Solution-phase oligosaccharide synthesis in a cycloalkane-based thermomorphic system

Article abstract of DOI:10.1039/b717446b

The cycloalkane-based thermomorphic (CBT) system is a convenient and practical method for oligosaccharide synthesis, and hydrophobically modified oligosaccharides have a remarkable affinity for CBT solutions composed of methylcyclohexane and propionitrile

Full text of DOI:10.1039/b717446b

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Solution-phase oligosaccharide synthesis in a cycloalkane-based
thermomorphic systemw
Shokaku Kim,a Ai Tsuruyama,b Akihiro Ohmorib and Kazuhiro Chiba*a
Received (in Cambridge, UK) 12th November 2007, Accepted 25th January 2008
First published as an Advance Article on the web 19th February 2008
DOI: 10.1039/b717446b
The cycloalkane-based thermomorphic (CBT) system is a con-
venient and practical method for oligosaccharide synthesis, and
hydrophobically modified oligosaccharides have a remarkable
affinity for CBT solutions composed of methylcyclohexane and
propionitrile.
various soluble polymer-bound catalysts and ligands can be
efficiently recovered, based on selective solubility, from
n-heptane or certain polar miscible biphasic solutions after
completion of a reaction under monophasic conditions.6 As an
alternative, we have found that a mixture of typical organic
solvents, composed of cyclohexane and aprotic polar organic
solvents, may be used as an effective thermomorphic system to
enable the practical application of liquid-phase peptide synth-
esis using a cyclohexane-soluble platform.7 With the use of an
appropriate thermomorphic combination of solvent and
cycloalkane-soluble phase tag for oligosaccharide preparation,
this methodology could overcome the substrate solubility and
interfacial reactivity issues associated with fluorous and other
biphasic conditions, which can cause kinetic limitations. We
envisioned that hydrophobic modification could render oligo-
saccharides highly soluble in cycloalkane-based thermo-
morphic (CBT) solutions, and also result in better reactivity
due to the lower molecular weight and size of a sequence of
small, hydrophobic CH2 units. Additionally, a wide variety of
lipophilic chains can readily be prepared, allowing efficient
construction of cycloalkane-soluble molecules with the desired
functions. As the CBT system allows for effective simplifica-
tion of oligosaccharide synthesis via LLE, it may prove useful
in certain areas of liquid-phase combinatorial chemistry and in
fully automated systems. Herein we report the development of
an effective oligosaccharide synthesis method using a CBT
system (Fig. 1).
Over the past few decades, oligosaccharide synthesis based on
solid-phase technology has been a powerful tool in automated
synthesis and combinatorial chemistry, allowing effective li-
brary construction of targeted carbohydrates.1 However, so-
lid-phase synthesis has serious shortcomings associated with
the nature of the heterogeneous conditions required. Recently,
solution-phase methodologies2 have gained significant atten-
tion as an alternative to traditional solid-phase oligosacchar-
ide synthesis. Their many advantages include high reactivity of
the soluble species and the possibility of using routine analy-
tical methods to monitor the reaction process and directly
determine structures, even when the molecules are attached to
phase-tags. In particular, the hydrophobic phase-tagging stra-
tegies based upon solid-phase-extraction (SPE) by C18 silica
have been well-recognized as useful methods that can permit
the effective generation of target carbohydrates.3 While most
procedures for the recovery of target molecules emphasize
SPE, liquid–liquid extraction (LLE) may also have broad
potential, because suitably phase-tagged compounds in a
biphasic system often show a particular affinity for one phase
without further supports. Among various methods, oligosac-
charide synthesis on fluorous supports with a high fluorine
content has opened the door to the implementation of simple
LLE techniques commonly used in classical organic synthesis.
This methodology is a useful approach toward automated and
large-scale synthesis of oligosaccharides.4 Since the target
products can be efficiently separated from other reaction
components by the highly fluorinated protecting groups, a
key structural feature, much effort has been made to address
the issues concerning cost, solubility and reactivity.5
Initially, a hydrophobic monosaccharide 1 was synthesized
and its solubility in thermomorphic solvents was tested to
evaluate the viability of the proposed approach and its
efficiency.4b The results are summarized in Fig. 2. Although
compound 1 displayed low solubility in n-heptane (8.1 mM),
its solubility increased to 84 mM in MCH (methylcyclohex-
ane). Surprisingly, it was found that the addition of EtCN to
the MCH solvent enhanced the solubility of 1 to 179 mM. As
expected, its solubility in EtCN was low. Because nitrile
solutions are known to promote glycosyl bond formation,8 it
was thought likely that a CBT solution composed of MCH
Bergbreiter et al. have successfully developed a thermo-
morphic liquid–liquid reaction–separation process in which
a Laboratory of Bio-organic Chemistry, Tokyo University of
Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-
8509, Japan. E-mail: chiba@cc.tuat.ac.jp; Fax: +81 42 360 7167;
Tel: +81 42 367-5667
b Yokohama Technical Center, MORITEX Corporation. 1-3-
3Azamino-Minami, Aoba-ku, Yokohama, Kanagawa 225-0012,
Japan
w Electronic supplementary information (ESI) available: Experimental
procedures and spectral data for all new compounds.details. See DOI:
10.1039/b717446b
Fig. 1 General view of CBT oligosaccharide synthesis.
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Fig. 2 Solubility of compound 1 in thermomorphic solvents.
Fig. 3 Effect of solvent composition on miscible temperature. The
graphs represent solvent mixtures composed of (A) a 1 : 1 (v/v) mixture
of MCH and nitriles with varying AN : PN ratios (v/v), and (B)
varying ratios of MCH–nitrile (v/v) at an MeCN : EtCN ratio of
1:1 (v/v).
and EtCN could be used for targeted glycosylation reactions
under miscible conditions, followed by spatial separation of
hydrophobic compound 1 in a biphase.
A glycosylation reaction was carried out using MCH–EtCN
(2 : 1, v/v) (Scheme 1). Compound 1 was allowed to react with
2 equivalents of trichloroacetimidate donor 2 in the presence
of TMSOTf (trimethylsilyl trifluoromethane sulfonate),
affording the corresponding disaccharide 3a (82% yield),
which was separated by extraction of the upper MCH layer
after cooling of the homogenous reaction mixture.
temperature and solvent ratio suitable for miscible–immiscible
phase separation according to target reaction conditions.
Using an MCH : EtCN ratio of 2 : 1 (v/v), we conducted a
further investigation of tetrasaccharide synthesis (Scheme 2).
The targeted di-tetrasaccharides 3a–c were successfully pre-
pared under the conditions described. Deprotection of TBDPS
(tert-butyldiphenyl silyl) groups by treatment with mild
TBAF–AcOH also proceeded under the CBT system to afford
the acceptors 4a and 4b in the MCH phase.
With the addition of MeCN, the product was recovered
almost quantitatively in MCH and was also readily isolated by
precipitation (via the addition of methanol to the MCH
solution, followed by filtration). In the absence of MCH, the
coupling reaction proceeded in unacceptable yield, even with
longer reaction times.
In this system, side-reactions such as the migration of
benzoyl groups via intramolecular transesterification were
not observed. Furthermore, saccharide derivatives attached
to phase-tags showed a significant improvement in terms of
partition into the separable phase, especially when compared
with fluorous modification (Table 1).9 Although the addition
of water was found to result in improved separation in the
MCH phase, this procedure was not employed because it is an
ineffective strategy in sequential glycosylation. It is notable
that simple LLE manipulations via the CBT process can
effectively facilitate sequential glycosylation and deprotection
steps, which should ensure its usefulness in solution-phase
oligosaccharide synthesis and in the realization of fully auto-
mated processes.
These results prompted us to investigate the relationship
between the MCN : MeCN : EtCN ratio and the miscible
temperature. As the content of EtCN increased, the miscible
temperature decreased in a linear fashion (Fig. 3(A)). As
shown in Fig. 3(B), the maximum miscible temperature was
observed at a MCH : nitrile ratio of 80 : 20 (v/v), and the
miscible temperature decreased as the MCH ratio decreased.
Clear phase separation was observed for MCH–MeCN–EtCN
mixtures between ca. 70 and 10 1C. In addition, although
MCH : EtCN (1 : 1, v/v) formed a homogenous phase at 25 1C,
addition of MeCN resulted in the formation of a biphase
without cooling. This allows for an effective CBT process at a
In conclusion, we have demonstrated that the CBT system
described here is a convenient and practical method for
oligosaccharide synthesis, and that hydrophobically modified
oligosaccharides have a remarkable affinity for cycloalkanes.
Scheme
synthesis.
2
Cycloalkane-based thermomorphic oligosaccharide
Scheme 1 Preparation of disaccharide 3a in a CBT system.
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Table 1 Partition ratio of hydrophobic oligosaccharide to MCH :
MeCN : EtCN 50 : 25 : 25 (v/v/v) in a biphasic solution
2720; (e) A. Dondoni, A. Marra and A. Massi, Angew. Chem., Int.
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Inazu, J. Org. Chem., 2004, 69, 5348; (c) T. Miura, Y. Hirose, M.
Ohmae and T. Inazu, Org. Lett., 2001, 3, 3947; (d) T. Miura, A.
Satoh and T. Inazu, Tetrahedron Asym., 2005, 16, 3.
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P. Curran, X. Wang and Q. Zhang, J. Org. Chem., 2005, 70, 3716; (c)
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Compound
Phase-tag contenta (%)
Partition ratiob (%)
1
80.8
47.0
36.8
30.2
96
98
88
71
3a
3b
3c
a
Tag content = phase-tag molecular weight/total molecular weight
b
Â100 (phase tag = 3 Â C17H35CO). Determined by HPLC analysis.
With the aim of preparing various carbohydrate derivatives,
the incorporation of cycloalkane-soluble chains into acceptor
and/or donor molecules while maintaining high levels of
partition in the cycloalkane phase is now in progress.
Notes and references
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9 Tetrasaccharide synthesis on fluorous supports (Bfp, TfBz =
1006.33, 1599.61) required about 70% phase-tag contents at least;
see ref. 3.
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