A practical method for the synthesis of sialyl α-glycosides

Article abstract of DOI:10.1021/ja960148b

Addition of 2,4-dimethylbenzenesulfenyl chloride to sialic acid glycal gives crystalline 2-chloro-3-thiosialic acid 3 in 85% yield. Reaction of 3 with sodium thiomethoxide in acetonitrile at 0°C affords the sialic acid donor α-2-(methylthio)-3-thiosialic

Full text of DOI:10.1021/ja960148b

VOLUME 118, NUMBER 35
SEPTEMBER 4, 1996
© Copyright 1996 by the
American Chemical Society
A Practical Method for the Synthesis of Sialyl R-Glycosides
Valeri Martichonok and George M. Whitesides*
Contribution from the Department of Chemistry, HarVard UniVersity, 12 Oxford Street,
Cambridge, Massachusetts 02138
ReceiVed January 18, 1996X
Abstract: Addition of 2,4-dimethylbenzenesulfenyl chloride to sialic acid glycal gives crystalline 2-chloro-3-thiosialic
acid 3 in 85% yield. Reaction of 3 with sodium thiomethoxide in acetonitrile at 0 °C affords the sialic acid donor
R-2-(methylthio)-3-thiosialic acid 4 in quantitative yield. Sialylation of glycosyl acceptors 9 and 10 with 4 in the
presence of phenylsulfenyl triflate (PST) as promotor in CH3CN at -40 °C gives R-sialosides in good yield and
excellent stereoselectivity. No â-sialosides are formed in either case. Removal of the auxiliary 3-(2,4-dimethylphenyl)-
thio group is achieved in high yield using Ph3SnH and AIBN in refluxing toluene. Protected GM3 trisaccharide and
6-sial-2-yllactose were obtained on a gram scale.
Introduction
of oligosaccharides. The most abundant of these linkages
include R2-3 to galactose, R2-6 to galactose or N-acetylga-
lactosamine, and R2-8 to Neu5Ac. Sialosides have been the
subject of extensive research due to their potential as therapeu-
tics, and recent reviews describe synthetic approaches to their
synthesis.5
Stereoselective R-sialylation is one of the unsolved problems
in carbohydrate chemistry. In this paper we describe a new
2-(methylthio)-3-thiosialic acid donor that is conveniently
prepared in two steps from sialyl glycal. We show that when
activated with phenylsulfenyl triflate (PST), this donor gives
R-sialosides in excellent yield and stereospecificity.
Practical and stereocontrolled chemical syntheses of R-sia-
losides are important, because sialic acid (Neu5Ac) frequently
terminates the oligosaccharide chains of glycoproteins and
glycolipids that play a central role in cell surface recognition
phenomena.1 Cell surface sialosides serve as ligands for
microbial toxins,2 microbial adhesins that mediate attachment
to host cells,3 and lectins that mediate intercellular recogni-
tion.4 Sialic acids are usually found in the terminal positions
Sialic acid possesses a C-3 deoxy structure and exists solely
as a 2R (equatorial) glycoside which is less favored in a
stereoelectronic sense than a corresponding 2â (axial) glycoside.
The C-2 carbon atom of sialic acid, to which sugar residues
must be attached in glycosylation reactions, is quaternary and
carries an electron-withdrawing carboxylate group; the formation
of the carbocation is disfavored sterically and electronically.
Classical methods in glycoside synthesis are, therefore, not
successful. Donors based on the methyl ester of 2-halosialic
acid give poor yield and stereoselectivity. Better stereoselec-
tivity is obtained with the benzyl ester; the yield, however,
remains low.6 Stereoselectivity can also be increased with
thioglycosides of sialic acid, when these compounds are
activated with equimolar amounts of thiophilic reagents in nitrile
X Abstract published in AdVance ACS Abstracts, August 1, 1996.
(1) Paulson, J. C. In Interactions of Animal Viruses with Cell Surface
Receptors; Conn, P. M., Ed.; Academic Press: New York, 1985; 131.
(2) Karlsson, K.-A. Molecular Mechanisms of Microbial Adhesion;
Springer-Verlag: New York, 1988; pp 77-96.
(3) Sharon, N.; Lis, H. Science 1989, 246, 227-234. Paulson, J. C. The
Receptors; Academic Press: New York, 1985; pp 131-219.
(4) Springer, T. A.; Lasky, L. A. Nature 1991, 349, 196-197. Paulson,
J. C. Adhesion: Its Role in Inflammatory Disease; Stockton Press: New
York, 1992; pp 19-42. Crocker, P. R.; Kelm, S.; Dubois, C.; Martin, B.;
McWilliam, A. S.; Shotton, D. M.; Paulson, J. C.; Gordon, S. EMBO J.
1991, 10, 1661-1669.
(5) (a) Ito, Y.; Gaudino, J. J.; Paulson, J. C. Pure Appl. Chem. 1993, 65,
753-762. (b) DeNinno, M. P. Synthesis 1991, 583-593.
(6) Ratcliffe, M. R.; Venot, A. P.; Abbas, Z. S. Eur. Pat. Appl. EP 319,-
253, 1990; Chem. Abstr. 1990, 112, 175281a.
S0002-7863(96)00148-5 CCC: $12.00 © 1996 American Chemical Society

8188 J. Am. Chem. Soc., Vol. 118, No. 35, 1996
Martichonok and Whitesides
Scheme 1
Scheme 2a
a Reagents and conditions: (a) (i) Me2C(OMe)2, CSA catalyst, DMF,
25 °C, 1 h; (ii) NaH, BnBr, DMF; (b) aqueous AcOH, 80 °C, overall
yield from 6: 9, 70%; 10, 11%.
Scheme 3
solvents at low temperature.7 A substantial amount of the sialic
acid-derived donor undergoes elimination to sialyl glycal during
glycosylation; this side reaction leads to modest yields. In
practice, an excess of these donors (they are expensive and
relatively difficult to prepare) is used in order to obtain
reasonable yields.
Sialyl donors that have an auxiliary Br, OH, SPh, or SePh
group have recently been introduced,5b in order to overcome
the problem of poor stereoselectivity and glycal formation. These
donors give exclusively R-sialosides and do not undergo
undesired elimination to sialyl glycal. They, however, have not
found wide use in carbohydrate chemistry due to the fact that
multistep syntheses are required for their preparation.
Enzymatic synthesis, on the other hand, can be used for
multigram synthesis of sialosides. It is, however, generally
limited to natural products and their analogs. In most cases
sialosides that can be obtained using enzymes are susceptible
to hydrolytic cleavage in ViVo by sialidases. Chemical synthesis
offers an advantage that analogs of sialosides can be prepared
that are stable to sialidases.
Scheme 2. The (trimethylsilyl)ethyl-protected lactose 69 was
reacted with acetone dimethyl acetal and subsequently benzy-
lated to give a mixture of acetonides 7 and 8 which were
separated by silica gel chromatography. Removal of the
acetonide group afforded acceptors 9 and 10 as crystalline
materials.
Results and Discussion
We reacted sialyl glycal 1 with 2,4-dimethylbenzenesulfenyl
chloride (10 equiv) in CH2Cl2 for two weeks at 0 °C under N2
and obtained a crystalline chloride, 3, in 85% yield.8 The
chloride 5 was formed as the only byproduct (4% yield) and
remained in the mother liquor during crystallization (Scheme
1). The ratio of 3 to 5 did not change when the reaction was
carried out at ambient temperature. The reaction of chloride 3
with sodium thiomethoxide (1.5 equiv) in acetonitrile at 0 °C
afforded methyl thioglycoside 4 in quantitive yield. Compound
4 was indefinetely stable at 4 °C.
Sialylation10 of acceptors 9 and 10 with donor 4 was
effectively achieved in CH3CN at -40 °C using PST11 as
promotor and 2,6-di-tert-butylpyridine (DTBP) as proton scav-
enger (Scheme 3). Sialylation of acceptor 9 occurred selectively
at the equatorial 3′-OH, rather than the axial 4′-OH, and
generated the R-product 11 in 78% yield. Sialylation of 10
occurred at the primary 6′-OH, giving 14 in 83% yield. The
only compounds in the product mixture were the unreacted
acceptors (used in excess) and a small amount of the unreacted
We prepared acceptors used for sialylation according to
(9) Jansson, K.; Ahlfors, S.; Frejd, T.; Kihlberg, J.; Magnusson, G.;
Dahmen, J.; Noori, G.; Stenvall, K. J. Org. Chem. 1988, 53, 5629-5647.
(10) Sialylation can be performed with chloride 3 using AgOTf as
promotor at -15 °C in THF for two days. The yield with acceptor 9,
however, is moderate (40-50%), the product is contaminated, and careful
chromatography is required in order to obtain pure 11.
(11) A similar procedure using methanesulfenyl triflate (MST) as
promotor has been described by Magnusson.8 MST, however, and some of
the intermediates and reagents used in its preparation are toxic and unstable.
Furthermore, a certain amount of O-alkylation products limit use of MST
to the small-scale reactions.7c
(7) (a) Lo¨nn, H.; Stenvall, K. Tetrahedron Lett. 1992, 33, 115-116. (b)
Sabesan, S.; Neira, S.; Davidson, F.; Duus, J.; Bock, K. J. Am. Chem. Soc.
1994, 116, 1616-1634. (c) Martichonok, V.; Whitesides, G. M. J. Org.
Chem. 1996, 61, 1702-1706.
(8) Addition of benzenesulfenyl chloride to sialic acid glycal gives a
2.3:1 mixture of diastereomeric chlorides. The desired major diastereomer
was isolated by gradient column chromatography and converted to ethyl
thioglycoside via a two-step procedure. See (a) Erce´govic, T.; Magnusson,
G. J. Chem. Soc., Chem. Commun. 1994, 831-832. (b) Erce´govic, T.;
Magnusson, G. J. Org. Chem. 1995, 3378-3384.

Synthesis of Sialyl R-Glycosides
J. Am. Chem. Soc., Vol. 118, No. 35, 1996 8189
mother liquor was concentrated in Vacuo. The residue was distilled in
Vacuo to give 2 (4.71 g, 75%), bp 58 °C, 0.5 mmHg (lit.16 bp 87 °C,
2 mmHg).
donor 4. No â-sialosides were detected in either case. Acety-
lation of 11, and subsequent reductive removal of the auxiliary
(2,4-dimethylphenyl)thio group using Ph3SnH (10 equiv)/AIBN
(1.2 equiv) in refluxing toluene, afforded 13sprotected GM3
trisaccharidesin 91% yield after two steps. The same procedure
for 14 afforded protected 6-sial-2-yllactose 16san element of
many gangliosidessin 94% total yield. We applied this method
for gram scale syntheses of 13 and 16 without any loss in yield
and stereoselectivity.
Methyl 5-Acetamido-4,7,8,9-tetra-O-acetyl-2-chloro-3-[(2,4-di-
methylphenyl)thio]-2,3,5-trideoxy-D-erythro-â-L-gluco-2-nonulopy-
ranosonate (3). To a solution of sialic acid glycal (3.76 g, 7.95 mmol)
in CH2Cl2 (10 mL) cooled to -10 °C was added dropwise 2,4-
dimethylbenzenesulfenyl chloride (5.43 g, 31.5 mmol), and the mixture
was left under N2 in the dark at 0 °C for 14 days. The reaction mixture
was loaded on a short silica gel column and eluted with 10% ethyl
acetate in CHCl3 to give a mixture of chlorides 3 and 5 in a 95:1 ratio
We identified the site of sialylation in the products 11 and
14 on the basis of observation of a downfield shift of the
C-4′ proton of their acetylation products. We determined
the stereochemistry of the glycosidic linkage in compound 3
to be â and in compounds 4, 11, and 14 to be R by the
measurement of the long-range 13C-1H coupling constants
1
as determined by H NMR. Compound 3 (4.38 g, 85% yield) was
obtained in pure form by crystallization of the mixture from CH2Cl2/
Et2O/hexane: mp 95-105 °C; [R]23 ) +2.3° (c 0.88, CHCl3); IR
D
(neat) 3504-3402 (br), 1753, 1749, 1665, 1372, 1221, 1035 cm -1; 1H
NMR (400 MHz, CDCl3) δ 1.84 (s, 3 H), 1.86 (s, 3 H), 2.02 (s, 3 H),
2.07 (s, 3 H), 2.10 (s, 3 H), 2.25 (s, 3 H, p-CH3 aromatic), 2.28 (s, 3
H, o-CH3 aromatic), 3.68 (s, 3 H, CO2CH3), 3.97 (dd, J ) 5.43, 12.58
Hz, 1 H, H-9), 3.98 (d, J ) 10.71 Hz, 1 H, H-3), 4.24 (dd, J ) 2.77,
12.58 Hz, 1 H, H-9), 4.34 (q, J ) 9.97 Hz, 1 H, H-5), 4.38 (dd, J )
2.45, 10.89 Hz, 1 H, H-6), 5.08 (ddd, J ) 2.70, 5.30, 8.11 Hz, 1 H,
H-8), 5.33 (dd, J ) 9.89, 10.60 Hz, 1 H, H-4), 5.41 (dd, J ) 2.34, J
) 8.15, 1 H, H-7), 5.45 (bd, J ) 9.81 Hz, 1 H, NHCOCH3), 6.95 (s,
1 H, aromatic), 6.97 (d, J ) 7.90 Hz, 1 H, aromatic), 7.41 (d, J ) 7.90
Hz, 1 H, aromatic); 13C NMR (125 MHz, CDCl3) δ 20.49, 20.66, 20.70,
20.87, 22.98, 49.26, 53.99, 57.35, 62.04, 66.56, 69.16, 73.74, 103.18,
127.72, 129.69, 131.01, 132.22, 137.64, 139.20, 164.03 (C-1, JC-1,H-3ax
< 1.00 Hz), 169.37, 169.86, 170.20, 170.47, 171.08; HRMS (FAB)
calcd for C28H36ClNO12SNa (M + Na) 668.1544, found 668.1562.
Compound 5 was obtained for analytical purposes by gradient chro-
matography (5% acetone in CHCl3 f 10% acetone in CHCl3) of the
fraction of the mother liquor: 1H NMR (400 MHz, CDCl3) δ 1.62 (s,
3 H), 1.92 (s, 3 H), 2.04 (s, 3 H), 2.05 (s, 3 H), 2.19 (s, 3 H), 2.25 (s,
3 H), 2.41 (s, 3 H), 3.82 (s, 3 H, CO2CH3), 4.15 (dd, J ) 5.86, 12.54
Hz, 1 H, H-9a), 4.19 (d, J ) 3.84 Hz, 1 H, H-3), 4.39 (dd, J ) 2.14,
10.83 Hz, 1 H, H-6), 4.47 (dd, J ) 2.54, 12.50 Hz, 1 H, H-9b), 4.73
(q, J ) 10.46 Hz, 1 H, H-5), 5.26 (ddd, J ) 2.54, 5.86, 6.43 Hz, 1 H,
H-8), 5.33 (bd, J ) 9.71 Hz, 1 H, NHCOCH3), 5.41 (dd, J ) 2.20, J
) 6.71, 1 H, H-7), 5.77 (dd, J ) 3.82, 10.66, 1 H, H-4), 6.94 (d, J )
7.90 Hz, 1 H), 6.95 (s, 1 H), 7.29 (d, J ) 7.88 Hz, 1 H); MS (FAB)
calcd for C28H36ClNO12SNa (M + Na) 668, found 668.
13
1
(J C(C-1)- H(H-3,axial) of the sialic acid residue using the method
described by Hori.12
Conclusions
Addition of 2,4-dimethylbenzenesulfenyl chloride to sialyl
glycal and subsequent substitution of chlorine with sodium
thiomethoxide is an effective and economic method for the
preparation of 2-(methylthio)-3-(phenylthio)sialic acid donor 4.
The reaction of this donor with glycosyl acceptors in the
presence of PST in CH3CN at low temperature affords R-sia-
losides in good yield, excellent stereoselectivity, and high purity.
Donor 4 does not undergo the elimination to sialyl glycal during
sialylation; good yields of R-sialosides are obtained using less
than an equimolecular amount of it. Phenylsulfenyl triflate that
is used as promotor for the sialylation is prepared in situ from
silver triflate and stable benzenesulfenyl chloride. In compari-
son with other promotors used for thioglycosides, phenylsulfenyl
triflate is nontoxic and does not give O-alkylated byproducts.
Sialosides prepared using this method contain the 3-(2,4-
dimethylphenyl)thio group, which is removed in high yield using
Ph3SnH.
The procedure described is superior in terms of stereospeci-
ficity, yield, simplicity, and reliability to any currently available
chemical method for the preparation of sialyl R-glycosides. In
terms of cost, in fact, it is superior to the enzymatic methods.
It opens new avenues for the preparation of bioactive stable
analogs of sialosides. We are currently studying the applications
of this method for the synthesis of complex sialyl oligosaccha-
rides.
Methyl 5-Acetamido-4,7,8,9-tetra-O-acetyl-2-chloro-3-[(2,4-di-
methylphenyl)thio]-2,3,5-trideoxy-D-erythro-â-L-gluco-2-nonulopy-
ranosonate (4). To a stirred suspension of MeSNa17 (408 mg, 5.83
mmol) in dried MeCN (molecular sieves 4 Å) (50 mL) cooled to 0 °C
was added chloride 3 (2.50 g, 3.87 mmol). The mixture was stirred at
0 °C for 3 h, after which the TLC (50% EtOAc in CHCl3) indicated
that the reaction was complete. The reaction mixture was diluted with
a suspension of silica gel (10 g) in EtOAc (50 mL), filtered through a
short silica gel column (2 × 10 cm), eluted with EtOAc, and
concentrated in Vacuo to give compound 4 (2.52 g, 100%) as a foam:
[R]23D ) +86.8° (c 1.14, CHCl3); 1H NMR (500 MHz, CDCl3) δ 1.84
(s, 3 H, SCH3), 1.88 (s, 3 H), 2.01 (s, 3 H), 2.11 (s, 3 H), 2.12 (s, 3 H),
2.14 (s, 3 H), 2.26 (s, 3 H, p-CH3 aromatic), 2.35 (s, 3 H, o-CH3
aromatic), 3.46 (d, J ) 11.09 Hz, 1 H, H-3), 3.79 (dd, J ) 2.00, 10.94
Hz, 1 H, H-6), 3.88 (s, 3 H, CO2CH3), 4.07 (dd, J ) 5.44, 12.51 Hz,
1 H, H-9a), 4.10 (q, J ) 10.36 Hz, 1 H, H-5), 4.29 (dd, J ) 2.68,
12.53 Hz, 1 H, H-9b), 5.22 (t, J ) 10.69 Hz, 1 H, H-4), 5.26-5.34
(m, 3 H, H-7, H-8, NH), 6.94 (s, 1 H, aromatic), 6.99 (d, J ) 8.02 Hz,
1 H, aromatic), 7.48 (d, J ) 8.00 Hz, 1 H, aromatic); 13C NMR (100
MHz, CDCl3) δ 12.09, 20.64, 20.68, 20.73, 20.84, 21.05, 23.09, 50.46,
52.93, 58.34, 62.06, 67.42, 69.12, 73.67, 74.19, 87.22, 127.73, 130.87,
131.47, 132.96, 136.93, 138.61, 167.44, 169.97, 170.06, 170.10, 170.57,
170.91; HRMS (FAB) calcd for C32H43NO14SNa (M + Na) 720.2302,
found 720.2276.
Experimental Section
General Methods. Anhydrous reagents and solvents were prepared
according to literature procedures.13 Sialic acid was obtained from
extraction of edible Chinese swiftlet’s nest.14 Sialic acid glycal was
prepared according to a published procedure.8b 2,4-Dimethylthiophenol
was obtained form Aldrich. Benzenesulfenyl chloride was synthesized
by reacting phenyl thioacetate with SO2Cl2.15 The proton chemical
1
shifts for all compounds were assigned using H homonuclear decou-
pling experiments.
2,4-Dimethylbenzenesulfenyl Chloride (2). To an ice-cooled sus-
pension of N-chlorosuccinimide (4.61 g, 34.5 mmol) in CH2Cl2 (50
mL) was added dropwise within 15 min a solution of 2,4-dimethyl-
thiophenol (4.75 g, 34.4 mmol) in CH2Cl2 (20 mL). The mixture was
stirred at rt for 2 h, diluted with pentane (200 mL), and left over-
night at 4 °C. The precipitate of succinimide was filtered, and the
2-(Trimethylsilyl)ethyl [2,6-bis-O-(phenylmethyl)-â-D-galactopy-
ranosyl]-(1,4)-2,3,6-tris-O-(phenylmethyl)-â-D-glucopyranoside (9)
and 2-(trimethylsilyl)ethyl [2,3-bis-O-(phenylmethyl)-â-D-galacto-
pyranosyl]-(1,4)-2,3,6-tris-O-(phenylmethyl)-â-D-glucopyranoside (10)
(12) Hori, H.; Nakajima, T.; Nishida, Y.; Ohrui, H.; Meguro, H.
Tetrahedron Lett. 1988, 29, 6317-6320.
(13) Perin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals; Pergamon Press: New York, 1980.
(14) Roy, R.; Laferriere, C. A. Can. J. Chem. 1990, 68, 2045-2054.
(15) Thea, S.; Cevasco, G. Tetrahedron Lett. 1988, 29, 2865-2866.
(16) Almasi, L; Hanz, A. Chem Ber. 1961, 94, 725-728.
(17) It is important to weigh sodium thiomethoxide in a glovebox and
to use dry CH3CN.

8190 J. Am. Chem. Soc., Vol. 118, No. 35, 1996
Martichonok and Whitesides
were prepared according to a modified published procedure.9 Reaction
of TMSE-protected lactose 6 with acetone dimethyl acetal and
subsequent benzylation afforded a mixture of acetonides 7 and 8, not
just 7 as reported.9 This mixture was separated by column chroma-
tography on silica gel eluting with 10% EtOAc in toluene. Treatment
of 7 with 80% aqueous acetic acid gave known7c acceptor 9 in 70%
overall yield. The same reaction of acetonide 8 afforded acceptor 10
in 11% overall yield. Compound 10 was crystallized from Et2O/
2-(Trimethylsilyl)ethyl 2,3,6-tri-O-benzyl-4-O-{2,3-di-O-benzyl-
6-O-[methyl [5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-3-[(2,4-
dimethyphenyl)thio]-D-erythro-r-L-gluco-2-nonulopyranosid]onate]-
â-D-galactopyranosyl}-â-D-glucopyranoside (14) was prepared in 83%
yield from 4 and 10 using the procedure for preparation of 11: Rf )
1
0.45 (20% acetone in CHCl3); [R]23 ) +17.2° (c 1.02, CHCl3); H
D
NMR (500 MHz, C6D6) δ 0.00 (s, 9 H, CH2Si(CH3)3), 1.00-1.10 (m,
2 H, CH2Si(CH3)3), 1.60 (s, 3 H), 1.66 (s, 3 H), 1.85 (s, 3 H), 1.86 (s,
3 H), 1.98 (s, 3 H), 2.21 (s, 3 H, CH3 aromatic), 2.35 (s, 3 H, CH3
aromatic), 2.75 (dd, J ) 4.03, 8.30, 1 H), 2.98 (d, J ) 3.06, 1 H, 4′-
OH), 2.36 (dd, J ) 3.99, 9.58 Hz, 1 H, H-5′), 3.31 (dd, J ) 3.11, 9.44
Hz, 1 H, H-3′), 3.36 (m, 1 H, H-5), 3.38 (d, J ) 10.86 Hz, 1 H, H-3′′),
3.52 (s, 3 H, CO2CH3), 3.61-3.66 (m, 2 H, H-2, OCH2CH2Si(CH3)3),
3.70 (t, J ) 8.85 Hz, 1 H, H-3), 3.73 (t, J ) 9.46 Hz, 1 H, H-4), 3.91
(dd, J ) 8.03, 9.25 Hz, 1 H, H-2′), 3.99 (dd, J ) 4.06, 9.88 Hz, 1 H,
H-6a), 4.04 (t, J ) 9.33, Hz, 1 H, H-6b), 4.12 (m, 1 H, OCH2CH2Si-
(CH3)3), 4.18 (dd, J ) 7.02, 12.50 Hz, 1 H, H-9a′′), 4.20 (dd, J )
9.47, 11.20 Hz, 1 H, H-6b′), 4.30 (t, J ) 2.73 Hz, 1 H, H-4′), 4.42 (d,
J ) 12.21 Hz, 1 H), 4.44 (d, J ) 7.59 Hz, 1 H, H-1), 4.48 (d, J )
12.12 Hz, 1 H), 4.50 (dd, J ) 2.24, 10.67 Hz, 1 H, H-6′′), 4.51 (d, J
) 7.77 Hz, 1 H, H-1′), 4.56 (d, J ) 11.64 Hz, 1 H), 4.62 (dd, J )
2.73, 12.36 Hz, 1 H, H-9b′′), 4.69 (q, J ) 10.64 Hz, 1 H, H-5′′), 4.78-
4.84 (m, 3 H), 4.92 (d, J ) 10.49 Hz, 1 H), 5.04 (d, J ) 11.63 Hz, 1
H), 5.18 (d, J ) 11.62 Hz, 1 H), 5.24 (d, J ) 10.49 Hz, 1 H), 5.45 (dd,
J ) 2.21, 7.51 Hz, 1 H, H-7′′), 5.66 (dt, J ) 2.76, 7.24 Hz, 1 H, H-8′′),
5.77 (t, J ) 10.52 Hz, 1 H, H-4′′), 6.76 (s, 1 H), 6.79 (d, J ) 7.97 Hz,
1 H), 7.06-7.68 (m, 26 H, aromatic); 13C NMR (100 MHz, CDCl3) δ
-1.41, 18.46, 20.61, 20.66, 20.75, 20.81, 21.10, 23.12, 49.73, 52.51,
54.70, 60.33, 62.32, 64.35, 67.27, 68.29, 68.98, 71.13, 72.12, 72.25,
72.42, 73.10, 74.74, 74.78, 75.10, 75.20, 77.18, 79.32, 80.74, 81.74,
82.63, 101.29, 102.86, 103.10, 127.26, 127.38, 127.43, 127.47, 127.53,
127.57, 127.63, 127.77, 127.95, 128.00, 128.17, 128.22, 128.26, 128.29,
129.38, 130.96, 134.34, 138.36, 138.39, 138.42, 138.74, 139.30, 140.47,
167.83, 169.39, 170.00, 170.24, 170.62, 171.01; MS (FAB) calcd for
C80H99NO23SSiNa (M + Na) 1524, found 1524.
pentane: Rf ) 0.31 (33% EtOAc in hexanes); mp 112-114 °C; [R]23
D
) +23.8° (c 1.33, CHCl3); 1H NMR (500 MHz, C6D6) -0.14 (s, 9 H,
Si(CH3)3), 0.96-1.07 (m, 2 H, CH2TMS), 2.20-2.38 (br s, 1 H, OH-
6), 2.42 (d, J ) 9.16 Hz, 1 H, OH-4), 3.03 (m, 1 H, H-5′), 3.15 (dd,
J ) 3.27, 9.29 Hz, 1 H, H-3′), 3.41 (br dd, J ) 2.51, 9.84 Hz, 1 H,
H-5), 3.55-3.66 (m, 4 H, OCH2CH2TMS, H-2, H-6a′, H-6b′), 3.71 (t,
J ) 9.00 Hz, 1 H, H-3), 3.74-3.84 (m, 3 H, H-2′, H-6a, H-4′), 3.96
(dd, J ) 4.06, 10.94 Hz, 1 H, H-6b), 4.08-4.14 (m, 1 H, OCH2CH2-
TMS), 4.24 (dt, J ) 2.11, 9.38 Hz, 1 H, H-4), 4.36-4.41 (m, 3 H, 3
CH2Ph), 4.43 (d, J ) 7.71 Hz, 1 H, H-1), 4.52 (d, J ) 12.13 Hz, 1 H),
4.58 (d, J ) 7.83 Hz, 1 H, 1 H′), 4.78 (d, J ) 11.36 Hz, 1 H), 4.86 (d,
J ) 11.79 Hz, 1 H), 4.89 (d, J ) 11.46 Hz, 1 H), 4.94 (d, J ) 10.82
Hz, 1 H), 5.08 (d, J ) 11.51 Hz, 1 H), 5.26 (d, J ) 10.82 Hz, 1 H),
7.05-7.32 (m, 19 H, aromatic), 7.39 (d, J ) 7.40 Hz, 2 H, aromatic),
7.44 (d, J ) 7.60 Hz, 2 H, aromatic), 7.62 (d, J ) 7.62 Hz, 2 H,
aromatic); HRMS (FAB) calcd for C52H64O11SiNa (M + Na) 915.4116,
found 915.4110.
2-(Trimethylsilyl)ethyl 2,3,6-Tri-O-benzyl-4-O-{2,6-di-O-benzyl-
3-O-[methyl [5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-3-[(2,4-
dimethyphenyl)thio]-D-erythro-r-L-gluco-2-nonulopyranosid]onate]-
â-D-galactopyranosyl}-â-D-glucopyranoside (11). Toluene (2 × 20
mL) was evaporated from the mixture of 4 (591 mg, 0.9 mmol) and 9
(981 mg, 1.1 mmol). The mixture was dissolved in CH3CN (20 mL)
and DTBP (337 µL, 1.5 mmol) and dried, and crushed molecular sieves
4 Å (3.0 g) were added. The mixture was stirred at rt for 0.5 h, then
AgOTf (308 mg, 1.20 mmol) was added, and stirring was continued in
the dark for an additional 1 h. After cooling to -40 °C, PhSCl (116
µL, 0.19 mmol) was added dropwise and the mixture was stirred at
-40 °C for 1 h. Diisopropylamine (280 µL, 0.35 mmol) was added,
and the mixture was stirred at -40 °C for 0.5 h. A suspension of
silica gel (10 g) in EtOAc (75 mL) was added to the cold reaction
mixture. After filtration through a fritt and concentration in Vacuo,
the reaction mixture was chromatographed, eluting with 10% acetone
in CHCl3 f 15% acetone in CHCl3 to give trisaccharide 11 (1.05 g,
2-(Trimethylsilyl)ethyl 2,3,6-Tri-O-benzyl-4-O-{2,6-di-O-benzyl-
4-O-acetyl-3-O-[methyl [5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-
dideoxy-3-[(2,4-dimethyphenyl)thio]-D-erythro-r-L-gluco-2-nonu-
lopyranosid]onate]-â-D-galactopyranosyl}-â-D-glucopyranoside (12).
Compound 11 (1.43 g, 0.95 mmol) was acetylated in a mixture of CH2-
Cl2 (10 mL), pyridine (5 mL), Ac2O (2.5 mL), and a few crystals of
DMAP to give 12 (1.42 g, 97%): Rf ) 0.51 (33% CH3CN in toluene);
1
[R]23 ) +18.6° (c 1.61, CHCl3); a signal for H-4′ in H NMR (500
78%): Rf ) 0.48 (20% acetone in CHCl3); [R]23 ) +13.7°(c 0.99,
D
D
1
MHz, C6D6) was detected at δ 5.84 (d, J ) 3.26 Hz); MS (FAB) calcd
for C82H101NO24SSiNa (M + Na) 1566, found 1566.
CHCl3); H NMR (500 MHz, C6D6) δ -0.37 (s, 9 H, CH2Si(CH3)3),
0.99 (m, 2 H, CH2Si(CH3)3), 1.54 (s, 3 H), 1.69 (s, 3 H), 1.70 (s, 3 H),
1.83 (s, 3 H), 1.92 (s, 3 H), 2.01 (s, 3 H, CH3 aromatic), 2.20 (s, 3 H,
CH3 arom), 3.03 (br s, 1 H, 4-OH), 3.40 (m, 1 H, H-5), 3.47 (s, 3 H,
CO2CH3), 3.50 (d, J ) 11.37 Hz, 1 H, H-3′′), 3.58 (t, J ) 8.92 Hz, 1
H, H-2), 3.61 (m, 1 H, OCH2CH2Si(CH3)3), 3.69 (dd, J ) 5.55, 9.55
Hz, 1 H, H-6a′), 3.75 (t, J ) 9.10 Hz, 2 H, H-3), 3.77 (t, J ) 9.53 Hz,
1 H, H-2′), 3.88 (m, 1 H, H-5′), 3.89 (br s, 1 H, H-4′), 3.95 (dd, J )
4.33, 10.95 Hz, 1 H, H-6a), 3.99 (dd, J ) 7.20, 9.55 Hz, 1 H, H-6b′),
4.10 (dt, J ) 7.00, 9.50 Hz, 1 H, OCH2CH2Si(CH3)3), 4.16 (dd, J )
6.37, 12.52 Hz, 1 H, H-9a′′), 4.22 (dd, J ) 2.31, 10.85 Hz, 1 H, H-6′′),
4.22-4.34 (m, 4 H, H-6b, H-4, NH C NNH2Ph), 4.43 (d, J ) 7.55 Hz,
1 H, H-1), 4.44 (d, J ) 11.98 Hz, 1 H), 4.56 (d, J ) 11.89 Hz, 1 H),
4.63 (d, J ) 11.25 Hz, 1 H), 4.65 (m, 2 H, H-5′′, H-9′′), 4.69 (d, J )
12.55 Hz, 1 H), 4.73 (dd, J ) 3.24, 9.44 Hz, 1 H, H-3′), 4.86 (d, J )
11.57 Hz, 1 H), 4.90 (d, J ) 12.58 Hz, 1 H), 4.92 (d, J ) 7.89 Hz, 1
H, H-1′), 4.96 (d, J ) 10.95 Hz, 1 H), 5.07 (d, J ) 11.47 Hz, 1 H),
5.30 (d, J ) 10.94 Hz, 1 H), 5.42 (dd, J ) 2.30, 7.69 Hz, 1 H, H-7′′),
5.48 (t, J ) 10.77 Hz, 1 H, H-4′′), 5.69 (dt, J ) 2.50, 7.80 Hz, 1 H,
H-8′′), 6.70 (s, 1 H), 6.74 (d, J ) 7.99 Hz, 1 H), 7.05-7.68 (m, 26 H,
aromatic); 13C NMR (100 MHz, CDCl3) δ -1.43, 18.46, 20.51,
20.61, 20.64, 20.80, 20.93, 23.15, 49.89, 52.83, 57.37, 62.14, 67.09,
67.13, 67.25, 68.59, 68.71, 72.44, 72.51, 72.54, 72.96, 73.18, 74.10,
74.88, 76.36, 76.68, 77.00, 77.32, 78.40, 82.01, 82.78, 100.44, 102.49,
103.01, 126.83, 127.13, 127.18, 127.29, 127.33, 127.40, 127.42, 127.47,
128.17, 128.22, 128.35, 130.13, 131.11, 134.28, 138.21, 138.46, 138.72,
138.83, 139.15, 139.24, 140.18, 168.56, 169.31, 169.95, 170.21, 170.45,
170.95; MS (FAB) calcd for C80H99NO23SSiNa (M + Na) 1524,
found 1524.
2-(Trimethylsilyl)ethyl 2,3,6-Tri-O-benzyl-4-O-{2,3-di-O-benzyl-
4-O-acetyl-6-O-[methyl [5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-
dideoxy-3-[(2,4-dimethyphenyl)thio]-D-erythro-r-L-gluco-2-nonu-
lopyranosid]onate]-â-D-galactopyranosyl}-â-D-glucopyranoside (15).
Compound 15 was prepared in 96% yield as described for 12: Rf )
0.52 (33% CH3CN in toluene); [R]23 ) +19.8° (c 1.22, CHCl3); a
D
1
signal for H-4′ in H NMR (500 MHz, C6D6) was detected at δ 5.66
(d, J ) 3.59); MS (FAB) calcd for C82H101NO24SSiNa (M + Na) 1566,
found 1566.
2-(Trimethylsilyl)ethyl 2,3,6-Tri-O-benzyl-4-O-{4-O-acetyl-2,6-
di-O-benzyl-3-O-[methyl [5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-
dideoxy-D-erythro-r-L-gluco-2-nonulopyranosid]onate]-â-D-galac-
topyranosyl}-â-D-glucopyranoside (13). AIBN (170 mg, 1.04 mmol)
was added in small portions every 1 h over a period of 10 h to the
mixture of 12 (1.26 g, 0.82 mmol) and Ph3SnH (2.88 g, 0.82 mmol) in
refluxing toluene (60 mL) under N2. The reaction was then refluxed
for an additional 5 h, cooled, and applied directly to a silica gel column.
Elution (20% acetonitrile in toluene f 33% acetonitrile in toluene,
gradient) gave the unreacted starting material 12 (49 mg, 4%) and the
product 13 (1.08 g, 94%): Rf
) 0.38 (33% CH3CN in toluene), [R]23
D
) -7.2° (c 0.85, CHCl3); 1H NMR (500 MHz, C6D6) δ -0.29 (s, 9H,
Si(SH3)3), 0.98 (m, 2H, CH2Si(SH3)3), 1.59 (s, 3H), 1.60 (s, 3H), 1.74
(s, 3H), 1.76 (s, 3H), 1.81 (s, 3H), 2.00 (t, J ) 12.70 Hz, 1 H, H-3ax′′),
2.09 (s, 3H), 2.84 (dd, J ) 4.68, 12.71 Hz, 1 H, H-3eq′′), 3.38-3.47
(m, 3 H, H-5, H-5′, H-6a′), 3.57-3.63 (m, 3 H, H-2, H-6′′, OCH2CH2-
TMS), 3.74 (t, J ) 9.04, 1 H, H-3), 3.79 (s, 3 H, CO2CH3), 3.83 (dd,
J ) 7.88, 9.46 Hz, 1 H, H-2′), 3.92 (d, J ) 10.45, 1 H), 3.96 (dd, J )

Synthesis of Sialyl R-Glycosides
J. Am. Chem. Soc., Vol. 118, No. 35, 1996 8191
1.60, 10.94 Hz, 1 H, H-6a), 4.00 (dd, J ) 4.24, 10.94 Hz, 1 H, H-6b),
4.06-4.12 (m, 2 H, OCH2CH2TMS, H-6b′), 4.28 (dd, J ) 5.30, 12.55
Hz, 1 H, H-9a′′), 4.36-4.43 (m, 4 H, CH2Ph, H-5′′, H-1, H-4), 4.60
(bd, J ) 11.70 Hz, 2 H, CH2Ph, H-9b′′), 4.77 (d, J ) 12.03 Hz, 1 H),
4.83 (d, J ) 11.67 Hz, 1 H), 4.87 (d, J ) 12.57 Hz, 1 H), 4.90 (dt, J
) 4.53, 11.86 Hz, 1 H, H-4′′), 4.94-4.98 (m, 2 H, H-3′), 5.06 (bd, J
) 12.00 Hz, 1 H, two CH2Ph), 5.17 (d, J ) 7.60 Hz, 1 H, H-1′), 5.33
(d, J ) 10.88 Hz, 1 H), 5.43 (dd, J ) 2.64, 8.68 Hz, 1 H, H-7′′), 5.48
(d, J ) 3.28 Hz, 1 H, H-4′), 5.96 (ddd, J ) 2.63, 5.20, 8.25 Hz, 1 H,
H-8′′), 7.00-7.65 (m, 25 H, aromatic); 13C NMR (100 MHz, CDCl3)
δ -1.45, 18.43, 20.37, 20.69, 20.75, 21.21, 23.13, 37.52, 49.10, 53.02,
62.02, 66.99, 67.24, 67.53, 68.45, 68.60, 68.90, 69.48, 71.31, 72.13,
72.75, 73.09, 73.80, 74.82, 74.92, 74.95, 76.62, 76.69, 77.00, 77.32,
79.42, 81.98, 82.82, 97.26, 102.04, 102.92, 126.95, 127.04, 127.15,
127.27, 127.38, 127.43, 127.58, 127.70, 127.90, 127.99, 128.06, 128.14,
128.18, 138.13, 138.65, 138.72, 139.26, 139.36, 167.82, 169.84, 169.91,
170.27, 170.49, 170.75; MS (FAB) calcd for C74H93NO24SiNa (M +
Na) 1430, found 1430.
2-(Trimethylsilyl)ethyl 2,3,6-tri-O-benzyl-4-O-{4-O-acetyl-2,3-di-
O-benzyl-6-O-[methyl [5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-
D-erythro-r-L-gluco-2-nonulopyranosid]onate]-â-D-galactopyranosyl}-
â-D-glucopyranoside (16) was prepared in 98% yield from 15 (730
mg, 0.47 mmol) utilizing the procedure for the preparation of 13: Rf
) 0.38 (33% CH3CN in toluene); [R]23D ) -1.9° (c 2.62, CHCl3); 1H
NMR (500 MHz, C6D6) δ -0.27 (s, 9H, Si(CH3)3), 0.99 (m, 2H, CH2-
Si(CH3)3), 1.57 (s, 3H), 1.58 (s, 3H), 1.81 (s, 3H), 1.84 (s, 3H), 1.95
(s, 3H), 2.05 (s, 3H), 2.12 (t, J ) 12.73 Hz, 1 H, H-3ax′′), 2.80 (dd, J
) 4.65, 12.78 Hz, 1 H, H-3eq′′), 3.42 (dd, J ) 3.49, 9.70 Hz, 1 H,
H-3′), 3.44 (s, 3 H, CO2CH3), 3.51 (ddd, J ) 1.55, 4.85, 9.52 Hz, 1 H,
H-5), 3.60 (dd, J ) 7.76, 8.90 Hz, 1 H, H-2), 3.63 (m, 1 H, OCH2CH2-
TMS), 3.70 (m, 2 H, H-5′, H-6a′), 3.78 (t, J ) 8.96 Hz, 1 H, H-2′),
3.80 (dd, J ) 7.86, 9.50 Hz, 1 H, H-3), 3.91 (dd, J ) 1.46, 10.85 Hz,
1 H, H-6a), 3.99 (dd, J ) 4.94, 10.93 Hz, 1 H, H-6b), 4.10 (dd, J )
2.05, 10.23 Hz, 1 H, H-6′′), 4.12 (m, 1 H, OCH2CH2TMS), 4.18 (m,
2 H, H-6b′, NH), 4.29 (dd, J ) 6.56, 12.35 Hz, 1 H, H-9a′′), 4.34 (t,
J ) 9.52 Hz, 1 H, H-4), 4.35 (d, J ) 11.15 Hz, 1 H),4.40-4.47 (m, 3
H, CH2Ph, H-5′′, H-1), 4.56 (d, J ) 12.27 Hz, 1 H), 4.69 (dd, J )
2.63, 12.31 Hz, 1 H, H-9b′′), 4.79 (d, J ) 11.10 Hz, 1 H), 4.82 (d, J
) 12.43 Hz, 1 H), 4.83 (d, J ) 11.69 Hz, 1 H), 4.84 (d, J ) 7.80 Hz,
1 H, H-1′), 4.85 (d, J ) 10.33 Hz, 1 H), 4.86 (dt, J ) 4.65, 11.91 Hz,
1 H, H-4′′), 5.02 (d, J ) 11.18 Hz, 1 H), 5.06 (d, J ) 11.62 Hz, 1 H),
5.34 (d, J ) 11.15 Hz, 1 H), 5.47 (dd, J ) 2.25, 7.31 Hz, 1 H, H-7′′),
5.74 (d, J ) 3.43 Hz, 1 H, H-4′), 5.78 (dt, J ) 2.65, 6.87 Hz, 1 H,
H-8′′), 7.06 -7.44 (m, 23 H, aromatic), 7.68 (d, J ) 7.36 Hz, 2 H,
aromatic); 13C NMR (100 MHz, CDCl3) δ -1.43, 18.49, 20.69, 20.80,
20.82, 20.87, 20.99, 23.19, 37.75, 49.41, 52.87, 62.28, 62.69, 66.19,
67.30, 67.35, 68.49, 68.86, 68.90, 71.28, 71.98, 72.74, 73.05, 74.73,
75.18, 76.68, 77.00, 77.21, 77.32, 79.37, 80.09, 81.92, 82.83, 98.91,
102.12, 103.07, 127.16, 127.40, 127.45, 127.64, 127.67, 127.93, 128.03,
128.10, 128.20, 128.27, 128.28, 137.86, 138.45, 138.59, 138.74, 139.26,
167.81, 169.62, 170.00, 170.07, 170.19, 170.56, 170.95; MS (FAB)
calcd for C74H93NO24SiNa (M + Na) 1430, found 1430.
Acknowledgment. This work was supported by NIH Grant
GM30367. The NMR facilities at Harvard were supported by
NIH Grants 1-S10-RR04870-01 and CHE-8814019. The Har-
vard University Mass Spectrometry Facility was supported by
grants from NSF (CHE-9020043) and NIH (SIO-RR067116).
Supporting Information Available: 1H NMR and 13C NMR
1
spectra for compounds 3, 4, 11, 13, 14, and 16 and H NMR
for compound 10 (13 pages). See any current masthead page
for ordering and Internet access instructions.
JA960148B