Copper mediated iodoacetoxylation and glycosylation: Effective and convenient approaches for the stereoselective synthesis of 2-deoxy-2-iodo glycosides

Article abstract of DOI:10.1016/j.tetlet.2016.01.035

Copper(II) triflate catalyzed stereoselective glycosylation of 2-iodo-glycosyl acetate donor is reported. Anomeric activation of 2-deoxy-1-O-acetyl sugar employing Cu(OTf)₂ found to be an attractive as well effective alternative reagent to the most frequently used triflic acid (TfOH) source such as TMSOTf or TBSOTf. Scope of the reaction was explored for various aglycones. This protocol involves simple reaction operation, employs less expensive and non-toxic reagent system, and enables the stereoselective preparation of 2-deoxy-2-iodo-glycosides. Furthermore, CuI/NaIO₄ in the presence of AcOH at ambient temperature promoted the regioselective iodoacetoxylation of various glycals to access 2-iodo-glycosyl acetates.

Full text of DOI:10.1016/j.tetlet.2016.01.035

Tetrahedron Letters 57 (2016) 811–814
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Copper mediated iodoacetoxylation and glycosylation: effective
and convenient approaches for the stereoselective synthesis
of 2-deoxy-2-iodo glycosides
a
a,b,
⇑
Suresh Kumar Battina , Sudhir Kashyap
a
Discovery Laboratory, Organic and Biomolecular Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India
Academy of Scientific and Innovative Research (AcSIR), CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Copper(II) triflate catalyzed stereoselective glycosylation of 2-iodo-glycosyl acetate donor is reported.
Received 4 December 2015
Revised 6 January 2016
Accepted 10 January 2016
Available online 11 January 2016
2
Anomeric activation of 2-deoxy-1-O-acetyl sugar employing Cu(OTf) found to be an attractive as well
effective alternative reagent to the most frequently used triflic acid (TfOH) source such as TMSOTf or
TBSOTf. Scope of the reaction was explored for various aglycones. This protocol involves simple reaction
operation, employs less expensive and non-toxic reagent system, and enables the stereoselective prepa-
4
ration of 2-deoxy-2-iodo-glycosides. Furthermore, CuI/NaIO in the presence of AcOH at ambient temper-
ature promoted the regioselective iodoacetoxylation of various glycals to access 2-iodo-glycosyl acetates.
Keywords:
2
-Deoxy glycosides
Copper(II) triflate
-Deoxy-2-iodo-glycosyl acetate
Ó 2016 Elsevier Ltd. All rights reserved.
2
Glycals
Iodoacetoxylation
2
-Deoxy sugars and their derivatives have been recognized as
promoters such as TMS-OTf and TBS-OTf were found to be suitable
reagents for anomeric activation of 2-deoxy-1-O-acetyl donors,6
strong acidic behavior and excessive loading of catalyst, low
reaction temperature, and need of additives such as molecular
sieves have remained as the associated disadvantages. Therefore,
developing an efficient and convenient glycosylation method by
employing economical and non-toxic catalyst for incorporating
glycosidic linkage in deoxy sugars in a stereocontrolled manner
is highly desirable.
a
important synthetic intermediates for constructing several valu-
able glycoconjugates and natural products.2 The presence of
1
a
or b linkage in 2-deoxy glycosides plays a crucial role in molecular
recognition, reactivity, and adhesion of glycosubstances and linked
to several cellular processes.1 In this context, stereoselective gly-
cosylation strategies serve as an important chemical tool for
assembling venerable sugar molecules and related natural prod-
ucts to probe their biological activities. In view of their pharmaco-
logical properties, the stereoselective preparation of 2-deoxy
sugars has gained considerable attention and remains a challeng-
,2
Owing to the inherent moisture/air stability and unique charac-
teristic, Cu(II) triflate and its analogous catalysts have shown
remarkable applicability in synthetic organic chemistry.9 Recent
studies on the comparison of metal triflates and strong Brønsted
acids reveal that copper(II) triflate is the most effective and
promising catalytic system to generate in situ TfOH in highly effi-
3
–5
ing task.
The most common and reliable method for the preparation of
-deoxy glycosides involves glycosylation of 2-deoxy-2-halo-gly-
cosyl acetate donor with aglycones using Brønsted acid such as
triflic acid (TfOH). In addition, the oxidative iodoglycosylation of
2
6
,7
1
0
cient green transformations. Inspired by this fact and considering
the versatile reactivity of Cu(OTf) as an inexpensive and moisture
stable catalyst, we focused to investigate the glycosylation of 2-
deoxy-2-iodo- -mannopyranosyl acetate donor under mild reac-
tion conditions.
We recently demonstrated the efficiency of Cu(OTf)
acid catalyst in stereoselective glycosylation of glycals to generate
8
a–c
8d,e
8f,g
glycals using NIS,
IDCP
or molecular iodine
as
2
electrophilic iodonium ion equivalent provide 2-iodoglycosides.
Subsequent reductive elimination or deiodonation from C-2
position would generate the 2-deoxy glycoside.7 The C-2
substituent in 2-halo-glycosyl acetate play a crucial role in
chemical glycosylation as the stereodirecting group and induced
anchimeric assistance to control the stereoselectivity.7 Although
a
c
2
as Lewis
11a
various
functionalized
2,3-unsaturated
glycosides.
In
1
1
continuation of our research in glycochemistry, herein we report
Cu(II) triflate as an alternative and effective catalyst for the
anomeric activation of 2-iodo-glycosyl acetate enabling the
⇑
Corresponding author. Tel.: +91 402 716 1649; fax: +91 402 716 0387.
E-mail address: skashyap@iict.res.in (S. Kashyap).
http://dx.doi.org/10.1016/j.tetlet.2016.01.035
040-4039/Ó 2016 Elsevier Ltd. All rights reserved.
0

8
12
S. K. Battina, S. Kashyap / Tetrahedron Letters 57 (2016) 811–814
stereoselective preparation of 2-deoxy-2-iodoglycosides. Further-
more, the regioselective iodoacetoxylation of several glycals was
achieved by employing copper(I) iodide as the halide source and
2-deoxy glycosyl acetates 2f–2j (entries 6–11). In contrast, the
reaction of -glucal (1a) and -galactal (1e) with stoichiometric
CAN (2.6 equiv) and NaI/AcOH gives the corresponding glycosyl
D
D
stoichiometric NaIO
4
as the oxidant in acetic acid at room temper-
acetates 2a and 2e in 75% and 80% yield, respectively, with a slight
6
a
ature. We believe that copper mediated glycosylation and preced-
ing oxidative iodoacetoxylation transformations would find its
applicability in glycochemistry for synthesizing biologically impor-
tant deoxy-sugar derivatives.
The iodoacetoxylation of glycals to access 2-deoxy-2-iodo-gly-
copyranosyl acetate donor is an important transformation in car-
variation in dr. Indeed, the iodoglycosylation of 1e using NIS in
1
2d
AcOH at 110 °C gives 2e in a moderate yield 64%.
Further
comparison of iodoacetoxylation reactions of 1a and 1f using
polymer-bound iodate reagent (ꢁ4 equiv) highlights the
advantage of the present protocol in terms of selectivity and
1
2e,f
yields.
highlighted for disaccharide substrates such as
-maltal (1l) to generate the 2-deoxy-disacchaides 1-O-acetates
2k–2l in satisfactory yields with a good dr (entries 12 and 13).
However, -lactal (1k) gives 2k in 75% yield by employing I /Cu
(OAc)
ꢀ6H
The synthetic utility of this method was further
8
,12
bohydrate chemistry.
We envisioned that NaIO
4
oxidant
D-lactal (1k) and
would efficiently promote the umpolung of copper halide in the
presence of acetic acid to generate the electrophilic I-OAc interme-
diate. Subsequent reaction with an electron-rich double bond fol-
lowing intrinsic regioselective opening of resulting iodonium ion
intermediate with a nucleophile, OAc in this case, would provide
the 1–2-trans-iodo-acetate.
D
D
2
1
2g
2
2
O in AcOH at 80 °C.
Having identified a mild and facile method for the synthesis of 2-
deoxy-2-iodo-glycosyl acetates, we next considered the possibility
of copper triflate catalyzed glycosylation of 2-iodo-glycosyl acetate
donor. Accordingly, the chemical glycosylation of 2-deoxy-2-iodo-
To test this hypothesis, the 3,4,6-tri-O-acetyl-
subjected to iodoacetoxylation using equimolar amount of CuI
and NaIO in acetic acid as the solvent (Table 1). To our delight,
D-glucal (1a) was
4
a-mannopyranosyl acetate donor (2a) with menthol (3a) as the
reaction was completed in utmost 1 h at room temperature to
acceptor was performed using 10 mol % of Cu(OTf) as the promoter.
2
afford the desired glycosyl acetate 2a in 97% yield (entry 1). An
The initial experiment using DCM as the solvent resulted only 30%
conversion of the starting material at room temperature in 20 h
(Table 2, entry 1). Preliminary optimization employing common
improved diastereoselectivity in favor of
a-manno isomer (dr;
8
7:13) was observed when compared with our previous report
1
1k
(
dr; 80:20).
Other oxidants such as H
2
O
2
, oxone, and CuO in
3
organic solvents such as CH CN, toluene, and 1,4-dioxane resulted
combination with CuI were unsuccessful, however the use of
stoichiometric PIDA (phenyliodonium diacetate) as the oxidant
resulted in 56% yield albeit with lower dr, 76:24 (entry 2).
in poor to moderate conversion (entries 2–4). Switching the solvent
to 1,2-dichloroethane afforded the desired product 4a in 68% yields
in 20 h (entry 5). However, significant improvement in the rate was
realized when the reaction was performed at 60 °C for 1 h, furnish-
ing the glycoside 4a in 82% yield (entry 6).
Although 5 mol % of Cu(OTf) was effective and optimal cat-
2
alytic amount in the glycosylation of 2a with 3a and 2-deoxy-gly-
coside 4a was similarly isolated in 86% yield (Table 2, entry 7).
Notably, the iodoacetoxylation of 1a with NH
4
I/H
AcOH gives 2a in 85% with dr 83:17.1 On the other hand,
molecular iodine in combination with Cu(OAc) O produces
a with selectivity upto 92 and a decreased yield (88%) albeit at
2 2 2
O in Ac O/
2c
2
ꢀ6H
2
2
8
f
high temperature.
The generality and scope of reaction was further illustrated
with substrate comprising various protecting groups in glucals.
Thus, the iodoacetoxylation of glucals 1b–1d underwent smoothly
to access the corresponding 2-deoxy-2-iodo-glycosyl-1-O-acetates
2
Further decreasing the quantity of Cu(OTf) resulted in poor con-
version albeit at a longer reaction time (entries 8 and 9). No further
improvement was observed when the reaction was performed in
the presence of molecular sieves (4 Å MS). Importantly, the reac-
tion proceeded with complete selectivity furnishing the single
2
b–2d in good yields (Table 1, entries 3–5). Furthermore,
D-galactal
(
1e) and various 6-deoxy sugar derived glycals 1f–1j conveniently
diastereomer, menthyl 3,4,6-tri-O-acetyl-2-deoxy-2-iodo-a-D-
mannopyranoside (4a). The spectroscopic data correlated with
1
3
underwent regioselective iodoacetoxylation to deliver the desired
that of literature report, was found consistent in accordance with
8
g
the assigned structure.
Table 1
CuI/NaIO
promoted iodoacetoxylation of glycalsa
4
I
O
CuI, NaIO
4
O
O
I
OAc
Table 2
+
Optimization of Cu(OTf)2 catalyzed glycosylationa
AcOH r.t.
OAc
α-manno)
30 min-2 h
(
(β-gluco)
1
a-l
2b-l
AcO
I
AcO
AcO
AcO
I
AcO
O
O
Cu(OTf)2
AcO
Yieldb (%)
drc
+ HO
Entry
1
Glycal
Product
Conditions
O
OAc
3,4,6-Tri-O-acetyl-
D
-glucal (1a)
2a
2a
2b
2c
2d
2e
2f
2g
2h
2i
97
56
87
92
98
95
88
86
84
93
86
92
96
87:13
76:24
60:40
48:52
91:09
95:05
73:27
65:35
42:58
79:21
79:21
94:06
90:10
d
2
1a
2a
3a
4a
3
4
5
6
7
8
9
3,4,6-Tri-O-methyl-
3,4,6-Tri-O-benzyl-
3,4,6-Tri-O-benzoyl-
3,4,6-Tri-O-acetyl-
3,4-Di-O-acetyl-
3,4-Di-O-acetyl-
3,4-Di-O-acetyl-
3,4-Di-O-acetyl-
3,4-Di-O-acetyl-
D
-glucal (1b)
-glucal (1c)
-glucal (1d)
-galactal (1e)
-rhamnal (1f)
-rhamnal (1g)
D
Yieldb (%)
Entry
Catalyst
mol %)
Solvent
Temp
(°C)
Time
(h)
D
c
(
(Conv )
D
1
2
3
4
5
6
7
8
9
10
10
10
10
10
10
5
Dichloromethane
Acetonitrile
rt
rt
rt
rt
20
20
16
20
20
1
1
8
10
NR (30%)
38
NR (20%)
NR (20%)
57
D
L
Toluene
1,4-Dioxane
D
-xylal (1h)
-arabinal (1i)
-arabinal (1j)
-lactal (1k)
-maltal (1l)
10
11
12
13
D
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
rt
L
2j
2k
2l
60
60
60
60
82
86
Per-O-acetyl-
Per-O-acetyl-
D
D
2
1
45
NR (20%)
a
Reaction conditions: Glycal (1.0 equiv), CuI (1.1 equiv), NaIO
4
(1.1 equiv), acetic
acid (0.5 mL), room temperature.
a
b
c
b
Reaction conditions: 2a (0.37 mmol), menthol (3a) (0.40 mmol).
Isolated yields, NR = not recorded.
Progress of reaction was monitored by TLC analysis at given time.
Isolated yields.
c
Based on relative integration of anomeric proton in 1H NMR spectrum.
d
2 4
Reaction was performed with PhI(OAc) (1.1 equiv) instead of NaIO .

S. K. Battina, S. Kashyap / Tetrahedron Letters 57 (2016) 811–814
813
Table 3
Cu(OTf)
AcO
AcO
AcO
2
catalyzed synthesis of 2-deoxy-2-iodo-
a
-glycosidesa
3a
O
6
h
AcO
AcO
AcO
I
Cu(OTf)
(5 mol%)
DCE, 60 o
2
AcO
AcO
AcO
I
Cu(OTf)
(5 mol%)
DCE, 60 o
2
O
AcO
AcO
AcO
O
O
5a; 88% (80:20)
O
+
ROH
C
AcO
C
naphthalene
-2-thiol (
2a
OAc 3b-3j
4b-4j OR
t (h)
2a'
3k AcO
O
OAc
)
AcO
6
h
S
Entry
1
ROH
2-Deoxy-2-iodo-glycosides
Yieldb (%)
89
5b; 86% (65:35)
AcO
AcO
I
I
I
I
O
Scheme 1. Cu(II) triflate-catalyzed glycosylation of 2-deoxy glycosyl acetate.
3b
3c
3d
AcO
4b
1
O
AcO
AcO
AcO
glycosylation of amino acid derived aglycones. Thus, glycosylation
reaction of 2a with Fmoc-Ser-OMe (3g) in the presence of catalytic
O
2
3
4c
1.5
2
87
92
Cu(OTf)
yield (Table 3, entry 6). Likewise, N-Fmoc-trans-4-hydroxy-
line (3h) was incorporated in 2-deoxy-mannopyranoside to gener-
ate the venerable -linked mannosylated peptide glycoconjugate
2
resulted in the corresponding glycoconjugate 4g in a good
O
L
-pro-
AcO
AcO
AcO
O
4d
a
O
4h in satisfactory yield (entry 7). In addition, a nucleoside base, uri-
dine derivative (3i) and a natural product for instance cholesterol
(3j) were reacted smoothly affording the corresponding 2-deoxy
glycoconjugates 4i–4j in good yields (entries 8 and 9). Notably,
all the reactions were proceeded with complete selectivity provid-
AcO
AcO
AcO
OMe
OBz
O
O
O
4
e
4
3e
0.5
82
O
OBz
OBz
ing a-mannosides, endorsed to the participation of axially oriented
C-2 iodo group in stabilization and controlling the stereochemistry
AcO
AcO
AcO
I
I
OMe
O
at anomeric position.
BzO
4f
We further examined the reactivity of 1-O-acetyl donor without
any substituent at C-2 position and stereochemical outcome in
present protocol. Thus, glycosylation of 2-deoxy glycosyl acetate
5
6
7
3f
0.5
2
84
86
88
O
OBz
OBz
0
AcO
AcO
AcO
donor 2a , readily prepared from 2a using deiodonation method,
O
with 3a in the presence of Cu(OTf)
sponding 2-deoxy glycoside 5a in good yield with an
0:20 in favor of -anomer (Scheme 1). The feasibility of sulfur
2
(5 mol %) afforded the corre-
3g
3h
CO
2
Me
4g
4h
a
/b ratio of
O
NHFmoc
8
a
AcO
AcO
AcO
I
nucleophile was also tested under similar conditions. Accordingly,
naphthalene-2-thiol (3k) was allowed to react with glycosyl donor
a , furnishing the corresponding 2-deoxy thioglycoside 5b in a
O
0.5
0
O
2
CO
2
Me
good yield albeit as a mixture with a/b ratio of 65:35.
N
Fmoc
In summary, we have demonstrated an operationally simple
glycosylation for the stereoselective synthesis of 2-deoxy-2-iodo-
glycosides utilizing solid, moisture tolerant, ease to handle and
O
NH
AcO
AcO
AcO
I
non-toxic catalytic system. The Cu(OTf)
venient and amenable to a wide range structurally diverse accep-
tors. Moreover, copper(I) iodide in combination with NaIO
efficiently promoted the iodoacetoxylation of several glycals. Fur-
ther studies utilizing these protocols to access deoxy saccharides
bearing different linkages at anomeric center are currently under-
going in our laboratory.
2
catalyzed protocol is con-
N
8
9
3i
3j
O
4i
6
1
79
82
O
O
O
4
O
O
AcO
AcO
AcO
I
H
O
H
4
j
H
H
O
Acknowledgments
a
All reactions were performed with 2a (1 equiv), ROH (1.2 equiv), 5 mol % Cu
OTf) in 1,2-dichloroethane at 60 °C.
Isolated and un-optimized yields.
(
2
b
S.K. gratefully acknowledge the Department of Science and
Technology, India for the INSPIRE Faculty award (GAP0397) and
Start-up Research Grant for Young Scientists (GAP0471). S.K.B.
acknowledges a UGC fellowship. The authors are grateful to the
Director CSIR-IICT for providing necessary infrastructure.
With the optimal protocol, we next probed the scope of glyco-
sylation reaction with a wide range of acceptors to generate struc-
turally diverse 2-deoxy glycosides. As summarized in Table 3, the
glycosylation of 2a with benzyl alcohol (3b) and alicyclic substrate
bearing cyclopropyl (3c) and cyclohexyl (3d) ring proceeded
smoothly, allowing the facile synthesis of corresponding 2-
deoxy-glycosides 4b–4d in good yields (entries 1–3). Encouraged
by these results, we next attempted the glycosylation of carbohy-
drate derived alcohols to access the disaccharides containing 2-
deoxy-sugars. Thus, glucose (3e) and mannose (3f) derived glyco-
syl acceptors were successfully incorporated in 2-deoxy-2-iodo-
Supplementary data
Supplementary data (general synthesis information, proce-
1
13
dures, characterization data and H, C NMR spectra of glycosides)
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2016.01.035.
References and notes
a
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