Towards a Synthetic Strategy for the Ten Canonical Carrageenan Oligosaccharides – Synthesis of a Protected γ-Carrageenan Tetrasaccharide

Article abstract of DOI:10.1002/ejoc.201900592

Herein, we report a synthetic strategy aiming at synthesizing the ten canonical carrageenan oligosaccharides from one single precursor. The key β-(1→4)-linked disaccharide was synthesized from commercially available galactose pentaacetate. The notoriously difficult formation of β-(1→4)-d-galactan linkages was successfully optimized on the differentially substituted monosaccharides to afford the desired disaccharide in 55 % yield. Following a convergent strategy, two disaccharides were then glycosylated to form the fully protected α-(1→4)-linked tetrasaccharide backbone of the carrageenans. The careful selection of protecting groups provides the opportunity to access all ten carrageenan substructures identified in polysaccharides isolated from red algae. Here, we demonstrate how one such target oligosaccharide can be obtained in a protected form.

Full text of DOI:10.1002/ejoc.201900592

A Journal of
Accepted Article
Title: Towards a synthetic strategy for the ten canonical carrageenan
oligosaccharides – synthesis of a protected γ-carrageenan
tetrasaccharide
Authors: Christine Kinnaert and Mads Hartvig Clausen
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201900592
Link to VoR: http://dx.doi.org/10.1002/ejoc.201900592
Supported by

10.1002/ejoc.201900592
European Journal of Organic Chemistry
COMMUNICATION
Towards a synthetic strategy for the ten canonical carrageenan
oligosaccharides – synthesis of a protected γ-carrageenan
tetrasaccharide
Christine Kinnaert,[a] and Mads H. Clausen*[a]
occurring carrageenan is
a
mixture of non-homologous
polysaccharides, which makes it challenging to obtain samples of
homogenous oligosaccharides by either chemical or enzymatic
degradation of the algae cell wall components. We present herein
a synthetic strategy aiming at producing the ten canonical
carrageenan oligosaccharides from one single protected
oligosaccharide precursor.
Abstract: We herein report
a synthetic strategy aiming at
synthesizing the ten canonical carrageenan oligosaccharides from
one single precursor. The key β-(1→4)-linked disaccharide was
synthesized from commercially available galactose pentaacetate. The
notoriously difficult formation of β-(1→4)-
successfully optimized on the differentially
monosaccharides to afford the desired disaccharide in 55% yield.
Following convergent strategy, two disaccharides were then
D-galactan linkages was
a)
OR
RO
Enzymes
or
KOH
substituted
OR
RO
O
OR
O
O
O
O
O
O
OR
O
a
RO
OR
OR
RO
D
glycosylated to form the fully protected α-(1→4)-linked
tetrasaccharide backbone of the carrageenans. The careful selection
of protecting groups provides the opportunity to access all ten
carrageenan substructures identified in polysaccharides isolated from
red algae. Here, we demonstrate how one such target oligosaccharide
can be obtained in a protected form.
O
G
G
DA
-
R =H or SO3
b)
OH
OH
HO
HO
O
O
O
OSO3
O
O
OH
O
O
O
-carrageenan
G-DA
-carrageenan
G-D6S
OH
HO
OH
HO
O
Introduction
OH
OH
HO
HO
O
O
O
OSO3
O
O
O
O
O
-carrageenan
G-DA2S
-carrageenan
G-D2S,6S
In order to increase understanding of plant cell wall biology and
physiology, well-defined plant cell wall oligosaccharides have
been used as model compounds to represent domains from larger,
more complex polysaccharides. Therefore, synthetic chemists
have devoted attention to developing efficient strategies for the
chemical synthesis of such oligosaccharide fragments.[1]–[8]
However, as outlined by a recent review summarizing all plant and
algal cell wall oligosaccharides synthesized to date,[9] there are
relatively few examples of synthetic fragments representative of
marine polysaccharides. Therefore, we are interested in the
synthesis of carrageenans, which are sulfated polysaccharides
found in red algae seaweed.[10]–[13] Not only are these targets
potentially very valuable for understanding the algal cell wall, but
they would also help advance the development of new
methodologies in carbohydrate chemistry and more specifically in
relation to sulfated oligosaccharides.[14]–[17] Carrageenans are a
family of highly sulfated galactans found in the cell walls of red
seaweeds of the Rhodophyceae class. These polysaccharides
serve as gelling, stabilizing and viscosity-enhancing agents in
many sectors including the pharmaceutical and food industry and
represent one of the major texturizing ingredients in processed
foods.[13]-[18] Their use in pharmaceutical products continues to be
investigated, and recent applications in drug delivery systems are
summarized in a review by S. Liang Li et al.[19] Carrageenan are
divided into ten different idealized repeating disaccharides, which
differ mainly by their degree of sulfation and the presence or
absence of 3,6-anhydrogalactose residues (Figure 1). Naturally
OH
OH
O3SO
HO
O3SO
O
OH
OH
O3SO
O3SO
O
O
O
O
OSO3
O
-carrageenan
-carrageenan
O
O
O
G4S-D6S
G4S-DA
OH
OH
OH
HO
HO
O
OH
OH
O3SO
O3SO
O
O
O
OSO3
O
O
O
-carrageenan
G4S-D2S,6S
O
O
-carrageenan
G4S-DA2S
OH
OH
O3SO
HO
O3SO
O
OH
OH
HO
HO
O
O
O
O
OSO3
O
O
O
HO
O
-carrageenan
G2S-DA2S
-carrageenan
G2S-D2S,6S
O3SO
O3SO
O3SO
O3SO
O
Figure 1. a) Carrabiose unit. b) Ten types of idealized repeating units of
carrageenans (see nomenclature established by Knutsen, Myslabodski, Larsen
and Usov (1994)[20] and integrated into the IUPAC rules).[21]
Carrageenans share a common galactan backbone of
alternating 3-linked-β-D-galactopyranose (G) and 4-linked-α-D-
galactopyranose (D). The disaccharide repeating units, called
carrabioses, are shown in Figure 1 a). It has been shown that hot
alkaline treatment of the D6S carrageenans (namely carrageenan
having a 4-linked-α-D-galactopyranose residue sulfated at the 6
position) leads to cyclization to form the 3,6-anhydro ring
accompanied by a ring flip from 4C1 to 1C4 (see Figure 1 a).[22],[23]
In algae this reaction is catalyzed by enzymes such as
sulfohydrolases[24],[25] or galactose-6-sulfurylases,[26] which have
been isolated from algal cells. The activity of sulfurylases I and II
catalyzing the conversion of ν-carrageenan to ι-carrageenan was
demonstrated in vitro and proved to be specific to this type of
carrageenan as it was inactive on μ-carrageenan.[26]
[a]
Professor M. H. Clausen and Dr. C. Kinnaert
Center for Nanomedicine and Theranostics, Department of
Chemistry
Technical University of Denmark
Kemitorvet 207, DK-2800, Kgs. Lyngby, Denmark
E-mail: mhc@kemi.dtu.dk
www.kemi.dtu.dk/mhc
Supporting information for this article is given via a link at the end of
the document
The chemical synthesis of oligocarrageenans requires the
formation of
a
backbone of alternating 3-linked-β-D-
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10.1002/ejoc.201900592
European Journal of Organic Chemistry
COMMUNICATION
galactopyranose and 4-linked-α-
D
-galactopyranose units,
amount of p-TSA in methanol. Unfortunately, a substantial
amount of isomerization to the undesired α-anomer was observed
under the reaction condition, and therefore the yield was only 62%.
The diol 4 was activated with Bu2SnO in situ followed by treatment
with benzyl bromide in the presence of TBAI to afford the 3-OBn
product. The penultimate step in the sequence was allylation of
the 2-OH under neutral conditions using silver oxide and allyl
bromide, which afforded the fully protected monosaccharide in
84% yield. The last step to reach the desired acceptor was a
regioselective reductive opening of the of the naphthylidene
acetal giving a C-6 NAP ether and the free 4-OH. This reaction
was performed using triethylsilane and trifluoroacetic acid (TFA)
to afford the acceptor 5. However, hydrolysis of the naphthylidene
group was also observed and the corresponding diol was isolated
as the major byproduct.
followed by selective sulfation of various positions depending on
the targeted carrageenan. The strategy we investigated is shown
in Figure 2 and uses a blockwise assembly. This approach relies
on the synthesis of a key β-(1→4)-linked disaccharide carrying
protecting groups suitable for selective modifications later on.
This disaccharide will serve as a common building block and will
require formation of α-(1→3)-linkages to construct the
carrageenan backbone. In order to evaluate the strategy we
decided to first target carrageenan tetrasaccharides.
OA
R1
O
key -1,4 disaccharide
O
R2
O
R'
OA
R1
O
OA
R1
O
OB
O
O
OC
OC
O
R2
O
HO
O
+
OB
AO
O
OB
AO
O
B = Participating
R
R
OC
OD
OD
HO
AO
1) NH2NH2.AcOH
DMF, 55 oC, 80%
R1 = PG or H
1) 0,02M NaOMe in MeOH
-20 oC, 79%
OAc
OAc
AcO
AcO
O
O
O
R
OA
OAc
OTBDPS
AcO
AcO
R1
O
OD
2) 2-naphthaldehyde
trimethylorthoformate
cat. p-TSA, MeOH
MeCN, 62%
2) TBDPSCl
imidazole, DMF
92%
OAc
OAc
O
OC
R2
O
O
2
3
OB
AO
O
D = non-participating
LG
OD
OC
Naphth
O
ONAP
1) a) Bu2SnO, toluene, reflux
b) BnBr, TBAI, toluene, 70%
HO
OA
O
R1
O
O
OTBDPS
BnO
O
OA
O
2) AllBr, Ag2O, 4Å MS
CH2Cl2, 84%
3) Et 3SiH, TFA
HO
O
OTBDPS
HO
R2
O
O
OAll
O
OH
OC
OB
AO
O
OA
O
R1
O
CH2Cl2, 65%
4
5
OB
O
O
O
DO
OC
O
BnO
O
Scheme 1. Synthesis of protected monosaccharide acceptor 5.
DO
OB
AO
O
n
R
new -1,3 linkage
R1 = PG or H
The synthesis of the monosaccharide glycoside donor was more
straightforward and achieved in two steps from the known
thioglycoside 6,[28] which in turn is available in three steps from
galactose pentaacetate (see Scheme 2). It is worth mentioning
that the benzoylation of the remaining unprotected hydroxyl group
had to be performed in dichloromethane at low temperature (0 °C)
and using Et3N in order to avoid deprotection of the chloroacetyl
group.
OD
Protecting Group Requirements
A
B
- Permanent protecting group
- Participating group
- Can be selectively removed
- Can be selectively removed
- Non-participating group
- Can be selectively removed
C
D
R
- Stable under R' and LG activation
R1 - Stable under R' and LG activation
R
2 - Stable under R' and LG activation
LG - Leaving group for a new glycosyl donor
Figure 2. Synthetic strategy based on well-chosen protecting groups.
The protected tetrasaccharide 1a shown in Figure 3 conforms to
all the requirements and was chosen as a potential single
precursor for all ten types of carrageenans.
Ph
Scheme 2. Synthesis of the known[29] protected monosaccharide donor 8.
O
O
O
Ph
O
ONAP
O
As the synthesis of β-(1→4)-D-galactans is known to be very
O
ClAcO
challenging,[30],[31] the optimization of the glycosylation producing
the β-(1→4)-linked disaccharide was cumbersome and even
under optimized conditions,[32] the product could only be isolated
OBz
BnO
O
O
AllO
O
ONAP
O
O
OBz
BnO
OTBDPS
1a
in a 55% yield (see Scheme 3).
OAll
Ph
O
1M Me2S2/ Tf2O in CH2Cl2
(2.5 equiv.)
Figure 3. Protected tetrasaccharide precursor of the 10 canonical
O
carrageenans.
O
6
8
ONAP
O
+
O
ClAcO
CH2Cl2, 4Å MS
- 40 oC, 20 min
OBz
BnO
OTBDPS
OAll
55%
9
Results and Discussion
Scheme 3. Optimized conditions to form the key β-(1→4)-linked disaccharide
9.
Based on the criteria set up on the retrosynthetic analysis, we
decided to target tetrasaccharide 1a. The journey to 1a began
with the synthesis of monosaccharide acceptor 5, achieved in
seven steps from commercially available galactose pentaacetate,
Scheme 1. First, the anomeric position was deacetylated [27] and
protected with TBDPSCl. It was found that the silyl group is prone
to migrate under both acidic and basic conditions and as a
consequence, all the following steps from TBDPS protected
compound 3 had to be carried out under neutral conditions or with
very low concentration of acid or base. Zemplén deacetylation of
the remaining acetyl groups was performed at low temperature
using dilute sodium methoxide in methanol. The C-4 and C-6
positions were protected by reaction with naphthaldehyde
dimethyl_acetal, which was formed in situ by reacting 2-
Disaccharide 9 could subsequently be transformed into both the
imidate disaccharide donor 10[33] and a disaccharide acceptor,
see Scheme 4. Surprisingly, we found that it was impossible to
remove the chloroacetyl group selectively in the presence of the
2‘-O benzoyl group without migration of the later (see SI for a
detailed account of the conditions that were investigated). Thus,
we decided to remove both acyl groups to arrive at 11 and rely on
the higher reactivity of the 3-position over the more sterically
hindered 2-position for the subsequent glycosylation. Gratifyingly,
this proved to be efficient, as the desired α-linked tetrasaccharide
could be isolated in 55% yield following the glycosylation reaction.
While HMBC correlations confirmed the 1-3 linkage, the
configuration of the newly formed anomeric center was
naphthaldehyde with trimethylorthoformate and
a catalytic
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10.1002/ejoc.201900592
European Journal of Organic Chemistry
COMMUNICATION
established by a one bond 13C–1H coupling constant of 168 Hz
selectively removed under oxidative conditions using DDQ in a
high yield (90%).[35] Selective removal of the allyl group using
along with a 13C chemical shift at 92.2 ppm.[34]
Ph
Ph
tetrakis(triphenylphosphine)palladium(0)
gave
triol
15,
O
O
1) 1M TBAF in THF, THF
0 to 22 oC, 70%
O
O
importantly without migration of the TBDPS group or
anomerization. Selective sulfation of the C-6 position was
performed using 10 equiv. of SO3-pyridine complex. It was pivotal
to add 4Å MS as well as additional pyridine to the reaction mixture
to remove any traces of water and acid, respectively, to avoid
partial cleavage of the benzylidene protecting group. The
monosulfated disaccharide 16 was obtained in 70% yield.
Sulfation at C-6 was identified by a downfield shift of the NMR
signal of the corresponding carbon atom (~ 8 ppm) and protons
(~ 0.5 ppm). According to previous syntheses of heparin
oligosaccharides, a downfield shift of ring protons H-2 and H-2’ of
approximately 1 ppm would indicate the formation of sulfate
esters at the C-2 positions.[36],[37] As the chemical shift of H-2 and
H-2’ did not change, we concluded that sulfation at these two
positions did not occur. This result was further confirmed by mass
spectroscopy.
O
O
ONAP
O
ONAP
O
O
O
ClAcO
ClAcO
OBz
BnO
OBz
BnO
OTBDPS
2) PTFAICl, Cs2CO3
CH2Cl2, 85%
O
CF3
NPh
AllO
OAll
9
10
1M NaOMe, MeOH, THF
22oC, 80%
Ph
O
O
O
ONAP
O
HO
OH
BnO
O
OTBDPS
AllO
1.1 equiv.
11
Ph
O
O
O
Ph
O
ONAP
O
O
ClAcO
Ph
BzO
Ph
O
0.25 equiv. TMSOTf
BnO
O
O
O
O
10
+
11
1) BzCl, Et3N
cat DMAP, CH2Cl2
-40 °C, 75%
AllO
ONAP
O
O
CH2Cl2, - 40 o
4Å MS, 1 h
55%
C
O
O
OH
BnO
O
O
OH
ONAP
O
O
O
OTBDPS
BzO
HO
1b
2) DDQ, CH2Cl2 / H2
90%
O
OH
BnO
O
OH
BnO
OAll
OTBDPS
OTBDPS
3) Pd(PPh 3
)
4, glacial AcOH
Scheme 4. Synthesis of the α-(1→3)-linked tetrasaccharide 1b.
OH
OAll
93%
11
15
Ph
Since the selective deprotection was unfruitful, we wanted to
decrease the amount of steps by replacing the chloroacetyl group
with another benzoyl group. To our great surprise, glycosylation
with donor 12 under the optimized conditions only afforded 10%
of the desired α-(1→3)-linked tetrasaccharide 14 (Scheme 5). The
major product formed was instead the corresponding β-anomer
13 (60% of β-(1→3)-linked product). The anomeric configuration
of 13 and 14 was verified by their one-bond 13C–1H coupling
constants of 159 and 168 Hz, respectively. That changing the
nature of the 3’-ester protecting group can have such a dramatic
effect on glycosylation outcome demonstrates how subtle and
O
10 equiv. SO3.Py
12 equiv. pyridine
4 Å MS
Na
O
OSO3
O
O
BzO
DMF, 1 h, 22 °C
70%
O
OH
BnO
OTBDPS
OH
16
OH
HO
Na
O
OSO3
1) 0.04 M NaOMe, MeOH
-20 °C, 48 h, 70%
O
HO
O
OH
HO
OTBDPS
2) Pd(OH)2/C, H2
H2O / AcOH (100 / 5)
70%
OH
17
Scheme 7. Protection/deprotection and optimized conditions for selective
sulfation of disaccharide 11.
remote differences can affect reactivity and stereoselectivity.
Ph
O
Ph
O
O
These successful results were then translated to the
tetrasaccharide 1b. Deprotection of the two acyl groups under
Zemplén conditions afforded the corresponding tetrasaccharide
triol. Regioselective benzoylation of the free C-3’’’ position under
the conditions optimized for the disaccharide analogue gave the
monobenzoylated adduct 18 in 76% yield. Oxidative cleavage of
the NAP groups using DDQ, followed by deallylation with
Pd(PPh3)4 afforded compound 19 in 74% yield (Scheme 8).
O
ONAP
O
O
O
BzO
BzO
O
ONAP
O
O
BnO
O
AllO
OH
BnO
OTBDPS
Ph
AllO
13
O
11
O
60%
+
1.1 equiv.
O
ONAP
O
O
BzO
Ph
OBz
0.25 equiv. TMSOTf
CH2Cl2, - 40 o
BnO
O
O
C
O
CF3
NPh
AllO
4Å MS, 1 h
12
1.0 equiv.
O
Ph
O
ONAP
Ph
O
Ph
BzO
BzO
BnO
O
O
O
O
O
1) 1M NaOMe,MeOH
MeOH/THF, 22 °C
84%
O
O
AllO
ONAP
O
O
O
Ph
O
Ph
O
O
ONAP
O
ONAP
O
O
O
ClAcO
BzO
OH
BnO
O
2) BzCl, Et3N, CH2Cl2
-40 °C, 76%
OBz
BnO
OH
BnO
OTBDPS
O
O
14
10%
AllO
O
O
AllO
ONAP
AllO
ONAP
O
O
O
O
O
OH
BnO
OH
O
OTBDPS
OTBDPS
BnO
Scheme 5. Unexpected outcome of [2+2] glycosylation reaction.
1b
18
OAll
OAll
With tetrasaccharide 1b in hand, the deprotection and sulfation
strategy to reach the selectively sulfated carrageenans (λ, γ, δ, μ
and ν) was envisioned as shown in Scheme 6. The five
corresponding anhydro-carrageenan (θ, β, α, κ and ι) could be
reached by performing an alkali treatment of their respective
precursor or by incubation with the enzymes mentioned in the
introduction.
Ph
O
O
O
Ph
O
OH
O
BzO
1) DDQ,CH2Cl2
22 °C, 97%
/ H2O 9 / 1
OH
BnO
O
O
O
HO
OH
O
O
2) Pd(PPh 3) , glacial AcOH
4
OH
BnO
22 °C, 76%
O
OTBDPS
19
OH
Scheme 8. Deprotection of tetrasaccharide 1b.
In order to prove the suitability of the protecting group and test the
sulfation step, we first applied the transformations to disaccharide
11, see Scheme 7. Regioselective benzoylation of diol 11 at the
C-3’ position was achieved at - 40 °C, using BzCl, Et3N and a
catalytic amount of DMAP in 75% yield with less than 15 % of di-
benzoylated adduct isolated. The 6-O-NAP protecting group was
As regioselective sulfation of the two primary alcohols was
required, fine-tuning of the amount of SO3 Py complex used was
critical to avoid oversulfation (see conditions used in Scheme 9).
We observed large variation in the outcome for this reaction and
.
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10.1002/ejoc.201900592
European Journal of Organic Chemistry
COMMUNICATION
careful control of the quality of the sulfation reagent is very
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
S. Kaeothip, G. -J. Boons, Org. Biomol. Chem. 2013, 11, 5136–
5146.
important.
Ph
Na
O
O
P. Dallabernardina, F. Schuhmacher, P. H. Seeberger, F. Pfrengle,
Org. Biomol. Chem. 2016, 14, 309–313.
O
Ph
OSO3
O
BzO
O
O
OH
BnO
O
Na
D. Schmidt, F. Schuhmacher, A. Geissner, P. H. Seeberger, F.
Pfrengle, Chem. Eur. J. 2015, 21, 5709–5713.
P. Dallabernardina, F. Schuhmacher, P. H. Seeberger, F. Pfrengle,
Chem. A Eur. J. 2017, 23, 1–7.
O
HO
OSO3
O
Ph
O
O
O
OH
BnO
OTBDPS
SO3.Py (22 equiv.)
Pyridine (24 equiv.)
4 Å MS
20
O
OH
O
Ph
O
OH
60%
+
O
BzO
OH
BnO
O
O
DMF
S. A. Nepogodiev, R. A. Field, I. Damager, Annu. Plant Rev. 2010,
41, 65–92.
O
22 °C
HO
OH
Ph
O
O
O
OH
BnO
O
OTBDPS
O
A.-L. Chauvin, S. A. Nepogodiev, R. A. Field, Carbohydr. Res. 2004,
339, 21–27.
OH
19
O
Ph
O
OH
O
BzO
OH
BnO
O
Na
O
O
N. A. Jones, S. A. Nepogodiev, C. J. MacDonal, D. L. Hughes, R. A.
Field, J. Org. Chem. 2005, 70, 8556–8559.
HO
OSO3
O
O
O
OH
OTBDPS
BnO
21
10%
OH
C. Kinnaert, M. Daugaard, F. Nami, M. H. Clausen, Chem. Rev.
2017, 117, 11337–11405.
Scheme 9. Conditions used for the selective sulfation of the C-6 positions.
[10]
[11]
A. Usov, Food Hydrocoll. 1998, 12, 301–308.
S. A. Stortz, Carrageenans: Structural and Conformational Studies.
In Handbook of Carbohydrate Engineering; Francis, T. and, Ed.;
2005; pp 211–245.
Current efforts include further investigation of the sulfation
reaction as well as the selective deprotection to reach all
canonical carrageenans. The results of these studies will be
reported in due course.
[12]
[13]
[14]
D. Jouanneau, M. Guibet, P. Boulenguer, J. Mazoyer, M. Smietana,
W. Helbert, Food Hydrocoll. 2010, 24, 452–461.
F. Van de Velde, S. H. Knutsen, A. Usov, H. S. Rollema, A. S.
Cerezo, Trends Food Sci. Technol. 2002, 13, 73–92.
S. Arungundram, K. Al-Mafraji, J. Asong, F. E. Leach, I. J. Amster,
A. Venot, J. E. Turnbull, G.-J. Boons, J. Am. Chem. Soc. 2009, 131,
17394–17405.
Conclusions
In conclusion, a synthesis strategy to afford the alternating 3-
linked-β-
D-galactopyranose
(G)
and
4-linked-α-D-
galactopyranose (D) backbone of carrageenans was developed
and a protected tetrasaccharide precursor for ten different
carrageenans was prepared using this approach. Furthermore,
the suitability of the protecting group scheme was demonstrated
by the synthesis of sulfated D6S di-carrageenan. Selective
sulfation of the tetrasaccharide was tested and the results
[15]
[16]
[17]
[18]
M. Lahaye, A. Robic, Biomacromolecules 2007, 8, 1765–1774.
O. Berteau, B. Mulloy, Glycobiology 2003, 13, 29–40.
M. Lahaye, C. Rochas, Hydrobiologia 1991, 221, 137–148.
M. F. De Jesus Raposo, A. M. B. De Morais, Mar. Drugs 2015, 13,
2967–3028.
.
highlighted that fine-tuning of the amount of SO3 pyridine as well
as careful control of the quality of this reagent are key to achieve
selective sulfation. We are currently in the process of
implementing those results to the synthesis of the other
carrageenan targets.
[19]
[20]
L. Li, R. Ni, Y. Shao, S. Mao, Carbohydr. Polym. 2014, 103, 1–11.
S. H. Knutsen, D. E. Myslabodski, B. Larsen, A. Usov, Bot. Mar.
1994, 37, 163–169.
[21]
[22]
J. Necas, L. Bartosikova, Vet. Med. (Praha). 2013, 58, 187–205.
S. D. Wiratni, M. Fahrurrozi, Rochmadi. Int. J. Chem. Mol. Nucl.
Mater. Metall. Eng. 2011, 5, 487–491.
Experimental Section
[23]
R. A. Hoffmann, M. J. Gidley, D. Cooke, W. J. Frith, Food Hydrocoll.
1995, 9, 281–289.
Experimental details can be found in the Supporting Information.
[24]
[25]
K. F. Wong, J. S. Craigie, Plant Physiol. 1978, 61, 663–666.
M. Zinoun, M. Diouris, P. Potin, J. Y. Floch, E. Deslandes, Bot. Mar.
1997, 40, 49–53.
Acknowledgements
[26]
[27]
[28]
[29]
[30]
[31]
S. Genicot-Joncour, A. Poinas, O. Richard, P. Potin, B. Rudolph, B.
Kloareg, Helbert, W. Plant Physiol. 2009, 151, 1609–1616.
G. Excoffier, D. Gagnaire, J.-P. Utille, Carbohydr. Res. 1975, 39,
368–373.
We acknowledge financial support from the Villum Foundation
(PLANET project) and the Carlsberg Foundation (grant no. CF14-
0564). NMR data were recorded at the NMR Center at DTU
supported by the Villum Foundation. We thank DTU for a PhD
fellowship for Christine Kinnaert.
A. Liptak, I. Jodal, J. Harang, P. Nanasi, Acta Chim. Hungarica
1983, 113, 415–422.
T. Kanaya, F. Schweizer, T. Takeda, F. Kiuchi, N. Hada, Carbohydr.
Res. 2012, 361, 55–72.
Keywords:
Plant/algal
oligosaccharides
•
sulfated
oligosaccharides • carrageenans • α-(1→3)-galactoside • β(1→4)-
galactoside
F. W. Lichtenthaler, M. Oberthur, S. Peters, Eur. J. Org. Chem.
2001, 3849–3869.
M. C. F. Andersen, S. K. Kračun, M. G. Rydahl, W. G. T. Willats, M.
H. Clausen, Chem. - Eur. J. 2016, 22, 11543–11548.
[1]
Y. Rao, G.-J. Boons, Angew. Chem. Int. Ed. Engl. 2007, 46, 6148–
6151.
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201900592
European Journal of Organic Chemistry
COMMUNICATION
[32]
[33]
[34]
J. Tatai, P. Fügedi, Org. Lett. 2007, 9, 4647–4650.
[36]
[37]
Y. P. Hu, Y. Q. Zhong, Z. G. Chen, C. Y. Chen, Z. Shi, M. M. L.
Zulueta, C. C. Ku, P. Y. Lee, C. C. Wang, S. C. Hung, J. Am. Chem.
Soc. 2012, 134, 20722–20727.
B. Yu, J. Sun, Chem. Commun. 2010, 46, 4668–4679.
K. Bock, C. Pedersen, J. Chem. Soc., Perkin Trans. 2 1974, 293–
297.
C. Zong, A. Venot, O. Dhamale, G.-J. Boons, Org. Lett. 2013, 15,
342–345.
[35]
K. Horita, T. Yoshioka, T. Tanaka, Y. Oikawa, O. Yonemitsu,
Tetrahedron 1986, 42, 3021–3028.
Ph
O
O
O
Ph
O
ONAP
O
ClAcO
O
OBz
BnO
O
O
AllO
ONAP
O
O
O
OH
BnO
OTBDPS
1
b
1) NaOMe,MeOH
2) BzCl, Et3N, DCM
3) DDQ
OAll
4) Pd(PPh3
)
4
BzCl
Ph
Ph
OBn
O
HO
ClAcO
O
O
O
O
1) Et3SiH, TFA
2) DDQ
OH
O
O
O
Ph
O
Ph
OH
ONAP
O
BzO
O
ClAcO
O
OBz
BnO
O
OBn
HO
O
O
OH
BnO
OBz
BnO
O
O
O
AllO
OH
O
O
O
O
HO
AllO
O
OH
ONAP
O
O
O
O
OBz
BnO
O
OH
BnO
OBz
BnO
OTBDPS
O
OTBDPS
OTBDPS
OAll
OH
OAll
SO3.pyridine
(large excess)
1) Pd(PPh3)
4
1) DDQ
2) Pd(PPh3
SO3 pyridine
2) SO3.pyridine
)
(large excess) for R1
=
R2 = SO3Na
4
3) SO3.pyridine
(large excess)
(10 equiv.) for R1 = SO3Na and R2 = H
(large excess)
Ph
Ph
OBn
OBn
O3SO
ClAcO
O3SO
ClAcO
O
O
O
O
O
O
OSO3
OSO3
O
O
O
O
O
OR1
O
OSO3
O
Ph
Ph
OBz
BnO
OBz
BnO
O
OBn
OBn
O
BzO
O
ClAcO
O
O3SO
O3SO
OR2
BnO
OBz
BnO
O
O
O
O
O
AllO
O3SO
OSO3
O
OTBDPS
OSO3
OSO3
O
O
O
O
O
R2
O
O3SO
O
O
O
OR1
O
OSO3
O
OBz
BnO
OBz
BnO
O
OR2
BnO
OBz
BnO
OTBDPS
O
O
OAll
OTBDPS
OTBDPS
OSO3
OR2
11 : R1 = R2
=
SO3Na
12 : R1 = SO3Na and R2 = H
1) NaOMe, MeOH
2) Pd(PPh3
3) H2, Pd(OH)2/C
1) NaOMe/MeOH
2) H2, Pd(OH)2/C
1) NaOMe, MeOH
1) NaOMe, MeOH
2) H2, Pd(OH)2/C
)
4
2) H2,
Pd(OH)2/C
OH
HO
OH
OH
HO
HO
OH
O
OH
O3SO
O3SO
HO
O
O
O
OSO3
O
OSO3
O
OSO3
O
HO
O
HO
O
HO
O
OSO3
O
OSO3
O
HO
O
O
O3SO
HO
OH
HO
OH
HO
OH
OH
OH
HO
O
OH
HO
HO
HO
OH
HO
OH
OH
O
O3SO
O3SO
O
O
O
O3SO
HO
O3SO
OSO3
O
OSO3
O
OSO3
O
O
O
O
O
OH
O
O3SO
O
O
O
OSO3
OSO3
O
O
O3SO
HO
OH
HO
OH
HO
O
OH
OH
HO
O
OTBDPS
OTBDPS
OTBDPS
O
HO
OTBDPS
OTBDPS
OSO3
OSO3
OH
OSO3
OH
l-carrageenan
-carrageenan
-carrageenan
-carrageenan
-carrageenan
(from 11)
(from 12)
Scheme 6. Deprotection and sulfation strategy for tetrasaccharide 1b.
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