Backbone-modified amphiphilic cyclic di- and tetrasaccharides

Article abstract of DOI:10.1039/c3cc48794f

Synthesis of amphiphilic, cyclic di- and tetrasaccharides, which incorporate a methylene moiety at the inter-glycosidic bond, is reported. The amphiphilic properties of the new cyclic tetrasaccharide host were identified through assessing the solubilities of guests in aqueous and in organic solvents. The glycosidic bond stability of the cyclic tetrasaccharide under aqueous acidic condition was also verified. the Partner Organisations 2014.

Full text of DOI:10.1039/c3cc48794f

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Backbone-modified amphiphilic cyclic di- and
tetrasaccharides†
Cite this: Chem. Commun., 2014,
50, 8554
Gour Chand Daskhan and Narayanaswamy Jayaraman*
Received 18th November 2013,
Accepted 9th May 2014
DOI: 10.1039/c3cc48794f
www.rsc.org/chemcomm
Synthesis of amphiphilic, cyclic di- and tetrasaccharides, which disaccharide derivative 3. The a-anomeric configuration of 3
incorporate a methylene moiety at the inter-glycosidic bond, is was confirmed by the appearance of H-10 of non-reducing gluco-
reported. The amphiphilic properties of the new cyclic tetrasac- pyranoside moiety at 4.80 ppm ( J = 3.6 Hz). The following
charide host were identified through assessing the solubilities of sequence of reactions was performed on 3 to secure the glycosyl
guests in aqueous and in organic solvents. The glycosidic bond donor 4: (i) removal of PMB-group (CF3CO2H);13 (ii) acetate
stability of the cyclic tetrasaccharide under aqueous acidic condi- protection of lactal (Ac2O/pyridine); (iii) thioglycoside for-
tion was also verified.
mation (EtSH, BF3ꢀEt2O at 0 1C) and (iv) O-deacetylation
(Scheme 1). The glycosidic bond expanded donor 4 was
Synthetic hosts, such as cyclodextrins (CDs),1 calix[n]arenes,2 obtained as an anomeric mixture (a : b B 1 : 1.7). Anomeric
cucurbiturils,3,4 cavitands,5,6 cyclophanes7 and glycophanes,8 Hb-1 of the reducing end of 4 appeared at 5.49 ppm ( J = 5.6 Hz)
and supramolecular assemblies exemplified by catenanes, and the corresponding Ha-1 resonated at 4.50 ppm, as an
rotaxanes, and molecular machines,9–11 have greatly contribu- apparent singlet, in the H NMR spectrum.
1
ted to uncovering many facets of molecular recognition. Among
The monomer 4, equipped with an activated thioglycoside
synthetic hosts, the poor solubility of CDs in aqueous solutions moiety at the reducing end and hydroxymethyl acceptor moiety
and their practical insolubility in organic solvents limit their at C-4 of the non-reducing end, was subjected subsequently to a
utilities, leading to their post-modification by functionalizing cyclo-oligomerization reaction.14 A thiophilic activator, either
the hydroxyl groups. Synthetic macrocylic hosts soluble in both N-iodosuccinimide (NIS)/silver triflate (AgOTf) or trimethylsilyl
organic solvents and aqueous solutions would, in principle, trifluoromethane sulfonate (TMSOTf) in CH2Cl2, was used for
expand their molecular recognition properties. With this the cyclo-condensation and the reaction was conducted in
perspective, we undertook synthesis of
a new type of CH2Cl2 at 0 1C for 2 h and at room temperature for 4 h, then
backbone-modified cyclic oligosaccharide. Backbone modifica- quenched with Et3N and worked up. The reaction mixture was
tion of the inter-glycosidic bond with a methylene moiety was then purified (SiO2) (pet. ether–EtOAc) to afford glycosidic bond
anticipated to result in hydrophobicity of such cyclic oligosac- expanded cyclic disaccharide 5 and cyclic tetrasaccharide 6 in
charides, differing from that of CDs, which, in turn, would 11 and 64%, respectively (Scheme 2). Concentrations of either
modify their solubility properties. This report describes our 3 mM or 20 mM of 4 did not result in much difference in the ratio
achievement of this objective.
of product formation. Structures of 5 and 6 were established
The synthesis of cyclic oligosaccharides, incorporating a
methylene moiety at the inter-glycosidic bond, was initiated
by identifying a disaccharide monomer suitable for cyclo-
oligomerization. Synthesis of the disaccharide monomer is
shown in Scheme 1. The protected derivative 112 was subjected
to a glycosylation with glycosyl donor 2 to afford the required
Indian Institute of Science, Bangalore 560012, India.
E-mail: jayaraman@orgchem.iisc.ernet.in; Fax: +91-80-2360-0529;
Tel: +91-80-2293-2578
Scheme 1 Reagents and conditions: (i) TMSOTf, Et2O, MS (4 Å), rt, 30 min,
66%; (ii) (a) CF3CO2H, CH2Cl2–H2O, 0 1C–rt, 2 h, 72%; (b) Ac2O/pyridine,
0 1C, 12 h, 92%; (iii) (a) EtSH, BF3OEt2, CH2Cl2, 0 1C, 30 min; (b) NaOMe,
MeOH, rt, 4 h, 85% (after two steps).
† Electronic supplementary information (ESI) available: Experimental section,
spectra and calculations. See DOI: 10.1039/c3cc48794f
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Fig. 2 13C NMR spectrum of 7 (D2O, 100 MHz).
Scheme 2 Reagents and conditions: (a) NIS, AgOTf or TMSOTf (Cat.),
CH2Cl2, MS (4 Å), 0 1C–rt, 6 h (5: 11%, 6: 64%); (b) H2, Pd/C (10%), MeOH/
EtOH (1 : 1), 24 h, 91%.
1
through H and 13C, COSY and HSQC NMR spectroscopic and
mass spectrometric analyses. The ESI-MS spectrum of 5 showed a
molecular ion peak at 915.4088 ([M + Na]+), along with a peak at
1
469.1786 as [M/2 + Na]+ adduct. In H NMR spectrum of 5, the
anomeric H-1 appeared at 4.92 ppm ( J = 2.0 Hz), and in 13C NMR
spectrum, the anomeric carbon appeared at 90.7 ppm. MALDI-
TOF mass spectral analysis of 6 showed the molecular ion peak at
m/z = 1807.6888 ([M + Na]+), along with peaks at m/z 1362.650,
915.533, corresponding to [M-(one monomer unit) + Na]+ and
[M/2 + Na]+ ion peaks, respectively. In 1H NMR spectrum of 6, H-1
nucleus was observed at 5.04 ppm, as an apparent singlet,
whereas in 13C NMR spectrum, C-1 signal resonated at 91.2 ppm.
Removal of benzyl protecting group in 6 (H2, Pd/C (10%)) afforded a
free hydroxyl group-containing cyclic tetrasaccharide 7. In 1H NMR
Fig. 3 Energy minimized structure of 7, derived from molecular modelling
simulation.
Whereas chemical modifications, such as per-O-alkylation,
spectrum of 7, the H-1 nucleus was observed at 4.89 ppm (app. s) are commonly performed in order to improve the solubility
and in 13C NMR spectrum, the C-1 nuclei appeared at 92.3 ppm behavior of CDs16 in water and in organic solvents, cyclic
(Fig. 1 and 2). ESI-MS analysis showed molecular ion peak at tetrasaccharide 7 is soluble in aqueous and in most organic
727.2617, as [M + Na]+ adduct.
solvents, as a result of its inherent amphiphilicity. Aqueous
Energy-minimized molecular modelling was performed in solubility of 7 is 74 ꢁ 3 mg mLꢂ1, whereas that of a- and
order to identify the size and shape of the backbone cyclic b-CDs are 145 and 18.5 mg mLꢂ1, respectively.1 Given its
tetrasaccharide 7, using Gaussian 03 software at B3LYP/6-311G* amphiphilic nature, further studies were undertaken to assess
level.15 In the optimized structure derived from modelling the solubilization properties of 7. Pyrene solubilization studies in
(Fig. 3), the pyranoside moieties were seen to adopt 4C1 con- aqueous solution were undertaken, in order to identify the extent
formation. The distances between alternate glycosidic oxygen of the microenvironment present in the host, for which UV-Vis
atoms in 7 were 8.42 and 6.80 Å. Interestingly, it was observed absorption and fluorescence spectroscopies were used. In order
from the optimized structure that all the primary hydroxyl groups to assess solubilization, an aqueous solution of 7 in varying
were placed on the top of upper rim and all the secondary concentrations was admixed with pyrene and stirred at 45 1C for
hydroxyl groups placed on the lower rim, in contrast to CDs, 24 h in the dark. The solutions were then filtered and the extent
where the reverse occurs.
of solubilization of pyrene in each aqueous solution of host was
determined by UV-Vis spectroscopy (e335 50 730 molꢂ1 cmꢂ1).17
Pyrene solubilization in water was found to be 0.7 mM, whereas in
aqueous solutions of the host this was 1.5, 2.3, 7.7, and 14.7 mM
with 0.2 mM; 0.5 mM; 0.7 mM and 0.9 mM of host 7, respectively.
Increased solubility of pyrene illustrated higher hydrophobicity
of the host. Further, in order to assess the polarity of the
microenvironment, fluorescence spectra were recorded. Emis-
sion spectra (Fig. 4) showed an enhancement of fluorescence
intensity of pyrene upon increasing concentration of 7 in aqu-
eous solutions. An excimer emission of pyrene was also apparent
at higher concentrations (0.7 mM and 0.9 mM) of 7.
The ratio of the intensities of third to first bands (I3/I1) in the
fluorescence spectrum, which is a measure of the polarity of the
Fig. 1 1H NMR spectrum of 7 (D2O, 400 MHz).
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Fig. 6 ROESY spectrum of the complex of
7
with adamantane-1-
Fig. 4 Emission spectra of pyrene: (a) in H2O; (b) in solutions of 7 of
carboxylic acid (D2O, 400 MHz).
concentration (b) 0.2 mM; (c) 0.5 mM; (d) 0.7 mM and (e) 0.9 mM.
was thus assessed through 1H NMR analysis in CDCl3. A mixture
microenvironment,18 was found to increase with increasing
concentrations of 7, reflecting the hydrophobicity of 7 in aq.
solutions. Further, a molar ratio of 4 : 1 of 7 to pyrene was
identified from the integration of the 1H NMR spectrum in
CDCl3, which reflected the creation of a hydrophobic micro-
environment in aqueous solution of 7 and thus its solubiliza-
tion property. Association constant (Ka) of the host–guest
interaction was derived by utilizing the aggregation number
(n), as is used with micelle host–guest interaction.19 By utilizing
the equation [guest]withhost/1 ꢂ [guest]without host p [7] ꢂ critical
aggregation concentration of 7/n with the proportionality con-
stant being Ka, a Ka of 1.77 ꢃ 105 Mꢂ1 was derived. A detailed
description of the calculation is given in the ESI.†
Solubilization of adamantane-1-carboxylic acid (AdCA) in
aqueous solution of 7 was also evaluated. For this purpose, an
aq. solution of guest and host (5: 1 host-to-guest molar equiv.)
was stirred for 12 h at 30 1C, filtered, and the filtrate concen-
trated in vacuo. The 1H NMR spectrum of the resulting solution
in D2O showed new peaks at 2.01, 1.87 and 1.73–1.65 ppm,
corresponding to AdCA protons (Fig. 5), and broadening of the
resonances was observed. The ROESY spectrum of the complex
(Fig. 6) showed cross-peaks between H-C of AdCA and Ha,b-6 of 7,
from which the complexation of AdCA appeared to occur on the
primary hydroxyl group side of the host. Complexation of AdCA
with 7 causes up-field shift of H-B and H-C by 0.04 and 0.02 ppm.
Analysis of 1H NMR spectrum and the proton integration values
revealed a molar ratio of 2 : 1 of 7-to-AdCA.
of 7 and L-tyrosine (5: 1 molar equiv.) was stirred in CDCl3 for
1
24 h at room temperature, filtered and the H NMR spectrum
was recorded. The appearance of signals of the aromatic moiety
of the guest at 7.52 and 6.98 ppm as doublets and methylene
protons at 2.12 ppm as a multiplet in the spectrum reflected the
solubilization of L-tyrosine in CDCl3 solution. Integration of
1H NMR values revealed a 1 : 1 molar ratio of 7-to-L-tyrosine in
the complex.
Further, an effort was undertaken to identify the stability of
the glycosidic bond in 7, for which an acid-catalyzed hydrolysis
1
was performed, followed by H NMR spectroscopy. The hydro-
lysis of 7 and a-cyclodextrin was performed using DCl in D2O
1
(2 N) at 60 1C and H NMR spectra were recorded periodically.
The analysis showed complete hydrolysis of 7 within 30 min,
whereas in the case of a-cyclodextrin this required 6 h. The
deoxygenation at C-4 led to faster hydrolysis, following a
general trend that glycosides undergo faster hydrolysis when
the hydroxyl group is substituted by a carbon substituent.12,20,21
In conclusion, synthesis of new cyclic di- and tetra-
saccharides, incorporating a methylene moiety at the inter-
glycosidic bond, has been achieved in good yields through a
one-pot condensation of a disaccharide monomer. Solubility
of free hydroxyl group-containing amphiphilic cyclic tetrasac-
charide in aqueous solution and in organic solvents provides a
new platform for host–guest studies.
Notes and references
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12 Experimental procedure is given in the ESI†.
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