Direct α-Selective glycosylations of acetyl-protected 2-deoxy- and 2,6-dideoxythioglycosides by preactivation protocol

Article abstract of DOI:10.1055/s-0029-1219943

An efficient preactivation protocol for the highly -stereoselective glycosylation of 2-deoxy- and 2,6-dideoxysugars has been developed using acetyl-protected 2-deoxy- and 2,6-dideoxythioglycosides as glycosyl donors. The approach allows a wide range of glycosyl acceptors and donors to be used. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0029-1219943

LETTER
1519
Direct a-Selective Glycosylations of Acetyl-Protected 2-Deoxy- and
2,6-Dideoxythioglycosides by Preactivation Protocol
Glycosylations of
i
A
cetyl-
n
Protected 2-
-
D
eoxy-
a
S
nd 2,6-Dideox
u
ythioglycos
o
ides Lu,a,b Qin Li,a,b Yuan Wang,a,b Xin-Shan Ye*a,b
a
The State Key Laboratory of Natural and Biomimetic Drugs, Peking University, Xue Yuan Rd No. 38, Beijing 100191, P. R. of China
School of Pharmaceutical Sciences, Peking University, Xue Yuan Rd No. 38, Beijing 100191, P. R. of China
Fax +86(10)62014949; E-mail: xinshan@bjmu.edu.cn
b
Received 4 March 2010
Dedicated to Professor Henry N. C. Wong on the occasion of his 60th birthday
also been examined as leaving groups in the 2-deoxy sys-
Abstract: An efficient preactivation protocol for the highly a-stereo-
selective glycosylation of 2-deoxy- and 2,6-dideoxysugars has been
developed using acetyl-protected 2-deoxy- and 2,6-dideoxythiogly-
cosides as glycosyl donors. The approach allows a wide range of
glycosyl acceptors and donors to be used.
tems.14 Lewis acid or metal-catalyzed syntheses of 2-
deoxyglycosides from glycals15 have additionally been
developed. Although b-anomeric selectivity of 2-deoxy-
glycosides is more difficult to achieve, the direct synthesis
of 2-deoxyglycopyranosides from 2-deoxy-glycosyl do-
nors, with high a-stereoselectivity, still remains an incon-
venient task.
Key words: preactivation, a-stereoselective glycosylation, 2-deoxy-
glycosides, 2,6-dideoxysugars, carbohydrate
In recent years, ‘preactivation’ as a new glycosylation ap-
proach has attracted considerable interest.16 ‘Preactiva-
tion’ was developed as an effective method for the
iterative one-pot synthesis of oligosaccharides in Huang’s
laboratory as well as in our own group.17 Very recently,
when this protocol was applied to the glycosylations of
oxazolidinone-protected glucosamine donors18 and car-
bonate-protected deoxyglycosyl donors,19,20 stereoselec-
tive coupling reactions were realized. In the synthesis of
2-deoxysugars and 2,6-dideoxysugars, this approach en-
joyed a large scope in terms of glycosyl acceptors and do-
nors, and the a-selectivity of glycosylations was good to
excellent. By comparison with the routine (non-preactiva-
tion) glycosylation approach, it was found that the preac-
tivation protocol played an important role in the outcomes
and stereochemistry of glycosylations. And it was pre-
sumed that the conformational constraint of 3,4-O-car-
bonate in glycosyl donors appeared to enhance a-
selectivity. In order to investigate how much the 3,4-O-
carbonate group might have influenced the glycosylation
results, we used acetyl-protected 2-deoxythioglycosides
and 2,6-dideoxythioglycosides as glycosyl donors to per-
form glycosylations by preactivation protocol, and herein
we report the high a-selectivity results by the use of these
simple glycosyl donors.
2-Deoxyglycosides are essential constituents of a variety
of biologically active natural products such as antitumor
antibiotics.1 The nonavailability of neighboring-group
participation from substituents at the C-2 position and the
enhanced conformational flexibility derived from the re-
duced number of substituents make it difficult to achieve
glycosylation in a stereoselective manner. Furthermore,
the lack of an electron-withdrawing C-2 substituent
makes the resulting glycosides more acid labile. Strategies
for the synthesis of 2-deoxyglycosides with good a- or b-
anomeric selectivities rely frequently on indirect glycosy-
lation. The indirect method involves the introduction of a
temporary directing group at the C-2 position of the donor
which is reductively removed after the glycosylation step
has been completed.2 Indirect approaches to 2-deoxygly-
cosides mainly exploit 1,2-migration,3 introduce auxiliary
groups at the C-2 position,2 use 2,6-anhydro-2-thio sugars
as donors,4 or cyclize acyclic sulfanyl alkenes.5 In some
cases, glycals are used as donors.6 These methods normal-
ly need a subsequent reduction step which might be not
suitable for the construction of some complex natural
products. Therefore, a direct strategy for the preparation
of 2-deoxyglycopyranosyl linkages that uses 2-deoxy-
glycopyranosyl donors would be more efficient and prac-
tical than the indirect strategies. Although some direct
methods such as glycosylation promoted by silver sili-
cate,7 I2-Et3SiH,8 AgPF6,9 or polymer-bound iodate(I)
complexes10 have been used, other approaches have been
explored, including the conformational assistance ap-
proach,11 the use of S-(2-deoxyglycosyl)phosphoro-
dithioates and 2-pyridylthioglycosides as donors,12 and
the anomeric O-alkylation/arylation method.13 Phos-
phites, phosphoramidites, and (2-carboxy)benzyl have
The ‘preactivation’ approach was conducted in a manner
where the glycosyl donor was completely activated and
consumed (by TLC detection) prior to the addition of a
glycosyl acceptor. First, 2-deoxy-galactopyranosyl donor
was examined. The combination of benzenesulfinyl mor-
pholine (BSM)21 and triflic anhydride (Tf2O) was used as
the promoter system in the preactivation operations. Thus,
the triacetyl-protected thioglycoside 1a was preactivated
at –72 °C in anhydrous dichloromethane using BSM–
Tf2O. After disappearance of donor 1a (TLC detection in
around ten minutes), the acceptor 2a was added to the re-
action mixture. Fortunately, the coupling reaction of 1a
and 2a exhibited complete a-selectivity and proceeded as
SYNLETT 2010, No. 10, pp 1519–1524
x
x
.x
x
.2
0
1
0
Advanced online publication: 19.05.2010
DOI: 10.1055/s-0029-1219943; Art ID: W03610ST
© Georg Thieme Verlag Stuttgart · New York

1520
Y.-S. Lu et al.
LETTER
shown in Table 1 (entry 1).22 Next, our investigation was pyranosides as well as furanosides. The glycosyl accep-
expanded to other glycosyl acceptors 2b–h, and the re- tors 2a and 2b, which differ only in their anomeric config-
sults are listed in Table 1. As displayed, all the glycosyla- urations, provided the same excellent a-stereoselectivity
tions proceeded very smoothly in high yields and with during the glycosylations (Table 1, entries 1 and 2). It was
excellent a-selectivity except glycosyl acceptors 2g and shown that the a-selectivity of acceptors 2f and 2h were
2h, in which the 6-OH was exposed (Table 1, entries 7 better than that of acceptors 2e and 2g (Table 1, entry 5 vs.
1
and 8). The a-anomers were identified by their H NMR entry 6, entry 7 vs. entry 8). One might therefore conclude
coupling constants for the anomeric protons or the cou- that the acceptors with lower reactivity can improve the
pling constants for the axial protons at the C-2 position of stereoselectivity of the glycosylations.23
deoxysugars (J1,2 = 3.0–4.0 Hz). It was found that donor
1a coupled with diverse glycosyl acceptors, including
Table 1 Glycosylation of Donor 1a with Various Acceptors by Preactivation
OAc
AcO
OAc
AcO
AcO
BSM, Tf2O
ROH
O
O
AcO
CH2Cl2, –72 °C
2a–h
STol
OR
1a
3a–h
Entry
1
Donor
Acceptor
Product
Yield (%)
84
Ratio a/b
a only
1a
OAc
AcO
O
Ph
O
O
O
BnO
AcO
HO
MeO
OMe
O
O
O
2a
OBn
O
O
O
Ph
3a
2
1a
93
a only
OAc
Ph
O
O
AcO
O
O
BnO
OMe
AcO
HO
2b
MeO
OBn
O
O
O
Ph
3b
3
1a
91
a only
BnO
OAc
OMe
AcO
O
OBn
O
AcO
OH
2c
BnO
O
OBn
MeO
3c
4
1a
87
a only
OAc
O
AcO
Ph
O
O
O
HO
AcO
BnO
MeO
OMe
OBn
2d
O
O
O
O
Ph
3d
5
1a
86
8:1
OBn
OAc
AcO
O
O
OBn
HO
BnO
AcO
O
BnO
O
BnO
OMe
BnO
2e
OMe
3e
Synlett 2010, No. 10, 1519–1524 © Thieme Stuttgart · New York

LETTER
Glycosylations of Acetyl-Protected 2-Deoxy- and 2,6-Dideoxythioglycosides
1521
Table 1 Glycosylation of Donor 1a with Various Acceptors by Preactivation (continued)
OAc
AcO
AcO
OAc
AcO
AcO
BSM, Tf2O
ROH
O
O
CH2Cl2, –72 °C
2a–h
STol
OR
1a
3a–h
Entry
6
Donor
Acceptor
Product
Yield (%)
87
Ratio a/b
a only
1a
OBn
OAc
AcO
O
O
HO
BzO
OBn
AcO
BzO
O
OMe
OMe
OMe
O
2f
BzO
BzO
OMe
3f
7
1a
85
2:1
OAc
OH
AcO
O
O
BnO
BnO
O
AcO
BnO
O
BnO
2g
BnO
BnO
OMe
3g
8
1a
91
4:1a
OAc
OH
AcO
O
O
BzO
BzO
AcO
BzO
O
O
2h
BzO
BzO
BzO
OMe
3h
a Determined from 1H NMR spectrum.
Encouraged by the results described above, we then syn- 2b and 2e exhibited moderate a-selectivity (Table 3, en-
thesized donor 1b in order to extend the scope of our tries 2 and 4). And the acceptor 2h in which the 6-OH was
methodology. Methyl glycosides 2b,d,f,h, in which the 2- exposed showed poor a-selectivity towards glycosylation
OH, 3-OH, 4-OH, and 6-OH were exposed, respectively, (Table 3, entry 5).
were again used as the glycosyl acceptors. The glycosyla-
tions were conducted under the above-mentioned preacti-
vation conditions, and the results are listed in Table 2. The
Comparing with the results we reported previously,19 it
can be concluded that the 3,4-O-carbonate group did in-
fluence the glycosylation results by enhancing the a-se-
glycosyl acceptors 2d and 2f proceeded with excellent a-
lectivity. The high a-selectivity observed might originate
selectivity (Table 2, entries 2 and 3), whereas the a-selec-
from the species formed as intermediates after preactiva-
tivity of the coupling reaction of 2b and 1b was just mod-
tion. Once activated, the glycosyl donor becomes an oxac-
arbenium ion, which can be trapped by triflate anion to
erate (Table 2, entry 1).
2,6-Dideoxysugars are also found in a plethora of biolog- give either a-triflate or b-triflate intermediate.20b As a re-
ically important natural products.1 To further check the ef- sult of the higher reactivity of the acetyl-protected
fectiveness of our method, the construction of 2,6- thioglycosides than the cyclocarbonate derivatives,24 both
dideoxyglycosyl linkages was investigated. For this pur- a-triflate and b-triflate intermediates formed from acetyl-
pose, the acetyl-protected thioglycoside donor 1c was protected thioglycosides will undergo glycosylation fast-
synthesized, and monosaccharide building blocks er. By lowering the reactivity of glycosyl acceptors,16c,23,25
2a,b,d,e,h were chosen as the glycosyl acceptors. The they might prefer to undergo an SN2-like reaction with the
glycosyl coupling reactions between donor 1c and accep- more reactive b-triflate intermediate, resulting in the for-
tors 2a,b,d,e,h by the preactivation protocol were carried mation of a-glycosides. This process might shift the ano-
out. The results are listed in Table 3. As shown, the glyc- merization equilibrium from the a- to b-triflate
osyl acceptors 2a and 2d displayed excellent a-selectivity intermediate. This might also explain why the higher ste-
(Table 3, entries 1 and 3), whereas the glycosyl acceptors
Synlett 2010, No. 10, 1519–1524 © Thieme Stuttgart · New York

1522
Y.-S. Lu et al.
LETTER
Table 2 Glycosylation of Donor 1b with Various Acceptors by Preactivation
OAc
OAc
O
BSM, Tf2O
O
ROH
AcO
AcO
AcO
AcO
CH2Cl2, –72 °C
2b,d,f,h
OR
STol
4b,d,f,h
1b
Entry
1
Donor
Acceptor
Product
Yield (%) Ratio a/b
1b
91
5.3:1
OAc
O
Ph
O
O
O
AcO
AcO
BnO
OMe
HO
O
2b
MeO
OBn
O
O
Ph
O
4b
2
1b
67
a only
OAc
O
O
HO
Ph
O
O
AcO
AcO
MeO
BnO
OMe
OBn
2d
O
O
O
O
Ph
4d
3
4
1b
1b
92
86
12:1
OAc
OBn
O
O
OBn
AcO
HO
BzO
AcO
O
BzO
O
BzO
OMe
BzO
2f
OMe
4f
2:1a
OH
OAc
O
O
AcO
AcO
BzO
BzO
BzO
O
OMe
O
2h
BzO
BzO
BzO
OMe
4h
a Determined from 1H NMR spectrum.
reoselectivity was achieved when less reactive acceptors ing (S)-(phenylthiomethyl)benzyl moiety at C-6, whereas
were employed.
2,6-dideoxyglycosides could be prepared by allyl glyco-
syl donors.28 Although several same disaccharides were
prepared independently in Boons’ and our own work and
both of the results were good, our method enjoyed using
the same type of simple glycosyl donors, and the glycosy-
lations were conducted in the same promoter system. In
addition, among the various glycosyl donors, thioglyco-
sides that we used are one type of the most enduring and
widely used donors due to their stability, accessibility, and
compatibility, and they can be activated by a variety of
methods.29
When the above-mentioned glycosylation reactions were
performed at –72 °C in CH2Cl2 preactivated by the BSM–
Tf2O system in the presence of a hindered base 2,4,6-tri-
tert-butylpyrimidine (TTBP), we obtained the glycosylat-
ing products in the same yields and the same a/b ratios as
that of glycosylation in the absence of TTBP. These re-
sults appeared to exclude the possibility that the a-selec-
tive glycosylation in the absence of TTBP may be due to
the in situ b → a anomerization under acidic condi-
tions.26,27
In conclusion, a new simple and efficient method for high-
ly a-stereoselective glycosylations of 2-deoxysugars and
2,6-dideoxysugars using acetyl-protected thioglycosides
as donors has been uncovered. To the best of our knowl-
edge, these glycosyl donors are the simplest thioglycoside
Recently, Boons and co-workers published their investi-
gation on direct and stereoselective synthesis of a-linked
2-deoxyglycosides by BF3·OEt2-promoted activation of
trichloroacetimidate glycosyl donors having a participat-
Synlett 2010, No. 10, 1519–1524 © Thieme Stuttgart · New York

LETTER
Glycosylations of Acetyl-Protected 2-Deoxy- and 2,6-Dideoxythioglycosides
1523
Table 3 Glycosylation of Donor 1c with Various Acceptors by Preactivation
O
AcO
AcO
BSM, Tf2O
ROH
O
AcO
AcO
CH2Cl2, –72 °C
OR
2a,b,d,e,h
STol
5a,b,d,e,h
1c
Entry
1
Donor
Acceptor
Product
Yield (%)
57
Ratio a/b
a only
1c
O
Ph
O
AcO
AcO
O
HO
O
BnO
MeO
O
OMe
OBn
O
2a
Ph
2b
O
O
Ph
5a
2
3
1c
1c
85
73
4.7:1
O
O
BnO
O
AcO
AcO
O
OMe
HO
O
MeO
OBn
O
O
O
Ph
5b
a only
O
O
O
Ph
AcO
AcO
O
HO
MeO
BnO
OBn
OMe
O
2d
O
O
O
Ph
5d
4
5
1c
1c
86
82
5:1
OBn
O
AcO
OBn
O
AcO
HO
O
BnO
O
BnO
BnO
OMe
BnO
OMe
2e
5e
1.5:1a
OH
O
AcO
O
AcO
BzO
O
BzO
BzO
O
OMe
BzO
BzO
2h
BzO
OMe
5h
a Determined from 1H NMR spectrum.
donors that have been used in the stereoselective construc-
tion of deoxyglycosides. Due to the good a-stereoselectiv-
ity obtained and the advantages of thioglycosides, it is
expected that the disclosed method might be a concise and
efficient supplement in the synthesis of a-linked 2-deoxy-
glycopyranose-containing complex structures with im-
portant biological functions. Further extension of this
protocol to b-selective glycosylations is still under inves-
tigation.
Acknowledgment
This work was financially supported by the National Natural
Science Foundation of China, and ‘863’ grant from the Ministry of
Science and Technology of China.
References and Notes
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LETTER
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Triflic anhydride (11.0 mL, 0.061 mmol) was added to a
stirred solution of p-methylphenyl 3,4,6-tri-O-acetyl-2-
deoxy-1-thio-a-D-galactopyranoside (1a, 30.0 mg, 0.076
mmol), benzenesulfinyl morpholine (BSM, 12.9 mg, 0.061
mmol), and 4 Å MS (350 mg, activated powder) in CH2Cl2
(3.0 mL) at –72 °C under nitrogen atmosphere. The reaction
mixture was stirred for 10 min. After loss of 1a detected by
TLC, a solution of methyl 3-O-benzyl-4,6-O-benzylidene-a-
D-glucopyranoside (2a, 18.8 mg, 0.051 mmol) in CH2Cl2
(0.5 mL) was added dropwise to the reaction mixture. The
mixture was stirred for 30 min, and the reaction was
quenched by Et3N (8.0 mL). The precipitate was filtered off,
and the filtrate was concentrated. The residue was purified
by column chromatography on silica gel (PE–EtOAc, 2.5:1)
to give methyl 3-O-benzyl-4,6-O-benzylidene-2-O-(3,4,6-
tri-O-acetyl-2-deoxy-a-D-galactopyranosyl)-a-D-gluco-
pyranoside (3a, 27.6 mg, 84% yield) as a foam. Rf = 0.35
(PE–EtOAc, 1.5:1). 1H NMR (500 MHz, CDCl3): d = 7.50
(dd, 2 H, J = 2.0, 7.5 Hz), 7.40–7.33 (m, 7 H), 7.29–7.27 (m,
1 H), 5.60 (s, 1 H), 5.37 (ddd, 1 H, J = 3.0, 5.0, 12.5 Hz),
5.15 (d, 1 H, J = 3.0 Hz), 5.14 (d, 1 H, J = 3.5 Hz), 4.93 (d,
1 H, J = 10.5 Hz), 4.89 (d, 1 H, J = 3.5 Hz), 4.71 (d, 1 H,
J = 10.5 Hz), 4.36 (t, 1 H, J = 6.5 Hz), 4.31 (dd, 1 H, J = 4.5,
10.0 Hz), 3.98 (t, 1 H, J = 9.5 Hz), 3.90 (dd, 1 H, J = 6.5,
11.0 Hz), 3.87–3.74 (m, 4 H), 3.65 (t, 1 H, J = 9.5 Hz), 3.43
(s, 3 H), 2.13–2.08 (m, 4 H), 2.00 (s, 3 H), 1.96–1.92 (m, 4
H). 13C NMR (125 MHz, CDCl3): d = 170.13, 170.23,
169.79, 138.06, 137.26, 128.96, 128.54, 128.47, 128.25,
127.86, 125.95, 101.27, 97.12, 94.10, 82.71, 76.85, 75.74,
73.45, 69.00, 66.61, 65.87, 62.25, 61.87, 55.23, 29.86,
20.81, 20.70. MS (ESI-TOF, positive): m/z = 667 [M + Na]+.
Anal. Calcd for C33H40O13: C, 61.48; H, 6.25. Found: C,
61.65; H, 6.40.
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