A new phenoxyacetate-based linker system for the solid-phase synthesis of oligosaccharides.

Article abstract of DOI:10.1021/ol007062e

[structure: see text]. A novel linker system has been designed, and its first application to solid-phase oligosaccharide synthesis is described. The use of the highly reactive o-nitro-phenoxyacetate linker allows a fast and quantitative cleavage using mild basic conditions. This method combined with the trichloroacetimidate glycosylation exhibits highly promising results as demonstrated for the synthesis of tetrasaccharide 1 (n = 3) containing glucose beta(1 --> 4) and beta(1 --> 6) linkages.

Full text of DOI:10.1021/ol007062e

ORGANIC
LETTERS
2001
Vol. 3, No. 5
747-750
A New Phenoxyacetate-Based Linker
System for the Solid-Phase Synthesis of
Oligosaccharides
Xiangyang Wu, Matthias Grathwohl, and Richard R. Schmidt*
Fachbereich Chemie, UniVersita¨t Konstanz, Fach M 725, D-78457 Konstanz, Germany
richard.schmidt@uni-konstanz.de
Received December 28, 2000
ABSTRACT
A novel linker system has been designed, and its first application to solid-phase oligosaccharide synthesis is described. The use of the highly
reactive o-nitro-phenoxyacetate linker allows a fast and quantitative cleavage using mild basic conditions. This method combined with the
trichloroacetimidate glycosylation exhibits highly promising results as demonstrated for the synthesis of tetrasaccharide 1 (n ) 3) containing
glucose â(1 f 4) and â(1 f 6) linkages.
The solid-phase synthesis of oligosaccharides has gained
considerable interest during the last years1 as a result of its
possible advantages over conventional technologies, and
many different linker systems have been developed.2 To
employ the powerful trichloroacetimidate glycosylation
method,3 temporary protective groups and the linker moiety
have to be designed to be stable in the presence of catalytic
amounts of Lewis acid and to permit orthogonal cleavage
under mild conditions. Most of the commonly used linker
types in synthetic organic chemistry or peptide chemistry4
do not fulfill this demand. Therefore, the design of advanced
protective groups and new efficient linker systems for the
solid-phase synthesis of oligosaccharides remains a chal-
lenging task.
We present here our results concerning the preparation
and first application of a new o-nitro-substituted phenoxy-
acetate linker system on Merrifield resin. Since it is known
from solution-phase chemistry that an unsubstituted phe-
noxyacetate protecting group can be easily cleaved using
methylamine, we chose the o-nitro-substituted derivative,
which as a result of its electron-withdrawing effect should
permit a particularly short reaction time for cleavage from
the solid support.5 The system proved to meet all the
requirements given above, leading to target tetrasaccharide
molecule 1 in an excellent yield (Scheme 1). To the best of
our knowledge the structural motif 1 was until now only
(1) For some recent reviews, see: (a) Haase, W. C.; Seeberger, P. H.
Curr. Org. Chem. 2000, 4, 481-511. (b) Osborn, H. M. I.; Khan, T. H.
Tetrahedron 1999, 55, 1807-1850.
(2) (a) Ester type linker: Roussel, F.; Knerr, L.; Grathwohl, M.; Schmidt,
R. R. Org. Lett. 2000, 2, 3043-3046. Zhu, T.; Boons, G. J. J. Am. Chem.
Soc. 2000, 122, 10222-10223. (b) Silyl ether type linker: Doi, T.; Sugiki,
M.; Yamada, H.; Takahashi, T.; Porco, J. A., Jr. Tetrahedron Lett. 1999,
40, 2141-2144. Danishefsky, S. J.; McClure, K. F.; Randolph, J. T.;
Ruggeri, R. B. Science 1993, 260, 1307-1309. (c) Thioether type linker:
Rademann, J.; Geyer, A.; Schmidt, R. R. Angew. Chem. 1998, 110, 1309-
1313; Angew. Chem., Int. Ed. 1998, 37, 1241-1245. Yan, L.; Taylor, C.
M.; Goodnow, R., Jr.; Kahne, D. J. Am. Chem. Soc. 1994, 116, 6953-
6954. (d) Benzyl ether type linker: Mehta, S.; Whitfield, D. M. Tetrahedron
Lett. 1998, 39, 5907-5910. (e) Sulfone linker: Hunt, J. A.; Roush, W. R.
J. Am. Chem. Soc. 1996, 118, 9998-9999. (f) Wang type linker: Manabe,
S.; Ito, Y.; Ogawa, T. Synlett 1998, 628-630. (g) Metathesis cleavable
linker: Knerr, L.; Schmidt, R. R. Eur. J. Org. Chem. 2000, 2803-2808.
Andrade, R. B.; Plante, O. J.; Melean, L.; Seeberger, P. H. Org. Lett. 1999,
1, 1811-1814. (h) Selenium based linker: Nicolaou, K. C.; Mitchell, H.
J.; Fylaktakidou, K. C.; Suzuki, H.; Rodr´ıguez, R. M. Angew. Chem. 2000,
112, 1131-1135; Angew. Chem., Int. Ed. 2000, 39, 1089-1093.
(3) (a) Schmidt, R. R. Angew. Chem. 1986, 98, 213-236; Angew. Chem.,
Int. Ed. Engl. 1986, 25, 212-235. (b) Schmidt, R. R.; Kinzy, W. AdV.
Carbohydr. Chem. Biochem. 1994, 50, 21-123.
(4) For a recent review on linkers, see: James, I. W. Tetrahedron 1999,
55, 4855-4946.
(5) For the use of this type of system in a reverse manner in
oligosaccharide synthesis on Wang resin, see: Manabe, S.; Nakahara, Y.;
Ito, Y. Synlett 2000, 9, 1241-1244.
(6) Kono, H.; Kawano, S.; Erata, T.; Takai, M. J. Carbohydr. Chem.
2000, 19, 127-140.
10.1021/ol007062e CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/15/2001

conditions into the corresponding primary alcohol 5 by a
highly regioselective p-methoxy-benzylidene ring opening
(Scheme 2).
Scheme 1
Scheme 2a
synthesized by an enzymatic approach under regioselective
transglycosylation conditions in low yield.6
a (a) 0.1 equiv Bu2BOTf, BH3‚THF, CH2Cl2, 0 °C. (b) DCC,
DMAP, CH2Cl2, rt. (c) DDQ, CH2Cl2/H2O, rt. (d) 2 equiv FmocCl,
pyridine, rt. (e) HF‚pyridine, THF, rt.
Retrosynthetic analysis of 1 led to the polymer-bound
linker 3 and to the 9-fluorenylmethyloxycarbonyl (Fmoc)
group bearing O-glucosyl trichloroacetimidate 2.7 Recently,
we reported the efficient preparation of O-glycosyl trichlo-
roacetimidates bearing Fmoc-protected hydroxy groups and
their use as glycosyl donors both in solution and on solid
phase.8 Other types of Fmoc-containing glycosyl donors have
been recently also reported.9,10 Resin 3 led to the required
building blocks 4-6 as well as to the activated trichloro-
acetimidate Merrifield resin 7,11 which was synthesized
starting from commercially available hydroxymethyl-sub-
stituted Merrifield resin 8. We describe here the first
application of polymer reagent 7 in an acid-catalyzed benzyl
ether synthesis. Compound 7 and all derivatives thereof
exhibited higher stability against acids, as required for the
solid-phase oligosaccharide synthesis, than the corresponding
Wang resin.12
Starting from commercially available 4-hydroxy-3-nitro-
benzaldehyde 10 the o-nitro-substituted phenoxyacetyl linker
molecule 6 was prepared following a four-step literature
procedure.5 Ester formation between primary alcohol 5 and
carboxylic acid 6 was achieved by treatment with a catalytic
amount of DMAP (0.1 equiv) and with DCC (2.0 equiv) as
condensing agen, affording fully protected 11. Cleavage of
the MPM ether moiety of 11 was performed in the presence
of DDQ (1.5 equiv) in a mixture of CH2Cl2/H2O to obtain
12 in an excellent yield. Because of its high stability toward
Lewis acid as well as its convenient cleavage under weak
basic conditions, which can be readily followed by UV, for
temporary protection the Fmoc group was employed. In our
The synthesis of 1 begins with suitably protected known
precursor 9,13 which is converted under reductive reaction
(12) For Wang trichloroacetimidate resin, see: Hanessian, S.; Xie, F.
Tetrahedron Lett. 1998, 39, 733-736.
(7) Wu, X.; Schmidt, R. R. Unpublished results.
(8) Roussel, F.; Knerr, L.; Grathwohl, M.; Schmidt, R. R. Org. Lett.
2000, 2, 3043-3046.
(9) For Fmoc-containing phosphoramidites, see: Freese, S. J.; Vann, W.
F. Carbohydr. Res. 1996, 281, 313-319.
(13) Joniak, K. Chem. ZVesti 1974, 110-112.
(14) Trost, B. M.; Caldwell, C. G.; Murayama, E.; Heissler, D. J. Org.
Chem. 1983, 48, 3252-3265.
(15) Preparation of Trichloroacetimidate Resin 7. After swelling of
dry Merrifield resin 8 (10.0 g, 14.6 mmol) in CH2Cl2 (100 mL),
trichloroacetonitrile (15 mL, 10 equiv) was added. The resulting suspension
was cooled under argon to 0 °C and shaken for 10 min. DBU (0.44 mL,
0.2 equiv) was added, and the resulting mixture was shaken for 40 min
under an inert gas atmosphere at 0 °C. The resin was rinsed off, switching
three times between CH2Cl2 and THF (each 100 mL), and dried under high
vacuum to afford resin 7 (12.9 g, 14.6 mmol, quant).
(10) For Fmoc-containing thioglycoside donors, see: (a) Nicolaou, K.
C.; Winssinger, N.; Pastor, J.; DeRoose, F. J. Am. Chem. Soc. 1997, 119,
449-450. (b) Nicolaou, K. C.; Watanabe, N.; Li, J.; Pastor, J.; Winssinger,
N. Angew. Chem. 1998, 110, 1636-1638; Angew. Chem., Int. Ed. 1998,
37, 1559-1561. (c) Zhu, T.; Boons, G.-J. Tetrahedron: Asymm. 2000, 11,
199-205.
(11) Grathwohl, M. Dissertation, Universita¨t Konstanz, Germany, 2001.
748
Org. Lett., Vol. 3, No. 5, 2001

hands this group has emerged as a very suitable protecting
group for both solution- and solid-phase synthesis.8 Instal-
lation of the Fmoc group to 12 using pyridine and FmocCl
(f 13) was followed by desilylation using 5 equiv of HF‚
py in THF14 to afford linker 4, ready for loading onto the
resin. Attachment of linker 4 to the hydroxymethyl-
substituted Merrifield resin 8 was achieved via formation of
trichloroacetimidate resin 7 and its activation with a catalytic
amount of TMSOTf (0.3 equiv) at 0 °C (Scheme 3).
Scheme 4a
Scheme 3a
a (a) (i) MeNH2, CH2Cl2, rt. (ii) Ac2O/pyridine, rt. (b) NEt3,
CH2Cl2, rt. (c) 0.3 equiv TMSOTf, 4 Å, CH2Cl2, 0 °C.
lamine until the generated UV spot completely disappeared.17
Solid-phase glycosylation with the Fmoc-bearing O-glucosyl
trichloroacetimidate 2 (3.0 equiv) at 0 °C and repetition of
the described sequence in a cyclic manner afforded resin-
bound oligosaccharides 16 (n ) 1-3) and 17 (n ) 1, 2).18
All glycosylations were performed only once with the
exception of the first one, which required repetition in order
to reach a complete â(1 f 4) linkage.19
The described solid-phase synthesis cycle was performed
three times. After each run preparative cleavage of the
polymer-bound structures 16 (n ) 1-3) using the conditions
described above16 furnished the corresponding oligosaccha-
rides 18, 19, and 1 in excellent yields (Scheme 5). Cellobiose
derivative 1820 was isolated in 85% overall yield over four
steps starting from 14 (96% per step). Up to the tetrameric
stage no significant drop in terms of efficiency of this new
a (a) 10 equiv CCl3CN, CH2Cl2, 0 °C. (b) (i) 0.3 equiv TMSOTf,
CH2Cl2, 0 °C. (ii) MeOH.
It is to be noted that conversion of 8 to 7 proceeds
smoothly and very quickly in a quantitative manner. The
reaction can be readily performed up to a 10-g scale, and
the resulting activated trichloroacetimidate polymer reagent
7 showed a high stability and was storable for at least 6
months.15 After linker attachment affording resin 14 the
remaining activated benzyl functions were quenched by direct
addition of methanol. Loading of resin 14 was determined
by a very fast and clean preparative cleavage of 15 by excess
MeNH2 (10 min) followed by acetylation to be 0.213 mmol/g
(Scheme 4).16 Under these conditions the linker was selec-
tively cleaved without affecting any O-acetyl groups; loading
was also determined by recycling unreacted linker molecule
4 from the washing solution.
(19) Solid-phase glycosylation to resin bound disaccharide 16 (n ) 1)
was repeated once under the same conditions.
(20) Analytical Data of Compound 18. MALDI-TOF (DHB/THF): m/z
calcd M (C32H42O17) 698.67; (M + Na)+ 721.66; (M + K)+ 737.77; found
721.9, 737.9. 1H NMR (600 MHz, CDCl3): δ 2.06 (m, 6 COCH3), 3.36 (s,
Generation of polymer-bound acceptor 3 was performed
by treatment with a mixture of dichloromethane and triethy-
3
3
(16) General Procedure for Cleavage. Dry resin was swollen in CH2-
Cl2 (10 mL/g resin), and the resulting suspension was shaken for 10 min
under an inert gas atmosphere. A solution of ∼8 M MeNH2 in EtOH (∼100
equiv) was added, and the resulting mixture was shaken for 10 min under
argon. The resin was rinsed off, switching three times between CH2Cl2
and THF. The evaporated filtrates were treated by a mixture of acetic
anhydride and pyridine (1:1, 20 mL/g) for 2 h. The resulting residues were
purified by flash chromatography.
(17) General Procedure for Deprotection. Dry resin was swollen in a
mixture of CH2Cl2/NEt3 (4:1), and the resulting suspension was shaken for
4 h. The resin was treated as described above and dried under high vacuum.
(18) General Procedure for Glycosylation. Dry acceptor loaded resin
was directly swollen in a CH2Cl2 solution (15 mL/g resin) containing donor
2 (3 equiv) and 4 Å molecular sieves. The resulting suspension was cooled
under argon to 0 °C and shaken for 10 min. A solution of a freshly prepared
0.5 M TMSOTf solution in CH2Cl2 (0.3 equiv) was added, and the resulting
mixture was shaken for 1 h under an inert gas atmosphere. The resin was
treated as described above.
OCH3), 3.55 (m, 5b-H), 3.65 (t, J ) 3.3 Hz, 4b-H), 3.71 (t, J ) 3.2 Hz,
4a-H), 3.87 (m, 5a-H), 4.13 (dd, J6,6′ ) 12.0 Hz, J5,6 ) 4.8 Hz, 6a-H), 4.27
3
3
(2d, J ) 3.7 Hz, 6′b-H, 6b-H), 4.49 (d, J1,2 ) 7.9 Hz, 1b-H), 4.50 (d, J
) 3.0 Hz, 6′a-H), 4.53-4.59 (m, CH2Ph), 4.80 (dd, J1,2 ) 3.7 Hz, J2,3
10.2 Hz, 2a-H), 4.84 (m, 2b-H), 4.85 (d, J1,2 ) 3.7 Hz, 1a-H), 5.19 (t, J
) 9.2 Hz, 3b-H), 5.43 (t, 3J ) 9.7 Hz, 3a-H), 7.21-7.31 (m, Ar). 13C NMR
(150.9 MHz, CDCl3): δ ) 62.2 (C-6a), 63.1 (C-6b), 65.4 (C-5a), 70.1 (C-
3a), 71.3 (C-2a), 72.5 (C-2b), 73.3 (C-5b), 75.5 (C-4b), 75.7 (C-3b), 77.0
(C-4a), 97.1 (C-1a), 101.1 (C-1b).
)
3
(21) Analytical Data of Compound 19. MALDI-TOF (DHB/THF): m/z
calcd M (C49H62O24) 1035.00; (M + Na)+ 1057.99; (M + K)+ 1074.10;
found 1057.3, 1073.3. 1H NMR (600 MHz, CDCl3): δ 2.03 (m, 8 COCH3),
3.36 (s, OCH3), 3.40 (m, 5b-H), 3.55 (m, 5c-H), 3.62 (m, 4b-H, 4c-H),
3.66 (m, 6b-H), 3.69 (m, 4a-H), 3.76 (d, 3J ) 7.3 Hz, 5a-H), 4.10 (m,
3
6′b-H), 4.13 (d, J ) 5.0 Hz, 6a-H), 4.17 (dd, J6,6′ ) 12.0 Hz, J5,6 ) 4.6
Hz, 6c-H), 4.34 (d, 2J ) 2.2 Hz, 6′c-H), 4.44 (d, J1,2 ) 7.9 Hz, 1b-H), 4.48
(d, J1,2 ) 7.9 Hz, 1c-H), 4.49 (d, 2J ) 2.1 Hz, 6′a-H), 4.53-4.59 (m, 2
CH2Ph), 4.80 (dd, J1,2 ) 7.9 Hz, J2,3 ) 6.2 Hz, 2b-H), 4.81 (m, 2a-H),
Org. Lett., Vol. 3, No. 5, 2001
749

Scheme 5a
a (a) (i) MeNH2, CH2Cl2, rt. (ii) Ac2O/pyridine, rt.
method was observed. Thus, trisaccharide 1921 (68% overall
over six steps, average 95% per step) and tetrasaccharide
122 (34% overall over eight steps, average 93% per step)
were isolated in similar remarkable yields.
In summary, we have presented the synthesis of a new
highly reactive linker system and its first successful applica-
tion to solid-phase oligosaccharide synthesis. This novel
o-nitro-phenoxyacetate linker proved to be completely stable
under glycosylation and Fmoc removal conditions and could
be cleaved very quickly at the end of the synthesis using
only volatile reagents. Thus, Fmoc as the temporary hydroxy
protecting group of O-glycosyl trichloroacetimidates was
found to be a very powerful combination leading to tetrasac-
charide 1 in an excellent overall yield. Currently we are
developing the application of this new linker system toward
higher oligomers and more complex carbohydrate structures.
4.86 (d, J1,2 ) 3.5 Hz, 1a-H), 4.89 (m, 2c-H), 5.17 (t, 3J ) 9.3 Hz, 3b-H),
5.23 (t, 3J ) 9.4 Hz, 3c-H), 5.42 (t, 3J ) 9.7 Hz, 3a-H), 7.22-7.32 (m,
Ar). 13C NMR (150.9 MHz, CDCl3): δ 62.2 (C-6a), 62.8 (C-6c), 68.1 (C-
6b), 70.0 (C-3a), 71.4 (C-2a), 72.3 (C-2c), 72.7 (C-2b), 73.4 (C-5c), 74.7
(C-5b), 75.6 (C-3c), 75.7 (C-3b), 76.0 (C-4b, C-4c), 77.0 (C-4a), 97.0 (C-
1a), 100.9 (C-1c), 101.3 (C-1b).
(22) Analytical Data of Compound 1. MALDI-TOF (DHB/THF): m/z
calcd M (C66H82O31) 1371.34; (M + Na)+ 1394.33; (M + K)+ 1410.44;
found 1393.7, 1410.3. 1H NMR (600 MHz, CDCl3): δ 2.02 (m, 11 COCH3),
3.35 (s, OCH3), 3.48 (m, 4c-H), 3.50 (m, 5b-H), 3.51 (m, 5c-H), 3.58 (m,
5d-H), 3.64 (d, J ) 2.3 Hz, 6c-H), 3.65 (d, J ) 2.4 Hz, 6b-H), 3.67 (m,
3
4b-H), 3.68 (m, 4d-H), 3.72 (d, J ) 9.6 Hz, 4a-H), 3.85 (m, 5a-H), 4.03
2
Acknowledgment. We thank the European Community
(grant no. FAIR-CT97-3142), the Bundesministerium fu¨r
Bildung und Forschung (grant no. 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.
(d, J ) 10.8 Hz, 6′b-H), 4.12 (t, 6a-H), 4.20 (m, 6′c-H, 6d-H), 4.37 (dd,
J6,6′ ) 12.0 Hz, J5,6′ ) 2.1 Hz, 6′d-H), 4.46 (m, 1c-H, 6′a-H), 4.49 (d, J1,2
) 8.0 Hz, 1b-H), 4.52-4.59 (m, 3 CH2Ph), 4.58 (m, 1d-H), 4.81 (d, J1,2
)
3.2 Hz, 1a-H), 4.82 (m, 2b-H), 4.84 (m, 2c-H), 4.86 (m, 2a-H), 4.89 (m,
3
2d-H), 5.18 (m, 3b-H), 5.20 (m, 3c-H), 5.21 (m, 3d-H), 5.40 (t, J ) 9.7
Hz, 3a-H), 7.22-7.35 (m, Ar). 13C NMR (150.9 MHz, CDCl3): δ 62.3
(C-6a), 62.9 (C-6d), 67.6 (C-6c), 68.5 (C-6b, C-5a), 70.1 (C-3a), 71.2 (C-
2a), 72.1 (C-2d), 72.4 (C-2c), 72.9 (C-2b), 73.6 (C-5d), 74.2 (C-5b), 75.3
(C-5c, C-3d), 75.6 (C-3c), 75.8 (C-3, C-4d), 75.9 (C-4b), 76.7 (C-4a, C-4c),
96.9 (C-1a), 100.8 (C-1c), 101.1 (C-1b), 101.4 (C-1d).
OL007062E
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Org. Lett., Vol. 3, No. 5, 2001