Efficient synthesis of a sialic acid α(2→3)galactose building block and its application to the synthesis of ganglioside GM3

Article abstract of DOI:10.1021/jo800138p

(Chemical Equation Presented) Glycosylation of various galactose derivatives with O-acetylated sialic acid N-phenyltrifluoroacetimidate as the donor was investigated. Efficient α(2,3)sialylation of galactose, with up to 73% yield and 8.4:1 stereoselectivity, was realized when 2,3,4-unprotected galactose derivatives and TBSOTf were used as acceptors and promoter, respectively. Sialylation of 2-(trimethylsilyl)ethyl 6-O-tert- butyldiphenylsilyl-β-D-galactopyranoside (3f) gave the best result, and the resultant Neu5Ac α(2→3)Gal disaccharide was successfully used in the synthesis of ganglioside GM3.

Full text of DOI:10.1021/jo800138p

leading to R-glycoside. In addition, the presence of the deoxy
moiety in combination with the electron-withdrawing carboxy-
late group at the anomeric center make these derivatives prone
to elimination to give 2,3-dehydro derivatives. These combined
factors disfavor the desired R-glycoside formation, especially
with secondary sugar hydroxyls. Ganglioside GM3 (NeuAcR-
3Galꢀ4Glcꢀ1-Cer) containing a sialic acid R(2f3)galactose
moiety is a ubiquitous metabolite with a variety of cellular
activities.4 The preparation of this ganglioside is important for
further biophysical and biological studies. There are a number
of the reports on the synthesis of ganglioside GM3.5 Although
the chemo-enzymatic approaches control the regio- and stereo-
selectivity of glycosylation, they have generally been demon-
strated on smaller scales.6 Chemical synthetic methods offer
the flexibility to prepare analogues. However, the stereoselec-
tivity is generally poor. Thus, highly regio- and stereoselective
syntheses are desired.
Efficient Synthesis of a Sialic Acid
r(2f3)Galactose Building Block and Its
Application to the Synthesis of Ganglioside GM3
Yunpeng Liu, Xiaohong Ruan, Xiangpeng Li, and
Yingxia Li*
Key Laboratory of Marine Drugs, The Ministration of
Education of China, School of Medicine and Pharmacy,
Ocean UniVersity of China, Qingdao 266003, China
liyx417@ouc.edu.cn
ReceiVed January 20, 2008
Recently, Yu and co-workers have successfully enhanced the
reactivity of sialic acid donors by utilizing N-phenyltrifluoro-
acetimidates as the leaving groups, and an R(2f3)-Neu5Ac-
Gal synthesis has been realized in 81% yield, but the R-selec-
tivity (R:ꢀ ) 3:1) is not good enough for further chemical
synthesis of complex sialylated oligosaccharides.7 Fukase and
co-workers8 and, more recently, Seeberger’s group9 have
evaluated different C-5-N-protecting groups of N-phenyltrif-
luoroacetimidate derivatives as the sialic acid donors and have
shown good yields and selectivities in glycosylations. However,
the modification of the C-5-N-protecting groups of the sialic
acid donors required the additional chemical steps for introduc-
tion and removal of the C-5-N-protecting groups. Thus, finding
a suitable galactose acceptor for improving the sialylation
reaction becomes very important and practical in the synthesis
of oligosaccharides containing a sialic acid R(2f3)galactose
moiety. Here, we report the efficient synthesis of the sialic acid
R(2f3)galactose building block with a N-phenyltrifluoroace-
timidate donor by optimizing the galactose acceptor and its
application to the synthesis of ganglioside GM3.
Glycosylation of various galactose derivatives with O-
acetylated sialic acid N-phenyltrifluoroacetimidate as the
donor was investigated. Efficient R(2,3)sialylation of galac-
tose, with up to 73% yield and 8.4:1 stereoselectivity, was
realized when 2,3,4-unprotected galactose derivatives and
TBSOTf were used as acceptors and promoter, respectively.
Sialylation of 2-(trimethylsilyl)ethyl 6-O-tert-butyldiphenyl-
silyl-ꢀ-D-galactopyranoside (3f) gave the best result, and the
resultant Neu5Ac R(2f3)Gal disaccharide was successfully
used in the synthesis of ganglioside GM3.
N-Acetylneuraminic acid (Neu5Ac), the most abundant sialic
acid congener in nature, is found at the termini of glycoproteins
and glycolipids on mammalian cell surfaces.1 In humans,
Neu5Ac occurs frequently via an R(2f3)glycosidic linkage to
galactose or R(2f6) linkages to galactose and N-acetylgalac-
tosamine.2 Since Neu5Ac on the cell surfaces plays diverse and
important roles in cell-cell interaction processes, such as
pathogen-host recognition, tumor metastasis, toxin-receptor
interaction, malignant alteration, cell differentiation, and pro-
liferation, much effort has been devoted to the efficient and
stereoselective synthesis of R(2f3)- and R(2f6)Neu5Ac-Gal
units in order to further investigate their biological functions.3
However, sialylation to make the equatorial 2-R-ketosidic
linkages is one of the most challenging tasks in glycosylation
chemistry. The electron-withdrawing C-1 carboxylic function,
disfavoring the oxocarbenium formation, restricts glycosidation
both electronically and sterically. The lack of a substituent at
C-3 precludes the suitable neighboring participation group
Coupled with our continuing interest in the N-phenyltrifluo-
roacetimidate method because of its accessibility, stability, and
reactivity,10 we first explored TMSOTf-promoted coupling of
sialyl N-phenyltrifluoroacetimidate 2 (1.5 equiv) with the ethyl
2-O-benzoyl-6-O-benzyl-1-thio-ꢀ-D-galactopyranoside 3a, an
ideal acceptor for the subsequent coupling with neighboring
(4) (a) Saito, M. AdV. Lipid Res. 1993, 25, 303. (b) Ledeen, R. W.; Wu, G.
Trends Glycosci. Glycotech. 1992, 4, 174. (c) Hakomori, S. I.; Igarashi, Y. AdV.
Lipid Res. 1993, 25, 147. (d) Ladisch, S.; Hasegawa, A.; Li, R.; Kiso, M.
Biochemistry 1995, 34, 119. (e) Fredman, P. AdV. Lipid Res. 1993, 25, 213.
(5) (a) Murase, T.; Ishida, H.; Kiso, M.; Hasegawa, A. Carbohydr. Res. 1989,
188, 71. (b) Tomoo, T.; Kondo, T.; Abe, H.; Tsukamoto, S.; Isobe, M.; Goto, T.
Carbohydr. Res. 1996, 284, 207. (c) Liu, K. K.-C.; Danishefsky, S. J J. Am.
Chem. Soc. 1993, 115, 4933. (d) Ito, Y.; Pauson, J. C. J. Am. Chem. Soc. 1993,
115, 1603. (e) Duclos, D. I., Jr Carbohydr. Res. 2000, 328, 489. (f) Sakamoto,
H.; Nakamura, S.; Tsuda, T.; Hashimoto, S. Tetrahedron. Lett. 2000, 41, 7691.
(6) Gilbert, M.; Bayer, R.; Cunningham, A. M.; Defrees, S.; Gao, Y.; Watson,
D. C.; Young, N. M.; Wakarchulc, W. W. Nat. Biotechnol. 1998, 16, 769.
(7) Cai, S.; Yu, B. Org. Lett. 2003, 5, 3827.
(8) Fukase, K.; Goi, T.; Tanaka, K. Synlett 2005, 19, 2958.
(9) Seeberger, P. H.; Nokami, T.; Esposito, D.; Castagner, B.; Hanashima,
S. Org. Lett. 2007, 9, 1777.
(1) Varki, A. Glycobiology 1993, 3, 97.
(2) Rosenberg, A. Biology of Sialic Acids; Plenum Press: New York, London,
1995.
(3) Boons, G. J.; Demchenko, A. V Chem. ReV. 2000, 100, 4539.
(10) (a) Yu, B.; Tao, H. Tetrahedron Lett. 2001, 42, 2405. (b) Yu, B; Tao,
H. J. Org. Chem. 2002, 67, 9099. (c) Ding, N.; Wang, P.; Zhang, Z; Liu, Y.; Li,
Y Carbohydr. Res. 2006, 341, 2769.
10.1021/jo800138p CCC: $40.75
Published on Web 04/24/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 4287–4290 4287

TABLE 1. Glycosylation of Galactose Nucleophiles 3a-g with
Sialic Acid Building Block 2
FIGURE 1. 2,3-Dehydro Compound 5.
produced by the C-2-O-protection group of 3a. In an attempt
to enhance the reactivity of the acceptor alcohol and diminish
the supposed steric hindrance, benzoyl protection on 3a was
removed with sodium methoxide in methanol to provide a new
galactose acceptor 3b.12 As expected, TBSOTf-promoted cou-
pling of 3b with 2 at -70 °C displayed good chemo- and
regioselectivity to give disaccharide 4b in 53% yield and with
a moderate R-selectivity (R:ꢀ ) 2.6:1) (Table 1, entry 2).
Encouraged by the promising result, we further tested other
triol acceptors 3c-f and galactal acceptor 3g. First, 4-methox-
yphenyl and 2-(trimethylsilyl)ethyl (SE) protections were
adopted to replace the anomeric thiol protection on 3b, and the
corresponding acceptors 3c and 3d13 were designed. Second,
the tert-butyldiphenylsilyl (TBDPS) group instead of benzyl
group was introduced as the C-6-O-protection group to form
another two acceptors 3e14 and 3f. In addition, bearing in mind
that galactal, with more nucleophilic C-3-hydroxyl group is an
attractive acceptor in building R(2f3)-Neu5Ac-Gal unit;9,15
here, galactal acceptor 3g15a was also surveyed.
All glycosylation reactions were carried out at -70 °C in
the presence of TBSOTf, and MeCN/CH2Cl2 (1:1) was used as
the optimal solvent by taking advantage of “nitrile solvent
effects”. To our delight, when thiol protection on 3b was
switched to 4-methoxyphenyl protection on 3c, the glycosylation
proceeded with significantly increased R-selectivity (R:ꢀ ) 6.1:
1) and in good yield (53%, Table 1, entry 3). The reaction of
acceptor 3d with donor 2 (1.5 equiv) provided the corresponding
disaccharide 4d in an even better yield (64%) and good
R-selectivity (R:ꢀ ) 5.9:1, table 1, entry 4). More interestingly,
when the TBDPS group was selected to protect the OH-6 of
galactose acceptors, the sialylation yields of sialic acid N-
phenyltrifluoroacetimidate 2 further increased. The glycosylation
yield of acceptor 3e with donor 2 increased up to 62% (Table
1, entry 3 vs 5), and the glycosylation of 3f with donor 2
provided the desired 4f in 73% yield and with very good
R-selectivity (R:ꢀ ) 8.4:1) (Table 1, entry 4 vs 6). Moreover,
the latter result was retained on a gram-scale synthesis (70%,
R:ꢀ ) 8.3:1). All of the desired R-isomer products 4b-f could
be easily separated by column chromatography. However, under
the same reaction conditions, the use of 1.5 equiv of donor 2
with 6-O-monoprotected galactal 3g provided an inseparable
mixture of R- and ꢀ-(2f3)-linked glycosylation products (R:ꢀ
9.3:1) in 70% yield (Table 1, entry 7). This observation is in
accordance with that of Danishefsky and co-workers.15a
As can be seen from the results shown in Table 1, the triol
galactose acceptor 3f is optimal for constructing the sialic acid
R(2f3)galactose building block by using N-phenyltrifluoro-
acetimidate as the leaving group. It is worthy of note that the
sialylation yield of galactose acceptor markedly increased with
a D/A
)
the ratio of donor and acceptor. b The R/ꢀ-ratio was
determined by NMR analysis.
group participation, using the combination of MeCN/CH2Cl2
(1:1) as the solvent at -70 °C.7 However, the reaction did not
work well. The desired coupling product (4a) was provided in
only 16% yield due to the decomposition of donor 2. Then, we
turned to survey the bulky promoter TBSOTf which was
reported to be a milder activator than TMSOTf.11 Examination
of the reaction of 2 with 3a under the influence of TBSOTf
was carried out. Indeed, this coupling displayed a higher yield
to give disaccharide 4a in 28% yield, and the ꢀ-isomer was not
isolated in sufficient quantity to identify (Table 1, entry 1). The
R configuration of 4a was implied by the NMR signals of H-3eq
(δ ) 2.55 ppm), H-4 (δ ) 4.74-4.95 ppm), JH-7, H-8 (9.1 Hz),
and ∆δ {H-9a-H-9b} (0.32-0.53 ppm), and the 2f3 linkage
was determined by the correlation of HMBC between the signal
of H-3 of the galactose unit and the signal of C-2 of the sialic
acid moiety.3
However, besides the desired product 4a, 2,3-dehydro
compound 5 (Figure 1) as a major product was observed
resulting from the competing elimination reaction of donor 2.
We assumed that the low sialylation yield was due to the poor
nucleophilicity and steric hindrance of the C-3-hydroxyl group
(12) Sherman, A. A.; Mironov, Y. V.; Yudina, O. N.; Nifantiev, N. E.
Carbohydr. Res. 2003, 338, 697.
(13) Eicher, E.; Jennings, H. J.; Whitfield, D. M. J. Carbohydr. Chem. 1997,
16, 385.
(14) Ovaa, H.; Lastdrager, B.; Codee, J. D. C.; Marel, G. A.; Overkleeft,
H. S.; Boom, J. H J. Chem. Soc., Perkin Trans. 1 2002, 2370.
(15) (a) Gervay, J.; Peterson, J. M.; Oriyama, T.; Danishefsky, S. J. J. Org.
Chem. 1993, 58, 5465. (b) Martichonok, V.; Whitesides, G. M. J. Org. Chem.
1996, 61, 1702.
(11) (a) Roush, W. R.; Narayan, S. Org. Lett. 1999, 1, 899. (b) Roush, W. R.;
Hartz, R. A.; Gustin, D. J J. Am. Chem. Soc. 1999, 121, 1990.
4288 J. Org. Chem. Vol. 73, No. 11, 2008

1221 afforded the ꢀ-linked glycosyl sphingosine derivative 13
in 83% yield. Reduction of the azido group with triphenylphos-
phine and subsequent coupling with octadecanoic acid in the
presence of DIPEA, EDC, and HOBt provided amide 14 in good
yield.22 Finally, one-pot deprotection of 14 with potassium tert-
butoxide in methanol and saponification of the methyl ester gave
the ganglioside GM3 in quantitative yield.20 The 1H NMR data
of the synthetic 1 were consistent with those reported for this
natural product.24
SCHEME 1. Transformation of 4f into Sialyl Galactose
Building Block 6
SCHEME 2. Linear Synthesis of Ganglioside GM3
In conclusion, an optimal triol galactose acceptor 2-(trim-
ethylsilyl)ethyl 6-O-tert-butyldiphenylsilyl-ꢀ-D-galactopyrano-
side 3f has been found to construct a sialic acid R(2f3)galactose
building block in good yield (73%) and R-selectivity (8.4:1),
in which N-phenyltrifluoroacetimidate was used as the leaving
group and TBSOTf as the promotor. This practical procedure
was applied successfully to the synthesis of ganglioside GM3.
The results of the present investigation should be of value to
synthesize complex sialylated oligosaccharides.
Experimental Section
Typical Procedure for Sialylation with TrifluoroacetimidateDo-
nors 2. A mixture of the donor 2 (1.5 equiv), acceptors 3a-g (1.0
equiv), and 4 Å molecular sieves in dry CH2Cl2/CH3CN (1:1, 5
mL) was stirred at room temperature under argon for 30 min and
then cooled to -70 °C. TBSOTf (0.4 equiv) was added. After being
stirred at -70 °C for 1.5 h, the mixture was warmed to room
temperature and quenched with a few drops of triethylamine. The
resulting mixture was filtered and concentrated. The residue was
chromatographed on a silica gel column to afford the desired
coupling products 4a-g.
2-(trimethylsilyl)ethyl protection of OH-1 and tert-butyldiphe-
nylsilyl (TBDPS) protection of OH-6 compared with 4-meth-
oxyphenyl protection and benzyl protection. This might be
attributed to that 2-(trimethylsilyl)ethyl and tert-butyldiphenyl-
silyl (TBDPS) protection, which enhanced the nucleophilicity
of galactose acceptors. Thus, the reactivity of the acceptor
nucleophiles strongly affects the stereoselectivity.5f,16
Ethyl [Methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-
ꢀ-D-glycero-r-D-galacto-2-nonulopyranosonate]-(2f3′)-6′-O-ben-
zyl-1′-thio-ꢀ-D-galactopyranoside (4a). Following the above-
mentioned general procedure, coupling of 2 (118 mg, 0.23 mmol)
with 3a (50 mg, 0.12 mmol) afforded 4a (30 mg, 28%) as a white
solid. R Anomer: mp 104-106 °C (EtOAc/hexanes); [R]20D +4.67
1
(c 1.45, CH2Cl2); Rf 0.18 (3:1 toluene/CH3CN); H NMR(CDCl3)
The disaccharide 4f was equipped with a suitable anomeric
leaving group (Scheme 1). Here N- phenyltrifluoroacetimidate
was still adopted to prepare donor 6 easily from disaccharide
4f in excellent yield (70% for four steps), involving the
following procedures: (1) removal of the TBDPS protection by
treatment with SnCl2,14 (2) peracetylation, (3) cleavage of the
anomeric 2-(trimethylsilyl)ethyl protection with TFA,15 and (4)
introduction of the N-phenyltrifluoroacetimidate leaving group.
With the disaccharide donor 6 in hand, the synthesis of
GM3 was then carried out (Scheme 2). A glucose acceptor
8 equipped with a neighboring participating group was
synthesized using known procedures from 719 and glycosy-
lated with 6 in the presence of TMSOTf. The desired 9 was
obtained as a single ꢀ-anomer in excellent yield. Debenzylation
of the trisaccharide by hydrogenolysis with 10% Pd-C and
peracetylation gave the desired product 10. Deprotection of the
4-methoxyphenyl group using CAN in CH3CN/H2O,20 followed
by introduction of the anomeric N-phenyltrifluoroacetimidate,
furnished trisaccharide donor 11 in 86% yield, and its condensa-
tion with (2S,3R,4E)-2-azido-3-O-benzoyl-4-eicosene-1,3-diol
δ 7.09-8.14 (m, 10 H, ArH), 5.49-5.51 (m, 1 H, H-8), 5.42 (t, 1
H, J ) 9.5 Hz, H-2′), 5.21 (dd, 2 H, J ) 9.1, 2.2 Hz, H-7), 5.01 (d,
1 H, J ) 9.5 Hz, NH), 4.74-4.76 (m, 1 H, H-4), 4.68 (d, 1 H, J
) 9.9 Hz, H-1′), 4.59 (d, 2 H, J ) 3.7 Hz, Ph-CH2), 4.45 (dd, 1 H,
J ) 9.5, 3.3 Hz, H-3′), 4.27 (dd, 1 H, J ) 12.1, 2.2 Hz, H-9a),
3.74-3.95 (m, 7 H, H-5, H-6, H-9b, H-4′, H-5′, H-6a′, H-6b′), 3.73
(s, 3 H, COOCH3), 2.66-2.78 (m, 2 H, CH2), 2.55 (dd, 1 H, J )
12.8, 4.4 Hz, H-3eq), 2.14, 2.06, 1.98, 1.81, 1.57 (5s, 15 H, 4 OAc
and NHAc), 1.90-1.91 (m, 1 H, H-3ax), 1.23 (t, 3 H, J ) 7.3 Hz,
CH3); 13C NMR (CDCl3): δ 170.8, 170.6, 170.3, 170.2, 170.0, 168.4,
165.3, 138.0, 133.0, 130.1, 128.3, 127.6, 127.5, 97.0, 83.5, 76.5,
74.8, 73.4, 72.1, 68.9, 68.8, 67.7, 67.6, 66.7, 62.3, 53.0, 48.9, 37.4,
29.6, 23.8, 23.0, 21.2, 20.7, 20.2, 14.9; ESIHRMS calcd for
C42H53NO18SNa [M + Na]+ 914.2899, found 914.2876.
Methyl 4,7,8,9-Tetra-O-acetyl-3,5-dideoxy-5-acetamidyl-D-glyc-
ero-ꢀ-D-galacto-2-nonulopyranosylonate-(2f3)-2,4,6-tri-O-acetyl-
D-galactopyranosyl N-Phenyltrifluoroacetimidate (6). To the so-
lution of 4f (300 mg, 0.3 mmol) in acetonitrile (5 mL) was added
SnCl2 until the pH was 2. After the mixture was stirred for 10 h at
room temperature, pyridine (10 mL) and acetic anhydride (5 mL)
were added. The reaction mixture was stirred at room temperature
for 8 h. Removal of the solvent yielded a crude mixture, which
was then dissolved in EtOAc and washed with 1 M HCl, saturated
(16) Hasegawa, A.; Kiso, M. In PreparatiVe Carbohydrate Chemistry;
Hanessian, S., Ed.; Dekker: New York, 1997; p 357.
(17) Cort, A. D. Synth. Commun. 1990, 20, 757.
(21) Schmidt, R. R.; Zimmermann, P. Angew. Chem. 1986, 98, 722.
(22) Liu, Y.; Ding, N.; Xiao, H.; Li, Y. J. Carbohydr. Chem. 2006, 25, 471.
(23) Tomoo, T.; Kondo, T.; Abe, H.; Tsukamoto, S.; Isobe, M.; Goto, T.
Carbohydr. Res. 1996, 284, 207.
(18) Yamaguchi, M.; Ishida, H.; Galustian, C.; Feizi, T.; Kiso, M. Carbohydr.
Res. 2002, 337, 2111.
(19) Palmacci, E. R.; Seeberger, P. H. Tetrahedron 2004, 60, 7755.
(20) Georg, G. I.; Mashava, P. M.; Akgun, E.; Milstead, M. W. Tetrahedron
Lett. 1991, 32, 3151.
(24) Koerner, T. A. W., Jr.; Prestegard, J. H.; Demou, P. C.; Yu, R. K.
Biochemistry 1983, 22, 2676.
J. Org. Chem. Vol. 73, No. 11, 2008 4289

NaHCO3, and brine. The organic phase was dried over Na2SO4,
filtered, and concentrated to dryness. The resulting residue was
dissolved in CH2Cl2 (4 mL) and TFA (2 mL). After the reaction
mixture was stirred for 2 h and concentrated, the crude mixture
was dissolved in CH2Cl2 and washed with saturated NaHCO3 and
brine. The organic phase was dried over Na2SO4, filtered, and
concentrated. The resulting residue was dissolved in acetone (10
mL) and K2CO3 solution (124 mg, 0.9 mmol), and CF3C(NPh)Cl
(370 mg, 1.8 mmol) was added. After being stirred for 2 h at room
temperature, the reaction mixture was filtered through Celite and
concentrated to dryness. Purification with flash column silica gel
chromatography (60:1 CH2Cl2/CH3OH) gave N-phenyl trifluoro-
acetimidate 6 (200 mg, 70% for four steps): [R]20D +27.6 (c 0.4,
CH2Cl2); 1H NMR (CDCl3) δ 7.29-7.31 (m, 2 H, ArH), 7.10-7.12
(m, 1 H, ArH), 6.80-6.88 (m, 2 H, ArH), 5.54-5.58 (m, 1 H),
5.48-5.51 (m, 1 H), 5.26-5.41 (m, 3 H), 5.15-5.17 (m, 1 H),
4.88-5.05 (m, 2 H), 4.42-4.44 (m, 1 H), 3.95-4.25 (m, 6 H),
3.85 (s, 3 H, OCH3), 3.72-3.75 (m, 1 H), 3.66-3.68 (m, 1 H),
2.60-2.64 (m, 1 H, H-3eq), 2.16 (s, 3 H), 2.13 (s, 3 H), 2.10 (s, 3
H), 2.18 (s, 3 H), 2.07 (s, 3 H), 2.04-2.06 (m, 1 H, H-3ax), 2.03
(s, 3 H), 2.02 (s, 3 H), 2.01 (s, 3 H); 13C NMR (CDCl3) δ 170.9,
170.7, 170.6, 170.5, 170.4, 170.3, 169.9, 168.0, 167.9, 129.4, 128.8,
128.7, 124.4, 120.5, 119.3, 96.9, 96.6, 72.3, 71.6, 71.1, 70.7, 69.5,
69.2, 68.7, 68.2, 67.4, 67.3, 66.9, 62.6, 61.8, 53.2, 49.3, 49.0, 38.0,
37.5, 31.9, 29.7, 29.2, 20.6-23.2; ESIHRMS calcd for
C40H49N2O21F3Na [M + Na]+ 973.2679, found 973.2672.
quenched with Et3N, and the solid was then filtered off. The filtrate
was concentrated under vacuum to yield a syrupy residue, which
was purified by column chromatography on silica gel (80:1 CH2Cl2/
CH3OH) to give trisaccharide 9 (32 mg, 93%): [R]20D -5.6 (c 0.5,
CH2Cl2); 1H NMR (CDCl3) δ 7.21-7.35 (m, 11 H, ArH),
6.95-6.96 (m, 2 H, ArH), 6.74-6.76 (m, 1 H, ArH), 5.60-5.62
(m, 1 H, H-8), 5.35-5.37 (m, 1 H, H-7), 5.28 (t, 1 H, J ) 8.4 Hz,
H-2′′), 5.09 (d, 1 H, J ) 10.2 Hz, N-H), 5.04 (t, 1 H, J ) 8.4 Hz,
H-2′), 5.00 (d, 1 H, J ) 11.0 Hz, PhCH2), 4.94 (d, 1 H, J ) 7.7
Hz, H-1′), 4.89 (dd, 1 H, J ) 11.3, 4.4 Hz, H-4), 4.86 (d, 1 H, J
) 3.3 Hz, H-4′), 4.84 (d, 1 H, J ) 8.0 Hz, H-1′′), 4.57-4.66 (m,
4 H, H-3′, PhCH2), 3.35-4.36 (m, 1 H, H-9a), 4.04-4.09 (m, 2
H, H-5, H-5′), 3.89-3.96 (m, 3 H, H-4′′, H-6a′′, H-9b), 3.84 (s, 3
H, -OCH3), 3.75-3.79 (m, 2 H, H-3′′, H-6a′), 3.75 (s, 3 H,
-OCH3), 3.62-3.69 (m, 4 H, H-5′′, H-6b′, H-6b′′, H-6), 2.58 (dd,
1 H, J ) 12.8, 4.3 Hz, H-3eq), 2.20 (s, 3 H, Ac), 2.15 (s, 3 H, Ac),
2.06 (s, 3 H, Ac), 2.05 (s, 3 H, Ac), 2.01 (s, 3 H, Ac), 1.98 (s, 3
H, Ac), 1.87 (s, 3 H, Ac), 1.86 (s, 3 H, Ac), 1.72 (t, 1 H, J ) 12.5
Hz, H-3ax), 1.16 (brs, 9 H); 13C NMR (CDCl3) δ 176.8, 170.9,
170.6, 170.4, 170.3, 170.2, 170.1, 170.0, 167.9, 155.3, 151.5, 138.5,
138.4, 128.3, 128.1, 127.4, 127.3, 127.2, 127.0, 118.3, 114.5, 100.6,
96.9, 81.3, 75.4, 74.2, 73.3, 72.5, 72.1, 71.5, 70.8, 70.6, 69.3, 68.7,
67.7, 67.4, 67.2, 62.4, 61.6, 55.6, 53.2, 49.1, 38.8, 37.5, 31.9, 29.7,
29.4, 29.2, 27.1, 23.2, 22.7, 21.4, 21.0, 20.8, 20.7, 20.6, 20.5; TOF
MS ES+ calcd for C64H81NO28Na [M + Na]+ 1334.4857, found
1334.4837.
4-Methoxyphenyl (Methyl 4,7,8,9-tetra-O-acetyl-3,5-dideoxy-5-
acetamidyl-D-glycero-ꢀ-D-galacto-2-nonulopyranosylonate)-(2f3)-
2,4,6-tri-O-acetyl-ꢀ-D-galactopyranosyl-(1f4)-2-pivaloyl-3,6-di-O-
benzyl-ꢀ-D-glucopyranoside (9). To a solution of sialyl galactose
donor 6 (66 mg, 0.07 mmol) and glucose acceptor 8 (25 mg, 0.05
mmol) in CH2Cl2 (5 mL) was added powdered molecular sieves (4
Å, 100 mg). The reaction mixture was stirred for 30 min at room
temperature, and then TMSOTf (1.9 uL, 0.007 mmol) was added
at -20 °C. The stirring continued at -20 °C until TLC revealed
full conversion of acceptor (about 30 min). The reaction was
Acknowledgment. This work is supported by the National
Basic Research Program of China (2003CB716400).
Supporting Information Available: Experimental proce-
dures and copies of NMR spectra of 4a-c,e-g, 6, 8, 9-11,
13, 14, and GM3 1. This material is available free of charge
via the Internet at http://pubs.acs.org.
JO800138P
4290 J. Org. Chem. Vol. 73, No. 11, 2008