A novel 5-thioglycosylation method with 1,5-dithioglycosyl donors: relevance to exo- versus endocyclic activation

Article abstract of DOI:10.1016/j.tetlet.2007.09.056

5-Thioglucosylation of a 1,6-anhydro-2-azido-2-deoxy-β-d-glucopyranose derivative was carried out with various 1,5-dithioglucosyl donors. The use of per-O-acetyl donors resulted in poor yields of an α-disaccharide. On the other hand, per-O-benzyl donors s

Full text of DOI:10.1016/j.tetlet.2007.09.056

Tetrahedron Letters 48 (2007) 7953–7956
A novel 5-thioglycosylation method with 1,5-dithioglycosyl
donors: relevance to exo- versus endocyclic activation
Hideya Yuasa,* Osamu Tsuruta and Hironobu Hashimoto
Department of Life Science, Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology,
4259 J2-10, Nagatsutacho, Midoriku, Yokohama 226-8501, Japan
Received 7 August 2007; revised 7 September 2007; accepted 10 September 2007
Available online 12 September 2007
Abstract—5-Thioglucosylation of a 1,6-anhydro-2-azido-2-deoxy-b-D-glucopyranose derivative was carried out with various 1,5-
dithioglucosyl donors. The use of per-O-acetyl donors resulted in poor yields of an a-disaccharide. On the other hand, per-O-benzyl
donors selectively gave an a-disaccharide in good to excellent yields regardless of the anomeric configuration of the donors. To study
the relevance of the glycosylation results to the pre-equilibrium of exo- versus endocyclic sulfide activation, the relative nucleophi-
licities of exo- and endocyclic sulfides were estimated from the regioselectivities in the electrophilic oxidation of the per-O-benzyl
donors with m-chloroperbenzoic acid. The results obeyed stereoelectronic effects and the endocyclic sulfides were more nucleophilic
than the exocyclic sulfides for both anomers of per-O-benzyl-1,5-dithioglucosyl donors, the relative nucleophilicities of the endocy-
clic over exocyclic sulfides being 67% and 100%, respectively, for the a- and b-anomers. The results of the glycosidation and oxi-
dation suggest that the glycosidation proceeded through an exocyclic cleavage mechanism despite the preferential endocyclic
activation of 1,5-dithioglucosides.
Ó 2007 Elsevier Ltd. All rights reserved.
The development of oligosaccharide-based drugs is one
of the less explored frontiers in drug discovery history.1
As are commonly important for biooligomer-based
drugs,2 the oligosaccharide drugs should be resistant
to degrading enzymes in the digestive system or blood.
Replacing the ring oxygen atom of a non-reducing sugar
unit with sulfur atom is a promising way of making an
oligosaccharide glycosidase-resistant.3 Thus a number
of oligosaccharide analogs capped with a thiosugar at
the non-reducing end (S-oligosaccharides) have been
synthesized both chemically3c,4 and enzymatically.3b,5
natural oligosaccharides,6 there have been no reports on
the use of 1,5-dithioglycosyl donors for the synthesis of
S-oligosaccharides.
One might have hesitated to the use of 1,5-dithioglyco-
sides because of a potential endocyclic sulfur (Sendo
)
activation that might lead to an endocyclic cleavage
reaction to give an acyclic product, which is best exem-
plified by a dimethylboron bromide promoted ring
opening reaction of glycopyranosides,7 as opposed to
an exocyclic cleavage through an exocyclic sulfur (Sexo
)
activation (Scheme 1). In practice, we found that the
oxidation of a 1,5-thioglucopyranoside with m-chloro-
perbenzoic acid (mCPBA) gave both endo- and exocyclic
mono sulfoxides,8 which suggests that an electrophilic
glycosylation promoter would partly activate Sendo of
the same compound. On the other hand, Delorme and
co-workers found that the reaction of 1,5-dithio-L-arabi-
nopyranoside with bromine gave a corresponding glyco-
syl bromide, which is presumably produced through an
exocyclic cleavage mechanism.4e In this study, we stud-
ied the 5-thioglycosylation reaction using 1,5-dithio-
glucopyranosides from the standpoint of the relation
between the exo- versus endocyclic activation and the
exo- versus endocyclic cleavage.
Most chemical S-oligosaccharide syntheses have been
carried out through glycosylation reactions with the
trichloroacetimidate4c,d and bromide4e derivatives of
5-thioglycopyranosides. All of these chemical glycosida-
tions stereoselectively gave a-glycosides even in the
presence of 2-O-acyl groups, which are supposed to
assist b-glycoside formations through anchimeric
participation. Although thioglycosides have been gener-
ally used as versatile glycosyl donors in the synthesis of
*
Corresponding author. Tel./fax: +81 45 924 5850; e-mail: hyuasa@
bio.titech.ac.jp
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.09.056

7954
H. Yuasa et al. / Tetrahedron Letters 48 (2007) 7953–7956
donors (Scheme 2), since these glycosides have distinct
exocyclic activation
endocyclic activation
relative nucleophilicities regarding Sendo versus Sexo as
estimated by mCPBA oxidation,8 the relative Sendo
nucleophilicity, Sendo/(Sendo + Sexo), being 0 and 0.91,
respectively, for 2aEt and 2bPh. These relative nucleo-
philicities were interpreted by stereoelectronic effects.
The main frame of the effects is the stabilizing interac-
tion between a sulfur lone pair and the vacant r*-orbital
of the adjacent C–S bond (nS–rÃCS), which is maximized
when the lone pair is antiperiplanar to the C–S bond.
Among the four sulfur atoms in Fig. 1, only the Sendo
of b-glycoside fails the stabilizing interaction, escaping
E
E
E+
E+
X
XR
X
XR
X
XR
exocyclic cleavage
endocyclic cleavage
E
X
X
X
Nu
Nu-
XR
a nucleophilicity reduction. The nS–rÃ
interactions
CS
with a bias of the intrinsic congestion around Sendo leads
to the relative nucleophilicities: Sendo > Sexo for b-glyco-
sides and Sendo < Sexo for a-glycosides.
Nu-
E
X
XR
Nu
X: O or S
E+: Electrophile
Nu-: Nucleophile
We chose a 3-O-protected 1,6-anhydro-2-azido-2-deoxy-
b-D-glucopyranose derivative 111 as a glycosyl acceptor,
because this derivative has a relatively reactive axial sec-
ondary alcohol, which serves a good criterion as to
whether the glycosidation is generally applicable to
disaccharide syntheses. Glycosidation was carried out
in the presence of a glycosyl donor (1.0 equiv), 1
(1.5 equiv), NIS (1.2 equiv), trifluoromethanesulfonic
acid (TfOH, 0.25 equiv), and molecular sieves 4A in
ClCH2CH2Cl at À20 °C for 1 h, the conditions are en-
ough to deplete all the glycosyl donors we tested.
acyclic product
Scheme 1.
For the acid-catalyzed solvolyses of normal pyranosides,
there have long been discussions as to whether the reac-
tions are accompanied by endocyclic cleavages.9 Eluci-
dation of the reaction pathways is difficult because the
solvolysis products are usually identical for two path-
ways and the activated intermediates are unisolable.
On the other hand, the preferred activation sites of
1,5-dithioglucopyranosides are easily estimated, because
the products of the electrophilic oxidation with mCPBA
are isolable and its regioselectivity would indicate the
relative nucleophilicities of the sulfides (Sendo vs Sexo).8
The glycosidation promoter used in this study is N-iod-
osuccinimide (NIS), which is electrophilic toward sul-
fides10 and thus predicted to cause a pre-equilibrium
activation of Sendo and Sexo in favor of the more nucle-
ophilic site. Then we may be able to reduce the potential
acyclic products by using the 1,5-dithioglycoside with a
less Sendo nucleophilicity.
The glycosidation with 2aEt gave a-disaccharide 4 in
19% together with a complex mixture containing a 2-
OH derivative 6. The same disaccharide 4 was obtained
in 14% yield from 2bPh. The similar glycosidation re-
sults for two donors suggest that the reactions pro-
ceeded in the same mechanism regardless of the
different preferred activating sites, Sexo versus Sendo, in
the pre-equilibrium activation.
To circumvent the acyloxy migration and increase the
reactivity toward glycosidation,6b O-benzylated glycosyl
donors (3abEt, 3aPh, and 3bPh)12 were examined
(Scheme 2). These glycosyl donors were synthesized
from the corresponding per-O-acetylated derivatives
through deacetylation and benzylation. All the glycosyl
donors gave an a-disaccharide 513 in good yields-68%,
We first examined per-O-acetylated ethyl and phenyl
1,5-dithioglucopyranosides (2aEt and 2bPh) as glycosyl
R1O
R1O
S
R1O
O
R1O
OR
O
SR2
O
AcO
AcO
AcO
OR
O
S
R1O
R1O
2αEt: R1 = Ac, R2 = Et
3αβEt: R1 = Bn, R2 = Et
α
S
+
HO
N3
R1O O
R1O
HO
3
Ph: R1 = Bn, R2 = Ph
1: R = SiMe2tBu
OAc
N3
6
4: R1 = Ac
NIS
TfOH
R1O
R1O
R1O
(78% from 2 Ph)
β
α
β
(19% from 2 Et, 14% from 2 Ph)
S
R1O
SR2
5: R1 = Bn
(68% from 3αβEt, 89% from 3αPh,
β
αβ
β
2
3
3
Ph: R1 = Ac, R2 = Ph
Et: R1 = Bn, R2 = Et
β
71% from 3 Ph)
Ph: R1 = Bn, R2 = Ph
Scheme 2.

H. Yuasa et al. / Tetrahedron Letters 48 (2007) 7953–7956
7955
reactions of glycopyranosides with Me2BBr,3c,7,15
suggesting that the endocyclic cleavage pathway would
not be stereospecific.
α-glycoside
β-glycoside
E+
nS
S
S
In conclusion, 1,5-dithioglycopyranosides can be used
for the syntheses of S-oligosaccharides, if appropriate
protecting groups are used. The a-selectivity is expected
for this 5-thioglycosylation reaction as with the other
5S-glycosyl donors. Possibility for the formation of acy-
clic products is low, even though the relative nucleo-
philicity of Sendo is high and Sendo activation would
predominate over Sexo activation. Though the reaction
pathways with regard to exo- versus endocyclic cleavage
were not determined, the observed a-selectivity favors
the exocyclic cleavage mechanism.
SR
H
*
CS
σ
SR
H
E+
Figure 1. The stereoelectronic effects for the sulfur nucleophilicities of
1,5-dithioglycosides.
89%, and 71% from 3abEt, 3aPh, and 3bPh,
respectively.
As expected, the use of the alkyl protecting group gave
better glycosylation yields than those with acetyl protec-
tion. Again, a-selectivity was observed. The stereoelec-
tronic effects predict that Sendo of each benzyl
derivative is more nucleophilic than that of the acetate.
To ensure this hypothesis, we carried out the oxidation
of 3aPh and 3bPh using mCPBA (1.1 equiv) in CH2Cl2
at À20 °C for 30 min (Scheme 3). We obtained endo-
and exo-sulfoxides (7aendo and 7aexo) in 47% and 23%
Acknowledgments
This work was supported by the Grant of the 21st Cen-
tury COE Program and a Grant-in-Aid for Scientific
Research (No. 13680668) from the Japanese Ministry
of Education, Culture, Sports, Science and Technology.
yield, respectively, from 3aPh and only the endo-sulfox-
14
References and notes
ides (7bendo-ax and 7bexo-eq
)
in 36% and 30% yield,
respectively, from 3bPh. The results indicate that the
relative nucleophilicity of Sendo is 0.67 for 3aPh and
nearly 1.0 for 3bPh.
´
1. (a) Gornik, O.; Dumic, J.; Flo¨gel, M.; Lauc, G. Acta
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The electronic effect of protecting groups is tremendous
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overwhelmingly nucleophilic over Sexo. Thus the exocy-
clic activation by NIS is presumed to be less than 0.5%
in the pre-equilibrium stage for the glycosidation with
3bPh. On the other hand, the fact that only the a-glyco-
sides were obtained as with the other 5S-glycosyl donors
suggests that these glycosidations proceeded through an
exocyclic cleavages despite the unfavorable pre-equilib-
rium Sexo activations, because the a-selectivities are pre-
sumably due to the kinetic anomeric effect that is
available only for the exocyclic cleavage mechanism. If
the endocyclic cleavage were the principal mechanism,
we would most probably have obtained both anomeric
isomers. Indeed, the anomeric mixtures have been
obtained for the acyclic products in the ring-opening
ˇ
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S
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+
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O
SPh
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α
α
exo (23%)
7
endo (47%)
7
´
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S
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β
β
endo-eq (30%)
7
endo-ax (36%)
7
Scheme 3.

7956
H. Yuasa et al. / Tetrahedron Letters 48 (2007) 7953–7956
J = 11.6 Hz, 1H), 4.66 (d, J = 2.6 Hz, 1H), 4.63 (br s, 1H),
9. (a) Mohr, M.; Bryce, R. A.; Hillier, I. H. J. Phys. Chem. A
´
2001, 105, 8216–8222; (b) Kaczmarek, J.; Kaczynski, Z.;
4.55 (d, J = 10.6 Hz, 1H), 4.52 (d, J = 12.2 Hz, 1H), 4.48
(d, J = 12.2 Hz, 1H), 4.17 (d, J = 6.9 Hz, 1H), 4.17 (t,
J = 9.5 Hz, 1H), 3.95 (s, 1H), 3.89 (dd, J = 4.7, 9.8 Hz,
1H), 3.83 (dd, J = 2.6, 9.8 Hz, 1H), 3.81 (t, J = 9.8, 1H),
3.72 (t, J = 6.6 Hz, 1H), 3.62 (dd, J = 2.6, 9.8 Hz, 1H),
3.51 (s, 1H), 3.42 (ddd, J = 2.6, 4.7, 9.5 Hz, 1H), 2.84 (s,
1H), 0.87 (s, 9H), 0.04 (s, 3H), 0.01 (s, 3H); 13C NMR
(67.8 MHz, CDCl3) d 139.0, 138.5, 138.4, 137.9, 128.4,
128.3, 128.0, 127.8, 127.69, 127.66, 127.58, 127.5, 127.4,
100.7, 84.2, 82.7, 82.1, 81.2, 77.0, 76.0, 75.6, 75.3, 73.4,
73.2, 70.4, 67.9, 64.8, 60.9, 42.1, 25.6, 17.7. HR-ESIMS:
Calcd for C46H57 N3O8SSiNa m/z [M+Na]+: 862.3535;
found, 862.3511. Anal. Calcd for C46H57N3O8SSi: C
65.76, H 6.84, N 5.00. Found: C 65.46, H 7.08, S 4.68.
14. Compound 7bendo-ax: mp 102–106 °C; +18.1 (c 0.52,
CHCl3); 1H NMR (270 MHz, CDCl3) d 7.63–7.08 (m,
25H), 5.54–4.43 (m, 8H), 4.19 (dd, J = 9.2, 11.2 Hz, 1H),
4.12 (J = 9.2, 11.2 Hz, 1H), 4.00 (dd, J = 4.0, 11.2 Hz,
1H), 3.79 (t, J = 9.6 Hz, 1H), 3.77 (d, J = 11.2 Hz, 1H),
3.67 (t, J = 9.2 Hz, 1H), 2.73 (dt, J = 4.0, 11.2 Hz, 1H);
13C NMR (67.8 MHz, CDCl3) d 138.3, 137.7, 137.34,
137.30, 133.8, 123.0, 129.4, 128.6, 128.48, 128.39, 128.1,
128.03, 127.96, 127.89, 127.6, 127.2, 87.6, 78.0, 77.2, 76.3,
76.0, 75.4, 73.7, 70.4, 65.3, 61.1. (ESI-MS.): Calcd for
C40H41O5S2 m/z [M+H]+: 665.2397; found, 665.2424.
Anal. Calcd for C40H40O5S2: C 72.26, H 6.06. Found: C
72.05, H 6.20. Compound 7bexo-eq: +50.9 (c 0.43, CHCl3);
1H NMR (270 MHz, CDCl3) d 7.70–7.13 (m, 25H), 5.00–
4.60 (m, 6H), 4.65 (d, J = 11.6 Hz, 1H), 4.52 (d,
J = 11.6 Hz, 1H), 4.15 (dd, J = 2.3, 10.8 Hz, 1H), 4.06
(dd, J = 2.3, 10.8 Hz, 1H), 3.99 (d, J = 11.2 Hz, 1H), 3.87
(dd, J = 9.2, 11.2 Hz, 1H), 3.72 (t, J = 9.2 Hz, 1H), 3.87
(dd, J = 9.2, 11.2 Hz, 1H), 3.72 (t, J = 9.2 Hz, 1H), 3.58
(dd, J = 9.2, 11.2 Hz, 1H), 2.87 (dt, J = 2.3, 11.2 Hz, 1H);
13C NMR (67.8 MHz, CDCl3) d 138.1, 137.7, 137.5,
137.45, 133.9, 133.1, 129.2, 128.5, 128.1, 127.9, 127.8,
127.7, 127.4, 81.1, 77.3, 77.2, 76.5, 76.0, 75.8, 73.7, 73.6,
67.0, 62.8. HR-ESIMS: Calcd for C40H41O5S2 m/z
[M+H]+: 665.2397; found, 665.2391.
Trumpakaj, Z.; Szafranek, J.; Bogalecka, M.; Lo¨nnberg,
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Tetrahedron Lett. 1996, 37, 1809–1812.
12. Compound 3aPh: mp 112–114 °C; +194.1 (c 0.99, CHCl3);
1H NMR (270 MHz, CDCl3) d 7.48–7.15 (m, 25H), 4.98
(d, J = 10.6 Hz, 1H), 4.89 (d, J = 10.9 Hz, 1H), 4.79 (d,
J = 10.6 Hz, 1H), 4.75 (d, J = 11.6 Hz, 1H), 4.68 (d,
J = 11.6 Hz, 1H), 4.56 (d, J = 10.9 Hz, 1H), 4.49 (d,
J = 12.2 Hz, 1H), 4.44 (d, J = 5.3 Hz, 1H), 4.43 (d,
J = 12.2 Hz, 1H), 4.12–4.07 (m, 1H), 4.00 (dd, J = 4.3,
9.9 Hz, 1H), 3.88–3.80 (m, 2H), 3.62–3.54 (m, 2H); 13C
NMR (67.8 MHz, CDCl3) d 138.8, 138.3, 137.84, 137.79,
134.2, 133.1, 128.9, 128.4, 128.3, 128.0, 127.92, 127.89,
127.82, 127.76, 127.69, 127.5, 84.1, 83.9, 81.8, 76.2, 75.6,
73.2, 72.3, 67.8, 55.2, 42.6. HR-ESIMS: Calcd for
C40H40O4S2Na m/z [M+Na]+: 671.2268; found,
671.2279. Anal. Calcd for C40H40O4S2: C 74.04, H 6.21,
S 9.88. Found: C 74.15, H 6.06, S 9.37. Compound 3bPh:
mp 103–107 °C; +71.2 (c 1.2, CHCl3); 1H NMR
(270 MHz, CDCl3) d 7.60–7.09 (m, 25H), 4.99–4.49 (m,
8H), 4.11 (d, J = 8.9 Hz, 1H), 3.85 (t, J = 9.6 Hz, 1H),
3.78 (dd, J = 4.9, 9.6 Hz), 3.71 (dd, J = 8.9, 10.2 Hz, 1H),
3.64 (dd, J = 2.6, 9.6 Hz, 1H), 3.46 (t, J = 10.2 Hz, 1H),
2.95 (ddd, J = 2.6, 4.9, 10.2 Hz, 1H); 13C NMR
(67.8 MHz, CDCl3) d 138.6, 138.1, 137.7, 133.9, 132.5,
129.0, 128.4, 128.3, 128.2, 128.0, 127.8, 127.49, 127.46,
88.2, 85.1, 81.8, 76.5, 76.1, 75.6, 73.4, 67.8, 53.8, 47.4. HR-
ESIMS: Calcd for C40H40O4S2Na m/z [M+Na]+:
671.2268; found, 671.2270. Anal. Calcd for C40H40O4S2:
C 74.04, H 6.21. Found: C 73.77, H 6.60.
1
13. Compound 5: +91.4 (c 1.5, CHCl3); H NMR (270 MHz,
CDCl3) d 7.39–7.16 (m, 20H), 5.58 (s, 1H), 4.92 (d,
J = 10.5 Hz, 1H), 4.90 (d, J = 10.6 Hz, 1H), 4.80 (d,
J = 11.6 Hz, 1H), 4.77 (d, J = 10.5 Hz, 1H), 4.73 (d,
15. Hashimoto, H.; Kawanishi, M.; Yuasa, H. Carbohydr.
Res. 1996, 282, 207–221.