Helical Chirality Induces a Substrate-Selectivity Switch in Carbohydrates Recognitions

Article abstract of DOI:10.1021/acs.joc.8b00276

A new chiral hemicryptophane cage combining an electron-rich cyclotriveratrylene (CTV) unit and polar amine functions has been synthesized. The resolution of the racemic mixture has been performed by chiral HPLC, and the assignment of the absolute configu

Full text of DOI:10.1021/acs.joc.8b00276

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Article
Helical Chirality Induces a Substrate-Selectivity
Switch in Carbohydrates Recognitions
Augustin Long, Olivier Perraud, Muriel Albalat, Vincent Robert, Jean-Pierre Dutasta, and Alexandre Martinez
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00276 • Publication Date (Web): 08 May 2018
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Helical Chirality Induces a Substrate-Selectivity Switch in Carbohydrates
Recognitions
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Augustin Long†, Olivier Perraud‡, Muriel Albalat†, Vincent Robert§, JeanꢀPierre Dutasta‡, Alexandre
Martinez*,†
†
Aix Marseille Univ, CNRS, Centrale Marseille, iSm2, Marseille, France
Eꢀmail: alexandre.martinez@centraleꢀmarseille.fr
‡
Laboratoire de Chimie
École Normale Supérieure de Lyon, CNRS, UCBL
46, Allée d'Italie, Fꢀ69364 Lyon, France
§
Laboratoire de Chimie Quantique Institut de Chimie, UMR CNRS 7177,
Université de Strasbourg, 4, rue Blaise Pascal,
Fꢀ67070 Strasbourg, France.
Abstract. A new chiral hemicryptophane cage combining electron rich CTV unit and polar amine
functions has been synthesized. The resolution of the racemic mixture has been performed by chiral
HPLC, and the assignment of the absolute configuration of the two enantiomers has been achieved
using ECD spectroscopy. In contrast with other hemicryptophane receptors, the two enantiomeric
hosts display both remarkable enantioselectivities in the recognition of carbohydrates and good
binding constants. Moreover, by switching the chirality of the CTV unit from M to P, a strong
preference shift from glucose to mannose derivatives is observed.
Introduction
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The recognition of carbohydrates plays a crucial role in many biological processes, like tumor
metastases,1 protein folding,2 infection by pathogens3 or cellꢀcell recognition.4 Their chirality can
account for their versatile use in living systems: sugars can be considered as the most informationꢀrich
biological molecules as they present numerous stereogenic centers. Thus, biological systems have
developed selective receptors in order to exploit this structural variability. The selective recognition of
carbohydrates by synthetic receptors through nonꢀcovalent interactions arouse a considerable interest
as it could allow for a better understanding of these biological processes and lead to the establishment
of new diagnostics or therapies in medicine.5ꢀ7 However, carbohydrates stereoisomers are very difficult
to discriminate by synthetic hosts as they present complex threeꢀdimensional structures, which often
only differ by the configuration of a single stereogenic centre. Cage receptors, when compared to their
“openꢀshell” counterparts,8 appear as particularly appealing because they can maximize the
interactions with sugar guests in the heart of their cavity leading to a better selectivity.9 Among the
cage compounds, hemicryptophanes have been recently described as a new class of receptors capable
of complexing carbohydrates.10 These hosts, built from a cyclotriveratrylene (CTV) linked to another
C3 symmetrical moiety, display low to good binding constants and selectivities toward sugar guests.
However, this class of host suffers from a major drawback in carbohydrates recognition: either
hemicryptophanes display good binding constants but a low selectivity, or a high selectivity is reached
but associated with a modest affinity. For instance, host 1 exhibits good binding constants toward
mannose up to 103 Mꢀ1, but negligible enantioselectivity is observed (binding constants are 1410 and
1544 Mꢀ1 for the recognition of OctꢀαꢀMan with MꢀSSSꢀ1 and PꢀRRRꢀ1, respectively, Figure 1),11
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whereas host 2 shows an exclusive enantioselectivity toward OctꢀαꢀGlc derivative, but a much modest
association constant of around 155 Mꢀ1 is measured with the MꢀSSS enantiomer.12 Thus, there is a
challenge to design new hemicryptophane cages capable of combining both good affinity and
selectivity toward carbohydrates. Here we report on the synthesis of the enantiopure hemicryptophanes
(ꢀ)ꢀ3 and (+)ꢀ3. Resolution of the racemic mixture has been achieved by chiral HPLC and assignment
of their absolute configuration by electronic circular dichroism (ECD). Their recognition properties
towards mannose, glucose and galactose derivatives reveal that both good binding constants (up to 103
Mꢀ1) and enantioselectivity (> 99.5/0.5), can be achieved with host 3. Interestingly, the change in the
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helical chirality of the CTV unit induces a substrateꢀselectivity switch: the Mꢀ(ꢀ)ꢀ3 enantiomer shows a
better binding constant toward βꢀglucose derivative whereas the Pꢀ(+)ꢀ3 enantiomer exhibits a
remarkable selectivity for mannose.
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(a)
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OMe
MeO
O
OMe
MeO
OMe
MeO
O
MeO
O
O
O
MeO
O
MeO
O
O
O
O
O
O
O
O
NH
O
O
O
HN
NH
O
NH HN
N
NH
O
OH
HO
O
O
HO
N
M-3
M-SSS-1
SSS-Mꢀ2
OH
OH
OH
HO
HO
(b)
OH
O
O
O
HO
HO
HO
HO
OH
O
OH
O
Oct
Oct
O
Oct
Octꢀ ꢀGlc
Octꢀ ꢀMan
Octꢀ ꢀGal
HO
OH
OH
OH
HO
O Oct HO
O
O
O
HO
HO
HO
HO
Oct
O
O Oct
OH
OH
Octꢀ ꢀMan
Octꢀ ꢀGal
Octꢀ ꢀGlc
Figure 1. (a) Structures of hemicryptohane hosts 1ꢀ3. (b) Structures of the carbohydrate guests.
Results and discussion
Synthesis
Scheme 1 shows the synthetic pathway followed to synthesize the hemicryptophane racꢀ3.
Cyclotriveratrylene derivative 4 was obtained according to the previously reported two steps
procedure: vanillyl alcohol first reacts with 1,2ꢀdibromoethane and the subsequent cyclization
achieved with scandium triflate in CH3CN, provides the CTV racꢀ4 in 18 % overall yield.13 The
reaction between sodium azide and racꢀ4 affords racꢀ5 in 90% yield. The reduction of the azido
groups of racꢀ5 is performed with PPh3 in a mixture of THF and water at room temperature to give
racꢀ6 in 91% yield. The C3 symmetrical derivative 7 is obtained by reaction between 1,3,5ꢀ
tribromomethylꢀbenzene and 2ꢀhydroxybenzaldehyde with NaOH in EtOH. Reductive amination
between 7 and racꢀ6 is then achieved in a CHCl3/MeOH mixture providing racꢀ3 in 42% yield. Thus,
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hemicryptophane racꢀ3 was obtained with a sixꢀstep convergent synthesis, starting from the
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commercially available vanillyl alcohol with an overall yield of 6%.
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Scheme 1. Synthesis of racꢀ3; a) NaN3, DMF, 60°C, 16h, 90%; b) PPh3, THF/H2O (30/1), r.t., 16h, 91%; c)
NaOH, EtOH, reflux, 20h, 78%; d) (i) CHCl3/MeOH (1/1), r.t., 18h; (ii) NaBH4, 0°C to r.t.; 3h, 42%.
The resolution of the racemic mixture of hemicryptophane 3 is performed using chiral HPLC. The
enantiomers are separated on analytical Chiralpak ID (250 x 4,6 mm) column with resolution higher
than two (Table S1). The use of a semiꢀpreparative Chiralpak ID (250 x 10 mm) column with
EtOH+Et3N/CH2Cl2 (50/50) as eluent, affords the (ꢀ)ꢀenantiomer and (+)ꢀenantiomer with ee > 99.5%
and 98.5% respectively (Figures S16 and S17).
The hemicryptophanes (ꢀ)ꢀ3 and (+)ꢀ3 are on average of C3 symmetry in solution as indicated by
1
their H NMR spectra in CDCl3. That of (+)ꢀ3 is shown in Figure 2 and displays the expected signals
for the CTV moiety: the characteristic AB system for the ArCH2 bridges at 3.53 and 4.73 ppm, two
singlets for the aromatic protons at 6.90 and 6.75 ppm and one singlet for the OMe group. The
aromatic protons of the benzene ring in the lower part of the host appear as a singlet at 6.82 ppm and
the connected diastereotopic methylene bridges appear as two multiplets between 4.20 and 4.35 ppm.
The signals of the aromatic protons of the linkers give two doublets and two triplets between 6.5 and
7.2 ppm, whereas the diastereotopic aliphatic protons exhibit expected broad multiplets.
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Figure 2. 1H NMR spectra (400 MHz, CDCl3) of (+)ꢀ3.
Assignment of the absolute configuration
The ECD spectra of hosts Pꢀ3 and Mꢀ3 were recorded in CH2Cl2 at 298 K (Figure 3). Each spectrum
exhibits the expected behavior for hemicryptophanes and CTV based host compounds, namely two
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exciton patterns roughly centered on the isotropic absorption of the LB (290 nm) and LA (240 nm)
transitions. As the signs of the ECD spectra in the 1LA transition area are poorly sensitive to the nature
of the substituents, the assignment of the absolute configuration of CTV’s is usually performed by
comparison to the calculated ECD spectra of a reference CTV, previously obtained by Collet and coꢀ
workers.14 Based on these previous works, the first (respectively second) eluted compound (+)ꢀ3
(respectively (ꢀ)ꢀ3) corresponds to the P configuration (respectively M configuration).
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P-3
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M-3
-5
ꢀ5
ꢀ7
-7
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Wavelength [nm]
λ (nm)
Figure 3. Experimental ECD spectra of the enantiomers of 3: the first eluted enantiomer is represented with a
green solid line (0.404 mmol.Lꢀ1 in CH2Cl2) and the second one by a red dotted line (0.442 mmol.Lꢀ1 in CH2Cl2).
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Recognition properties
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The inner cavity of hemicryptophane 3 presents both apolar (north and south parts) and polar domains
(NH groups in the linkers) and thus, should provide appropriate matches for sugars complexation
through CꢀH···π interactions and hydrogen bonding respectively. Additions of a solution of OctꢀβꢀGlc
in CDCl3 to a solution of the enantiomer (ꢀ)ꢀ3 in the same solvent induce changes in the chemical shift
of the 1H signals of the cage, evidencing interactions between the sugar guest and the
hemicryptophane host. The formation of a hostꢀguest complex is also supported by mass spectrometry:
a peak with an exact mass corresponding to the 1:1 host:guest ratio was detected in a mixture of (ꢀ)ꢀ3
and OctꢀβꢀGlc (Figure S20). Insights into the positioning of the carbohydrate inside the cavity of the
hemicryptophane (ꢀ)ꢀ3 can be retrieved from the NOESY experiments performed on an equimolar
solution of OctꢀβꢀGlc and (ꢀ)ꢀ3.15,16 Although several signals overlap, precluding the assignment of the
whole intermolecular contacts, the following intermolecular host/guest NOEs contacts are
unambiguously evidenced in the NOESY map: OMe/H1 and CTVꢀHar/H3 (Figure 4). These data
strongly support the formation of an inclusion complex of OctꢀβꢀGlc inside the cavity of (ꢀ)ꢀ3. It can
be underlined that with other hemicryptophane derivatives, previously reported as receptors for
carbohydrates, NOEs were never observed, suggesting that this OctꢀβꢀGlc@(ꢀ)ꢀ3 complex is
kinetically more stable in solution, than the previously reported hemicryptophane complexes, although
the hostꢀguest exchange remains fast on the NMR time scale.9ꢀ12,17 Based on these intermolecular
NOEs contacts, DFT calculations were then performed in order to provide a three dimensional
structure of the complex. The resulting minimized structure is shown in Figure 5 and is in agreement
with the NOE experimental data (H1···OMe and H3···CTVꢀHar distances of 2.5 and 3.9 Å
respectively). Furthermore, the optimized geometry of the complex reveals that OctꢀβꢀGlc is partially
encaged in the hemicryptophane structure with the sugar unit trapped inside the cavity and the alkyl
chain left outside. Hydrogen bonding between the alcohol functions of the guest and the nitrogen of
the host 3 are observed (for instance, NH···O4 and NH···O1 distances of 2.68 and 2.65 Å are measured
respectively). CH···π interactions are also evidenced in the DFT structure with H1··· CTVꢀC and
H4···CꢀAr distances of 2.57 and 2.75 Å respectively.
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Figure 4. (a) 400 MHz NOESY spectrum of a 1:1 mixture of OctꢀβꢀGlc and (ꢀ)ꢀ3 in CDCl3 at 298 K, acquired
with a 500 ms mixing time. (b) Schematic representation of NOE contacts.
Figure 5. DFT optimized structure showing the encapsulation of OctꢀβꢀGlc guest within hemicryptophane (ꢀ)ꢀ3.
We then investigated the ability of the enantiomeric hosts to discriminate stereoisomeric
1
carbohydrates. H NMR titration experiments between the two enantiomers of receptor 3 and six
carbohydrates (αꢀ and βꢀmannose, αꢀ and βꢀglucose and αꢀ and βꢀgalactose, Figure 1) were thus
performed in CDCl3 at 298 K. In all cases only one set of signals was observed for the complex and
for the receptor, showing that the hostꢀguest exchange is fast on the NMR time scale. As the aromatic
protons of the CTV unit display sharp wellꢀdefined signals without overlap with other ones, their
complexationꢀinduced shifts were plotted as a function of the guest/host ratio (Figures S22ꢀS27). In
light of recent articles dealing with the stoichiometry of host/guest complexes and the determination of
the related binding constants, titration curves were fitted with different models, i.e. stoichiometry,
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using Bindfit program, instead of performing Job’s plot experiments.18,19 In all cases the better fit was
obtained with a 1:1 host:guest stoichiometry, which is in agreement with the mass analysis of the (ꢀ)ꢀ3
and OctꢀβꢀGlc mixture (Figure S20). From the resulting association constants reported in Table 1
several features can be underlined. (i) Firstly, good association constants up to 103 Mꢀ1 can be reached,
in particular for the binding of OctꢀβꢀGlc by (ꢀ)ꢀ3. (ii) Secondly, the enantioselectivity remains high:
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an enantioselectivity of 96:4 is reached between (ꢀ)ꢀ3 and (+)ꢀ3 for the recognition of OctꢀβꢀGlc, with
binding constants of 993 and 45 Mꢀ1 respectively. It is all the more remarkable that the
enantioselectivity of the recognition process is only controlled by the inherent chirality of the CTV
moiety. This is in sharp contrast with most of the previous hemicryptophane cages used to complex
enantioselectively carbohydrates, which involved additional stereogenic units (center or axis).17 This
enantioselectivity is even exclusive with galactose derivatives since no recognition occurs with the
receptor (+)ꢀ3 whereas host (ꢀ)ꢀ3 displays binding constants of 245 Mꢀ1 and 274 Mꢀ1 for OctꢀαꢀGal and
OctꢀβꢀGal guest, respectively. (iii) Good α/β discrimination can be also achieved with these receptors:
the binding constant is ten times higher for OctꢀβꢀGlc than for OctꢀαꢀGlc with (ꢀ)ꢀ3 as receptor (993
Mꢀ1 and 100 Mꢀ1, respectively), and 30 times higher for OctꢀαꢀMan than for OctꢀβꢀMan with (ꢀ)ꢀ3 as
receptor (218 Mꢀ1 and 7 Mꢀ1, respectively). (iv) Whereas the (ꢀ)ꢀ3 enantiomer is OctꢀβꢀGlcꢀselective
with a selectivity from moderate to high (a selectivity of 3.6:1 and 141:1 when compared to OctꢀβꢀGal
and OctꢀβꢀMan, respectively), the (+)ꢀ3 enantiomer displays an OctꢀMan selectivity, with a binding
constant up to 856 Mꢀ1, for OctꢀβꢀMan and less than 45 Mꢀ1 for other glucose and galactose
derivatives. This change in substrate selectivity suggests different orientations and interactions of
OctꢀβꢀGlc and OctꢀMan inside the cavities of (ꢀ)ꢀ3 and (+)ꢀ3 respectively. As the abilities of
carbohydrates to induce intermolecular hydrogen bonds is the following: OctꢀαꢀMan > OctꢀβꢀMan >
OctꢀβꢀGlc > OctꢀαꢀGlc > OctꢀαꢀGal ≥ OctꢀβꢀGal,20,21 hydrogen bonds play probably a more important
role in the formation of OctꢀαꢀMan@(+)ꢀ3 than in the in the case of the OctꢀβꢀGlc@(ꢀ)ꢀ3.
.
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Table 1. Binding constants Ka (M–1) for the 1:1 complexes formed between the different isomers of host 3 and
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the carbohydrate guests.a
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Guest
Host
(ꢀ)ꢀ3
(+)ꢀ3
(ꢀ)ꢀ3
(+)ꢀ3
(ꢀ)ꢀ3
(+)ꢀ3
(ꢀ)ꢀ3
(+)ꢀ3
(ꢀ)ꢀ3
(+)ꢀ3
(ꢀ)ꢀ3
(+)ꢀ3
Ka (L.molꢀ1)
993 ± 7.3%
45 ± 3.1%
274 ± 3.2%
ꢀb
Enantioselectivity
96:4
OctꢀβꢀGlc
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> 99.5:0.5
1:99
OctꢀβꢀGal
OctꢀβꢀMan
OctꢀαꢀGlc
OctꢀαꢀGal
OctꢀαꢀMan
7 ± 2.3%
856 ± 7.8%
100 ± 3.2%
30 ± 1.4%
245 ± 4.7%
ꢀ b
77:23
> 99.5:0.5
30:70
218 ± 2.1%
508 ± 5.2%
a Ka was determined by fitting 1H NMR titration curves (CDCl3, 500 MHz, 298 K) on the aromatic protons of the
CTV unit (δ at 6.90 ppm for the free host) with Bindfit program. More details on the calculations results
(covariance and RMS) can be found in Table S4; estimated error 10%. b No complexation detected.
Conclusions
In summary, we have described the synthesis of a new chiral hemicryptophane cage. Its resolution has
allowed investigating the stereoselective recognition properties of the two enantiomers toward six
carbohydrate stereoisomers. In sharp contrast with other hemicryptophane derivatives, this new host
combines both good binding constants (up to 103 Mꢀ1) and remarkable enantioselectivity (exclusive in
some cases). Moreover, depending on the enantiomer of the receptor used, a strong change in substrate
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selectivity is observed, allowing to choose the most appropriate hemicryptophane enantiomer to
complex selectively the desired sugar derivative.
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Experimental Section
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Chemicals and materials.
Starting material and solvents were of commercial grade and were used without further purification.
Chromatography was carried out with Merck 60 A (0.040 ꢀ 0.063 mm) silica gel. TLC was performed
with Merck silica gel 60 F254 plates. Melting points were determined with a Büchi Melting Point Bꢀ
545. IR spectra were obtained using a Bruker Alpha Platinium ATR. H NMR and 13C NMR were
1
recorded at 298 K on a Bruker Avance III HD 300 MHz spectrometer and a Bruker Avance III HD
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400 MHz. H NMR and 13C NMR chemical shifts δ are reported in ppm referenced to the protonated
residual solvent signal. HRMS were performed on a SYNAPT G2 HDMS (Waters) mass spectrometer
with API and spectra were obtained with TOF analysis. Measurements were realized with two internal
standards. ECD spectra were recorded on a Jasco Jꢀ815 spectrometer in CH2Cl2.
CTV-derivative 4.
Compound 4 was prepared according to the published procedure.13
Synthesis of 5.22
In a dry 100 mL roundꢀbottom flask, compound 4 (1.30 g, 1.78 mmol) was dissolved in dry DMF
(40 mL) and sodium azide (1.16 g, 17.8 mmol) was added portionꢀwise. The mixture was stirred at
60 °C overnight. Solvent was partially removed under reduced pressure and the resulting crude
mixture was diluted with CH2Cl2 (50 mL). Water was added (50 mL) and the layers were separated.
The aqueous layer was extracted with CH2Cl2 (30 mL). The organic layers were combined, washed
with water (3 x 30 mL), dried over MgSO4. A yellowish oil was obtained after evaporation of the
solvent under reduced pressure. Diethyl ether (70 mL) was added and the mixture was stored at 4°C
for about 6 hours to give a white solid which was filtered on a frit and washed with Et2O to yield 5
(990 mg, 90 %). 1H NMR (CDCl3, 300 MHz, 298 K) δ 6.93 (s, 3H), 6.85 (s, 3H), 4.75 (d, J = 13.7 Hz,
13
3H), 4.14 (t, J = 5.2 Hz, 6H), 3.83 (s, 9H), 3.59–3.52 (m, 9H); C NMR (CDCl3, 75 MHz, 298 K) δ
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149.2, 146.5, 133.9, 131.9, 118.1, 114.2, 69.2, 56.2, 50.3, 36.4; IR σ = 2931, 2110, 1511, 1265 cmꢀ1;
HRMS (ESIꢀTOF) m/z: [M + Na]+ Calcd for C30H33N9O6Na 638.2446; Found 638.2442; Rf = 0.23
(CH2Cl2); m.p. = 137–138 °C.
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Synthesis of 6.
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Compound 5 (490 mg, 0.796 mmol) was dissolved in THF (15 mL) and water (0.5 mL) and
triphenylphosphine (3.75 g, 14.3 mmol) was slowly added at 0 °C. The reaction mixture was stirred at
r.t. for 16 h and solvent was removed under reduced pressure. Diethyl ether (50 mL) was added to the
resulting solid and the mixture was stirred at r.t. for 24 h. Filtration afforded compound 6 as a white
powder (391 mg, 91 %). 1H NMR (CDCl3, 400 MHz, 298 K) δ 6.88 (s, 3H), 6.82 (s, 3H), 4.75 (d, J =
13.7 Hz, 3H), 4.07–3.92 (m, 6H), 3.82 (s, 9H), 3.53 (d, J = 13.4 Hz, 3H), 3.05 (t, J = 5.2 Hz, 6H); 13C
NMR (CDCl3, 100 MHz, 298 K) δ 148.5, 147.1, 132.6, 131.9, 116.1, 113.8, 72.0, 56.2, 41.6, 36.5; IR
σ = 3430, 2935, 1511, 1263 cmꢀ1; HRMS (ESIꢀTOF) m/z: [M + H]+ Calcd for C30H40N3O6 538.2912;
Found 538.2913; Rf= 0.05 (CH2Cl2/MeOH : 97/3); m.p. = 174–176 °C.
Synthesis of 7.23
To a solution of salicylaldehyde (158 µL, 1.52 mmol) in absolute ethanol (20 mL) was added NaOH
(66 mg, 1.65 mmol). The reaction mixture was stirred at r.t. for 40 minutes and a solution of 1,3,5ꢀ
tris(bromomethyl)benzene (151 mg, 0.423 mmol) in absolute ethanol (7 mL) was added. The mixture
was refluxed for 20 h and cooled to 4 °C overnight. The resulting solid was filtered off and washed
with cold ethanol and cold water to afford trialdehyde 7 as a white powder (165 mg, 78 %). 1H NMR
(CDCl3, 300 MHz, 298 K) δ 10.53 (s, 3H), 7.87 (dd, J = 7.7; 1.7 Hz, 3H), 7.58–7.52 (m, 6H), 7.07 (dd,
J = 17.2; 8.1 Hz, 6H), 5.25 (s, 6H); 13C NMR (CDCl3, 75 MHz, 298 K) 189.7, 160.9, 137.7, 136.2,
129.0, 126.0, 125.4, 121.5, 113.2, 70.2; IR σ = 3053, 2842, 1677, 1604, 1392, 1269 cmꢀ1; HRMS (ESIꢀ
TOF) m/z: [M + NH4]+ Calcd for C30H28NO6 498.1911; Found 498.1915; Rf= 0.32 (CH2Cl2); m.p. =
135–137 °C.
Synthesis of hemicryptophane 3.
To a solution of compound 6 (179 mg, 0.333 mmol) in a 1/1 mixture of CHCl3/MeOH (120 mL) was
added dropwise a solution of trialdehyde 7 (147 mg, 0.306 mmol) in the same mixture of solvents
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(80 mL). The reaction mixture was stirred overnight at r.t. and NaBH4 (407 mg, 10.7 mmol) was added
portionwise at 0 °C. The mixture was stirred at r.t. for 4 h and solvents were removed under reduced
pressure. The crude residue was dissolved in CHCl3 (60 mL) and washed with a 10 % NaOH solution
(30 mL). Aqueous phase was extracted with CHCl3 (2 x 20 mL) and combined organic layers were
washed with a 10 % NaOH solution (40 mL), dried over MgSO4. The organic solvent was removed
under reduced pressure to afford a white solid which was purified by column chromatography
(CHCl3/MeOH/Et3N, gradient from 97/1/2 to 90/8/2). Racemic hemicryptophane 3 was obtained as a
white solid (126 mg, 42 %).
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The resolution of the racemic mixture was performed by chiral HPLC using a Chiralpak ID (250
x 10 mm) column with EtOH + Et3N / CH2Cl2 (50/50) as mobile phase (see supporting information) to
yield the two enantiopure hemicryptohane (ꢀ)ꢀ3 (30 mg, ee > 99.5%) and (+)ꢀ3 (34 mg, ee > 98.5%).
1H NMR (CDCl3, 400 MHz, 298 K) δ 7.21 (dd, J = 7.3; 1.6 Hz, 3H), 7.08 (td, J = 8.0; 1.6 Hz, 3H),
6.97 (s, 3H), 6.87 (t, J = 7.1 Hz, 6H), 6.78 (s, 3H), 6.58 (d, J = 8.0 Hz, 3H), 4.73 (d, J = 13.7 Hz, 3H),
4.28–4.14 (m, 12H), 3.95 (d, J = 13.2 Hz, 3H), 3.88–3.81 (m, 3H), 3.62 (s, 9H), 3.53 (d, J = 13.8 Hz,
3H), 3.49 (br s, 3H), 3.13–3.05 (m, 3H), 2.99–2.92 (m, 3H); 13C NMR (CDCl3, 100 MHz, 298 K) δ
156.3, 149.0, 146.5, 137.1, 132.0, 131.0, 130.9, 129.5, 126.9, 121.0, 113.6, 112.1, 69.1, 68.6, 56.1,
48.8, 47.3, 36.4; IR σ = 3438, 2929, 1602, 1510, 1490, 1457, 1263, 1220 cmꢀ1; HRMS (ESIꢀTOF) m/z:
[M + H]+ Calcd for C60H64N3O9 970.4637; Found 970.4639; Rf= 0.06 (CHCl3/MeOH/Et3N: 96/2/2);
m.p. = 299–302 °C;
M (ꢀ): [ꢀ]ꢂꢁꢃ = ꢀ 79 (c 0.15, CH2Cl2).
P (+): [ꢀ]ꢂꢁꢃ = + 78 (c 0.235, CH2Cl2).
1H NMR titrations.
A solution of enantiopure host 3 (1.0 mM in CDCl3, 500 ꢁL) was titrated in NMR tubes with aliquots
of a concentrated solution (10 mM in the same solvent) of carbohydrates. The shifts ꢂδ of the host’s
protons signals at 6.90 ppm were measured after each addition and plotted as a function of the
guest/host ratio ([G]/[H]). Association constant Ka was obtained by nonlinear leastꢀsquares fitting of
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these plots using Bindfit program from Thordarson’s group19. Ka, covariance and RMS are reported for
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each carbohydrate in the supporting information.
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AUTHOR INFORMATION
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Corresponding Author
Eꢀmail: alexandre.martinez@centraleꢀmarseille.fr
Notes The authors declare no competing financial interest.
ASSOCIATED CONTENTS
Supporting Information
1H, 13C NMR spectra, CD and mass spectra of compounds 5, 6, 7, 3; HPLC analysis; titration
experiments; calculation methods.
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