O-Glycosyl trichloroacetimidates bearing Fmoc as temporary hydroxy protecting group: a new access to solid-phase oligosaccharide synthesis.

Article abstract of DOI:10.1021/ol006081l

Different O-glycosyl trichloroacetimidates bearing base sensitive Fmoc protected hydroxy groups were efficiently prepared with CCl(3)CN using a catalytic amount of sodium hydride. The resulting glycosyl donors were engaged in glycosylation reactions both

Full text of DOI:10.1021/ol006081l

ORGANIC
LETTERS
2000
Vol. 2, No. 20
3043-3046
O-Glycosyl Trichloroacetimidates
Bearing Fmoc as Temporary Hydroxy
Protecting Group: A New Access to
Solid-Phase Oligosaccharide Synthesis
Fabien Roussel, Laurent Knerr,† Matthias Grathwohl, and Richard R. Schmidt*
Fakulta¨t fu¨r Chemie, UniVersita¨t Konstanz, Fach M 725, D-78457 Konstanz, Germany
richard.schmidt@uni-konstanz.de
Received May 19, 2000
ABSTRACT
Different O-glycosyl trichloroacetimidates bearing base sensitive Fmoc protected hydroxy groups were efficiently prepared with CCl3CN using
a catalytic amount of sodium hydride. The resulting glycosyl donors were engaged in glycosylation reactions both in solution and on solid
support with a new ester-type linker with good results. In both approaches, Fmoc groups were afterward quantitatively cleaved using mild
basic conditions.
The synthesis of complex oligosaccharides requires extremely
elaborate protecting group strategies. Therefore the study of
new hydroxy and amino protecting groups is of great interest
in this field. Particularly, the synthesis of oligosaccharides
on polymer supports1 requires protecting groups that are
cleaved under mild conditions and are compatible with the
linker system. In the course of our work on the use of
O-glycosyl trichloroacetimidates2 for the synthesis of oli-
gosaccharides on solid support, we became interested in the
elaboration of a new temporary hydroxy protecting group
that is cleaved under mild basic conditions. Indeed, acetyl
is the most popular nucleophile labile temporary protecting
group applied in solid-phase oligosaccharide synthesis.
However, its cleavage on solid support requires rather strong
alkaline conditions precluding the use of simple ester-type
linker systems, which have already proven useful in the
field.3 Thus we decided to initiate a study of base labile
hydroxy protecting groups that are compatible with ester-
type linkers.
Among the possible protecting groups envisaged, the
9-fluorenylmethyloxycarbonyl (Fmoc) group appeared par-
ticularly attractive. Although very popular as an amino
protecting group in peptide solid-phase synthesis, we were
surprised to find only a few reports on its use as a hydroxy
protecting group4 in carbohydrate chemistry, whether in
solution5 or on solid phase.6 Moreover, no application of
Fmoc as hydroxy protecting group in oligosaccharide
synthesis using O-glycosyl trichloroacetimidates7 as glycosyl
donors was reported. We describe here the syntheses of
† Present adress: UCB Pharma S.A., Chemin du Foriest 1420 Braine
l’Alleud, Belgium.
(1) For a recent review see: Osborn, H. M. I.; Khan, T. H. Tetrahedron
1999, 55, 1807.
(2) (a) Knerr, L.; Schmidt, R. R. Synlett 1999, 1802. (b) Knerr, L.;
Schmidt, R. R. Eur. J. Org. Chem. 2000, 2803. (c) Rademann, J.; Geyer,
A.; Schmidt, R. R. Angew. Chem. 1998, 110, 1309; Angew. Chem., Int.
Ed. Engl. 1998, 37, 1241. (d) Rademann, J.; Schmidt, R. R. J. Org. Chem.
1997, 62, 3650. (e) Heckel, A.; Mross, E.; Jung, K. H.; Rademann, J.;
Schmidt, R. R. Synlett 1998, 171.
(3) (a) Shimizu, H.; Ito, Y.; Kanie, O.; Ogawa, T. Bioorg. Med. Chem.
Lett. 1996, 6, 2841. (b) Manabe, S.; Ito, Y.; Ogawa, T. Synlett 1998, 628.
(4) Gioeli, C.; Chattopadhyaya, J. B. J. Chem. Soc., Chem Commun.
1982, 672.
(5) Freese, S. J.; Vann, W. F. Carbohydr. Res. 1996, 281, 313.
(6) (a) Nicolaou, K. C.; Winssinger, N.; Pastor, J.; DeRoose, F. J. Am.
Chem. Soc. 1997, 119, 449. (b) Nicolaou, K. C.; Watanabe, N.; Li, J.; Pastor,
J.; Winssinger, N. Angew. Chem., Int. Ed. Engl. 1998, 37, 1559. (c) Zhu,
T.; Boons, G.-J. Tetrahedron: Asymmetry 2000, 11, 199.
10.1021/ol006081l CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/06/2000

O-glycosyl trichloroacetimidates donors 1a-g containing
Fmoc protection and their use as glycosyl donors in solution
and on solid support.
glycosyl trichloroacetimidate 1b11 in 97% yield after filtration
on a small pad of silica.12 The reaction proved to be very
fast and clean. The same procedure applied to a similar
building block, namely, N-DMM-protected 4a, confirmed the
efficiency of this method. â-O-Glycosyl trichloroacetimidates
1a11 and 1b are of great interest for the construction of
carbohydrates of the lacto-series of glycosphingolipids and
of N-glycans.
These good results prompted us to investigate the compat-
ibility of our methodology with other protective groups. At
first, the synthesis of glucosamine derivatives 1c11 and 1e11
was investigated (Scheme 2). These compounds were
The main difficulty associated with the preparation of these
O-glycosyl trichloroacetimidates was to find conditions
compatible with the presence of this base sensitive group
all along the synthesis. Most of all, the last step of
introduction of the trichloroacetimidate moiety had to be
performed in basic conditions7b and seemed to be problem-
atic. However, the synthetic sequence described in Scheme
1 gave excellent results in all cases studied until now. For
Scheme 1a
Scheme 2a
a (a) FmocCl, DMAP, pyridine. (b) Excess HF‚pyridine, THF,
rt. (c) 0.1 equiv NaH, (1a) CH2Cl2/CCl3CN (1/1), (1b) CCl3CN,
rt.
all the building blocks prepared, we have chosen to use the
thexyldimethylsilyl group (TDS) to protect the anomeric
position. Thus the O-TDS-protected compounds (2a-g)8
were prepared. In all cases, the Fmoc group was introduced
using FmocCl with a catalytic amount of DMAP in pyridine
at room temperature in very good yields. To optimize the
desilylation step, N-DMM-protected9 glucosamine derivative
3a and the N-phthalimido derivative 3b were chosen as
models. The best results were obtained using an excess of
HF‚pyridine complex in THF at room temperature10 (92%).
Among the methods available to perform the formation of
the trichloroacetimidate function,7b the use of a catalytic
amount of NaH as the base seemed particularly attractive to
avoid the cleavage of the Fmoc group via â elimination.
Thus, in the presence of less than 0.1 equiv of NaH in
trichloroacetonitrile, alcohol 4b was converted into â-O-
a (a) FmocCl, DMAP, pyridine. (b) NaBH3CN, HCl‚Et2O, THF.
(c) Ac2O, pyridine. (d) Excess HF‚pyridine, THF, rt. (e) 0.1 equiv
NaH, CH2Cl2/CCl3CN (1/1), rt. (f) 2.5 equiv BzCl, pyridine.
synthesized using 2c as starting material. After linkage of
the Fmoc group to the O3 position (93% yield), the
benzylidene acetal function was regioselectively opened
using NaBH3CN and a solution of HCl‚Et2O13 to afford 3c
(11) NMR selected data for compounds 1a-g: 1H NMR (600 MHz,
CDCl3) δ 1a 6.23 (d, J1,2 ) 8.8 Hz, 1-Hâ); 1b 6.41 (d, J1,2 ) 8.4 Hz,
1-Hâ); 1c 6.38 (d, J1,2 ) 8.7 Hz, 1-Hâ); 1d 6.36 (d, J1,2 ) 8.7 Hz, 1-Hâ);
1e 6.54 (d, J1,2 ) 8.9 Hz, 1-Hâ); 1f 6.85 (d, J1,2 ) 3.4 Hz, 1-HR); 1g 6.40
(d, J1,2 ) 3.5 Hz, 1-HR).
(12) Typical Procedure. Compound 4b (115 mg, 0.16 mmol) was
dissolved in CCl3CN (3 mL) under Ar. Sodium hydride (0.1 equiv, 0.5
mg) was added to the solution, and after 10 min of stirring, a TLC analysis
showed a complete consumption of the starting material. The solution was
adsorbed on silica, and the solvents were evaporated in vacuo. The residue
was purified by flash chromatography (very small amount of silica) to afford
imidate 1b in a 97% yield.
(7) (a) Schmidt, R. R. Angew. Chem., Int. Ed. Engl. 1986, 25, 212. (b)
Schmidt, R. R.; Kinzy, W. AdV. Carbohydr. Chem. Biochem. 1994, 50, 21.
(8) (a) For 2a and 2c see: Chiesa, V.; Schmidt, R. R. Eur. J. Org. Chem.,
accepted for publication. (b) For 2b see: Grathwohl, M. Diplomarbeit,
Universita¨t Konstanz, Germany, 1997. (c) For 2f see: Peter, J. Diplomarbeit,
Universita¨t Konstanz, Germany, 1994. (d) For 2g see: Knerr, L.; Roussel,
F.; Schmidt, R. R., unpublished results.
(9) Aly, M. R. E.; Castro-Palomino, J. C.; Ibrahim, E. I.; El-Ashry, E.
H.; Schmidt, R. R. Eur. J. Org. Chem. 1998, 2305.
(10) Trost, B. M.; Caldwell, C. G.; Murayama, E.; Heissler, D. J. Org.
Chem. 1983, 48, 3252.
(13) Garegg, P. J.; Hultberg, H.; Wallin, S. Carbohydr. Res. 1982, 108,
97.
3044
Org. Lett., Vol. 2, No. 20, 2000

in 91% yield. At first, we have introduced an acetyl group
in the O4 position using acetic anhydride in pyridine to give
compound 4c in a yield of 83%. During the desilylation step
with HF‚pyridine, partial loss of the acetyl group was
observed. Thus, an inseparable mixture of compounds 5c
and 6c (61/39) was obtained in 83% yield. The imidate
moiety was introduced in this mixture of sugars to afford
â-O-glycosyl trichloroacetimidate 1c and 1d,11 separable by
flash chromatography, with 90% yield. It is worthwhile to
note that no trace of trichloroacetimidate function at O4
position was observed. To improve the resistance of the
protective group at O4 position to the acidic conditions of
desilylation, a benzoyl group was attached instead of an
acetyl group. At first, we tried to introduce this ester group
on compound 3c by reaction with benzoic anhydride in
pyridine. Unfortunately, the major compound isolated had
after the loss of the Fmoc group benzoyl groups at O3 and
O4 positions. The transformation of 3c into 4d was optimized
by using benzoyl chloride in pyridine (best conditions, 2.5
equiv BzCl, pyridine, 12 h) to afford compound 4d with a
yield of 74%. Only 13% of the dibenzoylated compound was
obtained. After desilylation (4d f 5d, 83% yield) and
formation of the trichloroacetimidate function (92% yield)
under the previous conditions, only glycosyl donor of â
configuration 1e was produced. Glycosyl donors 1c and 1e
are very interesting for the synthesis of oligosaccharides of
the lactoneo-glycosphingolipid family.
compound 4f in 95% yield. In this case, no loss of the Fmoc
group was observed. The sensitivity of the benzylidene group
to acidic conditions was circumvented by performing the
desilylation in pyridine instead of THF at low temperature
(95% yield). Treatment with trichloroacetonitrile according
to the previous conditions gave R-O-galactosyl trichloro-
acetimidate 1f11 (containing only trace amounts of â-anomer)
in 86% yield.
This general methodology was also applied to lactose
derivative 2g to afford in three steps an anomeric mixture
(1:1) of trichloroacetimidate derivative 1g11 in 72% yield
over three steps (Scheme 4). To show the usefulness of
Scheme 4. Glycosylation in Solutiona
The same strategy was also applied to the galactosyl
derivative 2f bearing two hydroxy functions in C2 and C3
positions (Scheme 3). An interesting regioselective introduc-
a (a) FmocCl, DMAP, pyridine. (b) Excess HF‚pyridine, THF,
rt. (c) 0.1 equiv NaH, CH2Cl2/CCl3CN (1/1), rt. (d) HO(CH2)8CO2Me,
TMSOTf, CH2Cl2.
Scheme 3a
glycosyl donor 1g as building block, it was employed in a
glycosylation reaction. Thus, coupling with the Lemieux
spacer14 (methyl 9-hydroxynonanoate) in dichloromethane
with TMSOTf as catalyst was carried out. In a “one-pot”
procedure, triethylamine was added to afford after workup
derivative 3g as an anomeric mixture (1:1) in 92% yield.
Glycosyl donor 1g was also employed in a glycosylation
reaction on solid phase (Scheme 5). For this endeavor a new
ester-type linker was selected. To this end acid chloride
support 7, derived from a commercial carboxypolystyrene
resin (100-200 mesh, 1% cross-linked, 2 mmol/g)15 was
reacted with alcohol 8 (0.29 equiv) in pyridine at room
temperature followed by capping with anhydrous methanol.
Trityl groups were removed by treatment with trifluoroacetic
acid allowing the calculation of the loading by colorimetric
assay (0.53 mmol/g), thus affording linker-loaded resin 9.
Reaction of 9 with lactosyl donor 1g (3 equiv) upon
activation with TMSOTf (0.3 equiv) in dichloromethane gave
the corresponding glycosylated resin. This glycosylation step
a (a) FmocCl, DMAP, pyridine/CH3CN. (b) BzCl, CH2Cl2/
pyridine (2/1). (c) Excess HF‚pyridine, pyridine, 0 °C. (d) 0.1 equiv
NaH, CH2Cl2/CCl3CN (1/1), 0 °C.
tion of the Fmoc group in the more reactive O3 position of
the galactoside was observed to afford 3f in 67% yield (95%
of conversion). A first attempt to install a benzoyl group at
O2 position with benzoyl cyanide in acetonitrile with a
catalytic amount of DMAP resulted in the loss of the Fmoc
group. As previously, best conditions were the use of benzoyl
chloride in a mixture of CH2Cl2/pyridine (2:1) to give
(14) Lemieux, R. U.; Bundle, D. R.; Baker, D. A. J. Am. Chem. Soc.
1975, 97, 4076.
(15) Panek, S. P.; Zhu, B. Tetrahedron Lett. 1996, 37, 8151.
Org. Lett., Vol. 2, No. 20, 2000
3045

meric linkage in resin 3h. After analytical cleavage, not even
trace amounts of the preceding disaccharide were observed.
Cleavage from the solid support was carried out by using
basic conditions (5 equiv MeONa, CH2Cl2/MeOH 9:1, 10
h, rt) to give, after purification by flash chromatography, an
anomeric mixture (â/R, 6:4) of deacetylated trisaccharide 4h17
in 69% overall yield from 9 (average yield of 91% per step
over four steps).
Scheme 5. Application on Solid Phasea
In summary, we have synthesized O-glycosyl trichloro-
acetimidates 1a-g bearing Fmoc protected hydroxy groups
in very good yields. Their application, for instance, of
building block 1g, to construct oligosaccharides in solution
and on solid phase is very promising. For solid-phase
oligosaccharide synthesis a novel ester-type linker was
developed that allowed the construction of a trisaccharide
in very good yield. Thus, the Fmoc group, which exhibits
little sensitivity to acidic conditions, proved to be highly
suitable for oligosaccharide synthesis on solid phase with
ester-type linkers. We are currently developing the use of
Fmoc-bearing building blocks to the synthesis of more
complex oligosaccharides on solid phase.
Acknowledgment. We thank the European Community
(grant FAIR-CT97-3142), the Bundesministerium fu¨r Bil-
dung und Forschung (grant 0311 229), and the Deutsche
Forschungsgemeinschaft for financial support of this work.
We are grateful to Dr. A. Geyer for his help in the structural
assignments and to C. Gege for the gift of a compound.
a (a) TrCl, DMAP, pyridine, rt. (b) pyridine, rt, 30 h then MeOH,
16 h. (c) 5% TFA, CH2Cl2/MeOH 9:1. (d) TMSOTf, CH2Cl2, rt.
(e) CH2Cl2/NEt3 (4/1), rt. (f) TMSOTf, CH2Cl2/dioxane 1:1, -25
°C. (g) MeONa, CH2Cl2/MeOH 9:1, rt.
OL006081L
was carried out twice to ensure a maximum functionalization.
This step, as the next steps, were monitored by TLC and
MALDI-TOF analysis of the cleavage product (MeONa,
CH2Cl2/MeOH) from a small amount of resin (2 mg). The
Fmoc group was easily removed by treatment with a
dichloromethane/triethylamine mixture (4/1) at room tem-
perature to afford resin 2h. The disaccharide moiety was
elongated with known fucosyl donor 1016 using the previ-
ously described experimental conditions2d to obtain R-ano-
(16) (a) Windmu¨ller, R. Diplomarbeit, Universita¨t Konstanz, Germany,
1991. (b) Windmu¨ller, R.; Schmidt, R. R. Tetrahedron Lett. 1994, 35, 7927.
(17) Physical Properties of Compound 4h. MALDI-TOF (DHB/THF)
calcd M(C75H90O16) ) 1247.54 m/z, (M + Na)+ ) 1270.54 m/z, found
1270.4 m/z. 4hâ (selected data): 1H NMR (600 MHz, CDCl3) δ 1.26 (3H,
CH3), 3.10 (5a-H), 3.29 (2a-H), 3.38 (5b-H), 3.39 (3a-H), 3.46 (6a-H), 3.53
(2c-H), 3.64 (6a-H′), 3.66 (4c-H), 3.70 (4b-H), 3.71 (3b-H), 3.73 (2b-H),
3.83 (3c-H), 3.89 (5c-H), 3.90 (4a-H), 4.20 (1a-H), 4.36 (1b-H), 5.44-
5.45 (1c-H). 4hr (selected data): 1H NMR (600 MHz, CDCl3) δ 3.34 (5b-
H), 3.37 (2a-H), 3.42 (5a-H), 3.62 (3b-H), 3.70 (4b-H), 3.71 (2b-H), 3.86
(4a-H), 4.20 (1b-H), 4.60 (1a-H), 5.44-5.45 (1c-H).
3046
Org. Lett., Vol. 2, No. 20, 2000