Fucosylation of triethyleneglycol-based acceptors into 'clickable' α-fucosides

Article abstract of DOI:10.1016/j.carres.2014.06.002

Design of multivalent glycoconjugates can find applications such as in anti-adhesive therapy against bacterial infections. Nevertheless, the access to such macromolecules requires functionalized building blocks prepared in a minimum number of steps and on a multi-gram scale at least for the laboratory. Fucose is a representative epitope used by several bacteria for adhesion to their host cells. The stereoselective, rapid, and efficient access to two 'clickable' α-fucosides was re-investigated using PPh₃/CBr ₄-promoted glycosylation of chloro- (as precursors of azido-) and alkyne-functionalized triethyleneglycols with fully unprotected l-fucose. The convenient access to such building blocks paves the way to the design of new multivalent glycoconjugates functionalized with fucose epitopes and their applications.

Full text of DOI:10.1016/j.carres.2014.06.002

Carbohydrate Research 395 (2014) 15–18
Contents lists available at ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Fucosylation of triethyleneglycol-based acceptors into ‘clickable’
a-fucosides
Shuai Wang a, Nicolas Galanos a, Audric Rousset a, Kevin Buffet b, Samy Cecioni a, Dominique Lafont a,
Stéphane P. Vincent b, Sébastien Vidal a,
⇑
a Institut de Chimie et Biochimie Moléculaires et Supramoléculaires, CO2-Glyco, UMR 5246, CNRS, Université Claude Bernard Lyon 1, Université de Lyon,
43 Boulevard du 11 Novembre 1918, F-69622 Villeurbanne, France
b Department of Chemistry, University of Namur, Rue de Bruxelles 61, 5000 Namur, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 April 2014
Received in revised form 20 May 2014
Accepted 4 June 2014
Available online 14 June 2014
Design of multivalent glycoconjugates can find applications such as in anti-adhesive therapy against bac-
terial infections. Nevertheless, the access to such macromolecules requires functionalized building blocks
prepared in a minimum number of steps and on a multi-gram scale at least for the laboratory. Fucose is a
representative epitope used by several bacteria for adhesion to their host cells. The stereoselective, rapid,
and efficient access to two ‘clickable’
sylation of chloro- (as precursors of azido-) and alkyne-functionalized triethyleneglycols with fully
unprotected -fucose. The convenient access to such building blocks paves the way to the design of
new multivalent glycoconjugates functionalized with fucose epitopes and their applications.
Ó 2014 Elsevier Ltd. All rights reserved.
a-fucosides was re-investigated using PPh3/CBr4-promoted glyco-
Keywords:
L
Carbohydrate
Glycosylation
Fucoside
Click chemistry
Triethyleneglycol
1. Introduction
sialic acid. The recent identification of several pathogenic fucose-
binding lectins (e.g., LecB,17–19 BambL,20 and DC-SIGN21,22) has
prompted the design of a robust and reliable synthetic access to
Carbohydrates are playing a pivotal role in biology and more
specifically in recognition processes involving protein receptors
(lectins). While these monovalent interactions (i.e., 1:1 partners)
are rather weak in energy (dissociation constants typically in the
millimolar range), higher affinities or avidities (typically sub-
micromolar) can be reached through multivalent associations
between glycoconjugates and lectins.1 Intensive investigations for
the design of multivalent glycosylated architectures such as glyco-
dendrimers,2,3 glycoclusters,4–6 glyconanoparticles,7,8 or glyco-
polymers9 have been performed. The conjugation technique is a
keystone in the synthesis of multivalent glycoconjugates since
the reaction must be usually performed on several reactive ends
at once when using a divergent approach in which the carbohy-
drate epitope is introduced in the last steps of the synthesis. Mean-
while, Cu(I)-catalyzed azide-alkyne cycloaddition10–14 (CuAAC)
was developed and provided a rapid and efficient access to a large
number of glycosylated macromolecules.15,16
alkyne- or azido-functionalized
a-fucosides for the design of
potential anti-adhesives against bacterial infection.4,23
The choice for the spacer arm introduced at the anomeric posi-
tion of fucose is important since multivalent interactions require
the access of several lectins at once on the multivalent scaffold
and the steric hindrance generated by multiple copies of lectins
at the periphery of a multivalent glycoconjugate may hamper their
correct assembly. The flexibility and solubility of triethyleneglycol
in water make it an appropriate candidate in such design. The syn-
thesis of propargyl fucoside was reported24–27 but the introduction
of a longer linker was scarcely investigated to date.28–35 We report
herein on the rapid, reproducible, and efficient (40% isolated yield)
synthesis of fucosylated ‘clickable’ triethyleneglycols for the
further design of multivalent glycoconjugates.
2. Results and discussion
The typical carbohydrate epitopes used in such multivalent
design are mannose, galactose, N-acetylglucosamine, lactose, or
We have recently reported34 the synthesis of 1-azido-3,6-diox-
aoct-8-yl 2,3,4-tri-O-acetyl-a-L-fucopyranoside 1 in a three-step
process using Fischer glycosylation catalyzed by sulfuric acid
⇑
Corresponding author. Tel.: +33 472 448 349; fax: +33 472 448 109.
E-mail address: sebastien.vidal@univ-lyon1.fr (S. Vidal).
adsorbed on silica gel27 (Scheme 1, Path A). This process was pro-
http://dx.doi.org/10.1016/j.carres.2014.06.002
0008-6215/Ó 2014 Elsevier Ltd. All rights reserved.

16
S. Wang et al. / Carbohydrate Research 395 (2014) 15–18
41% /
/ = 70:30 / pure = 28%
α β α
Path A
H2SO4/SiO2
120°C then 65°C Ac2O / C5H5N NaN3 / nBu4NI
r.t. / 16 h
DMF / 90°C / 40 h
3.5 h
O
O
N3
2
OH
OH
O
Me
O
HO
Cl
Me
O
2
OAc
1
OH
OAc
PPh3 / CBr4
65°C / 4 h
Ac2O / C5H5N NaN3 / nBu4NI
r.t. / 16 h DMF / 90°C / 40 h
HO
AcO
L-Fucose
59% / α/β = 66:34 / α pure = 39%
1 g ~ 10 €
Path B
NaN3 / nBu4NI
DMF / 90°C
16 h
84%
2 steps
O
H2 (8 atm)
Pd-C 10%
O
Cl
CuBr2 / nBu4NBr
2
Ac2O / C5H5N
r.t. / 16 h
SEt
OBn
CH2Cl2 / r.t. / 84 h
EtOH / 48 h
Me
O
Me
O
OBn
3
O
HO
Cl
81%
OBn
2
OBn
2
BnO
BnO
Path C
76% /
/ = 71:29 / pure = 30%
α β α
1 g ~ 45 €
Scheme 1. Comparative study for the synthesis of the fucosylated azido-functionalized triethyleneglycol.
ducing valuable amounts of the desired
a
-fucoside (up to 5 g of
poorer selectivities (<70:30).40 The in situ anomerization strat-
egy41–43 was reported for the mono-
-fucosylation of ethylenegly-
col in good yield (82%) and with complete stereocontrol.44
Commercially available thioglycoside donor was therefore
applied to the same conditions (Scheme 1, Path C) providing a
71:29 /b mixture of fucoside anomers in 76% yield and the pure
-anomer 3 could be isolated in 30% yield. Nevertheless, isolation
of the pure compound required two consecutive silica gel chroma-
tographies for optimal purity and this long purification protocol
appears as a possible limitation in this approach. In a similar
approach using a triethyleneglycol-based acceptor, the separation
of anomers was not possible while yields and selectivities were
comparable, highlighting the difficulty for the separation process.30
Subsequent hydrogenolysis of benzyl ether was performed suc-
cessfully although necessitating high pressure (8 atm.) in dihydro-
gen while allowing the conservation of the chloroalkane moiety.
The hydroxylated fucoside intermediate could be readily charac-
terized by 1H NMR and mass spectrometry (see Supplementary
material) to verify the cleavage of the benzyl ethers and the pres-
ence of the chlorine atom. Acetylation and azidation provided the
isolated compound) but the purification of the material from the
complex reaction mixture required several difficult and long silica
gel column chromatographies. We have re-examined this particu-
lar synthetic procedure and have identified that the quality of sul-
furic acid adsorbed on silica gel was critical and the drying process
required an inert atmosphere. Also, the experimental setup needs
pre-heating of the triethyleneglycol and fucose solution at 120 °C
to obtain a clear solution prior to addition of the acid promoter.
These parameters were quite difficult to control in our previous
experimental design34 and the reaction has now been optimized
and is reproducible providing an overall yield of 41% (for the mix-
a
2
a
a
ture of both
selectivity of 70:30. The desired
28% overall yield from the reaction mixture.
a
and b anomers) after three synthetic steps with a
a/b
a
-fucoside 1 could be isolated in
A recent report36 discussed the protecting-group-free synthesis
of glycosides using the native carbohydrate and acceptors in the
presence of the Appel reagent37,38 (namely PPh3/CBr4). This study
provided high yields of the corresponding glycosides with high ste-
reocontrol and focused mainly on pentoses (e.g., ribose and arabi-
nose) and only mannose was exemplified as hexose. The
application to 6-deoxyhexoses was not reported and this reaction
was therefore applied to fucose using monochlorinated triethyl-
eneglycol (Scheme 1, Path B). The crude reaction mixture was puri-
fied to remove the excess of acceptor and subsequent acetylation
desired
Almost identical selectivity of nearly 2:1 was observed in favor
of the -anomer regardless of the three different glycosylation
a-fucoside 1 in high yield.
a
strategies applied (Table 1). The Fischer glycosylation strategy
(Path A) relies on the formation of the thermodynamic product
based on the anomeric effect favoring the axial substituent at the
anomeric position. Nevertheless, complex mixtures of furanoside
and pyranoside anomers were always obtained and their separa-
tion on silica gel column chromatography was rather troublesome
even using an optimal ternary eluent system.
and azidation provided the desired
a-fucoside 1. The overall yield
obtained for the mixture of and b anomers was 59% after three
a
synthetic steps with a
a/b selectivity of 66:34 and the pure
azido-functionalized fucoside 1 was isolated in 39% yield.39
Benzyl ethers as non-participating groups are generally applied
for the stereoselective 1,2-cis-glycosylation including
a
-fucosyla-
The Appel reagent strategy (Path B) usually provides better
selectivities.38,36 The limited stereoselectivity observed here is
probably due to the primary character of the alcohol acceptor used,
in comparison with the secondary positions of the carbohydrates
used in previous examples.38,36 Nevertheless, the experimental
tion of saccharides. A detailed study of the parameters influencing
the fucosylation of linear alcohols was reported.40 The higher ste-
reoselectivities observed for bulky oligosaccharide acceptors were
attributed to their steric effect, while linear alcohols provided
Table 1
Comparison of the three different synthetic strategies toward a-fucoside 1
Method
Equivalents of acceptor
Number of steps
Ratio
a
/ba
Yieldb
a
+ b (%)
Isolated yield a
pureb (%)
Path A
Path B
Path C
5
5
10
3
3
4
70:30
66:34
71:29
41
59
28
39
76c
30d
a
b
c
Calculated after three or four synthetic steps from the amount of each isolated fraction and 1H NMR data for fractions containing mixture of anomers.
Calculated after three or four synthetic steps from the amount of starting material ( -fucose or donor 2).
Yield obtained for the synthesis of -fucoside 3.
After two column chromatographies (see Supplementary material) with 7% and 23% of pure
L
a
d
a-anomer 3 isolated consecutively.

S. Wang et al. / Carbohydrate Research 395 (2014) 15–18
17
Path D
O
O
Ac2O / C5H5N
OH
HO
O
O
O
2
r.t. / 16 h
2
Me
O
OH
Me
O
PPh3 / CBr4
65°C / 4h
OAc
OH
4
HO
OAc
AcO
L-Fucose
56% / α/β = 72:28 / α pure = 40%
OAc
Ac2O / C5H5N
BF3 OEt2 / CH2Cl2
r.t. / 3 d
46% /
/ = 52:48 / pure = 24%
α β α
99%
DMAP / r.t. / 16 h
Me
O
OAc
O
OAc
HO
O
5
Path E
2
AcO
Scheme 2. Comparative study for the synthesis of the fucosylated alkyne-functionalized triethyleneglycol.
setup was much easier than for the Fischer glycosylation and also
the overall yield of the desired -fucoside 1 was quite high. The
synthetic procedure was then repeated on a 10 g-scale of -fucose
and provided the same yields and selectivity and nearly 10 g of the
desired compound could be isolated.
The thioglycoside donor 2 was next converted in situ into a
mixture of glycosyl bromides that was subjected to copper(II) bro-
mide-promoted glycosylation. Kinetic selection of the b-bromo-
is gratefully acknowledged. Funding for this project was provided
by a grant from la Région Rhône-Alpes (PhD stipend to N.G.) and
the FNRS (FRIA grant to KB). This work was funded by the French
Fond Unique Interministériel (managed by Oseo and DGCIS), the
Conseil Régional Rhône-Alpes, and the Conseil Général des Bouches
du Rhône. The authors also thank Lyonbiopôle and Eurobiomed for
their support. Dr F. Albrieux, C. Duchamp and N. Henriques are
gratefully acknowledged for mass spectrometry analyses.
a
L
anomer being more reactive than its
a-counterpart against SN2
substitution with the acceptor led to the corresponding fucoside
3 in good yield and moderate stereoselectivity (Path C). Neverthe-
less, the commercially available donor 2 is about 4.5 times more
Supplementary data
Supplementary data (Detailed experimental procedures and
NMR spectra for all new compounds) associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.carres.2014.06.002.
expensive than L-fucose and preparation in the laboratory would
require four synthetic steps. Moreover, this strategy requires
10 equiv of triethyleneglycol acceptor instead of 5 equiv for the
Fischer or Appel glycosylations.
The Appel strategy was therefore identified as the most efficient
and reliable methodology for such glycosylation. The alkyne-func-
References
1. Lee, Y. C.; Lee, R. T. Acc. Chem. Res. 1995, 28, 321–327.
2. Chabre, Y. M.; Roy, R. Chem. Soc. Rev. 2013, 42, 4657–4708.
3. Shiao, T. C.; Roy, R. New J. Chem. 2012, 36, 324–339.
4. Bernardi, A.; Jimenez-Barbero, J.; Casnati, A.; De Castro, C.; Darbre, T.; Fieschi,
F.; Finne, J.; Funken, H.; Jaeger, K.-E.; Lahmann, M.; Lindhorst, T. K.; Marradi,
M.; Messner, P.; Molinaro, A.; Murphy, P. V.; Nativi, C.; Oscarson, S.; Penades, S.;
Peri, F.; Pieters, R. J.; Renaudet, O.; Reymond, J.-L.; Richichi, B.; Rojo, J.; Sansone,
F.; Schaffer, C.; Turnbull, W. B.; Velasco-Torrijos, T.; Vidal, S.; Vincent, S.;
Wennekes, T.; Zuilhof, H.; Imberty, A. Chem. Soc. Rev. 2013, 42, 4709–4727.
5. Martinez, A.; Ortiz Mellet, C.; Garcia Fernandez, J. M. Chem. Soc. Rev. 2013, 42,
4746–4773.
6. Sansone, F.; Casnati, A. Chem. Soc. Rev. 2013, 42, 4623–4639.
7. Marradi, M.; Chiodo, F.; Garcia, I.; Penades, S. Chem. Soc. Rev. 2013, 42, 4728–
4745.
8. Marradi, M.; Martin-Lomas, M.; Penades, S. Adv. Carbohydr. Chem. Biochem.
2010, 64, 211–290.
9. Kiessling, L. L.; Grim, J. C. Chem. Soc. Rev. 2013, 42, 4476–4491.
10. Hein, J. E.; Tripp, J. C.; Krasnova, L. B.; Sharpless, K. B.; Fokin, V. V. Angew. Chem.,
Int. Ed. 2009, 48, 8018–8021.
11. Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004–
2021.
12. Meldal, M.; Tornoe, C. W. Chem. Rev. 2008, 108, 2952–3015.
13. Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew. Chem., Int. Ed.
2002, 41, 2596–2599.
14. Tornoe, C. W.; Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057–3064.
15. Dondoni, A. Chem. Asian J. 2007, 2, 700–708.
tionalized
a-fucoside 4 was consequently prepared from 2-[2-[2-
(2-propyn-1-yloxy)ethoxy]ethoxy]ethanol45 (Scheme 2, Path D)
using the optimized reaction conditions and slightly improved
selectivity was observed (a/b 72:28) while the yield of pure a-ano-
mer 4 isolated was similar (40%). The 1-O-acyl methodology is also
usually applied for such syntheses but rather for the preparation of
1,2-trans glycosides. Nevertheless, fucoside 4 was also synthesized
from the peracetate 546,47 using an excess of BF3ÁOEt2 as a pro-
moter (Scheme 2, Path E). A 1:1
a
/b mixture was obtained in 46%
yield and the desired
a-anomer could be isolated in 24% yield.
Once again, the methodology involving PPh3/CBr4 proved superior
to the standard 1-O-acyl method. This ‘clickable’ fucoside 4 will
provide access to conjugations with poly-azido core scaffolds for
the design of multivalent glycoconjugates.
In conclusion, the rapid access to two ‘clickable’
a-fucosides
was investigated and a reliable and reproducible method for their
preparation was identified using PPh3/CBr4-promoted glycosyla-
tion of chloro- (as precursors to azido-) and alkyne-functionalized
triethyleneglycols from L-fucose. This three-step procedure was
applied on a large scale (up to 10 g) and provided similar selectiv-
ities and isolated yields. The rapid access to such building blocks is
now opening the way to additional design of multivalent glycocon-
jugates functionalized with fucose epitopes toward studies of lec-
tin–carbohydrate interactions26,29,31–34,48–50 but also for the
multivalent inhibition of enzymes.51
16. Witczak, Z. J.; Bielski, R. Click Chemistry in Glycoscience. New Developments and
Strategies; Wiley: Weinheim, 2013.
17. Gilboa-Garber, N. Methods Enzymol. 1982, 83, 378–385.
18. Imberty, A.; Wimmerova, M.; Mitchell, E. P.; Gilboa-Garber, N. Microbes Infect.
2004, 6, 222–229.
19. Mishra, N. K.; Kriz, Z.; Wimmerova, M.; Koca, J. Carbohydr. Res. 2010, 345,
1432–1441.
20. Audfray, A.; Claudinon, J.; Abounit, S.; Ruvoën-Clouet, N.; Larson, G.; Smith, D.
F.; Wimmerova, M.; Le Pendu, J.; Römer, W.; Varrot, A.; Imberty, A. J. Biol. Chem.
2012, 287, 4335–4347.
Acknowledgments
21. Geijtenbeek, T. B.; Torensma, R.; van Vliet, S. J.; van Duijnhoven, G. C.; Adema,
G. J.; van Kooyk, Y.; Figdor, C. G. Cell 2000, 100, 575–585.
22. Sanchez-Navarro, M.; Rojo, J. Drug News Perspect. 2010, 23, 557–572.
23. Sharon, N. Biochim. Biophys. Acta 2006, 1760, 527–537.
24. Béha, S.; Giguère, D.; Patnam, R.; Roy, R. Synlett 2006, 2006, 1739–1743.
25. Deguise, I.; Lagnoux, D.; Roy, R. New J. Chem. 2007, 31, 1321–1331.
The authors thank the Université Claude Bernard Lyon 1 and the
CNRS for financial support. Partial financial support was provided
by the Agence Nationale de la Recherche (ANR-12-BSV5-0020).
Financial support from the COST Action CM-1102 MultiGlycoNano

18
S. Wang et al. / Carbohydrate Research 395 (2014) 15–18
26. Nierengarten, J.-F.; Iehl, J.; Oerthel, V.; Holler, M.; Illescas, B. M.; Munoz, A.;
Martin, N.; Rojo, J.; Sanchez-Navarro, M.; Cecioni, S.; Vidal, S.; Buffet, K.; Durka,
M.; Vincent, S. P. Chem. Commun. 2010, 3860–3862.
27. Roy, B.; Mukhopadhyay, B. Tetrahedron Lett. 2007, 48, 3783–3787.
28. Bardonnet, P.-L.; Faivre, V.; Pirot, F.; Boullanger, P.; Falson, F. Biochem. Biophys.
Res. Commun. 2005, 329, 1186–1192.
29. Chevolot, Y.; Bouillon, C.; Vidal, S.; Morvan, F.; Meyer, A.; Cloarec, J.-P.; Jochum,
A.; Praly, J.-P.; Vasseur, J.-J.; Souteyrand, E. Angew. Chem., Int. Ed. 2007, 46,
2398–2402.
30. Faivre, V.; Rosilio, V.; Boullanger, P.; Martins Almeida, L.; Baszkin, A. Chem.
Phys. Lipids 2001, 109, 91–101.
dried (Na2SO4). The solvent was removed by evaporation and the crude
mixture (1.014 g) was then subjected to azidation by adding NaN3 (747 mg,
11.50 mmol, 5 equiv), nBu4NI (170 mg, 0.46 mmol, 0.2 equiv) and DMF
(10 mL). The reaction was stirred at 90 °C for 40 h. The reaction mixture was
then diluted with EtOAc (80 mL), filtered through Celite and washed with
EtOAc (3 Â 20 mL). The filtrate was washed with H2O (40 mL) and the aqueous
layer was extracted with EtOAc (3 Â 20 mL). The solvent of the combined
organic layers were evaporated then co-evaporated with toluene (8 Â 20 mL).
The crude mixture (946 mg) was then purified by silica gel column
chromatography (PE/CHCl3/CH3COCH3, 7:1.2:1.2) to afford the desired
fucoside
1 (798 mg, a/b = 66:34, 59% overall yield after three synthetic
31. Gerland, B.; Goudot, A.; Pourceau, G.; Meyer, A.; Dugas, V.; Cecioni, S.; Vidal, S.;
Souteyrand, E.; Vasseur, J.-J.; Chevolot, Y.; Morvan, F. Bioconjugate Chem. 2012,
23, 1534–1547.
32. Gerland, B.; Goudot, A.; Pourceau, G.; Meyer, A.; Vidal, S.; Souteyrand, E.;
Vasseur, J.-J.; Chevolot, Y.; Morvan, F. J. Org. Chem. 2012, 77, 7620–7626.
33. Ligeour, C.; Audfray, A.; Gillon, E.; Meyer, A.; Galanos, N.; Vidal, S.; Vasseur, J.-
J.; Imberty, A.; Morvan, F. RSC Adv. 2013, 3, 19515–19524.
34. Morvan, F.; Meyer, A.; Jochum, A.; Sabin, C.; Chevolot, Y.; Imberty, A.; Praly, J.-
P.; Vasseur, J.-J.; Souteyrand, E.; Vidal, S. Bioconjugate Chem. 2007, 18, 1637–
1643.
steps). Details for the purification and characterization are provided in the
Supplementary material.
40. Vermeer, H. J.; van Dijk, C. M.; Kamerling, J. P.; Vliegenthart, J. F. G. Eur. J. Org.
Chem. 2001, 193–203.
41. Lemieux, R. U.; Driguez, H. J. Am. Chem. Soc. 1975, 97, 4063–4069.
42. Lemieux, R. U.; Driguez, H. J. Am. Chem. Soc. 1975, 97, 4069–4075.
43. Lemieux, R. U.; Hendriks, K. B.; Stick, R. V.; James, K. J. Am. Chem. Soc. 1975, 97,
4056–4062.
44. Borbas, A.; Szabovik, G.; Antal, Z.; Feher, K.; Csavas, M.; Szilagyi, L.; Herczegh,
P.; Liptak, A. Tetrahedron: Asymmetry 2000, 11, 549–566.
45. Lu, G.; Lam, S.; Burgess, K. Chem. Commun. 2006, 1652–1654.
46. Sardzik, R.; Noble, G. T.; Weissenborn, M. J.; Martin, A.; Webb, S. J.; Flitsch, S. L.;
Beilstein J. Org. Chem. 2010, 6, 699–703.
35. Sanki, A. K.; Mahal, L. K. Synlett 2006, 455–459.
36. Schmalisch, S.; Mahrwald, R. Org. Lett. 2013, 15, 5854–5857.
37. Appel, R. Angew. Chem., Int. Ed. 1975, 14, 801–811.
38. Nishida, Y.; Shingu, Y.; Dohi, H.; Kobayashi, K. Org. Lett. 2003, 5, 2377–2380.
47. Wolfrom, M. L.; Orsino, J. A. J. Am. Chem. Soc. 1934, 56, 985–987.
48. Chevolot, Y.; Zhang, J.; Meyer, A.; Goudot, A.; Rouanet, S.; Vidal, S.; Pourceau,
G.; Cloarec, J.-P.; Praly, J.-P.; Souteyrand, E.; Vasseur, J.-J.; Morvan, F. Chem.
Commun. 2011, 8826–8828.
49. Durka, M.; Buffet, K.; Iehl, J.; Holler, M.; Nierengarten, J.-F.; Taganna, J.;
Bouckaert, J.; Vincent, S. P. Chem. Commun. 2011, 1321–1323.
50. Zhang, J.; Pourceau, G.; Meyer, A.; Vidal, S.; Praly, J.-P.; Souteyrand, E.; Vasseur,
J.-J.; Morvan, F.; Chevolot, Y. Chem. Commun. 2009, 6795–6797.
51. Durka, M.; Buffet, K.; Iehl, J.; Holler, M.; Nierengarten, J.-F.; Vincent, S. P. Chem.
Eur. J. 2012, 18, 641–651.
39. Appel glycosylation methodology (Path B):
L-Fucose (500 mg, 3.04 mmol,
1 equiv), 2-[2-(2-chloroethoxy)ethoxy]ethanol (2.2 mL, 15.2 mmol, 5 equiv),
PPh3 (79 mg, 0.3 mmol, 0.1 equiv) and CBr4 (100 mg, 0.3 mmol, 0.1 equiv) were
placed in a 10-mL round-bottom flask. The mixture was heated at 65 °C for 4 h.
The solvent-free reaction was then cooled to rt and loaded on a short silica gel
column to remove the excess of acceptor (EtOAc, then 10% MeOH in EtOAc).
The crude fucoside (729 mg) was then subjected to acetylation with Ac2O
(4.5 mL) and pyridine (9 mL). The reaction was stirred at rt overnight then
diluted with EtOAc (80 mL). The organic layer was washed with H2O (20 mL),
1 N HCl (20 mL), satd NaHCO3 (20 mL) and brine (20 mL). The organic layer was