ethylamine 15 (50 mg, 0.29 mmol) and
triethyl amine (90 mL, 0.65 mmol) in DMF (0.5 mL) and stirred for
more 24 h. DMF was evaporated and solids were washed with
water (4 mL). Crude product was purified by silica gel column
chromatography (eluent DCM:MeOH, 9:1). Compound 13 was
isolated as white solid (50 mg, 0.11 mmol, 69%).
13C NMR (126 MHz, DMSO-d6) δ 165.34, 160.92, 157.44, 142.36,
140.75, 133.44, 132.83, 132.01, 127.81, 127.60, 127.57, 127.46,
127.39, 126.05, 125.53, 124.96, 124.01, 67.29, 65.14, 48.54, 43.77,
27.73, 22.01.
HR-MS (APCI+): m/z [C25H25N5O3 +H]+ observed: 444.2029;, calcu-
lated: 444.2030.
1H NMR (500 MHz, DMSO-d6) δ 8.85 (d, J=8.0 Hz, 1H), 7.91–7.84 (m,
6H), 7.57 (dd, J=8.5, 1.6 Hz, 1H), 7.54–7.44 (m, 4H), 7.39 (d, J=
8.3 Hz, 2H), 5.33 (m, 1H), 5.14–4.88 (m, 3H), 2.65 (s, 3H), 1.57 (d, J=
7.1 Hz, 3H), 1.54 (d, J=7.3 Hz, 3H).
Preparation of tetrabutylammonium salts of chiral acids
Chiral acid (6.84 mmol) was dissolved in methanol (1–2 mL).
Volumetric solution of tetrabutylammonium hydroxide (0.5009 M,
13.8 mL) was added. The solution was shaken and left to stand for
10 min. Then the methanol was evaporated under vacuum and the
oily residue was dried under high vacuum to provide correspond-
ing tetrabutylammonium salt.
13C NMR (126 MHz, DMSO-d6) δ 165.29, 161.15, 157.07, 144.16,
142.36, 133.45, 132.83, 132.01, 127.81, 127.61, 127.53, 127.39,
126.81, 126.05, 125.53, 124.96, 124.01, 67.57, 64.20, 50.32, 48.54,
27.62, 22.00, 18.37.
HR-MS (APCI+): m/z [C26H27N5O3 +H]+ observed: 458.2185, calcu-
lated: 458.2187.
Acknowledgements
Glycoluril amides 14a and 14b
Enantiomerically pure glycoluril (1R,5S)-8 (0.3 g, 0.8 mmol), NHS
(0.11 g, 1.0 mmol) and DCC (0.22 g, 1.1 mmol) were dissolved in
This work was supported by the Czech Science Foundation (No.
18-21801S). We thank the CETOCOEN EXCELLENCE Teaming 2
project (supported by MEYS CR: CZ.02.1.01/0.0/0.0/17_043/
0009632) and the RECETOC research infrastructure (LM2018121).
We acknowledge the CF X-ray diffraction and Bio-SAXS and CF
Proteomic supported by the CIISB research infrastructure
(LM2018127 funded by MEYS CR).
°
THF (4 mL). Solution was stirred at 25 C overnight. Solids were
removed by filtration and THF was evaporated. Remaining solid
was dissolved in DCM (20 mL) and washed with brine (10 mL).
Organic phase was dried over anhydrous MgSO4. The solution was
treated with 15 (140 mg, 0.8 mmol) and triethyl amine (0.11 mL,
0.8 mmol) and stirred for 2 h. After that more 15 (30 mg, 0.2 mmol)
was added and the solution was stirred for more 4 h. Last portion
of 15 (30 mg, 0.2 mmol) was added and the solution was stirred
overnight. Solution was then washed with brine (10 mL), dried over
anhydrous MgSO4 and DCM evaporated. Resulting solid was
dissolved in MeOH (20 mL), treated with powdered KOH (100 mg,
1.8 mmol) and stirred for 15 min. After that MeOH was evaporated
and solids were washed with water (8 mL). Crude product was
purified by silica gel column chromatography (eluent DCM:MeOH,
9:1). Compound 14a was isolated as white solid (210 mg,
0.5 mmol, 59%). Compound 14b was prepared analogically with
similar yield starting from compound (1R,5S)-8.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: anion receptors · enantioselective recognition ·
glycolurils · host-guest systems · macrocycles
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1H NMR (500 MHz, DMSO-d6) δ 8.85 (d, J=8.0 Hz, 1H), 7.87 (m, 6H),
50
51
52
53
54
55
56
57
7.60–7.53 (m, 3H), 7.51–7.44 (m, 2H), 7.34 (d, J=8.2 Hz, 2H), 5.33 (m,
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Manuscript received: April 1, 2020
Revised manuscript received: May 27, 2020
ChemPlusChem 2020, 85, 1307–1314
www.chempluschem.org
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