Stereoselective α-sialylation with sialyl xanthate and phenylsulfenyl triflate as a promotor

Article abstract of DOI:10.1021/jo951711w

Reaction α- and β-xanthates 2 and 3 of sialic acid with glycosyl acceptors 5-8 in the presence phenylsulfenyl triflate (PST) as a promoter in a 2:1 mixture of CH₃CN/CH₂Cl₂ at low temperature affords α-sialosides in good yield and stereoselectivity. PST is prepared-in. situ by reacting benzenesulfenyl chloride with silver triflate. Less reactive acceptors 5 and 6 give a higher α/β ratio than more reactive allylic alcohol 7 and primary alcohol 8; α-stereoselectivity is increased in a dilute solution. A possible mechanism of the reaction that involves intermediate α- and β-nitrilium cations 16 and 17 is discussed.

Full text of DOI:10.1021/jo951711w

1702
J . Org. Chem. 1996, 61, 1702-1706
Ster eoselective r-Sia lyla tion w ith Sia lyl Xa n th a te a n d
P h en ylsu lfen yl Tr ifla te a s a P r om otor
Valeri Martichonok and George M. Whitesides*
Department of Chemistry, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138
Received September 19, 1995X
Reaction R- and â-xanthates 2 and 3 of sialic acid with glycosyl acceptors 5-8 in the presence
phenylsulfenyl triflate (PST) as a promotor in a 2:1 mixture of CH3CN/CH2Cl2 at low temperature
affords R-sialosides in good yield and stereoselectivity. PST is prepared in situ by reacting
benzenesulfenyl chloride with silver triflate. Less reactive acceptors 5 and 6 give a higher R/â
ratio than more reactive allylic alcohol 7 and primary alcohol 8; R-stereoselectivity is increased in
a dilute solution. A possible mechanism of the reaction that involves intermediate R- and â-nitrilium
cations 16 and 17 is discussed.
In tr od u ction
MST is usually prepared in situ from methanesulfenyl
bromide and silver triflate. Methanesulfenyl bromide in
turn is synthesized from dimethyl disulfide and bromine
and is unstable. Both DMTST and MST are unstable and
Here we report an efficient R-stereoselective synthesis
of sialyl glycosides using phenylsulfenyl triflate (PST) as
a new promotor for sialyl xanthates 2 and 3 (Scheme 1).
Sialic acid (Neu5Ac) frequently terminates the oligosac-
chride chains of the 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 crucial in intercellular recogni-
tion.4 Sialosides have been the subject of extensive
research, and recent reviews describe synthetic ap-
proaches to their synthesis.5
Enzymatic synthesis has been used for multigram
syntheses of sialosides.6 It is, however, generally limited
to natural products, or to close structural analogs of
them, by the specificities of these enzymes. Practical and
stereocontrolled chemical syntheses are of particular
interest, especially for the preparation of analogs of
sialosides. Recent reports have established the potential
in oligosaccharide synthesis of thioglycosides and of sialic
acid xanthate 3 when these compounds are activated
with equimolar amounts of thiophilic reagents in nitrile
solvents at low temperature.7 Dimethyl(methylthio)-
sulfonium triflate (DMTST)8 and methylsulfenyl triflate
(MST)9 are the most effective activators; DMTST is
prepared from dimethyl disufide and methyl triflate.
toxic, and some of the intermediates and reagents used
in their preparation also are toxic, unstable, and expen-
sive. Toxicity and a certain amount of O-alkylation limits
their use to the small-scale reactions. Although the
desired R-product is formed predominantly in nitrile
solvents, as much as 30% of the undesired â-product has
been reported in some cases.10 A substantial amount of
the sialic acid-derived donor undergoes elimination to
sialyl glycal 4; this side reaction decreases yields of the
R-product to modest values.
In this paper we show that R-sialyl glycosides can be
efficiently prepared from sialyl xanthates 2 and 3 using
PST as a promotor. PST is, in fact, superior to both
DMTST and MST. We demonstrate that this procedure
can be used for a gram-scale synthesis of protected GM3
trisaccharide 10. We discuss a possible mechanism of
the reaction.
* Author to whom correspondence should be addressed.
X Abstract published in Advance ACS Abstracts, February 1, 1996.
(1) Schauer, R. Adv. Carbohydr. Chem. Biochem. 1982, 40, 131.
Hakomori, S. I. Annu. Rev. Immunol. 1984, 307, 560. Ravindranath,
M. H.; Higa, H. H.; Cooper, E. L.; Paulson, J . C. J . Biol. Chem. 1985,
260, 8850. Paulson, J . C. Interactions of Animal Viruses with Cell
Surface Receptors; Conn, P. M., Ed.; Academic Press: New York, 1985;
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Epithelial Surfaces; Savage, D. C., Fletcher, M., Eds.; Plenum Press:
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F.; Nsimba-Lubaki, M.; Peeters, B.; Peumans, W. J . J . Biol. Chem.
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Lo¨nn, H.; Stenvall, K. Tetrahedron Lett. 1992, 33, 115-116. Hasegawa,
A.; Ohki, H.; Nagahama, T.; Ishida, H.; Kiso, M. Carbohydr. Res. 1991,
212, 277-281. Sabesan, S.; Neira, S.; Davidson, F.; Duus, J .; Bock, K.
J . Am. Chem. Soc. 1994, 116, 1616-1634. Hasegawa, A.; Nagahama,
T.; Ohki, H.; Kiso, M. J . Carbohydr. Chem. 1992, 11, 699-714.
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Ravenscroft, M.; Roberts, R. M. G.; Tillett, J . G. J . Chem. Soc., Perkin
Trans. 2 1982, 1569-1572.
(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.
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65, 753-762. DeNinno, M. P. Synthesis 1991, 583-593.
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0022-3263/96/1961-1702$12.00/0 © 1996 American Chemical Society

Stereoselective R-Sialylation
J . Org. Chem., Vol. 61, No. 5, 1996 1703
Ta ble 1. Sia lyla tion w ith Sia lyl Xa n th a te 2
donor/acceptor
acceptor
ratio
promotor
conditionsa
product
yieldb (%)
R:â
5
5
6
6
7
7
8
6
1:1.3
1:1.3
1:1.5
1.5:1
1:1.2
1.5:1
1:1.5
1:1.3
PhSCl/AgOTf
PhSCl/AgOTf/DTBP
PhSCl/AgOTf
-70 °C f -40 °C/3 h
-70 °C/3 h
9
9
63
74
61
78c
48
54
59
31
94:6
95:5
96:4
-70 °C f -40 °C/3 h
-70 °C/3 h
10
10
11
11
12
10
PhSCl/AgOTf/DTBP
PhSCl/AgOTf/DTBP
PhSCl/AgOTf/DTBP
PhSCl/AgOTf
96:4d
83:17
90:10
84:16
86:14
-70 °C f -40 °C/3 he
-70 °C/15 mine
-70 °C f -40 °C/1 h
-70 °C/1 h
MeSBr/AgOTff
a
b
The concentration of 2 was ∼0.05 mol/L; the ratio 2:PhSCl:AgOTf:DTBP was 1:1.05:1.1:1.2. The yield was determined by 1H NMR
and refers to a mixture of R- and â-anomers. c The isolated yield. A ratio of >99:1 was obtained in a diluted solution, with the concentration
d
of the donor 2 ≈ 0.01 mol/L, albeit in 52% yield. e The ratio 2:PhSCl:AgOTf:DTBP was 1:1.05:1.1:2.5.
Sch em e 1
more reactive 3-OH and gave rise to the predominantly
R-products 9-11. Racemic alcohol 8 was used as a model
for NeuAc-2,6-Gal linkage that often occurs as an element
in gangliosides. Compound 12 was obtained as a dia-
stereomeric mixture.
Resu lts a n d Discu ssion
P r ep a r a t ion of Sia lyl Xa n t h a t es. When sialyl
chloride 1 was allowed to react with O-ethyl S-potassium
dithiocarbonate in acetone at ambient temperature, a
mixture of R- and â-sialyl xanthates 2 and 3 and glycal
4 was obtained (Scheme 1) in molar ratio 16:1:3, as
determined by 1H NMR. The proportion of glycal/product
was increased when the reaction was carried out at 0 °C.
R-Sialyl xanthate 2 was reported as the only a product
when the reaction was carried out in EtOH.11 We
obtained the same product ratio in this solvent as in
acetone. Chromatographic separation gave xanthates 2
and 3, which were contaminated with sialyl glycal 4,
probably due to their partial decomposition on silica gel.
Sia lyla tion . MST is less reactive than DMTST and,
according to literature data, is more effective than
DMTST for R-sialylation in terms of yield and stereose-
lectivity. We reasoned that PST, which is less reactive
then MST, would be even better. PST was prepared in
situ by reacting benzenesulfenyl chloride with silver
triflate. Benzenesulfenyl chloride was synthesized by
reacting phenyl thioacetate with SO2Cl2.12 It also can
be conveniently prepared by treatment of thiophenol or
diphenyl disulfide with Cl2 or SO2Cl2.13 We found that
unlike unstable methanesulfenyl bromide, benzenesulfe-
nyl chloride was stable for at least 10 months at 4 °C
under nitrogen. Sialylation of acceptors 5-8 with xan-
The yield and purity of the product increased when di-
tert-butylpyridine (DTBP) was used as a proton scaven-
ger. We note that less reactive alcohols 5 and 6 gave a
higher R/â ratio than more reactive allylic alcohol 7 and
primary alcohol 8. This ratio is reversed relative to that
thate 2 was studied in a 2:1 mixture of acetonitrile/
dichloromethane under conditions that are summarized
in Table 1. Sialylation of acceptors 5-7 occurred at the
(11) Marra, A.; Sinay, P. Carbohydr. Res. 1989, 187, 35-42.
which was observed with sialyl chloride 1 in in the
presence of silver or mercury salts. Furthermore, we
(12) Thea, S.; Cevasco, G. Tetrahedron Lett. 1988, 29, 2865-2866.
(13) Kuhle, E. The Chemistry of the Sulphenic Acids; Georg Thieme
Publishers: Stuttgart, 1973.
observed that R-stereoselectivity increased when the

1704 J . Org. Chem., Vol. 61, No. 5, 1996
Martichonok and Whitesides
by integration of H(3)eq signals in 1H NMR spectra.
Chemical shifts were smaller for â-glycosides (in the
range 2.58-2.42 ppm for â-anomers of compounds 2, 3,
9-12) than for R-glycosides (in the range 2.67-2.51 ppm
for compounds 2, 3, 9-12).16
Sch em e 2
Mech a n ism of th e Rea ction . We propose that in the
first step sialyl xanthate reacts with PST to give the
oxonium cation 14 via the formation of the intermediate
13 (Scheme 2). This conjecture is supported by the
following facts: (i) the R/â ratio of the final product is
independent of the stereochemistry of the starting xan-
thate; (ii) compound 15 was isolated in 86% as a major
byproduct of the reaction. In the second step oxonium
cation 14 is stabilized by reaction with acetonitrile to
form nitrilium cations 16 and 17. Nitrilium cations have
been evoked to explain the stereoselectivities of glycosi-
lation in acetonitrile solvents.17 According to our semiem-
pirical MO calculations (PM318 molecular model), 16 (R
) Me) is 1.46 kcal/mol more stable than 17. The
nitrilium cation 16 is probably formed first due to kinetic
and thermodynamic control. The attack of acetonitrile
from the re side of 14 to form 17 is hindered by
unfavorable steric interactions with protons at C-4 and
C-6 of the sialic acid; both 16 and 17 are probably formed
in this step. We assume that the nitrilium cations 16
and 17 are in equilibrium. In the third step acceptor
ROH reacts with the cation 17 to give R-product 18.
Attack on cation 16 is very hindered due to protons at
C-4 and C-6 and to the CO2Me group. A small amount
of the â-product is formed by reaction of the acceptor
ROH with oxonium cation 14. More reactive acceptors,
such allylic alcohol 7 and primary alcohol 8, are more
likely to compete with acetonitrile for 14 and give a lower
R/â ratio. The dilution of the reaction mixture with the
solvent increases the ratio [CH3CN]/[ROH] and, conse-
quently, increases the R/â ratio for the product of the
reaction.
reaction was conducted in a dilute solution, although the
yield was lower. Only R-product 10 was detected by H
1
NMR with 1.5 equiv of donor 2 relative to acceptor 6 in
a dilute solution. The yields of products 9, 10, and 12
were good; the yield of 11 was somewhat lower because
PST can add to the double bond of the galactosyl glycal
7. About 30% of the unreacted xanthate 2 was detected
in this reaction. Sialyl glycal 4 was formed as a byprod-
uct of the reaction.
We found that sialylation of lactosyl acceptor 6 with
pure â-xanthate 3 gave the same R/â ratio of the product
10 as sialylation with R-xanthate 2. For large-scale
sialylations compounds 2, 3, and 4 were not separated.
Using the crude reaction mixture of 2, 3, and 4 as sialyl
donor, we prepared 10sprotected GM3 trisaccharideson
a gram scale.
The site of sialylation in the product 10 was deter-
mined by acetylation and observation of a downfield shift
of the C-4′ proton. The stereochemistry of the new gly-
cosidic linkage in products 9-12 was determined to be
R on the basis of the occurrence of the NeuAc H-4 at ∼4.8
ppm as well as the J NeuAc7-8 > 7.0 Hz.14 The stereochem-
istry of 10 was also confirmed by the measurement of
the long-range coupling constant J C-1,H-3ax ) 5.10 Hz of
the sialic acid residue.15 In addition, the anomeric
configuration of 2 and 3 was confirmed by the chemical
shift difference between the two hydrogen atoms at
position 9 [∆δ(H(9) - H(9′))]: ∆δ ) 0.50 ppm for R-2, ∆δ
) 0.14 ppm for â-3.14 The R/â ratios were determined
Con clu sion
PST is superior to MST in terms of both yield and
stereoselectivity of sialylation. High stability of benze-
nesulfenyl chloride, combined with ease of its of prepara-
tion and its low toxicity, makes PST an effective reagent
for stereoselective R-sialylation. Important deficiencies
still remain in sialylation, however. Although PST is a
useful reagent, it is not a panacea in synthesis of car-
bohydrates. Among these residual problems are the
following: (i) overall yields are only moderate due to the
formation of sialyl glycal during preparation of sialyl
xanthates and during sialylation; (ii) some amount of the
undesired â-product is always formed, especially with
reactive alcohols; (iii) the product of the reaction is
contaminated with traces of byproducts, separation of
which requires careful chromatography.
(15) Hori, H.; Nakajima, T.; Nishida, Y.; Ohrui, H.; Meguro, H.
Tetrahedron Lett. 1988, 29, 6317-6320.
(16) Dabrowski, U.; Friebolin, H.; Brossmer, R.; Supp, M. Tetrahe-
dron Lett. 1979, 48, 4637-4640.
(17) Ratcliffe, A. J .; Fraser-Reid, B. J . Chem. Soc., Perkin Trans. 1
1990, 747-750. Schmidt, R. R.; Behrendt, M.; Toepfer, A. Synlett 1990,
694-696. Braccini, I.; Derouet C.; Esnault, J .; Herve´ du Penhoat, C.;
Mallet, J .-M.; Michon, V.; Sinay¨, P. Carbohydr. Res. 1993, 246, 23-
41.
(18) Stewart, J . J . P. J . Comput. Chem 1989, 10, 209-220. Stewart,
J . J . P. J . Comput. Chem 1989, 10, 221-264. PM3 was used from
within MOPAC 6.0 (QCPE 455).
(14) Okamoto, K.; Kondo, T.; Goto, T. Bull. Chem. Soc. J pn. 1987,
60, 637-643.

Stereoselective R-Sialylation
Exp er im en ta l Section
J . Org. Chem., Vol. 61, No. 5, 1996 1705
P r ep a r a tion of Accep tor s 5-7. Methyl(2,3)-O-2,6-bis-O-
(phenylmethyl)-â-D-galactopyranoside (5) was prepared in 66%
yield from methyl D-galactopyranoside following the procedure
for synthesis of 6. Rf ) 0.49 (50% EtOAc in hexanes); mp 76-
Gen er a l Meth od s. Anhydrous reagents and solvents were
prepared according to literature procedures.19 N-Acetyl-
neuraminic acid was obtained from extraction of edible Chi-
nese swiftlet’s nest.20 O-Ethyl S-potassium dithiocarbonate
was recrystallized before use from EtOH. Elemental analyses
were performed by Galbraith Laboratories, Knoxville, TN. The
proton chemical shifts for all compounds were assigned using
1H homonuclear decoupling experiments.
77 °C, [R]23D ) +11.0 (c 1.68, CHCl3); lit.22 mp 80-81 °C, [R]25
D
) +10.0 (c 0.40, CHCl3). 2-(Trimethylsilyl)ethyl 2,6-bis-O-
(phenylmethyl)-â-D-galactopyranosyl-(1,4)-2,3,6-tris-O-(phenyl-
methyl)-â-D-glucopyranoside (6) was prepared in total 26%
yield from lactose (six steps23 ). Compound 6 was crystallyzed
from Et2O/pentane: mp 88-89 °C, [R]23 ) +15.9 (c 1.13,
D
Sia lyl Xa n th a tes 2 a n d 3. To a suspension of sialic acid
methyl ester21 (4.90 g, 15.2 mmol) in AcCl (250 mL) cooled to
0 °C was slowly added absolute MeOH (3.0 mL). The mixture
was allowed to warm to rt, and stirring was continued for 3
days, during which time a clear solution was obtained. The
end of the reaction was determined by NMR on a small sample
(∼0.5 mL) of the reaction mixture that was periodically taken,
concentrated in vacuo, and redissolved in CDCl3. The reaction
mixture was concentrated in vacuo, and residual AcCl was
coevaporated three times with CHCl3. The obtained crude
product was dissolved in AcOEt (20 mL) and run through a
short silica gel column (4 × 10 cm), eluting with AcOEt.
Concentration in vacuo afforded chloride 1 as a white foam
(7.04 g, 91%). To a solution of chloride 1 (6.0 g, 11.8 mmol) in
dry acetone (250 mL) was added solid O-ethyl S-potassium
dithiocarbonate (2.14 g, 13.3 mmol) in small portions within
10 min. The mixture was stirred under N2 in the dark at 0
°C for 12 h and then at rt for additional 12 h. The reaction
mixture was diluted with pentane (150 mL), filtered, and
concentrated in vacuo. The residue was dissolved in CHCl3
(150 mL) and washed with H2O (2 × 50 mL), saturated
NaHCO3 (50 mL), and brine (30 mL). After drying (MgSO4)
and concentration in vacuo, the residue was run through a
short silica gel column (4 × 10 cm), eluting with 25% acetone
in CHCl3. The eluents were concentrated to afford a mixture
of 2 and 3 in molar ratio 16:1 contaminated with 19% w/w of
sialyl glycal 4 (6.21 g, 72% of 2 + 3, 21% of 4). Column
chromatography of 2.6 g of this mixture with 50% of EtOAc
in CHCl3 afforded xanthate 3, Rf ) 0.28, and known10b
xanthate 2, Rf ) 0.22). O-Eth yl S-(N-a cetyl-4,7,8,9-tetr a -
CHCl3), lit.20 mp 99.5-101 °C (after trituration); 1H NMR (400
MHz, C6D6): δ -0.26 (s, 9 H, Si(CH3)3), 1.02 (m, 2 H, CH2-
TMS, 2.27 (d, J ) 5.48 Hz, 1 H, HO-3), 2.47 (d, J ) 2.73 Hz,
1 H, HO-4), 3.25 (t, J ) 5.73 Hz, 1 H, H-5′), 3.34 (m, 1 H,
H-3′), 3.39 (broad, dd, J ) 2.30, 9.83 Hz, 1 H, H-5), 3.46 (dd,
J ) 5.30, 9.89 Hz, 1 H, H-6a′), 3.57 (dd, J ) 7.85, 9.25 Hz, 1
H, H-2′), 3.59-3.67 (m, 3 H, H-2, H-6b′, OCH2CH2TMS), 3.71-
3.77 (m, 3 H, H-3, H-6a, H-4′), 3.95 (dd, J ) 3.92, 10.97 Hz, 1
H, H-6b), 4.12 (dt, J ) 7.10, 9.54 Hz, 1 H, OCH2CH2TMS),
4.23 (d, J ) 12.09 Hz, 1 H), 4.30 (t, J ) 9.37 Hz, 1 H, H-4),
4.30 (d, J ) 12.14 Hz, 1 H), 4.38 (d, J ) 12.15 Hz, 1 H), 4.44
(d, J ) 7.70 Hz, 1 H, H-1), 4.51 (d, J ) 12.10 Hz, 1 H), 4.60 (d,
J ) 7.77 Hz, 1 H, H-1′), 4.69 (d, J ) 11.60 Hz, 1 H), 4.86 (d,
J ) 10.64 Hz, 1 H), 4.89 (d, J ) 11.03 Hz, 1 H), 4.97 (d, J )
11.13 Hz, 1 H), 5.08 (d, J ) 11.51 Hz, 1 H), 5.27 (d, J ) 11.12
Hz, 1 H), 7.04-7.61 (m, 25 H, aromatic); 13C NMR (100 MHz,
CDCl3) δ -1.48, 18.37, 67.24, 68.29, 68.58, 68.68, 72.82, 73.05,
73.34, 73.44, 74.76, 75.02, 75.06, 76.61, 79.96, 81.82, 82.75,
102.51, 103.05, 127.11, 127.39, 127.44, 127.49, 127.54, 127.67,
127.76, 127.87, 127.93, 128.15, 128.19, 128.28, 128.37, 137.95,
138.21, 138.31, 138.66, 139.10; HRMS (FAB) calcd for
C34H49N4O8Na (M + Na) 335.3678, found 335.3678. 1,5-
Anhydro-6-O-(tert-butyldimethylsilyl)-2-deoxy-D-lyxo-hex-1-
enopyranose (7) was prepared from D-galactal and tert-
butyldimethylsilyl chloride according to Danishefsky et al.24
Ben zen esu lfen yl Ch lor id e. To a solution of phenyl
thioacetate (25 g, 164 mmol) in CCl4 (10 mL) cooled to 0 °C
was added SO2Cl2 (13.3 mL, 164 mmol) within 10 min. The
mixture was stirred at rt for 30 min, concentrated in vacuo,
and distilled using Kugelrohr. Distillation with a Vigreux
column afforded benzenesulfenyl chloride as a red liquid (19.3
g, 81%), bp 51-53 °C/3 mmHg, d 1.25 g/cm3 (lit.25 bp 61.5 °C/5
mmHg).
O-a cetyl-1-m eth yl-â-n eu r a m in osyl)d ith ioca r bon a te (3)
1
(98 mg) as a white foam: [R]23 ) +79.2 (c 0.96, CHCl3); H
D
NMR (400 MHz, CDCl3) δ 1.36 (t, J ) 7.07, 3 H), 1.85 (s, 3 H),
1.94 (t, J ) 12.63 Hz, 1 H, H-3ax), 2.00 (s, 3 H), 2.11 (s, 6 H,
two C(O)CH3), 2.13 (s, 3 H), 2.57 (dd, J ) 4.64, 12.96 Hz, 1 H,
H-3eq), 3.51 (dd, J ) 7.99, 12.16 Hz, 1 H, H-9a), 3.79 (s, 3 H,
CO2CH3), 4.01 (dd, J ) 2.80, 12.11 Hz, 1 H, H-9b), 4.01 (q, J
) 10.38 Hz, 1 H, H-5), 4.53 (m, 1 H), 4.63 (dd, J ) 2.07, 10.73
Hz, 1 H, H-6), 4.77 (m, 1 H), 4.83 (dt, J ) 4.60, 12.73 Hz, 1 H,
H-4), 5.21 (ddd, J ) 2.81, 4.13, 8.00 Hz, 1 H, H-8), 5.33 (dd, J
) 2.13, 4.08 Hz, 1 H, H-7), 5.58 (bd, J ) 10.10 Hz, 1 H, NH);
13C NMR (100 MHz, CDCl3) δ 13.26, 20.68, 20.75, 20.83, 20.89,
23.08, 36.89, 42.58, 49.05, 53.37, 68.61, 70.89, 73.41, 86.41,
168.72, 170.03, 170.21, 170.77, 206.92; HRMS (FAB) calcd for
C23H33NO13S2Na (M + Na) 618.1291, found 618.1298. Accord-
ing to its 1H NMR spectrum, compound 3 was contaminated
with ∼7% w/w of glycal 4. O-Eth yl S-(N-a cetyl-4,7,8,9-tetr a -
O-a cetyl-1-m eth yl-r-n eu r a m in osyl)-d ith ioca r bon a te (2)
(2.32 g) as a white foam, lit.10b mp 102-104 °C (benzene/
hexane): 1H NMR (500 MHz, CDCl3) δ 1.35 (t, J ) 7.13, 3 H,
OCH2CH3), 1.87 (s, 3 H), 1.99 (dd, J ) 11.89, 12.73 Hz, 1 H,
H-3ax), 2.00 (s, 3 H), 2.01 (s, 3 H), 2.10 (s, 3 H), 2.12 (s, 3 H),
2.61 (dd, J ) 4.65 12.95 Hz, 1 H, H-3eq), 3.78 (s, 3 H, CO2CH3),
4.00 (q, J ) 10.40 Hz, 1 H, H-5), 4.17 (dd, J ) 5.22, 12.39 Hz,
1 H, H-9a), 4.31 (dd, J ) 2.43, 12.46 Hz, 1 H, H-9b), 4.50-
4.58 (m, 2 H, OCH2CH3, H-6), 4.79 (m, 1 H, OCH2CH3), 4.87
(ddd, J ) 4.63, 10.36, 11.90 Hz, 1 H, H-4), 5.17 (d, J ) 10.05
Hz, 1 H, NH), 5.26-5.32 (m, 2H, H-7, H-8). According to the
1H NMR spectrum compound 2 was contaminated with ∼15%
w/w of glycal 4 which had the same Rf value as 2. This mixture
was used for the sialylations shown in Table 1.
Gen er a l P r oced u r e for Sia lyla tion . We demonstrate the
general procedure on the synthesis of 10 using the mixture of
xanthates 2 and 3 and glycal 4. The same basic procedure
was applied for all sialylations. A mixture of 2 and 3 and 4
(1.06 g, contained 1.44 mmol of 2 and 3), 6 (0.86 g, 0.96 mmol),
powdered molecular sieves (3.0 g, 4 Å), dry CH3CN (20 mL),
and dry CH2Cl2 (10 mL) was stirred under N2 for 1 h. AgOTf
(0.41 g, 1.58 mmol) and DTBP (381 µL, 1.70 mmol) were added,
and the mixture was cooled to -70 °C and kept protected from
light. PhSCl (180 µL, 1.55 mmol) in dry CH2Cl2 (1 mL) was
added by running the solution down the cold wall of the
reaction flask, and the stirring was continued for 2 h at -70
°C. The mixture was diluted with a suspension of silica gel
(5 g) in EtOAc (30 mL), filtered (Celite), washed (saturated
aqueous NaHCO3 and water), dried (Na2SO4), and concen-
trated. The residue was chromatographed (10% acetone in
CHCl3 f 20% acetone in CHCl3) to give 2-(tr im eth ylsilyl)-
eth yl (N-acetyl-4,7,8,9-tetr a-O-acetyl-1-m eth yl-r-n eu r am i-
n osyl)-(2,3)-2,6-bis-O-(p h en ylm eth yl)-â-D-ga la ctop yr a n o-
s y l -( 1 ,4 ) -2 ,3 ,6 -t r i s -O -( p h e n y l m e t h y l ) -â-D -g l u c o -
p yr a n osid e (10) as a foam (0.97 g, 0.71 mmol, 74%): [R]23
D
1
) +3.0 (c 0.98, CHCl3); H NMR (500 MHz, C6D6) δ -0.18 (s,
9H, Si(SH3)3), 1.01 (m, 2H, CH2Si(SH3)3), 1.56 (s, 3H), 1.60 (s,
3H), 1.70 (s, 3H), 1.87 (s, 3H), 2.03 (s, 3H), 2.12 (t, J ) 12.50
(22) Paulsen, H.; Hasenkamp, T.; Paal, M. Carbohydr. Res. 1985,
144, 45-55.
(23) J ansson, K.; Ahlfors, S.; Frejd, T.; Kihlberg, J .; Magnusson, G.;
Dahmen, J .; Noori, G.; Stenvall, K. J . Org. Chem. 1988, 53, 5629-
5647.
(19) Perin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals; Pergamon Press: New York, 1980.
(20) Roy, R.; Laferriere, C. A. Can. J . Chem. 1990, 68, 2045-2054.
(21) Kuhn, R.; Lutz, P.; MacDonald, D. L. Chem. Ber. 1966, 99, 611-
617.
(24) Danishefsky, S. J .; Gervay, J .; Peterson, J . M.; Mcdonald, F.
E.; Koseki, K.; Griffith, D. A.; Oriyama, T.; Marsden, S. P. J . Am. Chem.
Soc. 1995, 117, 1940-1953.
(25) Almasi, L; Hanz, A. Chem Ber. 1961, 94, 725-728.

1706 J . Org. Chem., Vol. 61, No. 5, 1996
Martichonok and Whitesides
Hz, 1 H, H-3ax′′), 2.67 (dd, J ) 4.57, 12.81 Hz, 1 H, H-3eq′′),
2.86 (d, J ) 3.06 Hz, 1 H, 4-OH′), 3.36 (s, 3 H, CO2CH3), 3.43
(broad dd, J ) 2.62, 9.90 Hz, 1 H, H-5), 3.58-3.65 (m, 3 H,
H-6a′, H-2, OCH2CH2TMS), 3.74 (t, J ) 9.02 Hz, 1 H, H-3),
3.78 (bt, J ) 6.50 Hz, 1 H, H-5′), 3.83 (dd, J ) 7.87, 9.30 Hz,
1 H, H-2′), 3.84 (t, J ) 9.30 Hz, 1 H, H-6a), 3.95 (m, 4 H, H-6b′,
H-3′, H-6b, H-6′′), 4.08-4.11 (m, 2 H, OCH2CH2TMS, H-4′),
4.20 (dd, J ) 6.58, 12.41 Hz, 1 H, H-9a′′), 4.29 (d, J ) 12.04
Hz, 1 H), 4.32-4.40 (m, 3 H, H-4, NH, H-5′′), 4.39 (d, J ) 12.05
Hz, 1 H), 4.43 (d, J ) 7.72 Hz, 1 H, H-1), 4.51 (d, J ) 12.11
Hz, 1 H), 4.57 (d, J ) 12.10 Hz, 1 H), 4.68 (dd, J ) 2.57, 12.39
Hz, 1 H, H-9b′′), 4.78 (dt, J ) 4.50, 11.90 Hz, 1 H, H-4′′), 4.70
(m, 3 H, three CH2Ph), 4.90 (d, J ) 7.74 Hz, 1 H, H-1), 4.97
(d, J ) 10.86 Hz, 1 H), 5.08 (d, J ) 11.58 Hz, 1 H), 5.30 (d, J
) 10.86 Hz, 1 H), 5.42 (dd, J ) 2.30, 7.61 Hz, 1 H, H-7′′), 5.78
(dt, J ) 2.56, 7.08 Hz, 1 H, H-8′′), 7.05 -7.28 (m, 17 H,
aromatic), 7.41 (d, J ) 7.14 Hz, 2 H, aromatic), 7.45 (d, J )
7.34 Hz, 2 H, aromatic), 7.47 (d, J ) 8.20 Hz, 2 H, aromatic),
7.67 (d, J ) 7.18 Hz, 2 H, aromatic); â-anomer δ 2.58 (dd, J )
4.95, 12.96 Hz, 1 H, H-3eq′); 13C NMR (100 MHz, CDCl3) δ
-1.49, 18.39, 20.42, 20.62, 20.73, 21.03, 23.04, 36.35, 49.10,
52.93, 62.20, 67.11, 67.20, 67.80, 68.33, 68.49, 68.84, 69.04,
72.36, 72.65, 72.93, 73.20, 74.79, 75.00, 75.25, 76.25, 76.56,
78.32, 81.91, 82.91, 98.34, 102.30, 102.98, 127.06, 127.19,
127.30, 127.38, 127.93, 127.99, 128.02, 128.10, 128.16, 138.31,
138.47, 138.72, 138.86, 139.09, 168.29 (C-1, J C-1,H-3ax ) 5.10
Hz), 169.85, 169.95, 170.21, 170.50, 170.73; MS (FAB) calcd
for C72H91NO23SiNa (M + Na) 1388, found 1388. Anal. Calcd
for C72H91NO23Si: C, 63.28; H, 6.71. Found: C, 62.97; H, 6.72.
Additional chromatography of the front fraction (17% of CHCl3
in hexanes) afforded O-eth yl SS-p h en yl ca r bon od ith iop -
er oxoa te (15) as a yellow liquid (285 mg, 86%), Rf ) 0.48: 1H
NMR (500 MHz, CDCl3) δ 1.43 (t, J ) 7.10 Hz, 3 H), 4.69 (q,
J ) 7.10 Hz, 2 H), 7.25-7.50 (m, 5 H); 13C NMR (100 MHz,
CDCl3) δ 13.60, 71.48, 128.23, 129.12, 129.93, 135.86, 211.10;
HRMS (EI) calcd for C9H10OS3 229.9894, found 229.9899.
Applying the above procedure for sialylation, the compounds
9, 11, and 12 were prepared. Conditions, reagent ratios, and
yields are given in Table 1. Compounds 9, 11, and 12 had Rf
values (20% acetone in CHCl3 ) close to the starting xanthate
and glycal that was formed as a byproduct of the reaction. The
fractions that contained xanthate, glycal, and the product were
combined and concentrated in vacuo. Yields and product ratios
were determined by 1H NMR. Only selected NMR data are
presented below.
Met h yl (N-a cet yl-4,7,8,9-t et r a -O-a cet yl-1-m et h yl-r-
n eu r a m in osyl)-(2,3)-O-2,6-b is-O-(p h en ylm et h yl)-â-D-ga -
la ctop yr a n osid e (9): 1H NMR (500 MHz, CDCl3) δ 2.51 (dd,
J ) 4.67, 12.78 Hz, 1 H, H-3eq′), 3.54 (s, 3 H, OCH3), 3.75 (s,
3 H, CO2CH3), 3.95 (dd, J ) 5.80, 12.51 Hz, 1 H, H-9a′), 4.00
(dd, J ) 2.21, 10.65 Hz, 1 H, H-6′), 4.29 (dd, J ) 2.50, 12.50
Hz, 1 H, H-9b′), 4.83 (dt, J ) 4.43, 11.88 Hz, 1 H, H-4′), 5.29
(dd, J ) 2.10, 8.04 Hz, 1 H, H-7′); â-anomer δ 2.43 (dd, J )
4.91, 12.90 Hz, 1 H, H-3eq′); MS (FAB) calcd for C41H53NO18
Na (M + Na) 870, found 870.
-
(N-Acet yl-4,7,8,9-t et r a -O-a cet yl-1-m et h yl-r-n eu r a m i-
n osyl)-(2,3)-O-1,5-a n h yd r o-6-O-(ter t-bu tyld im eth ylsilyl)-
2-deoxy-D-lyxo-h ex-1-en opyr an ose (11): 1H NMR (500 MHz,
CDCl3) δ 0.62 (s, 6 H, Si(CH3)2), 0.87 (s, 9 H, Si-t-Bu), 2.62
(dd, J ) 4.66, 12.91 Hz, 1 H, H-3eq′), 2.71 (d, J ) 3.38 Hz, 1
H, 4-OH), 3.84 (s, 3 H, CO2CH3), 6.36 (d, J ) 6.07 Hz, 1 H,
H-1). ); â-anomer δ 2.54 (dd, J ) 4.96, 12.98 Hz, 1 H, H-3eq′);
MS (FAB) calcd for C32H51NO16SiNa (M + Na) 756, found 756.
(1-Tetr a h yd r op yr a n yl)m eth yl (N-a cetyl-4,7,8,9-tetr a -
O-a cetyl-1-m eth yl)-r-n eu r a m in osid e (12) was obtained as
a mixture of two diastereomers. R-Diastereomers 1H NMR
(500 MHz, CDCl3) 2.59 (dd, J ) 4.66, 13.14 Hz, 1 H, H-3eq′),
2.61 (dd, J ) 4.68, 13.28 Hz, 1 H, H-3eq′); 13C NMR (100 MHz,
CDCl3) 98.60 and 98.82. â-Diastereomers: 1H NMR 2.42 (dd,
J ) 5.05, 12.92 Hz, 1 H, H-3eq′), 2.47 (dd, J ) 4.96, 12.98 Hz,
1 H, H-3eq′); 13C NMR (100 MHz, CDCl3) 98.14 and 98.28;
HRMS (FAB) calcd for C26H39NO14Na (M + Na) 612.2268,
found 612.2272.
Ack n ow led gm en t. 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 Harvard University Mass Spec-
trometry Facility was supported by grants from NSF
(CHE-9020043) and NIH (SIO-RR067116).
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra for
crude compounds 9, 11, and 12 (3 pages). This material is
contained in libraries on microfiche, immediately follows this
article in the microfilm version of the journal, and can be
ordered from the ACS; see any current masthead page for
ordering information.
J O951711W