Synthesis of the tetrasaccharide α-D-Glcp-(1→3)-α-D-Manp- (1→2)-α-D-Manp-(1→2)-α-D-Manp recognized by Calreticulin/Calnexin

Article abstract of DOI:10.1016/j.carres.2005.09.001

The title compound as its methyl glycoside was efficiently synthesized using a block synthesis approach. Halide-assisted glycosidations between 6-O-acetyl-2,3,4-tri-O-benzyl-α-d-glucopyranosyl iodide and ethyl 2-O-acetyl-4,6-di-O-benzyl-1-thio-α-d-mannopy

Full text of DOI:10.1016/j.carres.2005.09.001

Carbohydrate
RESEARCH
Carbohydrate Research 340 (2005) 2558–2562
Note
Synthesis of the tetrasaccharide a-D-Glcp-(1!3)-a-D-
Manp-(1!2)-a-D-Manp-(1!2)-a-D-Manp
recognized by Calreticulin/Calnexin
Emiliano Gemma,a Martina Lahmanna and Stefan Oscarsonb,*
aDepartment of Chemistry, Go¨teborg University, S-412 96 Gothenburg, Sweden
bDepartment of Organic Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden
Received 2 June 2005; accepted 1 September 2005
Available online 19 September 2005
Abstract—The title compound as its methyl glycoside was efficiently synthesized using a block synthesis approach. Halide-assisted
glycosidations between 6-O-acetyl-2,3,4-tri-O-benzyl-a-D-glucopyranosyl iodide and ethyl 2-O-acetyl-4,6-di-O-benzyl-1-thio-a-D-
mannopyranoside using triphenylphosphine oxide as promoter yielded, with complete a-selectivity, a disaccharide building block
in high yield. The perbenzylated derivative of this proved to be an excellent donor affording 88% of the protected target tetrasac-
charide in an NIS/AgOTf-promoted coupling to a known methyl dimannoside acceptor. Deprotection through catalytic hydrogen-
olysis then gave the target compound in 47% overall yield.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Calreticulin substrates; Iodide glycosyl donors; Triphenylphosphine oxide promotion; Thioglycoside block donor
The title tetrasaccharide is the important part of an N-
glycan structure recognized by the lectins Calreticulin/
Calnexin. This recognition is a major event of the fold-
ing procedure of all proteins glycosylated with N-glycan
chains.1–4 Thus, interaction studies between this carbo-
hydrate structure and the chaperone proteins are of im-
mense biological interest. We have earlier synthesized
the title tetrasaccharide as well as the glucose-containing
di- and trisaccharide parts.5 These compounds have
been used in ITC-studies with Calreticulin and various
mutants thereof.5,6 Other syntheses of the tetrasaccha-
ride have also been reported.7,8 Furthermore, the syn-
theses of monodeoxy analogues of the trisaccharide, to
pinpoint the importance of each hydroxyl group in the
interaction, have been performed.9 New mutants of
the proteins are continuously prepared to investigate
in further detail the binding and the binding site and
consequently more oligosaccharides are constantly
needed for various interaction experiments. Here, we
present an improved synthesis of the title tetrasaccharide
using a block synthesis approach, which also facilitates
the synthesis of the glucose-containing trisaccharide
part.
Although efficient, there were some drawbacks in our
earlier published syntheses,5 mainly the need for HPLC-
purification after introduction of the glucose moiety due
to incomplete stereoselectivity in these couplings. Also,
the preparation of the third mannose residue was cum-
bersome. Since we considered the stereospecific coupling
of the glucose residue to be the most problematic glyco-
sylation in the synthesis, a new approach was designed
where this linkage is formed early in the synthesis to
produce a disaccharide thioglycoside donor block,
which can then be used for the synthesis of both the
tri- and tetrasaccharide.
Orthoesterification of unprotected a-D-mannopyrano-
sides yields the 2,3;4,6-di-O-orthoester derivatives.10,11
However, simple regioselective acetylation of the pri-
mary hydroxyl group12 (!2) prior to orthoesterification
allowed efficient formation of the 2,3-O-monoorthoester
*
Corresponding author. Tel.: +46 8 16 24 80; fax: +46 8 15 49 08;
e-mail: s.oscarson@organ.su.se
0008-6215/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2005.09.001

E. Gemma et al. / Carbohydrate Research 340 (2005) 2558–2562
2559
1. CH3C(OEt)3, cat. CSA
2. NaH, BnBr
AcO
OH
O
HO
HO
HO
AcO
OH
O
AcCl, coll.
O
BnO
OAc
BnO
Ph3PO
CH2Cl2
-35°C
HO
3. 1N HCl
BnO
O
BnO
HO
RO
HO
BnO
85%
82%
I
O
BnO
BnO
SEt
BnO
rt, 3 days
SEt
4
OR
O
1
2
SEt
3
BnO
BnO
BnO
HO
OAc
O
70%
BnO
O
Scheme 1. Synthesis of thioglycoside acceptor.
SEt
5 R = Ac
1. NaOMe, MeOH
2. NaH, BnBr, DMF
6 R = Bn (84%)
SEt
3
(Scheme 1). This derivative was then directly 4,6-di-O-
benzylated and the orthoester subsequently opened to
give the desired 3-OH acceptor 3.
Scheme 2. Synthesis of the disaccharide donor block.
OAc
O
TMSCl
CH2Cl2
BnO
To obtain complete a-selectivity in the introduction of
the glucose residue, a rather elaborately protected glu-
cose donor, methyl 4,6-O-benzylidene-3-O-pivaloyl-2-
O-p-methoxybenzyl-1-thio-b-D-glucopyranoside, can be
utilized as has been shown by Ito and co-workers.13
We reasoned that halide-assisted conditions14 should
allow the use of simpler donors and also be compatible
with thioglycoside acceptors. However, the standard do-
nor (benzylated glucosyl bromide) used earlier for O-
glycoside acceptors15 did not work with the thioglyco-
side acceptor 3, wherefore alternatives were tested.
Iodo-sugars, easily obtained, for example, from the cor-
responding anomeric acetate, have recently received
increased attention as donors.16–18 When published
conditions,17 using tetrabutylammonium iodide as
promoter, were tried in the coupling between acceptor
3 and iodide donor 4, high yield was obtained in an
initial attempt, but it was not possible to reproduce this
first result. Similar problems have been encountered by
Mukayiama and Kobashi, who advocated the use of
triphenylphosphine oxide as promoter instead.19 Also,
in our coupling this afforded a high and reproducible
yield of the desired a-linked disaccharide building block
5 (Scheme 2).
BnO
OR
O
O
O
BnO
BnO
BnO
BnO
O
BnO
BnO
BnO
8
Cl
O
AgOTf
O
OH
O
BnO
CH2Cl2 BnO
BnO
NaOMe 10 R = Ac
MeOH 11 R = H
AcCl
MeOH
reflux
BnO
BnO
BnO
O
7
OMe
OMe
9
Scheme 3. Synthesis of the mono- and disaccharide acceptors.
from a common orthoester precursor according to the
published procedures (Scheme 3).20,21
The final glycosidations were attempted to give the
target structures using donor block 5 and NIS/AgOTf
as promoter, but the yields were disappointing. Thus,
the acetates in compound 5 were exchanged to benzyl
groups to afford derivative 6 (Scheme 4). This slight
change in the protecting group pattern of the donor
proved most efficient. Now coupling between donor
block 6 and the disaccharide acceptor 11 using NIS/
AgOTf promotion yielded the tetrasaccharide 12 in
88% yield and with complete a-selectivity (Scheme 4).
The coupling to monosaccharide acceptor 9 under the
same conditions afforded trisaccharide 13 in 75% yield.
One-step deprotection through catalytic hydrogenolysis
then smoothly yielded the known target molecules 14
(90%) and 15 (86%).5,7,8
The 2-OH monosaccharide acceptor 9 and a-D-Man-
(1!2)-a-D-Man disaccharide acceptor 11 were prepared
OH
O
BnO
BnO
BnO
HO
BnO
BnO
BnO
O
BnO
BnO
HO
HO
O
OBn
O
HO
HO
OH
O
BnO
O
HO
BnO
BnO
BnO
O
O
O
90%
88%
O
O
BnO
BnO
BnO
HO
HO
HO
O
O
OMe
11
NIS
AgOTf
CH2Cl2
H2, Pd/C
MeOH
O
O
BnO
BnO
HO
BnO
BnO
BnO
O
O
HO
O
BnO
BnO
OBn
HO
BnO
O
OMe
OMe
BnO
12
14
O
BnO
BnO
SEt
HO
HO
6
O
BnO
BnO
HO
HO
O
OBn
O
OH
O
BnO
HO
BnO
O
HO
O
75%
NIS
AgOTf
CH2Cl2
84%
OH
O
BnO
O
O
BnO
BnO
BnO
HO
HO
HO
H2, Pd/C
MeOH
O
BnO
O
BnO
9
OMe
OMe
OMe
13
15
Scheme 4. Synthesis of target tetra- and trisaccharides.

2560
E. Gemma et al. / Carbohydrate Research 340 (2005) 2558–2562
In conclusion, an efficient synthesis of the thioglyco-
After additional 10 min, benzyl bromide (3 mL,
25.0 mmol) was added and the reaction mixture stirred
at rt for 2 h. Ice was poured into the reaction mixture,
and the aqueous phase was extracted with EtOAc. The
organic phase was then left to stir for 1 h with 1 N
HCl, washed with water, dried (Na2SO4), filtered and
concentrated. The crude product was purified by silica
gel chromatography (6:1 toluene–EtOAc) to give 3
(2.90 g, 82%). NMR data of the product were in agree-
ment with the literature values.23
side disaccharide building block 6 has been performed.
This derivative proved to be an efficient donor and has
been used as a key intermediate in the synthesis of trisac-
charide 15 and the title compound 14, both valuable
structures in interaction studies with the Calreticulin/
Calnexin chaperone proteins.
1. Experimental
1.1. General
1.4. Ethyl 6-O-acetyl-2,3,4-tri-O-benzyl-a-D-gluco-
pyranosyl-(1!3)-2-O-acetyl-4,6-di-O-benzyl-1-thio-
a-D-mannopyranoside (5)
CH2Cl2 was distilled from calcium hydride. Organic
solutions were concentrated under diminished pressure
at <45 °C (bath temperature). NMR spectra were re-
Triphenylphosphine oxide (375 mg, 1.35 mmol) was
added to a stirred mixture of glucosyl iodide 4 (prepared
by reaction of 360 mg, 0.67 mmol, of the corresponding
glucosyl acetate with 1.1 equiv TMSI)24 and acceptor 3
1
corded at 400 MHz for H and at 100 MHz for 13C.
Chemical shifts are reported relative to CHCl3 [dH
7.26, dC (central of triplet) 77.0] or to CH3OH [dH
3.35, dC (central of septet) 49.0] or to acetone as internal
standard (D2O). TLC was performed on a E. Merck Sil-
ica Gel 60 F254 with detection by charring with 8%
H2SO4 acid. Silica gel (0.040–0.063 mm) was used for
column chromatography.
˚
(100 mg, 0.22 mmol) in CH2Cl2 (2 mL) containing 4 A
MS. The reaction mixture was stirred at rt for 3 days,
then filtered through a Celite pad and concentrated. Sil-
ica gel column chromatography (toluene–EtOAc 16:1)
of the residue yielded 5 (145 mg, 70%). [a]D +69 (c 1.0,
1
CHCl3); H NMR (CDCl3): d 1.29 (t, 3H, SCH2CH3),
1.2. Ethyl 6-O-acetyl-1-thio-a-D-mannopyranoside (2)
2.06 (s, 3H, CH3CO), 2.15 (s, 3H, CH3CO), 2.58–2.68
(q, 2H, SCH2CH3), 3.46 (t, 1H, J4,3 = J4,5 9.5 Hz, H-
Ethyl 1-thio-a-D-mannopyranoside (1) (2.5 g, 0.011 mol)
was dissolved in sym-collidine (25 mL) and the soln
cooled to À35 °C. Acetyl chloride (1.5 mL, 0.021 mol)
was then added dropwise over 30 min under vigorous
stirring. After 1 h, MeOH (5 mL) was added, and the
reaction was allowed to attain rt. The crude mixture
was co-concentrated under diminished pressure several
times with toluene. The residue was purified on a silica
gel column (2:1 CH2Cl2–acetone) to afford 2 (2.5 g,
4Glc), 3.49 (dd, 1H, J2,3 10.6 Hz, J2,1 3.3 Hz, H-2Glc),
3.68 (dd, 1H, J6b,6a 10.5 Hz, J6b,5 2 Hz, H-6bMan), 3.84
(dd, 1H, J6a,6b 10.5 Hz, J6a,5 4.0 Hz, H-6aMan), 3.88–
3.92 (m, 1H, H-5Glc), 3.99–4.09 (m, 4H, H-3Man, H-
4
Man, H-3Glc, H-6bGlc), 4.17–4.22 (m, 1H, H-5Man),
4.33 (dd, 1H, J6a,6b 12.1 Hz, J6a,5 4.4 Hz, H-6aGlc),
4.45–4.68 (m, 6H, CH2Ph), 4.82–4.89 (dd, 2H, CH2Ph),
4.94 (d, 1H, J1,2 3.3 Hz, H-1Glc), 4.99 (d, 1H, CH2Ph),
5.17 (d, 1H, CH2Ph), 5.22 (br s, 1H, H-2Man), 5.39 (d,
1H, J1,2 1.5 Hz, H-1Man), 7.15–7.40 (m, 25H, aromatic).
13C NMR (CDCl3): d 15.1 (SCH2CH3), 20.9 (CH3CO),
21.4 (CH3CO), 25.8 (SCH2CH3), 63.0, 68.9, 69.9, 72.0,
73.2, 73.5, 74.1, 74.2, 75.1, 75.2, 75.8, 77.3, 79.7, 80.5,
81.6 (C-2-6, 20-60, CH2Ph), 81.9 (C-1), 99.7 (C-10),
127.0–129.0, 137.9, 138.0, 138.2, 138.6, 138.8 (C-aro-
matic), 170.4, 170.9 (C@O). Anal. Calcd for
C53H60O12S: C, 69.11; H, 6.57. Found: C, 69.15; H,
6.64.
1
85%). [a]D +128 (c 1.0, CHCl3); H NMR (CD3OD): d
1.29 (t, 3H, SCH2CH3), 2.04 (s, 3H, CH3CO), 2.62 (q,
2H, SCH2CH3), 3.62–3.65 (m, 2H, H-3, H-4), 3.89
(dd, 1H, J2,3 3.0 Hz, J2,1 1.4 Hz, H-2), 4.08 (m, 1H, H-
5), 4.23 (dd, 1H, J6b,6a 12.0 Hz, J6b,5 6.6 Hz, H-6b),
4.37 (dd, 1H, J6a,6b 12.0 Hz, J6a,5 2.0 Hz, H-6a), 5.22
(d, 1H, J1,2 1.4 Hz, H-1). 13C NMR (CD3OD): d 14.0
(SCH2CH3), 19.4 (CH3CO), 24.6 (SCH2CH3), 63.8 (C-
6), 67.7 (C-4), 71.0 (C-5), 71.8 (C-3), 72.2 (C-2), 84.9
(C-1), 171.5 (C@O).
1.5. Ethyl 2,3,4,6-tetra-O-benzyl-a-D-glucopyranosyl-
1.3. Ethyl 2-O-acetyl-4,6-di-O-benzyl-1-thio-a-D-manno-
pyranoside (3)
(1!3)-2,4,6-tri-O-benzyl-1-thio-a-D-mannopyranoside
(6)
Compound 2 (2.10 g, 7.9 mmol) was reacted with tri-
ethyl orthoacetate following the protocol of Pozsgay.22
Once TLC showed complete conversion of the starting
material into the 2,3-cyclic orthoester, additional
DMF (10 mL) and sodium hydride (1.26 g of 60% dis-
persion in mineral oil, 31.0 mmol) were added at 0 °C.
To a solution of 5 (565 mg, 0.61 mmol) in MeOH (10 mL),
a catalytic amount of 1 M sodium methoxide in MeOH
was added. The reaction mixture was stirred for 1 day,
+
then neutralized with DowexH
ion exchange resin,
filtered and evaporated under diminished pressure. The
crude deacetylated product was dissolved in DMF

E. Gemma et al. / Carbohydrate Research 340 (2005) 2558–2562
2561
(15 mL) and sodium hydride (96 mg, 2.40 mmol) was
added at 0 °C. After 15 min, benzyl bromide
(1.80 mmol, 214 lL) was added and after another 3 h
ice-cold water was poured into the reaction mixture.
The aqueous phase was extracted with toluene and the
organic phase washed twice with water before being
dried and concentrated. After silica gel column chroma-
tography (30:1 toluene–EtOAc), disaccharide 6 was iso-
lated in 84% yield (500 mg). [a]D +79 (c 0.45, CHCl3);
1H NMR (CDCl3): d 1.28 (t, 3H, SCH2CH3), 2.58–
2.68 (q, 2H, SCH2CH3), 3.49–3.57 (m, 3H, H-2Glc, H-
OCH3), 3.38–3.56 (m, 3H), 3.68–3.86 (m, 7H), 3.91–
4.08 (m, 6H), 4.12–4.19 (m, 2H), 4.26–4.69 (m, 15H),
4.76–4.94 (m, 5H), 5.06 (br s, 1H, H-100), 5.14 (br s,
1H, H-1), 5.27 (d, 1H, J1,2 2.2 Hz, H-10), 7.1–7.4 (m,
0
50H, aromatic). 13C NMR (CDCl3): d 54.8 (OCH3),
68,4, 69.4, 69.8, 70.9, 71.6, 72.2, 73.3, 73.4, 73.5, 74.2,
74.9, 75.1, 75.2, 75.3, 77.6, 77.7, 79.9, 80.2, 81.9 (C-2-
6, 20-60, 200-600, CH2Ph), 100.0, 100.1, 100.2 (C-1, 10,
100), 127.0–129.0, 137.9, 138.3, 138.4, 138.4, 138.5,
138.7, 138.8, 138.9 (C-aromatic). Anal. Calcd for
C89H94O16: C, 75.29; H, 6.67. Found: C, 75.34; H, 6.61.
5
Glc, H-3Man), 3.62 (t, 1H, J4,3 = J4,5 9.5 Hz, H-4Glc),
3.70 (dd, 1H, J6b,6a 11.0 Hz, J6b,5 2.0 Hz, H-6bMan),
3.80 (dd, 1H, J6a,6b 11.0 Hz, J6a,5 3.7 Hz, H-6aMan),
1.8. Methyl a-D-glucopyranosyl-(1!3)-a-D-manno-
pyranosyl-(1!2)-a-D-mannopyranosyl-(1!2)-a-D-
mannopyranoside (14)
4.01–4.10 (m, 5H, H-4Man, H-3Glc, H-2Man, H-6aGlc
,
H-6bGlc), 4.17–4.21 (m, 1H, H-5Man), 4.40–4.95 (m,
13H, CH2Ph), 5.09 (d, 1H, J1,2 3.3 Hz, H-1Glc), 5.12
(d, 1H, CH2Ph), 5.45 (s, 1H, H-1Man), 7.1–7.3 (m, 35H,
aromatic). 13C NMR (CDCl3): d 15.1 (SCH2CH3),
25.5 (SCH2CH3), 68.7, 69.3, 71.2, 72.2, 73.0, 73.3,
73.6, 74.7, 74.8, 75.7, 76.4, 77.9, 79.3, 79.8, 81.0 (C-2-6,
20-60, CH2Ph), 81.9 (C-1), 99.4 (C-10), 127.0–129.0,
137.9, 138.2, 138.5, 138.6, 138.8, 139.0 (C-aromatic).
Anal. Calcd for C63H68O10S: C, 74.38; H, 6.74. Found:
C, 74.31; H, 6.63.
Compound 12 (185 mg, 0.10 mmol) was dissolved in a
mixture of EtOAc–MeOH (1:5, 10 mL). 1 N Hydrochlo-
ric acid (200 lL) and a catalytic amount of Pd/C were
added and the reaction mixture was stirred under an
H2 atmosphere (1 atm) for 2 h. The mixture was filtered
through Celite and the solvent co-evaporated with tolu-
ene. The residue was purified on a Bio-Gel P2 column
eluted with 1% butanol in water, followed by a short re-
versed phase column (eluted with water) to give 14
(61 mg, 90%), with NMR data in agreement with the
published values.7
1.6. Methyl 2,3,4,6-tetra-O-benzyl-a-D-glucopyranosyl-
(1!3)-2,4,6-tri-O-benzyl-a-D-mannopyranosyl-(1!2)-
3,4,6-tri-O-benzyl-a-D-mannopyranosyl-(1!2)-3,4,6-tri-
O-benzyl-a-D-mannopyranoside (12)
1.9. Methyl a-D-glucopyranosyl-(1!3)-a-D-manno-
pyranosyl-(1!2)-a-D-mannopyranoside (15)
To a suspension of 1121 (159 mg, 0.18 mmol) and 6
(150 mg, 0.15 mmol) in CH2Cl2 (3 mL) containing 4 A
Compound 13 (125 mg, 0.09 mmol) was deprotected as
described above for compound 12 to give 155,25
(40 mg, 86%). [a]D +49 (c 1.0, H2O); H NMR (D2O):
d 3.40 (s, 3H, OCH3), 3.41 (m, 1H), 3.54–3.96 (m,
16H), 4.24 (m, 1H), 4.99 (br s, 1H), 5.02 (br s, 1H),
5.25 (d, 1H, J1,2 3.7 Hz, H-1Glc). 13C NMR (D2O): d
55.0 (OCH3), 60.6, 60.8, 60.9, 65.5, 67.1, 69.8, 69.9,
70.4, 71.9, 72.5, 72.7, 73.0, 73.5, 78.5, 78.7 (C-2-6, 20-
60, 200-600), 99.5, 100.6, 102.3 (C-1, 10, 100).
˚
1
MS were added NIS (46 mg, 0.21 mmol) and a catalytic
amount of AgOTf (2 mg, 0.01 mmol). After 0.5 h, the
reaction mixture was neutralized with Et3N and filtered
through Celite. Silica gel chromatography of the crude
material (30:1!20:1 toluene–EtOAc) furnished product
12 (240 mg, 88%), whose NMR-spectra were in agree-
ment with the published data.7
1.7. Methyl 2,3,4,6-tetra-O-benzyl-a-D-glucopyranosyl-
(1!3)-2,4,6-tri-O-benzyl-a-D-mannopyranosyl-(1!2)-
3,4,6-tri-O-benzyl-a-D-mannopyranoside (13)
Acknowledgements
We are thankful to the Swedish Research Council and
EU (Glycotrain, Contract no. HPRN-CT-2000-00001)
for financial support.
Donor 6 (125 mg, 0.12 mmol) and acceptor 920 (86 mg,
0.18 mmol) were dissolved in 5 mL absolute CH2Cl2
˚
and stirred with 4 A MS for 15 min. NIS (41 mg,
0.18 mmol) and a catalytic amount of AgOTf (2 mg,
0.01 mmol) were subsequently added and the reaction
mixture was stirred for 1 h. Neutralization of the reac-
tion mixture with Et3N and removal of molecular sieves
through Celite followed. The mixture was then concen-
trated and purified on a silica gel column (20:1 tolu-
ene–EtOAc), to afford 13 (130 mg, 75%). [a]D +24 (c
0.36, CHCl3); 1H NMR (CDCl3): d 3.27 (s, 3H,
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