Synthesis of beta-D-Galf-(1-3)-D-GlcNAc by the trichloroacetamidate method and of beta-D-Galf-(1-6)-D-GlcNAc by SnCl4-promoted glycosylation.

Article abstract of DOI:10.1021/ol9905811

In a continuation of our studies on the characterization of the glycoproteins of T. cruzi new galactofuranosyl disaccharides were synthesized. Beta-D-Galf-(1-3)-D-GlcNAc was prepared by employing the trichloroacetamidate procedure for the glycosylation st

Full text of DOI:10.1021/ol9905811

ORGANIC
LETTERS
1999
Vol. 1, No. 2
245-247
Synthesis of â-D-Galf-(1−3)-D-GlcNAc by
the Trichloroacetamidate Method and of
â-D-Galf-(1−6)-D-GlcNAc by
SnCl4-Promoted Glycosylation
Carola Gallo-Rodriguez, Lucia Gandolfi, and Rosa M. de Lederkremer*
CIHIDECAR, Departamento de Qu´ımica Orga´nica, Facultad de Ciencias Exactas y
Naturales, UniVersidad de Buenos Aires, Ciudad UniVersitaria, Pabellon II,
Buenos Aires 1428, Argentina
lederk@qo.fcen.uba.ar
Received April 12, 1999
ABSTRACT
In a continuation of our studies on the characterization of the glycoproteins of T. cruzi new galactofuranosyl disaccharides were synthesized.
â-D-Galf-(1−3)-D-GlcNAc (1) was prepared by employing the trichloroacetamidate procedure for the glycosylation step. The mild conditions of
this reaction are appropriate for condensation of 2,3,5,6-tetra-O-benzoyl-â-D-galactofuranosyl trichloroacetamidate (6) with acid-labile benzyl
2-acetamido-4,6-O-benzylidene-2-deoxy-r-D-glucopyranoside (3). On the other hand, tin(IV) chloride promoted condensation of benzyl 2-acetamido-
3-O-benzoyl-2-deoxy-r-D-glucopyranoside (11) with penta-O-benzoyl-r,â-D-galactofuranose (4) gave the derivative of â-D-Galf-(1−6)-D-GlcNAc
(2) in 78% yield.
In connection with our studies on the structure of O-linked
oligosaccharides in the mucins of T. cruzi we synthesized
the new disaccharides â-D-Galf-(1-3)-D-GlcNAc (1) and
â-D-Galf-(1-6)-D-GlcNAc (2). These, together with â-D-
Galf-(1-4)-D-GlcNAc, obtained from mucins1 and synthe-
sized in our laboratory,2 are the possible disaccharides Galf-
GlcNAc, necessary for unequivocal identification of the unit
found in the mucins of T. cruzi. It is interesting that the
presence of galactofuranose is dependent on the protozoan
strain; for example, in the Y strain the O-linked oligosac-
charides contain only galactopyranose, whereas Galf and
Galp units are present in the G strain.1,3
Glycopyranosides have been extensively used for glyco-
sylation by the trichloroacetamidate method.4 A recent report5
on the use of this method for the preparation of some
galactofuranosyl disaccharides, as methyl glycosides, prompts
us to report our results.
We have extensively employed SnCl4-promoted glycosi-
dation to link galactofuranosyl units to many acceptors.2,6
(1) (a) Previato, J. O.; Jones, C.; Gonc¸alves, L. P. B.; Wait, R.; Travassos,
L. R.; Mendonc¸a-Previato, L. Biochem. J. 1994, 301, 151. (b) Acosta
Serrano, A.; Schenkman, S.; Yoshida, N.; Mehlert, A.; Richardson, J. M.;
Ferguson, M. A. J. J. Biol. Chem. 1995, 270, 27244.
(2) Gallo-Rodriguez, C.; Varela, O.; de Lederkremer, R. M. J. Org. Chem.
1996, 61, 1886.
(4) (a) Schmidt, R. R. In Modern Methods in Carbohydrate Synthesis;
Khan, S. H., O’Neill, R. A., Eds.; Harwood Academic Publishers:
Amsterdam, 1996; p 20. (b) Schmidt, R. R.; Kinzy, W. AdV. Carbohydr.
Chem. Biochem. 1994, 50, 21.
(5) Choudhury, A. K.; Roy, N. Carbohydr. Res. 1998, 308, 207.
(6) (a) Marino, C.; Varela, O.; de Lederkremer, R. M. Carbohydr. Res.
1989, 190, 65. (b) de Lederkremer, R. M.; Marino, C.; Varela, O. Carbohydr.
Res. 1990, 200, 227.
(3) Previato, J. O.; Jones, C.; Xavier, M. T.; Wait, R.; Travassos, L. R.;
Parodi, A. J.; Mendonc¸a-Previato, L. J. Biol. Chem. 1995, 270, 7241.
10.1021/ol9905811 CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/27/1999

This method has the advantage of using a peracylated
galactofuranose donor which can be obtained in a one-step
reaction from D-galactose.7 However, this facile procedure
fails when acid labile acceptors have to be glycosylated.
For the synthesis of disaccharide 1, a derivative of GlcNAc
with the free OH-3 was required. 2-Acetamido-4,6-O-
benzylidene-2-deoxy-R-D-glucopyranoside (3) was easily
prepared as described.7 Because of the lability of the
benzylidene group to acidic media, the trichloroacetamidate
method was employed to build the furanosic linkage. Thus,
penta-O-benzoyl-R,â-D-galactofuranose7 (4) was treated with
32% HBr in glacial AcOH and the resulting galactofuranosyl
bromide was further hydrolyzed with the assistance of
AgCO3,8 to give 2,3,5,6-tetra-O-benzoyl-R,â-D-galactofura-
nose (5) in 90% yield.
nosyl-(1-3)-2-acetamido-4,6-O-benzylidene-2-deoxy-R-D-
glucopyranoside (7) in 82% yield.10 Due to a â-furanosyl
linkage, the H NMR spectrum of 7 showed J1′,2′ < 1 Hz
1
and, the C-1′ resonance in the 13C NMR appeared at 105.9
ppm. The resonance of C-3 was shifted downfield to 77.5
ppm, as expected for the glycosidation of OH-3 (Table 1).
Hydrolysis of the benzylidene group of 7 was performed with
aqueous acetic acid at 85 °C, conditions which did not affect
the labile furanosyl linkage. Crystalline benzyl 2,3,5,6-tetra-
O-benzoyl-â-D-galactofuranosyl-(1-3)-2-acetamido-2-deoxy-
R-D-glucopyranoside (8; mp 196 °C) was obtained in 82%
yield. O-Debenzoylation of 8 with sodium methoxide af-
forded crystalline benzyl glycoside 9 (mp 193-195 °C) in
98% yield. Hydrogenolysis of the benzyl group of 9 by
treatment with ammonium formate-10% Pd/C in hot
methanol afforded free â-D-Galf-(1-3)-D-GlcNAc (1) in 98%
yield as a crystalline solid (mp 170-172 °C). The anomeric
region of the 13C NMR spectrum showed the resonances of
C-1′ (109.1 and 109.3 ppm) and C-1 of GlcNAc (91.9 and
95.4 for the R- and â-anomers, respectively) with an R:â
ratio of 3:2, as indicated by the integrals of the anomeric
region in the proton NMR. Sodium borohydride reduction
of 1 yielded â-D-galactofuranosyl-(1-3)-2-acetamido-2-
deoxy-D-glucitol (10) in 97% yield.
It is known that the use of a strong base such as DBU, in
the synthesis of galactopyranosyl trichloroacetamidates, leads
to the more stable R-anomer.4 However, treatment of 5 with
DBU and Cl3CCN at 0 °C yielded 2,3,5,6-tetra-O-benzoyl-
â-D-galactofuranosyl trichloroacetamidate (6) as a very
reactive syrup in 85% yield (Scheme 1).9 This compound
Scheme 1a
On the other hand, disaccharide 2 could be obtained by
the easy SnCl4-promoted glycosylation. As precursor of the
reducing end, benzyl 2-acetamido-3-O-benzoyl-2-deoxy-R-
D-glucopyranoside (11) was selected.7 Condensation of acid-
stable 11 with 1.1 equiv of 4 resulted in a regio- and
(10) Procedure for the Preparation of 7. To a solution of acetamidate
6 (700 mg, 0.94 mmol) in dry Cl2CH2 (30 mL) and CH3CN (1 mL) were
added anhydrous benzyl 2-acetamido-4,6-O-benzylidene-2-deoxy-R-D-
glucopyranoside (3; 360 mg, 0.90 mmol) and 4 Å powder molecular sieves.
The suspension was vigorously stirred for 5 min at room temperature under
Ar and then cooled to -20 °C. TMSOTf was added (150 µL, 0.83 mmol)
and the stirring continued for 1 h. After the mixture was warmed to room
temperature, solid NaHCO3 (200 mg) was added with vigorous stirring and
the suspension was filtered over Celite. The filtrate was diluted with Cl2-
CH2 (150 mL), washed with water (3 × 100 mL), dried (MgSO4), filtered,
and concentrated. Purification of the residue by silica gel column chroma-
tography (10:1 toluene-EtOAc) yielded 7 (720 mg, 82%) as a foamy solid.
Unreacted 3 was also recovered (52 mg, 14%). Compound 7: Rf 0.56, 2:1
toluene-EtOAc; [R]D +32.6° (c ) 1, CHCl3); 1H NMR (200 MHz, CDCl3)
δ 8.1-7.1 (m, 30H), 5.86 (dt, 1H, J ) 8.8, 2.9 Hz, H-5′), 5.80 (d, 1H, J )
9.5 Hz, NH), 5.48 (d, 1H, J ) 4.8 Hz, H-3′), 5.41 , 5.40 (2s, 2H, H-1′,
PhCH), 5.33 (br s, 1H, H-2′), 4.93 (d, 1H, J ) 3.7 Hz, H-1), 4.77, 4.51
(2d, 2H, J ) 11.7 Hz, PhCH2), 4.62 (dd, 1H, J ) 4.8, 2.9 Hz, H-4′), 4.51
(m, 1H, H-2), 4.45 (t, 1H, J ) 8.8 Hz, H-6′a), 4.26 (dd, 1H, J ) 10.2, 4.4
Hz, H-6a), 4.09 (t, 1H, J ) 9.7 Hz, H-3), 3.96 (m, 1H, H-5), 3.90 (dd,
1H, J ) 8.8, 2.9 Hz, H-6′b), 3.73 (t, 1H, J ) 10.2 Hz, H-6b), 3.67 (t, 1H,
J ) 9.1 Hz, H-4), 1.98 (s, 3H, CH3). A 2D-COSY 1H NMR correlation
spectrum allowed the assigment of the 1H signals. Anal. Calcd for C56H51-
NO15: C, 68.77; H, 5.26. Found: C, 68.76; H, 5.22.
aLegend: (a) 32% HBr in glacial AcOH, 2 h, room temperature;
(b) acetone-H2O, Ag2CO3, 40 °C, 40 min, 90%; (c) Cl3CCN, DBU,
0 °C, 85%.
has to be stored under an argon atmosphere at -20 °C. In
1
the H NMR, the signal for H-1 appears at low fields (δ
6.71) with J1,2 < 1 indicating the â-configuration (a trans
relationship between H-1 and H-2) for trichloroacetamidate
6. The 13C NMR spectrum showed signals at 102.9 (C-1),
84.6 (C-4), and 80.8 (C-2), confirming that the â-D-
galactofuranosyl trichloroacetamidate was formed (Table 1).
Condensation of 6 with 3 proceeded smoothly, by employ-
ing TMSOTf as catalyst (Scheme 2), with â-stereoselective
formation of benzyl 2,3,5,6-tetra-O-benzoyl-â-D-galactofura-
(11) Procedure for the Preparation of 12. To an externally cooled (0
°C) solution of 1,2,3,5,6-penta-O-benzoyl-R,â-D-galactofuranose7 (4; 0.65
g, 0.92 mmol) in dry Cl2CH2 (14 mL) was added tin(IV) chloride (0.11
mL, 0.93 mmol). After 15 min of stirring at 0 °C, a solution of 11 (0.35 g,
0.84 mmol) in dry CH3CN (1 mL) was slowly added, and stirring was
continued for 15 h at room temperature. The mixture was diluted with Cl2-
CH2 (40 mL) and poured into saturated aqueous NaHCO3 with vigorous
stirring. The aqueous layer was extracted with Cl2CH2 (2 × 50 mL), and
the combined organic solutions were washed with water until pH 7, dried
(MgSO4), filtered, and concentrated. The resulting syrup was purified by
column chromatography (9:1 and then 6:1 toluene-EtOAc). First, benzyl
4,6-di-O-(2,3,5,6-tetra-O-benzoyl-â-D-galactofuranosyl)-2-acetamido-3-O-
benzoyl-2-deoxy-R-D-glucopyranoside (Rf 0.35, 4:1 toluene-EtOAc; 0.06
g, 4%) was eluted. Next, a fraction from the column (Rf 0.23, 4:1 toluene-
EtOAc) afforded 12 (0.65 g, 78%), with the same spectroscopic and physical
properties as already described.
(7) Gallo-Rodriguez, C.; Varela, O.; de Lederkremer, R. M. Carbohydr.
Res. 1998, 305, 163.
(8) Allen, P. Z. In Methods in Carbohydrate Chemistry; Whistler, R. L,
Wolfrom, M. L., Eds.; Academic Press: New York, 1962; Vol 1, p 372.
(9) Procedure for the Preparation of 6. To a solution of 5 (700 mg,
1.17 mmol) in dry CH2Cl2 (13 mL) was added trichloroacetonitrile (0.58
mL, 5.8 mmol). After the mixture was cooled at 0 °C, DBU (0.05 mL,
0.33 mmol) was added; this solution was stirred for 30 min and concentrated
in vacuo. The crude product was purified on a short silica gel column to
afford 740 mg of trichloroacetamidate 6 as a syrup (85%, Rf 0.65, 10:1
toluene-EtOAc) which was stored at -20 °C under an argon atmosphere:
1
[R]D -6.6 (c ) 2, CHCl3); H NMR (200 MHz, CDCl3) δ 8.73 (s, NH),
8.1-7.1 (m, 20H), 6.71 (s, 1H, H-1), 5.93 (m, 1H, H-5), 5.79 (d, 1H, J )
4.4 Hz, H-3), 5.77 (s, 1H, H-2), 4.87 (t, 1H, J ) 4.0 Hz, H-4), 4.82 (dd,
1H, J ) 12.1, 5.1 Hz, H-6a), 4.75 (dd, 1H, J ) 12.1, 6.6 Hz, H-6b).
246
Org. Lett., Vol. 1, No. 2, 1999

Table 1. 13C NMR (50.3 MHz) Chemical Shifts for Compounds 1, 2, 5-10, 13, and 14
compd
C-1
C-2a
C-3
C-4
C-5
C-6
CH2Ph
CHPh
1b
GlcNAc
Galf
GlcNAc
Galf
Galf
Galf
GlcNAc
Galf
GlcNAc
Galf
GlcNAc
Galf
GlcNAc-ol
Galf
GlcNAc
Galf
GlcNAc-ol
Galf
91.9 (R), 95.4 (â)
109.1, 109.3
91.8 (R), 95.9 (â)
108.9
101.0 (â), 96.0 (R)
102.9
97.8
105.9
96.9
108.8
97.0
109.0
61.3
109.5
96.8
108.8
61.8
108.7
54.1
82.0
54.9
81.9
81.7
80.8
52.9
82.0**
52.2
83.6**
53.7
82.0
54.0
82.1
54.5
81.9
54.6
81.9
79.6
77.3
71.6*
77.7
77.7
77.0
73.6
77.5
81.2
76.6
79.7
77.3
77.2
77.2
71.7*
77.6
70.8*
77.6
69.4
83.4
71.2
83.7
82.7
84.6
80.5
82.4**
70.4*
84.0**
70.4*
83.5
71.0*
83.9
70.9#
83.6
72.4
71.3
71.5*
71.7*
70.6
70.1
64.2
70.2#
71.6
70.3*
72.9
61.7
63.7
68.0
63.7
63.2
63.4
68.9
63.4
62.6#
62.5#
61.4
63.7
63.6#
63.7#
67.6
63.7
69.3*
63.7
2b
5c
6c
7c
70.4#
69.9*
69.2*
102.0
8c
9b
71.3
10b
13b
14b
71.8*
71.4*
72.0*
71.7*
71.7*
71.7*
70.7#
70.0*
83.8
a Signals marked *, #, and ** indicate that they may be interchanged. b Recorded in D2O. c Recorded in CDCl3.
Scheme 2
stereoselective glycosylation of the primary OH-6 of 11 to
afford benzyl 2,3,5,6-tetra-O-benzoyl-â-D-galactofuranosyl-
(1-6)-2-acetamido-3-O-benzoyl-2-deoxy-R-D-glucopyrano-
side (12) in 78% yield.11 This compound was previously
obtained as a minor product in the synthesis of benzyl 4,6-
di-O-(2,3,5,6-tetra-O-benzoyl-â-D-galactofuranosyl)-2-aceta-
mido-3-O-benzoyl-2-deoxy-R-D-glucopyranoside and pre-
sented the same spectroscopic and physical data.7 Further
O-debenzoylation of 12 with methanolic sodium methoxide
gave benzyl â-D-galactofuranosyl-(1-6)-2-acetamido-2-
deoxy-R-D-glucopyranoside (13; mp 107-108 °C) in 99%
yield. Crystalline â-D-Galf-(1-6)-D-GlcNAc (2; mp 159-
160 °C) was obtained in 98% yield by treatment of 13 with
ammonium formate-10% Pd/C in hot methanol. Sodium
borohydride reduction of 2 gave the corresponding alditol
14 as a hygroscopic syrup in 99% yield.
sides5 which are not amenable to hydrolysis, leading to the
free sugar, without cleavage of the galactofuranosyl linkage.
We have now described the preparation of two free galacto-
furanosyl disaccharides employing two different procedures
for the glycosylation step, depending on the substituents in
the acceptor. Moreover, the glycosyl alditols obtained by
NaBH4 reduction could be useful for comparison with alditols
liberated by reductive â-elimination from mucins of T. cruzi.1
Acknowledgment. We are indebted to CONICET (Con-
sejo Nacional de Investigaciones Cient´ıficas y Te´cnicas) and
the University of Buenos Aires for financial support.
Supporting Information Available: Full characterization
and proton spectra for compounds 1/2/5/8-10/13/14 and a
detailed description of experimental procedures. This material
is available free of charge via the Internet at http://pubs.acs.org.
The trichloroacetamidate method was recently employed
to obtain galactofuranosyl disaccharides as methyl glyco-
OL9905811
Org. Lett., Vol. 1, No. 2, 1999
247