Iterative one-pot synthesis of oligosaccharides

Article abstract of DOI:10.1002/anie.200460176

Straight to the point! Preactivation of a p-tolyl thioglycoside donor, followed by sequential addition of p-tolyl thioglycosyl acceptors in one reaction flask allowed rapid syntheses of oligosaccharides independent of anomeric reactivities of donors and a

Full text of DOI:10.1002/anie.200460176

Angewandte
Chemie
Carbohydrates
anomeric reactivities, extensive protective group manipula-
tions and/or aglycon adjustments must be carried out.[5] This
excessive synthetic manipulation on building blocks compli-
cates the synthetic process and decreases overall efficiency.
To overcome limitations of existing approaches, we have
investigated the possibility of designing a general one-pot
method independent of differential glycosyl donor reactiv-
ities. This can be achieved by pre-activating the donor,[6]
which generates a reactive intermediate in the absence of
the acceptor (Scheme 1). Upon addition of the second
building block to the pre-activated donor, a disaccharide
will be formed with an identical activatable aglycon at the
reducing end. This process can be repeated in the same
reaction vessel allowing rapid assembly of oligosaccharides.
Several prerequisites, however, must be satisfied for a
successful iterative one-pot synthesis: 1) the promoter utilized
must be stoichiometric in activation of a wide range of
glycosyl donors and be completely consumed by the donor to
prevent activation of following building blocks; 2) the
intermediate generated after pre-activation must be stable
till addition of acceptor, yet reactive for rapid high-yielding
glycosylations; and 3) side products formed from activation
must not interfere with glycosylations.
Iterative One-Pot Synthesis of Oligosaccharides**
Xuefei Huang,* Lijun Huang, Haisheng Wang, and
Xin-Shan Ye*
Traditional oligosaccharide synthesis is a time-consuming
process, primarily due to tedious protective group manipu-
lation and intermediate separation. To reduce the number of
synthetic and purification steps, many innovative method-
ologies have been developed, such as automated solid-phase
synthesis,[1] orthogonal glycosylation,[2] iterative glycosyla-
tion,[3] and chemoselective glycosylation,[4] among which the
reactivity-based one-pot method is particularly noteworthy.[5]
The reactivity-based one-pot method refers to one in which
glycosyl donors with decreasing anomeric reactivities are
allowed to react sequentially in a single reaction flask. Large
oligosaccharides can be assembled in this fashion without
tedious purification of intermediates or adjustment of
anomeric leaving groups, as witnessed by total syntheses of
complex oligosaccharides such as fucosyl GM1,[5a] LeY,[5c] and
Globo H,[5d] as well as assembly of oligosaccharide librar-
ies.[5b,e] However, to obtain building blocks with suitable
After much experimentation examining the effects of
various promoters,[7] aglycon leaving groups,[8] and additives,[9]
Scheme 1. Iterative one-pot synthesis of oligosaccharides.
general reaction conditions were established by using p-tolyl
thioglycosides as building blocks, p-toluenesulfenyl triflate (p-
TolSOTf),[10] formed in situ from p-toluenesulfenyl chloride
(p-TolSCl) and silver triflate (AgOTf), as the stoichiometric
promoter, in the presence of the dehydrating reagent MS-
AW300. Chemoselective glycosylation of donors was
observed independent of the reactivities of donors and
acceptors, producing disaccharides bearing an anomeric p-
thiotolyl moiety in satisfactory yields (Table 1).
Introduction of one equivalent of p-TolSCl to a mixture of
armed donor 1, AgOTf, and MS-AW300 in diethyl ether at
À608C led to instantaneous complete activation of the
glycosyl donor.[11] Addition of the acceptor 4 to the pre-
activated donor rapidly formed disaccharide 9 in just a few
minutes, which was isolated in 87% yield as the a anomer due
to the anomeric effect (Table 1, entry 1). The p-tolyl disulfide
generated from activation did not perturb the glycosylation.
[*] Prof. X. Huang, Dr. L. Huang
Department of Chemistry
University of Toledo
2801 W. Bancroft St. MS 602, Toledo, OH 43606 (USA)
Fax: (+1)419-530-4033
E-mail: xuefei.huang@utoledo.edu
H. Wang, Prof. X.-S. Ye
The State Key Laboratory of Natural and Biomimetic Drugs
School of Pharmaceutical Sciences
Peking University
Xue Yuan Road 38, Beijing, 100083 (China)
Fax: (+86)10-62014949
E-mail: xinshan@mail.bjmu.edu.cn
[**] This work was supported by the University of Toledo (X.H.), the
National Natural Science Foundation of China, and Peking
University (X.Y.).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 5221 –5224
DOI: 10.1002/anie.200460176
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Table 1: Results of chemoselective glycosylations of thioglycoside donors.
selectively glycosylated with the
more reactive acceptor, is not pos-
sible with the traditional reactivity-
based approach.
Entry
1
Donor + Acceptor
Product
Yield [%]
87
To explore the scope of the
current method, glycosylations
with poorly nucleophilic acceptors
are examined, which are known to
be challenging due to reduced
glycosylation rates and/or compe-
tition of other more nucleophilic
compounds in the reaction mix-
ture.[5f,12] With our protocol, donor
2 reacted readily with a disarmed
acceptor 6 to give disaccharide 11
in a few minutes in 72% yield
(Table 1, entry 3). Two poorly
nucleophilic acceptors 7 and 8[12c]
were also glycosylated smoothly by
pre-activated donor 3 in 67% and
65% yields, respectively (Table 1,
entries 4 and 5).[13]
2
3
4
69
72
67
We next focused on applying
this strategy to the one-pot syn-
thesis of oligosaccharides. A sub-
stoichiometric amount (0.9 equiv)
of acceptor was utilized for each
coupling to ensure complete con-
sumption of the acceptor. The
5
65
p-TolSOTf is a powerful promoter capable of stoichiometri-
cally activating highly disarmed donors as well. Disarmed
donor 2 with the participating benzoyl moiety at C-2 reacted
smoothly with the more reactive armed acceptor 5 to give
disaccharide 10 in 69% as the b anomer (Table 1; entry 2). No
products due to the self-coupling of acceptor 5 were isolated.
This reversal of reactivity, that is, the less reactive donor is
reaction mixture was warmed up to room temperature after
each glycosylation to decompose the slight excess of activated
donor that had been present to prevent the formation of
deletion sequences.
The trisaccharide motif 14 occurs in complex N-glyco-
protein structures containing the demanding Gal-b1,4-
GlcNAc and GlcNAc-b1,2-Man linkages. Because our strat-
Scheme 2. One-pot synthesis and deprotection of trisaccharide 18. The values given in parentheses denote the number of equivalents of each
reagent. Reagents and conditions: 1) ethylenediamine, EtOH, reflux; 2) Ac2O, MeOH; 3) H2, Pd/C.
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Angewandte
Chemie
Scheme 3. One-pot syntheses of oligosaccharides 19 and 20. The values given in parentheses denote the number of equivalents of each reagent.
egy does not require tuning of reactivities, this allowed us to
select the readily available building blocks 15–17 (Scheme 2).
Pre-activation of the disarmed thiogalactoside 15 (1 equiv) by
p-TolSOTf (1 equiv) at À608C was followed by addition of
the more reactive armed glucosamine building block 16
(0.9 equiv) (Scheme 2a). The reaction mixture was warmed
up to room temperature for 15min, and then cooled down
back to À608C. Activation of the newly formed disaccharide
with p-TolSOTf followed by addition of acceptor 17 produced
the trisaccharide 18[14] within one hour in 54% yield. The
progress of the reaction was readily monitored by TLC. The
trisaccharide 18 was then deprotected in 70% yield by using a
three-step sequence to give 14 (Scheme 2b). Trisaccharide 14
has also been synthesized by employing automated solid-
phase methodology using an excess of each glycosyl donor
(12 equiv) in 37.2% overall yield for assembly and depro-
tection.[1a] Our iterative one-pot method has the advantages
of using a near-stoichiometric amount of building blocks, ease
in reaction monitoring, and simplicity of operation while
achieving similar overall synthetic efficiencies in the synthe-
ses of medium-sized oligosaccharides.
To illustrate the generality of our methodology, trisac-
charide 19[14] and tetrasaccharide 20,[14] which contain bio-
logically relevant glycosidic linkages such as Fuc-a1,3-
GlcNAc, GlcNAc-b1,3-Gal, Man-a1,2-Man, Man-a1,6-Glc,
and Glc-b1,6-Glc, were assembled as outlined in Scheme 3.
One-pot sequential coupling of fucosyl thioglycoside 21,
glucosamine 22 and thiogalactoside 4 produced trisaccharide
19 in 59% yield in only one hour (Scheme 3a). With its
anomeric p-thiotolyl moiety, the trisaccharide 19 can be
immediately utilized as a donor in the synthesis of Lex
oligosaccharides without any aglycon modifications. Consec-
utive condensation of building blocks 23, 24, 6, and 25
promoted by p-TolSOTf led to formation of tetrasaccharide
20 in 55% yield in less than two hours (Scheme 3b). It should
be noted that both a and b linkages were constructed in one
pot within the same oligosaccharide with excellent stereo-
selectivities.
In summary,
a
new iterative one-pot glycosylation
approach is developed for the efficient assembly of oligosac-
charides, which is based upon pre-activation of a p-tolyl
thioglycoside donor, followed by sequential addition of
building blocks in a single reaction flask. This strategy
obviates the need for the extensive adjustment of protective
groups required for traditional reactivity-based one-pot syn-
thesis, significantly reducing the amount of time needed for
preparing building blocks. Furthermore, a single glycosylation
protocol was found applicable for the assembly of a wide
range of oligosaccharides. This will be particularly advanta-
geous for designing oligosaccharide libraries. Studies are
ongoing to further explore the scope of this method and apply
it to syntheses of complex oligosaccharides as well as
carbohydrate libraries.
Received: March 31, 2004
Keywords: carbohydrates · glycosylation · iterative synthesis ·
.
synthetic methods
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Angew. Chem. Int. Ed. 2004, 43, 5221 –5224
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1
Angew. Chem. 2001, 113, 428; Angew. Chem. Int. Ed. 2001, 40,
[14] Compound 18: H NMR (599.87 MHz, CDCl3): d = 0.80 (t, J =
7.2 Hz, 3H), 1.10–1.26 (m, 4H), 1.36 (quin, J = 7.2 Hz, 2H), 2.94
(dd, J = 6.6, 10.8 Hz, 1H), 3.11 (dt, J = 7.2, 9.0 Hz, 1H), 3.38–3.52
(m, 5H), 3.55(d, J = 10.2 Hz, 1H), 3.70–3.76 (m, 2H), 3.94–4.08
(m, 4H), 4.22–4.43 (m, 8H), 4.65(d, J = 12.0 Hz, 1H), 4.72 (d,
J = 12.0 Hz, 1H), 4.73 (d, J = 12.0 Hz, 1H), 4.74 (d, J = 10.8 Hz,
1H), 4.98–5.02 (m, 2H), 5.17 (d, J = 7.8 Hz, 1H), 5.43 (dd, J =
3.6, 10.8 Hz, 1H), 5.79 (dd, J = 7.8, 10.8 Hz, 1H), 5.85 (d, J =
3.6 Hz), 6.78–6.82 (m, 2H), 7.06–7.68 (m, 39H), 7.74–7.78 (m,
2H), 7.89–7.94 (m, 4H), 8.06–8.42 ppm (m, 2H); 13C NMR
(100.5MHz, CDCl 3): d = 14.20, 22.62, 28.41, 29.21, 55.77, 60.64,
61.97, 67.89, 68.32, 68.43, 70.24, 70.62, 70.83, 71.40, 71.92, 71.97,
73.08, 73.75, 73.87, 74.49, 74.94, 75.07, 76.80, 77.25, 78.03, 78.37,
97.16, 100.82, 123.32, 127.28, 127.49, 127.56, 127.64, 127.74,
127.98, 128.06, 128.12, 128.38, 128.45, 128.48, 128.52, 128.55,
128.61, 128.68, 129.17, 129.32, 129.71, 129.98, 130.06, 132.02,
133.47, 133.51, 133.68, 138.25, 138.69, 138.76, 138.81, 165.22,
165.53, 165.69, 166.26 ppm; HRMS C94H91NNaO21 [M+Na+]
calcd 1952.5981 found 1952.5961. Compound 19: 1H NMR
(599.87 MHz, CDCl3): d = 0.81 (d, J = 6.6 Hz, 3H), 2.27 (s,
3H), 3.38 (d, J = 1.8 Hz, 1H), 3.52–3.65 (m, 6H), 3.79 (d, J =
9.6 Hz, 1H), 3.93 (q, J = 6.6 Hz, 1H), 4.01–4.08 (m, 3H), 4.23 (d,
J = 11.4 Hz, 1H), 4.28–4.38 (m, 4H), 4.42–4.47 (m, 2H), 4.66 (d,
J = 3.0 Hz, 1H), 4.68 (d, J = 9.6 Hz, 1H), 4.73 (d, J = 11.4 Hz,
1H), 5.31 (t, J = 9.6 Hz, 1H), 5.48 (s, 1H), 5.51 (s, 1H), 5.55 (d,
J = 9.6 Hz, 1H), 6.88–6.92 (m, 2H), 6.93–6.96 (m, 2H), 7.08–7.50
(m, 32H), 7.62–7.66 ppm (m, 2H); 13C NMR (100.5MHz,
CDCl3): d = 16.67, 21.46, 55.50, 66.27, 67.45, 68.77, 68.99, 69.37,
70.25, 72.62, 73.32, 74.88, 75.61, 76.28, 76.33, 78.13, 79.68, 80.06,
82.08, 86.02, 99.77, 100.16, 101.19, 101.33, 122.92, 126.19, 126.80,
127.49, 127.58, 127.66, 127.67, 127.71, 127.89, 128.01, 128.27,
128.38, 128.43, 128.49, 128.72, 129.68, 129.84, 129.86, 132.81,
133.45, 134.19, 137.21, 137.94, 138.26, 138.54, 138.71, 139.10,
164.55 ppm; HRMS C75H71NNaO16S [M+Na+] calcd 1296.4391
found 1296.4384. Compound 20: 1H NMR (599.87 MHz,
CDCl3): d = 1.91 (s, 3H), 1.92 (s, 3H), 1.97 (s, 3H), 1.98 (s,
3H), 2.10 (s, 3H), 3.46–3.92(m, 17H), 4.32–4.53 (m, 8H), 4.60 (d,
J = 10.8 Hz, 1H), 4.68 (d, J = 12.0 Hz, 1H), 4.78–4.85(m, 4H),
4.87 (t, J = 9.0 Hz, 1H), 4.93 (dd, J = 8.4, 9.6 Hz, 1H), 4.99 (d, J =
1.8 Hz, 1H), 5.10 (t, J = 9.6 Hz, 1H), 5.44 (dd, J = 7.8, 9.6 Hz,
1H), 5.46–5.49 (m, 1H), 5.50 (d, J = 9.6 Hz, 1H), 5.57 (d, J =
8.4 Hz, 1H), 5.80 (t, J = 9.6 Hz, 1H), 7.08–7.42 (m, 42H), 7.48–
7.52 (m, 1H), 7.78–7.82 (m, 2H), 7.87–7.90 (m, 2H), 7.96–
8.02 ppm (m, 2H); 13C NMR (100.5MHz, CDCl 3): d = 20.76,
20.91, 21.38, 66.93, 67.46, 68.80, 68.92, 69.05, 69.27, 70.40, 70.59,
71.83, 72.05, 72.08, 72.88, 73.16, 73.35, 73.57, 74.27, 74.47, 74.50,
74.89, 75.18, 75.24, 77.44, 78.33, 79.69, 91.80, 99.22, 99.76, 101.06,
127.56, 127.63, 127.70, 127.74, 127.77, 127.83, 127.99, 128.09,
128.30, 128.38, 128.45, 128.48, 128.52, 128.55, 128.64, 129.17,
129.26, 129.59, 129.98, 130.17, 133.33, 133.35, 133.49, 138.25,
138.55, 138.64, 138.71, 138.77, 138.87, 165.22, 165.36, 166.01,
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acceptor, extra rearrangement steps, additional quenching
reagent, or intermediate aglycon adjustments.
[7] Other promoters such as N-iodosuccinimide (NIS)/TMSOTf,
dimethyl (methylthio) sulfonium triflate (DMTST), p-nitrophe-
nylsulfenyl chloride/AgOTf, phenylselenyl bromide/AgOTf, and
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also examined. None of them gave desired oligosaccharides
consistently in satisfactory yields.
[8] Thioglycosyl donors of varying electron-withdrawing power and
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tBu have been examined, and have been found to give lower
yields compared with those obtained by using p-tolyl thioglyco-
side donors. Other excellent donors such as glycosyl sulfoxide, n-
pentenyl glycoside, glycosyl fluoride, and glycosyl iodide may be
potentially utilized in iterative one-pot synthesis as well.
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1
168.94, 169.38, 169.61, 170.22, 170.32 ppm; J(13C,1H): 166.4 Hz
(d = 91.80 ppm, b linkage), 171.8 Hz (d = 99.22 ppm, a linkage),
172.3 Hz (d = 99.76 ppm, a linkage), 163.9 Hz (d = 101.06 ppm,
b linkage); HRMS C97H100NaO29 [M+Na+] calcd 1751.6248
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[13] Pre-activated donor 2 did not react with acceptors 7 or 8, and we
are currently investigating the reasons for this.
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