Synthesis of the Lewis a trisaccharide based on an anomeric silyl fluorous tag

Article abstract of DOI:10.1021/ol048474g

(Chemical Equation Presented) The synthesis of the trisaccharide Lewis a was performed using an anomeric fluorous silyl protective group. This methodology allowed us to fully characterize each product (NMR, MS) and monitor each synthetic step (TLC). Although the product purifications could be performed by fluorous-solid-phase extraction (F-SPE) technology, standard chromatography could be used to effect purification if necessary. Trichloroethoxy carbonyl (Troc) protection of the amino group of the glucosamine moiety was found essential to allow protecting group manipulation of the fluorous protected sugar.

Full text of DOI:10.1021/ol048474g

ORGANIC
LETTERS
2004
Vol. 6, No. 23
4195-4198
Synthesis of the Lewis a Trisaccharide
Based on an Anomeric Silyl Fluorous
Tag
Leonardo Manzoni*,† and Riccardo Castelli‡
C.N.R.-Istituto di Scienze e Tecnologie Molecolari (ISTM), Dipartimento di Chimica
Organica e Industriale, and Centro Interdisciplinare Studi biomolecolari e applicazioni
Industriali (CISI), Via Venezian 21, I-20133, Milano, Italy,
leonardo.manzoni@istm.cnr.it
Received August 2, 2004
ABSTRACT
The synthesis of the trisaccharide Lewis a was performed using an anomeric fluorous silyl protective group. This methodology allowed us to
fully characterize each product (NMR, MS) and monitor each synthetic step (TLC). Although the product purifications could be performed by
fluorous-solid-phase extraction (F-SPE) technology, standard chromatography could be used to effect purification if necessary. Trichloroethoxy
carbonyl (Troc) protection of the amino group of the glucosamine moiety was found essential to allow protecting group manipulation of the
fluorous protected sugar.
Rapid and reliable access to specific oligosaccharides is criti-
cal to the development of glycobiology. However, traditional
oligosaccharide synthesis is a time-consuming business pri-
marily because of the tedious chromatographic purifications
often required of the multiple synthetic intermediates. To
overcome these problems the solid-phase synthesis of oli-
gosaccharides has been actively studied,1 and the synthesis
of oligosaccharides using a peptide synthesizer has been
reported.2 However, current solid-phase synthetic technolo-
gies suffer from disadvantages, such as difficulty in monitor-
ing the reaction progress and in characterizing the reaction
outcome by NMR analysis or mass spectrometry. Recently,
fluorous chemistry has become an attractive alternative to
solid-phase synthesis.3 This technology is attractive for
strategic separation of reaction mixtures since fluorous-tagged
compounds can be quickly separated from nontagged com-
pounds in binary liquid-liquid and solid-liquid extractions.
Highly fluorinated acetal, silyl, benzyl, thiol, and novel acyl
protecting groups for hydroxy functions used for oligosac-
charides synthesis have already been reported.4 However,
such highly fluorinated molecules can have limited or no
solubility in organic solvents. Therefore, finding suitable
reaction conditions for working with such materials is not
trivial. On the other hand, lightly fluorinated compounds are
soluble in common organic solvents and can be analyzed
by NMR and MS, and the reactions can be monitored by
TLC on conventional plates.
† C.N.R.-Istituto di Scienze e Tecnologie Molecolari (ISTM) and Centro
Interdisciplinare Studi biomolecolari e applicazioni Industriali (CISI).
‡ Dipartimento di Chimica Organica e Industriale and Centro Interdis-
ciplinare Studi biomolecolari e applicazioni Industriali (CISI).
(1) (a) Manabe, S.; Ito, Y. J. Am. Chem. Soc. 2002, 124, 12638. (b)
Roussel, F.; Takhi, M.; Schmidt, R. R. J. Org. Chem. 2001, 66, 8540. (c)
Wu, X.; Grathwohl, M.; Schmidt, R. R. Org. Lett. 2001, 3, 747. (d) Roussel,
F.; Knerr, L.; Grathwohl, M.; Schmidt, R. R. Org. Lett. 2000, 2, 3043. (e)
Rademann, J.; Schmidt, R. R. Carbohydr. Res. 1995, 269, 217.
(2) (a) Plante, O. J.; Palmacci, E. R.; Seeberger, P. H. Science 2001,
291, 1523 and references therein. (b) Palmacci, E. R.; Hewitt, M. C.;
Seeberger, P. H. Angew. Chem., Int. Ed. 2001, 40, 4433.
(3) (a) Luo, Z.; Zhang, Q.; Oderatoshi, Y.; Curran, D. P. Science 2001,
291, 1766. (b) Barret, A. G. M.; Braddock, D. C.; Catterick, D.; Chadwick,
D.; Henschke, J. P.; McKinnell, R. M. Synlett 2000, 847. (c) Curran, D. P.
Pure Appl. Chem. 2000, 72, 1649. (d) Curran, D. P. Angew. Chem., Int.
Ed. 1998, 37, 1175 and references therein. (e) Zhang, W. Tetrahedron 2003,
59, 4475. (f) Zhang, W. Chem. ReV. 2004, 104, 2531.
10.1021/ol048474g CCC: $27.50
© 2004 American Chemical Society
Published on Web 10/19/2004

Furthermore, a lightly fluorinated compound is readily pur-
ified from nonfluorinated compounds by a simple fluorous-
solid-phase extraction (F-SPE)5 on FluoroFlash silica gel.
Recently, we reported on the fluorous disaccharide syn-
thesis of Galâ(1-3)GlcNAc using an anomeric silyl fluorous
tag as a protective group on the glycosylation acceptor.6
We have shown that all non-fluorinated compounds can
be removed from glycosylation crudes by eluting with
MeOH/H2O on FluoroFlash silica, and the fluorinated
materials can be recovered using MeOH as the eluant. Thus,
with this method the compounds can be purified by a simple
F-SPE, and the reaction can be driven to completion by
multiple cycles, in analogy to the solid-phase synthesis.
Furthermore, all the intermediates can be characterized by
NMR and MALDI-TOF analysis, and the reactions can be
monitored by TLC.
Scheme 1. Cleavage of the Benzylidene Groupa
With the aim of demonstrating the utility of this strategy
for speeding up the synthesis of oligosaccharides, we have
now performed the synthesis of the Lewis a trisaccharide
employing reagents, protecting-group manipulations, and
glycosylation conditions that are standard in the field.
Dramatically important for this approach was the selection
of the nitrogen protective group for the glucosamine moiety.
Our initial approach began with the known fluorous-tag-pro-
tected disaccharide 1.6 We began the protecting group manip-
ulation with the aim of obtaining the free 4-OH acceptor 2
ready for the following planned glycosylation (Scheme 1).
To our disappointment we found that the standard, widely
used procedures for regioselective cleavage of the ben-
zylidene group (HCl, NaCNBH3 or Et3SiH, TFA)7,8 afforded
the desired product in a very poor yield (5-10%), the main
isolated product being the tagged monosaccharide 3. To test
the reactivity/stability of this disaccharide, we tried to
completely remove the benzylidene group via hydrogenolysis
(H2/Pd). To our surprise 4 was again the main isolated
product.9,10 A similar pattern of behavior was observed when
the disaccharide 1 was submitted to a transacetalyzation
reaction (1,3-propanditiol, CSA or MeOH, CSA). These
findings suggested that the fluorous tag produced an inversion
of the normal reactivity11 between the benzylidene acetal and
the glycosyl linkage.12
a Regioselective cleavage: NaCNBH3, HCl or Et3SiH, TFA.
Hydrogenolytic cleavage: H2, Pd/C. Transacetalyzation: 1,3-pro-
panditiol, CSA or MeOH, CSA.
purpose the N-trichloroethoxycarbonyl derivative (Troc), as
this is a useful protecting group in oligosaccharide synthe-
sis.13 The Troc group can be removed in high yield by
reduction.14 In addition, this group is stable under a range
of standard conditions used for carbohydrate synthesis, and
N-Troc-protected glycosyl donors have been used for a
convergent synthesis of oligosaccharides.
Starting from the known N-Troc-protected glucosamine
511 the fluorous tag was attached to the anomeric hydroxy
group by using triethylamine and DMAP; the fluorous
compound 6 was obtained in quantitative yield (Scheme 2).
Removal of the acetyl groups from 6 under Zemple´n con-
ditions15 followed by treatment of the crude product with
benzaldehyde dimethylacetal in the presence of CSA afforded
the fluorous glycosyl acceptor 7. As a result of the limited
number of fluorine atoms 6 and 7 show normal chromato-
Recognizing that the glycosyl linkage might be stabilized
by stereoelectronic effects, we used a different amino pro-
tecting group for the glucosylamine. We chose for our
(4) (a) Wipf, P.; Reeves, J. T. Tetrahedron Lett. 1999, 40, 5139. (b)
Wipf, P.; Reeves, J. T. Tetrahedron Lett. 1999, 40, 4649. (c) Wipf, P.;
Reeves, J. T.; Balachandran, R.; Giuliano, K. A.; Hamel, E.; Day, B. W. J.
Am. Chem. Soc. 2000, 122, 9391. (d) Curran, D. P.; Ferritto, R.; Hua, Y.
Tetrahedron Lett. 1998, 39, 4937. (e) Miura, T.; Hirose, Y.; Ohmae, M.;
Inazu, T. Org. Lett. 2001, 3, 3947. (f) Miura, T.; Inazu, T. Tetrahedron
Lett. 2003, 44, 1819. (g) Miura, T.; Goto, K.; Hosaka, D.; Inazu, T. Angew.
Chem., Int. Ed. 2003, 42, 2047. (h) Jing, Y.; Huang, X. Tetrahedron Lett.
2004, 45, 4615.
(5) (a) Curran, D. P.; Luo, Z. J. Am. Chem. Soc. 1999, 121, 9069. (b)
Zhang, Q.; Luo, Z.; Curran, D. P. J. Org. Chem. 2000, 65, 8866.
(6) Manzoni, L. Chem. Commun. 2003, 2930.
(7) (a) Horne, D. A.; Jordan, A. Tetrahedron Lett. 1978, 19, 1357. (b)
Garegg, P. J.; Hultberg, H.; Wallin, S. Carbohydr. Res. 1982, 108, 97.
(8) DeNinno, M. P.; Etienne, J. B.; Duplantier, K. C. Tetrahedron Lett.
1995, 36, 669.
(10) Sakagami, M.; Hamana, H. Tetrahedron Lett. 2000, 41, 5547.
(11) Manzoni, L.; Lay, L.; Schmidt, R. R. J. Carbohydr. Chem. 1998,
17, 739.
(12) We have tried, with no success, to modulate this reactivity by
changing the 4,6-protection from a benzylidene to an acetonide group.
(13) (a) Dullenkopf, W.; Castro-Palomino, J. C.; Manzoni, L.; Schmidt,
R. R. Carbohydr. Res. 1996, 296, 135. (b) Ellervik, U.; Magnusson, G.
Carbohydr. Res. 1996, 280, 251. (c) Saha, U. K.; Schmidt, R. R. J. Chem.
Soc., Perkin Trans. 1 1997, 1855. (d) Fukase, K.; Fukase, Y.; Oikawa, M.;
Liu, W.; Suda, Y.; Kusumoto, S. Tetrahedron 1998, 54, 4033. (e) Ellervik,
U.; Magnusson, G. Tetrahedron Lett. 1997, 38, 1627 and references therein.
(f) Mitchell, S. A.; Pratt, M. R.; Hruby, V. J.; Polt, R. J. Org. Chem. 2001,
61, 2327. (g) Mong, K. T.; Wong, C. H. Angew. Chem., Int. Ed. 2002, 41,
4087; Angew. Chem. 2002, 114, 4261.
(14) (a) Woodward, R. B.; Heusler, K.; Gosteli, J.; Naegeli, P.; Oppolzer,
W.; Ramage, R.; Ranganathan, S.; Vorbru¨ggen, H. J. Am. Chem. Soc. 1966,
88, 852. (b) Just, G.; Grozinger, K. Synthesis 1976, 457. (c) Dong, Q.;
Anderson, C. E.; Ciufolini, M. A. Tetrahedron Lett. 1995, 36, 5681 and
references therein. (d) Zhu, X.; Schmidt, R. R. Synthesis 2003, 1262.
(9) When a reductive cleavage was used the galactose moiety was
detected as a pyran derivative, whereas in the case of hydrolytic cleavage
the hydrolyzed galactose has been detected.
4196
Org. Lett., Vol. 6, No. 23, 2004

Scheme 2. Synthesis of the Trisaccharide Lewis a
graphic behavior on standard silica gel, which allows moni-
In conclusion, we have expanded the possibilities for using
a fluorous tag as a protecting group at the anomeric position
of a glycosyl acceptor. The methodology has allowed rapid
synthesis of a trisaccharide by a F-SPE purification. The
trisaccharide 14 was obtained in only 3 days, in 23% overall
yield, from 5 with a reduced number of silica gel chromato-
graphic purification (average of 81% every step). All fluori-
nated compounds synthesized retain normal chromatographic
behavior on standard silica gel, allowing easy monitoring
of the course of reactions by TLC. Purification of the desired
products obtained from multistep synthesis was significantly
simplified by the fluorous moiety. The reaction mixtures were
subjected to F-SPE on fluorous silica gel. The fluorous com-
pounds could also be purified by normal silica gel chroma-
tography if necessary, which was advantageous compared
to solid-phase synthesis.
In addition, each synthetic intermediate could be easily
characterized by NMR and mass spectroscopy (MALDI-
TOF). The NMR signals originating from the fluorous tag
did not interfere with the carbohydrate region, as is often
observed in the case of PEG-supported synthesis. The
optimization of the reaction conditions using the fluorous
technology is easier than with the solid-phase synthesis be-
cause the reactions are run in liquid phase. Only standard
apparatus is required for such oligosaccharides syntheses,
and the reactions are easily scalable.
toring of the course of the reactions by TLC. Starting from
7, compound 10 was obtained with the same procedure
described for the synthesis of compound 1.6
Using Troc for glucosamine N-protection the normal
reactivity was restored, and the regioselective cleavage of
the benzylidene acetal proceeded smoothly with Et3SiH in
the presence of TFA and TFAA. The main product observed
(TLC, MALDI, NMR) was the disaccharide 11. Regiose-
lective opening was confirmed by acetylating the free 4-OH
and observing the downfield shift of the 4-H proton in the
glucosamine moiety. A very small quantity of the 4,6-diol
byproduct derived from hydrolysis of the benzylidene group
was easily removed by standard chromatography.
The suitably protected disaccharide 11 was then submitted
to a glycosylation reaction with the fucosyl donor 12, in the
presence of TMSOTf, affording the Lewis a trisaccharide
13 in 60% yield. The crude product was purified by F-SPE
with the procedure described above, and the reaction was
driven to completion by a second cycle of glycosylation with
3 equiv of the fucosyl donor 12, affording the trisaccharide
13 in 60% unoptimized yield. The relatively low yield in
this step is due to the low stability of 13 for which we
observed 30% decomposition in 3 days at -18 °C.
Finally, the fluorous silyl protecting group of 13 was
removed by reaction with TBAF to afford crude 14, which
was purified by fluorous silica gel chromatography, eluting
with 80% MeOH-H2O.
(15) Zemple´n, G. Ber. Dtsch. Chem. Ges. 1927, 60, 1555.
Org. Lett., Vol. 6, No. 23, 2004
4197

The reactivity of the fluorous tagged compounds appears
to depend critically on the protecting group on the gluco-
samine moiety. Using the appropriate group, standard manip-
ulation of the carbohydrate functionality and glycosylation
conditions could be adopted.
The study of new silyl fluorous tags and further applica-
tions to the synthesis of several complex carbohydrates and
glycoconjugates are now in progress.
Acknowledgment. We thank CNR and Fluorous Tech-
nologies, Inc. (FTI) (Whitepaper program) for financial
support.
Supporting Information Available: Experimental pro-
cedures. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL048474G
4198
Org. Lett., Vol. 6, No. 23, 2004