Use of iodine for efficient and chemoselective glycosylation with glycosyl ortho-alkynylbenzoates as donor in presence of thioglycosides

Article abstract of DOI:10.1016/j.tetlet.2012.11.101

A novel and high yielding glycosylation protocol with glycosyl ortho-alkynylbenzoates as donors and iodine as promoter is described. The donors are stable and can be chemoselectively activated in the presence of thioethyl and thiophenyl glycosides. The application of this methodology in one-pot consecutive glycosylation reaction is described.

Full text of DOI:10.1016/j.tetlet.2012.11.101

Tetrahedron Letters 54 (2013) 865–870
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Use of iodine for efficient and chemoselective glycosylation with glycosyl
ortho-alkynylbenzoates as donor in presence of thioglycosides
⇑
Samrat Dutta, Swarbhanu Sarkar, Shyam Ji Gupta, Asish Kumar Sen
Chemistry Division, CSIR-Indian Institute of Chemical Biology, 4 Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 June 2012
Revised 22 November 2012
Accepted 24 November 2012
Available online 1 December 2012
A novel and high yielding glycosylation protocol with glycosyl ortho-alkynylbenzoates as donors and
iodine as promoter is described. The donors are stable and can be chemoselectively activated in the pres-
ence of thioethyl and thiophenyl glycosides. The application of this methodology in one-pot consecutive
glycosylation reaction is described.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Glycosylation
ortho-Alkynylbenzoates
Iodine
Chemoselective
One-pot
Oligosaccharides and glycoconjugates play an important role in
numerous biological processes1 such as cell–cell interaction,2
inflammation,3 signal transduction4 and many others.5 Easy and
rapid access to pure oligosaccharides and glycoconjugates is, there-
fore, a prerequisite for further investigation of the precise role of
carbohydrates in biology. Thus, the need for development of new
and efficient glycosylation protocols has received the attention of
organic chemists for many years.6
Glycosylation reaction, central to the problem of oligosaccha-
ride and glycoconjugate syntheses, entails a reaction between a
glycosyl donor, a protected monosaccharide with a leaving group
at the anomeric position, and a glycosyl acceptor, usually carrying
one or more free hydroxyl groups. One of the many challenges that
carbohydrate chemists face is to attain high yields and complete
diastereoselectivity in glycosylation reactions.7 The development
of various glycosylation techniques since the discovery of Koe-
nigs–Knorr reaction,8 has addressed these problems and allowed
for the syntheses of complex oligosaccharides and glycoconjugates.
The different glycosyl donors employed in these techniques are
glycosyl halides, thioglycosides, trichloroacetimidates, sulfoxides,
thioimidates, phosphate/phosphite derivatives, glycals, 1-acyl,
1-carbonate, 4-pentenyl glycosides, 1-hydroxy sugars and orthoes-
ters.9 Most of these donors require promoters which are toxic,
carcinogenic or light and moisture sensitive. Therefore, convenient
to use, non-toxic promoters are in high demand.
Studies on C–C triple bond activation by metals, to promote
glycosylation have introduced new classes of stable and easily
synthesized glycosyl donors such as
x-alkynoic acids, propargyl
glycosides and glycosyl o-alkynylbenzoate.10,11 These exploit the
alkynophilic character of Hg(II),10a Au(III)10b,10c and Au(I).11
Although Hg(II) and Au(III) salts are proved to be potent promoters
for these donors, the application is limited to armed donors only.
AuPPh3OTf complex, generated from AuPPh3Cl and AgOTf, was used
for the activation of both armed and disarmed donors. Unfortu-
nately, use of metal Lewis acid catalyst results in the disposal of
enormous amounts of harmful waste. Recent researches in metal-
free organic reactions led us to look for an alternative environmen-
tally benign and cost effective promoter.
Recent reports revealed that iodolactonization of o-(1-Alky-
nyl)benzoates produces substituted 4-iodoisocoumarins.12 This
prompted us to apply the iodocyclization on glycosyl-o-(1-Alky-
nyl)benzoates (1); initiated by iodine it stimulates a tandem
glycosylation with alcohols (2), to produce glycoside (3) and 4-iod-
oisocoumarin (4) (Scheme 1). Iodine used to promote this reaction
is inexpensive, convenient to use, environmentally benign and
non-toxic. The mild reaction condition can be applied to
a
one-pot chemoselective consecutive glycosylation reaction. It is
worthwhile to mention that the use of iodonium ion for remote
activation of n-pentenyl glycosides and glycosyl halides is well
known.13,14
o-(1-Alkynyl)benzoic acids
5 were prepared as reported
earlier.12b,12c Glycosyl o-(1-Alkynyl)benzoates 1 were prepared by
EDCI coupling of the corresponding lactols 6 with 5 in high yield
(entries 1–10, Table 1). In the gluco- and galacto-configurations
⇑
Corresponding author. Tel.: +91 33 24995720; fax: +91 33 24735197.
E-mail address: aksen@iicb.res.in (A.K. Sen).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.101

866
S. Dutta et al. / Tetrahedron Letters 54 (2013) 865–870
R2OH
O
I
O
PO
O
R1
O
(2)
O
O
PO
OR2
R1
I2, CH3CN
3
4
1
Scheme 1. Glycosylation with glycosyl o-(1-alkynyl)benzoate donors.
the 1,3-diaxial interaction of the bulky ortho-alkynylbenzoate (in
the -face) with axial H-atoms may be responsible for the forma-
of 1,3-diaxial interaction in a-configuration favours b-product
and (ii) steric interaction between equatorial ortho-alkynylbenzo-
a
tion of b-product exclusively. In the case of manno-configuration
ate and the axial C-2 acetyl group in b-configuration, favours for-
(Table 1, entry 4) two types of interactions are possible; (i) effect
mation of
a-product. These two interactions are probably more
Table 1
Preparation of glycosyl o-(1-alkynyl)benzoatesa
R1
CO2H
O
PO
O
5a-c
R1
O
O
PO
OH
EDCI, HOBt, Et3N, DCM,
0oC to r.t, overnight
6a-f
1a-j
Entry
1
o-(1-Alkynyl)benzoic acid 5a–c
5a: R1 = C6H5
Lactol 6a–f
Ester 1a–jb
Yieldc (%) (
a
:b)d
OBn
O
BnO
1a
94 (0:1)
BnO
OH
OH
BnO
6a
OAc
O
AcO
AcO
2
5a: R1 = C6H5
1b
95 (0:1)
AcO
6b
OAc
OAc
O
3
4
5a: R1 = C6H5
1c
96 (0:1)
94 (5:4)
AcO
OH
AcO
6c
OAc
AcO
O
5a: R1 = C6H5
1d
AcO
AcO
OH
6d
OH
O
AcO
5
6
5a: R1 = C6H5
1e
1f
93 (1:4.8)
AcO
OAc
6e
OAc
AcO
O
OAc
AcO
AcO
5a: R1 = C6H5
89 (0:1)
O
O
AcO
OH
AcO
6f
7
8
9
10
5b: R1 = p-OMePh
5b: R1 = p-OMePh
5c: R1 = cyclopropyl
5c: R1 = cyclopropyl
6b
6c
6b
6c
1g
1h
1i
94 (0:1)
93 (0:1)
91 (0:1)
90 (0:1)
1j
a
All reactions were performed with lactols 3 (1 equiv), o-(1-alkynyl)benzoic acids 1 (1.1 equiv), EDCI (1.1 equiv), triethylamine (1.1 equiv) and HOBt (1.1 equiv) in 3 mL
DCM.
b
All the products were characterized by NMR and mass spectroscopy.
c
Yield refers to products isolated by column chromatography.
d
All a
:b ratios are determined by 1H NMR spectroscopy.

S. Dutta et al. / Tetrahedron Letters 54 (2013) 865–870
867
Table 2
or less equal resulting in the formation of
a
- and b-product in
Model glycosylation reaction of glucosyl o-(1-alkynyl)benzoate 1b with 1-naphthyl
nearly equal quantities (5:4). The mixture of anomers was used di-
rectly in the subsequent glycosylation reactions without
separation.
methanola
Entry Temperature Amount of iodine Solvent Time (h) Yieldb (%)
(equiv)
Initially we attempted glycosylation of per-O-acetyl-D-gluco-
1
2
3
4
5
6
25 °C
10 °C
25 °C
25 °C
25 °C
25 °C
1.2
1.2
1.2
1.2
1.0
1.5
CH3CN 1.5
CH3CN 1.5
88 (30c, 78d)
pyranosyl-o-(phenylethynyl)benzoate 1b (1 equiv) with 1-naph-
thylmethanol 2c (1.2 equiv), as the model reactants to optimize
the reaction parameters (Table 2). Reaction was complete within
1.5 h in the presence of iodine (1.2 equiv) in dry acetonitrile at
25 °C, yielding the desired b glycoside 3c in high yield (entry 1).
Lower reaction temperature (entry 2) and use of CH2Cl2, Et2O as
solvents produced inferior yields (entries 3–4). A substantial de-
crease in reaction yield was also observed when the amount of io-
dine was decreased below 1 equiv (entry 5). However, an increase
in the amount of iodine to 1.5 equiv did not change the yield con-
20
23
68
72
88
Et2O
1.5
CH2Cl2 1.5
CH3CN 1.5
CH3CN 1.5
a
All reactions were performed with 1.0 equiv of glycosyl donor, 1.2 equiv of
aglycone.
b
Yield refers to products isolated by column chromatography.
Reaction was performed with 1.2 equiv N-iodosuccinimide.
Reaction was performed with 1.2 equiv iodine monochloride.
c
d
Table 3
Glycosidic coupling of glycosyl o-(1-alkynyl)benzoates with alcohols using iodine as promotera
Entry
Donor 1
Acceptor 2
Product 3b
OBn
Yield of 3c
(a
:b)d
Product 4b (yield)c
4a: R1 = Ph (97)
Ref.
—
O
BnO
BnO
1
1a
Methanol 2a
95 (1.5:1)
OMe
OMe
OBn
3a
OAc
O
AcO
AcO
2
3
1b
1b
2a
94 (0:1)
87 (0:1)
4a (95)
4a (86)
15a
15b
OAc
3b
OAc
O
AcO
AcO
O
2b
7
OAc
3b
HO
OAc
O
AcO
AcO
O
4
1b
88 (0:1)
4a (90)
—
OAc
2c
3c
5
6
1g
1i
2c
2c
3c
3c
89 (0:1)
77 (0:1)
4b: R1 = p-OMePh (90)
4c: R1 = cyclopropyl (75)
—
—
OAc
HO
O
AcO
O
AcO
7
8
1b
71 (0:1)
4a (73)
15c
OAc
2d
3d
OH
Cl
OAc
O
N
AcO
AcO
O
Cl
1b
73 (0:1)
4a (76)
—
N
Cl
OAc
Cl
3e
2e
2e
9
10
1g
1i
3e
3e
74 (0:1)
62 (0:1)
4b (74)
4c (65)
—
—
2e
OH
OAc
OAc
O
O
11
12
1c
65 (0:1)
76 (0:1)
4a (69)
4a (79)
15d
AcO
OAc
NO2
2f
NO2
3f
O
OAc
HO
O
O
O
AcO
O
AcO
1b
—
O
OAc
2g
3g
(continued on next page)

868
S. Dutta et al. / Tetrahedron Letters 54 (2013) 865–870
Table 3 (continued)
Entry
13
Donor 1
Acceptor 2
Product 3b
OAc
Yield of 3c
(a
:b)d
Product 4b (yield)c
Ref.
—
OAc
O
O
AcO
1c
2c
86 (0:1)
4a (89)
OAc
3h
14
15
1h
1j
2c
2c
3h
3h
88 (0:1)
73 (0:1)
4b (95)
4c (79)
—
—
OAc
OAc
O
HO
16
1c
73 (0:1)
4a (76)
16a
O
AcO
2h
OAc
3i
OAc
OAc
O
AcO
AcO
17
18
1d
1e
2c
2c
87 (1:0)
83 (1:0)
4a (89)
—
—
O
3j
O
O
4a (82)
AcO
AcO
AcO
OAc
3k
OAc
O
OAc
AcO
19
1f
2i
72 (0:1)
4a (73)
—
O
OAc
O
O
AcO
OAc
3l
OH
OAc
OAc
O
O
BnO
BnO
AcO
AcO
OBn
20e
1d
O
BnO
BnO
75 (1:0)
4a (77)
16b
OMe
O
2j
OBn
OMe
3m
OH
OAc
OAc
OBz
OAc
O
O
BzO
BzO
AcO
AcO
SEt
21e
1d
78 (1:0)
4a (79)
17
O
OBz
O
2k
BzO
BzO
SEt
3n
OAc
OAc
O
OAc
O
OAc
AcO
HO
AcO
AcO
OAc
O
22e
1d
67 (1:0)
4a (67)
17
AcO
SPh
O
2l
SPh
3o
a
b
c
All reactions were performed with 1.2 equiv of iodine, 1 equiv of glycosyl donor, 1 equiv of aglycone in dry 5 mL acetonitrile at 25 °C.
All the products were characterized by NMR and mass spectroscopy.
Yield refers to products isolated by column chromatography.
d
e
All
Reactions were performed with 1.4 equiv of iodine, 1.2 equiv mmol of glycosyl donor, 1 equiv of acceptor in 5 mL dry acetonitrile at 25 °C.
a
:b ratios are determined by 1H NMR spectroscopy.
siderably (entry 6). Use of N-iodosuccinimide and iodine mono-
To investigate the feasibility of the above glycosylation method-
chloride instead of iodine gave relatively lower yields.
ology, the coupling of donors 1a–j with various alcohols–aliphatic,

S. Dutta et al. / Tetrahedron Letters 54 (2013) 865–870
869
O
O
PO
PO
R1
I
O
O
O
O
O
R1
I
O
PO
R2OH
PO
OR2
7
3
I
O
I
O
R1
4
Scheme 2. Plausible mechanism for glycosylation with glycosyl o-(1-alkynyl)benzoates donors using iodine as promoter.
(R1 = cyclopropyl) led to a significant decrease in reaction yields
(entries 6, 10, 15).
Table 4
Competitive glycosylation reaction using ethyl thioglycoside
alkynylbenzoate (1d) as donors
8 and glycosyl o-
To evaluate the utility of our approach in oligosaccharide syn-
thesis, we focused our attention on glycosylation with sugar accep-
tors. Coupling with 6-hydroxyl sugar derivatives (entries 20 and
21) and 3-hydroxyl mannose derivative (entry 22) proceeded
smoothly. It is worth noting that the disarmed thioglycosides re-
mained intact under this reaction condition (entries 21 and 22)
indicating chemoselective activation of glycosyl o-(1-Alkynyl)ben-
zoate donors.
The plausible mechanism for glycosylation with glycosyl o-(1-
Alkynyl)benzoate donors using iodine as promoter is depicted in
Scheme 2. Initially the C–C triple bond was activated by iodine
which resulted in a nucleophilic attack on the triple bond by car-
bonyl oxygen. This forced cleavage of glycosidic bond produced
2-substituted-4-iodoisocoumarin 4 and oxocarbenium ion 7. Next
the oxocarbenium ion underwent nucleophilic attack by R2OH to
form glycoside 3.
OBz
OBz
2.5 equiv. I2
O
BzO
BzO
2j
3m
+ 4a
1d
CH3CN, rt
SEt
8
Glycosyl acceptor
Glycosyl donors
Entry
Time (h)
Product, yielda (%)
Recovered, yielda (%)
3m
84
87
89
4a
8
1
2
3
1.5
2
6
86
90
91
96
97
96
For glycosyl donors having non-participating benzyl protecting
a
Isolated yields.
group at C-2 the glycoside produced is a mixture of
a and b ano-
mers. Whereas the donors with participating acetyl group at C-2,
1,2-trans glycoside are produced as the only product.
A competitive glycosylation reaction between the two donors
1d and ethyl 2,3,4,6-tetra-O-benzoyl-1-thio-a-D-mannopyranoside
benzylic, aromatic, heteroaromatic and sugar acceptors was tried.
The results are summarized in Table 3. The armed donor per-O-
benzyl-D-glucopyranosyl-o-(phenylethynyl)benzoate 1a reacted
(8), 1.2 equiv of each, with acceptor 2j (1 equiv) was carried out in
the same pot in the presence of iodine (2.5 equiv) for 1–6 h
(Table 4). Of the two possible disaccharides, only 3m (derived from
donor 1d) was isolated while thioglycoside 8 remained intact even
after 6 h at rt and was recovered (ꢀ97%) indicating no activation of
ethyl thioglycosides donor 8. The possible reason for this may be
the presence of deactivating benzoyl protecting groups on donor
8.18
faster than the disarmed donor 1b (entry 1 vs entry 2) and the
reactions were completed within 50 min and 1.5 h respectively.
Reaction with aliphatic alcohol resulted in high yield (entry 3). Gly-
cosylation with benzylic alcohol (entry 4) also proceeded satisfac-
torily. Heteroaromatic alcohols (entry 8) as well as aromatic
alcohols with electron withdrawing substituent (entry 11) reacted
efficiently.
On replacing the phenylethynyl group (R1 = Ph) on the substrate
After establishing the optimal glycosylation conditions, we fur-
ther explored the scope of the reaction to synthesize trisaccharide
10, the key intermediate for the synthesis of biologically important
1, with p-methoxy phenylethynyl (R1 = p-OMeC6H4),
a slight
improvement in yield was observed (entries 5, 9, 14) but replacing
the phenylethynyl (R1 = Ph) group with cyclopropylethynyl
OAc
OAc
OAc
OAc
O
OTBDPS
OH
AcO
AcO
O
AcO
O
AcO
HO
AcO
OH
O
O
O
Ph
OBz
O
OBz
O
OBn
9
1d
BzO
BzO
BzO
BzO
OTBDPS
OH
3n
I2, CH3CN
-20 °C, AgOTf
SEt
2k
O
AcO
O
10
OBn
Scheme 3. One-pot synthesis of trisaccharide by successive chemo- and regioselective manner.

870
S. Dutta et al. / Tetrahedron Letters 54 (2013) 865–870
pentasaccharide
({
a
-
D
-Manp-(1 ? 3)-
a
-
D
-Manp-(1 ? 6)-[
a
-
D
-
11.101. These data include MOL files and InChiKeys of the most
important compounds described in this article.
Manp-(1 ? 6)-
a
-D
-Manp-(1 ? 3)]-a-D
-Manp}).19 The chemo- and
regioselective glycosylation was carried out in one-pot as de-
scribed in Scheme 2. Glycosylation of donor 1d (1.2 equiv) with
acceptor 2k20 (1 equiv) was achieved using the iodine (2.5 equiv)
promoted approach described earlier. Chemoselective activation
of benzoate donor 1d produced 1 ? 6 disaccharide 3n having an
anomeric thioethyl group. On addition of diol 917 (0.7 equiv) along
with AgOTf (0.3 equiv) in the same reaction medium, regioselec-
tive 3-O-glycosylation was achieved to generate desired trisaccha-
ride 10 in good yield (54%). The activation of the intermediate
thioglycoside donor 3n was achieved with AgOTf and the excess io-
dine present in the one-pot system.
The second glycosylation step was highly regioselective and no
2-O-glycosylated product was detected. The reason may be attrib-
uted to the increased reactivity of 3-OH induced by free 2-
OH.17,21(see Scheme 3)
In conclusion, we have developed a simple and highly efficient
glycosylation protocol for glycosyl ortho-alkynylbenzoates, which
are easily prepared and stable under standard laboratory condi-
tions. Glycosylation occurs at room temperature utilizing iodine
as the mild promoter, which is cheap, easily available, non-toxic
and convenient to use. The protocol can chemoselectively activate
glycosyl ortho-alkynylbenzoate donors in the presence of thiogly-
cosides. This chemoselective glycosylation strategy has also been
applied to design a new one-pot glycosylation protocol.
References and notes
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Acknowledgment
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The authors express their gratitude to Professor S. Roy, Director,
IICB for continuous encouragement, and CSIR & DBT, Govt. of India,
for providing fellowships to S.D., S.S & S.J.G. The authors are in-
debted to Mr. K. K. Sarkar for recording ESI-MS and Mr. T. Sarkar,
Mr. E. Padmanaban for recording NMR spectra.
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