A facile and convenient synthesis of disarmed glycosyl fluorides using in situ-generated iodine monofluoride (see Ref. 1)

Article abstract of DOI:10.1016/j.tetlet.2012.08.024

A facile and convenient synthesis of disarmed glycosyl fluorides using in situ-generated iodine monofluoride is reported. The method is tolerant to most of the popularly used protecting groups and gives exclusively the α-anomeric product in very good yields.

Full text of DOI:10.1016/j.tetlet.2012.08.024

Tetrahedron Letters 53 (2012) 5631–5634
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A facile and convenient synthesis of disarmed glycosyl fluorides using
in situ-generated iodine monofluoride (see Ref. 1)
⇑
G. Mugunthan, K. P. Ravindranathan Kartha
Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research, S.A.S. Nagar, Punjab 160062, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile and convenient synthesis of disarmed glycosyl fluorides using in situ-generated iodine monoflu-
oride is reported. The method is tolerant to most of the popularly used protecting groups and gives exclu-
sively the a-anomeric product in very good yields.
Received 14 June 2012
Revised 4 August 2012
Accepted 6 August 2012
Available online 19 August 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Iodine monofluoride
Silver fluoride–iodine
Thioglycosides
Glycosyl fluorides
Applications of interhalogens
Glycosyl fluorides are versatile building blocks in the synthesis
of oligosaccharides. Though known since 1923,2 they gained
importance only after their successful application as a glycosyl do-
nor by Mukaiyama and co-workers using SnCl2–AgClO4 as the acti-
In the latter category of reactions, the fluoride donor reagents
used are either corrosive, moisture sensitive, foul smelling, toxic,
and/or expensive. A cheaper, easy-to-handle alternative that can
serve the purpose under mild conditions must therefore be
welcome. Thioglycoside-activation has been one of the versatile
applications of iodine and its inter halogen compounds in carbohy-
drate chemistry. While armed thioglycosides can be easily acti-
vated by molecular iodine33,34 for glycosylation with an acceptor,
the disarmed thioglycosides require interhalogens35 for the
purpose; and this observation prompted us to investigate the pos-
sibility of converting the disarmed thioglycosides into the corre-
sponding fluorides using iodine monofluoride (IF). IF has been
previously reported for the conversion of glycals to 2-iodo glycosyl
fluorides.36 It can be conveniently generated in situ from molecular
iodine, a cheap and easy-to-handle reagent and AgF (Scheme 1).36
vator.3 Subsequently
a number of reaction conditions were
developed for their activation with ease.4 This, along with their rel-
ative stability, made them a favorable choice of many synthetic
glycochemists in the years followed.
Glycosyl fluorides can be prepared by a number of methods.
Sugars with a reducing end can be converted into the corresponding
glycosyl fluorides using any of the reagent systems, namely, 2-flu-
oro-1-methylpyridinium tosylate,5 diethyl-1,1,2,3,3,3-hexafluoro-
propylamine,6 pyridinium poly(hydrogen fluoride),7,8 DAST,9,10
– 11
DEAD/PPh3, and Et3O+BF4
,
N,N-diisopropyl(fluoro-2-methyl-1-
propenyl)amine,12 CF3ZnBr2ꢀ2CH3CN, and TiCl4,13 Selectfluor14 and
its analogs,15 N,N-diethyl- -difluoro-(m-methylbenzyl)amine,16
a,a
n-C4F9SO2F-NR3(HF)3-NR3,17 or 4-tert-butyl-2,6-dimethylphenyl-
sulfur trifluoride.18 They can also be accessed from glycosyl
acetates7 and orthoesters19 using pyridineꢀ(HF)x, or HF-MeNO2.20,21
Glycosyl triazoles22 and tetrazoles23 can also be converted into the
corresponding fluorides using (HF.pyridine). Alternatively, fluoride
displacement of other glycosyl halides can be achieved using
AgF,24,25 AgBF4,26 Et3N.3HF,27 ZnF2,28 or CF3ZnBr2ꢀ2CH3CN.13 Also,
thioglycosides have been converted into their corresponding
glycosyl fluorides using DAST/NBS,29 4-methyl(difluoroiodo)
benzene,30 selectfluor,14 or I(Py)2BF431,32 as the reagent.
O
F
AgF + I
I F
2
F
AgI
O
δ
δ
O
I F
CH2Cl2
I
O
F
S
S
R
R
RSI
F
O
F
⇑
Corresponding author. Tel.: +91 172 2214682 86; fax: +91 172 2214692.
E-mail address: rkartha@niper.ac.in (K.P.R. Kartha).
Scheme 1. In situ-generation of I–F and its reaction with thioglycosides.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.08.024

5632
G. Mugunthan, K.P.R. Kartha / Tetrahedron Letters 53 (2012) 5631–5634
Table 1
was not used in the current study. To examine the feasibility of
the reaction tolyl 2,3,4,6-tetra-O-acetyl-1-thio-b- -galactopyrano-
Reaction of a disarmed thiogalactoside with AgF-I2
D
side (1) was treated with a mixture of AgF and I2 at room temper-
ature in the dark. Based on the observations we have made earlier
in the context of the I2-promoted glycosylation reactions, the reac-
tion was carried out in three different solvent systems and the re-
sults are summarized in Table 1.
As can be seen from Table 1, promising results were obtained in
all the three cases. Thus, although the reaction was found to be fas-
ter in MeCN, the yield obtained in the case was inferior to those
employing both CH2Cl2 as well as dioxane. In CH2Cl2 the reaction
went to completion in 1 h with complete stereoselectivity giving
AcO
OAc
AcO
AcO
OAc
O
O
AgF, I2 (2 mol equiv each)
AcO
STol
Solvent
rt
F
AcO
AcO
1
1a
Solvent
Time (h)
Yield (%)a
1a (a:b)
MeCN
CH2Cl2
1,4-Dioxane
0.25
1.0
18
52
76
81
1:3
1:0
3:1
the desired
a-fluoride 1a-a in good isolated yield. In the case of
dioxane on the other hand the reaction was more effective (yield
81%) but required a longer time for completion. In the case of both
MeCN as well as dioxane fluoride 1a was obtained as a mixture of
a
Isolated yield.
a- and b-anomers, though opposing stereoselectivity was observed
in the two cases. As for the IF concentration, the use of less than
2 mol equiv of the reagents (each of AgF and I2) led to inferior re-
sults (values not shown). Thus, for further studies 2 mol equiv of
An alternate method reported in the literature for the genera-
tion of IF involves the use of anhydrous liquid HF, an extremely
corrosive reagent, in combination with N-bromosuccinimide and
Table 2
Reaction of various thioglycosides with AgF-I2 systema
Starting material
Product
Isolated yield (%)
[a] at 20 °C c = 1 (CHCl3)
D
Observed
Literature
OAc
OAc
OAc
OAc
O
O
81
+97.4
+96.538
AcO
AcO
AcO
AcO
STol
1
OAc
OAc
AcO
1a
F
OAc
O
O
76
+89.20
+90.082
AcO
AcO
SMe
2a
2
3
AcO
OAc
OAc
F
F
OAc
O
AcO
AcO
O
AcO
AcO
AcO
AcO
85
80
+24.0
+21.539
3a
SMe
STol
F
O
O
ꢁ27.0
ꢁ30.040
AcO
AcO
AcO
4a
AcO
4
OAc
OAc
AcO
F
AcO
OAc
O
OAc
O
AcO
OAc
STol
O
AcO
AcO
O
72
78
+42.3
+43.041
O
AcO
AcO
O
OAc
OAc
5
OAc
OAc
OAc
OAc
5a
O
O
O
O
+108.0
—
O
O
SMe
6
6a
OAc
AcO
F
STol
F
O
O
MsO
MsO
83
+46.2
—
O
O
O
O
7a
7

G. Mugunthan, K.P.R. Kartha / Tetrahedron Letters 53 (2012) 5631–5634
5633
Table 2 (continued)
Starting material
Product
Isolated yield (%)
[a] at 20 °C c = 1 (CHCl3)
D
Observed
Literature
Ph
Ph
O
O
O
O
67
+175.4
—
O
O
AcO
AcO
SPh
AcO
AcO
8
F
8a
OAc
OAc
O
O
AcO
BnO
AcO
BnO
59
ꢁ10.5
—
—
SPh
9
BnO
OBn
9a
F
AcO
AcO
OAc
O
1a
>80
—
AcO
SMe
α
10
AcO
AcO
OAc
O
1a
1a
>80
>80
—
—
—
—
SMe
AcO
β
10
10 a/b
a
Thioglycoside (1 mmol) was treated with AgF-I2 (2 mmol each) in anhydrous CH2Cl2 at rt in the dark.
Table 3
Method-comparison with literature methods
Substrate
Reaction condition
Yield (%)
Reagent
Time, temp
8
2,3,4,6-Tetra-O-acetyl-
D
-glucopyranose
-glucopyranosyl bromide
Pyꢀ(HF)n
10 h, rt
5.15 h, 82 °C
53 2a-
61 2a-b
65 (2a-
76 (2a-
a
2,3,4,6-Tetra-O-acetyl-
a
-
D
ZnF2, 2,20-bipyridine28
Phenyl 2,3,4,6-tetra-O-acetyl-1-thio-b-
2
D-galactopyranoside
4-Methyl (difluoroiodo) benzene30
AgF, I2 (current)
Over-night ꢁ78 °C
a
a
/b)
)
1 h, rt
the fluorinating agent in CH2Cl2 at rt was chosen as optimum for
the reaction.
When the glycosyl fluoride formation was attempted with p-
tolyl 2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-1-thio-b- -glucopy-
D
In order to establish wider acceptability of the method, the
reaction was extended to a range of thioglycoside-substrates. Thus,
ranoside (11), the formation of oxazoline 12, not surprisingly
though, was observed. The thioglycoside 13 that can serve as a pre-
cursor to the 2-acetamido-2-deoxy-glucopyranosyl analogues on
the other hand, failed to react owing to the highly ‘disarming’ nat-
ure of the azido substituent on the C-2 position of the hexosyl unit.
Likewise, the methylthiosialoside derivative 14 though was sus-
ceptible to the fluorinating agent, gave the expected sialosyl fluo-
ride 14a (dF ꢁ116 in the 19F NMR spectrum) only partially in
spite of using excess IF (in situ) for prolonged durations of reaction.
representative aryl and alkyl thioglycosides of various mono- (
D-
galactose, -glucose, -mannose and -rhamnose) and disaccha-
D
D
L
rides (lactose) bearing different protecting groups (acid-labile
groups such as the isopropylidene and benzylidene acetals and
the base-labile groups such as the acetate and mesylate) were
studied37 and the results are summarized in Table 2. It can be
clearly seen from Table 2 that all the nine thioglycoside-substrates
studied gave their respective
a-glycosyl fluorides in good yield
with complete functional group tolerance in all the cases. However,
in the case of substrate 9 the yield was found to be only 59% owing
to the partial cleave of the benzyl ether protection.
OAc
O
OAc
O
AcO
AcO
AcO
STol
AcO
Interestingly, in separate experiments when the two anomeric
N
NHAc
11
12
thioglycosides 10
with the fluorinating agent, the
a
or 10b, either alone or as a mixture, was treated
-fluoride 2a was the only product
O
a
OAc
O
OAc
SMe
O
AcO
obtained which was isolated in >80% yield. This clearly showed
that the reaction occurred via the oxocarbenium ion intermediate
following a path as shown in Scheme 1. In MeCN or dioxane on
the other hand there could be participation by the solvent mole-
cules and hence could lead to a mixture of isomeric fluorides as
can be seen from Table 1. Use of these solvents in the stereoselec-
tive synthesis of 1,2-cis-oriented glycosides is well known in the
literature.
AcO
AcO
O
SPh
O
AcHN
AcO
N3
13
AcO
14
OAc
F
AcO
O
O
AcHN
AcO
O
AcO
14a

5634
G. Mugunthan, K.P.R. Kartha / Tetrahedron Letters 53 (2012) 5631–5634
23. Palme, M.; Vasella, A. Helv. Chim. Acta 1995, 78, 959–969.
24. Helferich, B.; Gootz, R. Ber. 1929, 61, 2505–2507.
A comparison of the current method with those reported in the lit-
erature for the synthesis of 2,3,4,6-tera-O-acetyl- -glucopyranosyl
D
25. Micheel, F.; Klerner, A.; Flitsch, R. Chem. Ber. 1958, 91, 663–667.
26. Igarashi, K.; Honma, T.; Irisawa, J. Carbohydr. Res. 1970, 13, 49–55.
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106, 4189–4192.
fluoride as the example (where most comparison was possible) has
been provided in Table 3. It is evident from Table 3 that the current
method proves faster and high yielding among the methods com-
pared. The method using Py.(HF)n not only required longer reaction
time but also suffered from poorer yield.
30. Caddick, S.; Gazzard, L.; Motherwell, W. B.; Wilkinson, J. A. Tetrahedron 1996,
52, 149–156.
The ZnF2/2,20-bipyridine-method required heating while the 4-
methyl(difluoroiodo)benzene-method required cooling for the
methods to be effective. Besides, it was also found that the latter
reagent is not available commercially. The only limitation in the
case of the current method noted was that the required stirring
in the dark, which could be achieved easily by wrapping the flask
with aluminum foil.
In conclusion, a facile and effective method for the conversion of
various thioglycosides to their respective glycosyl fluorides using
in situ-generated IF as the fluorinating agent has been developed
with good functional group tolerance and isolated yields. The fluo-
31. Huang, K.-T.; Winssinger, N. Eur. J. Org. Chem. 2007, 1887–1890.
9
9
32. Lopez, J. L.; Bernal-Albert, P.; Uriel, C.; Valverde, S.; Gomez, A. M. J. Org. Chem.
2007, 72, 10268–10271.
33. Reviewed in K. P.
R Kartha and Robert Field, Best synthetic methods:
Carbohydrates 2003, 121–145.
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35. Kartha, K. P. R.; Field, R. A. Tetrahedron Lett. 1997, 47, 8233–8236.
36. Hall, L. D.; Manville, J. F. Can. J. Chem. 1969, 47, 361–377.
37. Typical experimental procedure: AgF (2 mmol) was added to a solution of the
thioglycoside (1 mmol) in anhydrous CH2Cl2 in the dark followed by addition
of iodine (2 mmol) at rt. After 60 min (when the reaction was found to be
complete), the mixture was diluted with excess CH2Cl2 and was quenched by
the addition of an ice-cold aqueous solution of Na2S2O7 and was filtered
through a Celite bed. The filtrate was washed with ice-cold aqueous solution of
NaHCO3 in a separating funnel and after drying over Na2SO4, was concentrated
under reduced pressure. The crude product was purified by column
chromatography (silica gel, 200–430 mesh; eluent, EtOAc-n-hexane, 2:3 to
1:1 depending upon the substrate) to afford the corresponding glycosyl
fluoride in pure form.
rides obtained were exclusively
a-anomers in all the cases when
the reaction was carried out in CH2Cl2. All thioglycosides except
those bearing a nitrogen substituent on the C-2 position of the
pyranosyl ring gave satisfactory yields.
38. Voznij, Y. V.; Koikov, L. N.; Galoyan, A. A. Carbohydr. Res. 1984, 132, 339–341.
39. Brauns, D. H. Bur. Standards J. Res. 1931, 7, 573–583.
40. Nishiyama, K.; Esaki, S.; Deguchi, I.; Sugiyama, N.; Kamiya, S. Biosci. Biotechnol.
Biochem. 1993, 57, 107–114.
Acknowledgment
M. G. sincerely acknowledges the financial support in the form
of a Research associate fellowship by CSIR-New Delhi.
41. Kent, P. W.; Dimitrijevich, S. D. J. Fluorine Chem. 1977, 10, 455–478.
42. Physical data and NMR characterisation:
(a) 2,6-Di-O-acetyl-3,4-O-isopropylidene-
a-D-galactopyranosyl fluoride (6a) Yield
78%; colorless solid; mp 82 °C; [
a
]
D
+108 (c 1, CHCl3); 1H NMR (400 MHz,
CDCl3) d 5.68 (dd, J1-F = 53 Hz, J1-2 = 2.8 Hz, 1H, H-1), 5.00 (ddd, J2-F = 23 Hz, J2-
1 = 2.8 Hz, J2-3 = 7.4 Hz, 1H, H-2), 4.44–4.29 (2 ꢂ m, 5H, H-3, H-4, H-5, H-6a and
Supplementary data
H-6b), 2.15, 2.11 (2 ꢂ s, 6H, 2 ꢂ ꢁCOCH3), 1.51, 1.35 (2 ꢂ s, 6H, ꢁC(CH3)2); 19
F
NMR (376 MHz, CDCl3) d ꢁ151.47 (dd, J1-F = 53 Hz, J2-F = 23 Hz, 1F); 13C NMR
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.08.
024.
(100 MHz, CDCl3) d 170.79, 170.18, 110.53, 105.35, 103.09, 72.65, 70.13, 69.90,
68.36, 68.32, 63.24, 27.56, 26.11, 20.87, 20.81; IR (Neat) mmax 2988, 1744, 1372,
1224, 1169, 1075, 1051, 926 cmꢁ1; HRMS: m/z calculated for C13H19FNaO7:
329.1013. Found: 329.1030.
References and notes
(b) 2,3-
O
-Isopropylidene-4-
O
-methanesulfonyl-
a
a
-
L
-rhamnopyranosyl fluoride (7a)
]
+46.2 (c 1, CHCl3); 1H NMR
D
Yield 83%; pale yellow solid; mp 111 °C; [
(400 MHz, CDCl3) d 5.76 (d, J1-F = 48 Hz, 1H, H-1), 4.42–4.32 (m, 3H, H-2, 3 and
4), 3.96–3.92 (m, 1H, H-5), 3.19 (s, 3H, ꢁSO2CH3), 1.58, 1.39 (2 ꢂ s, 6H,
ꢁC(CH3)2), 1.38 (d, J5-6 = 5.8 Hz, 3H, ꢁCH3); 19F NMR (376 MHz, CDCl3) d
ꢁ133.08 (dd, J1-F = 48 Hz, J2-F = 3.7 Hz, 1F); 13C NMR (100 MHz, CDCl3) d 110.90,
106.02, 103.90, 82.74, 75.30, 75.18, 74.71, 66.2, 39.14, 27.61, 26.24, 16.84; IR
1. Iodine and its interhalogen compounds: Versatile reagents in carbohydrate
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6. Araki, Y.; Watanabe, K.; Kuan, F.-H.; Itoh, K.; Kobayashi, N.; Ishido, Y. Carbohydr.
Res. 1984, 127, C-5–C-9.
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13. Miethchen, R.; Hager, C.; Hein, M. Synthesis 1997, 159–161.
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11743–11746.
15. L’Heureux, A.; Beaulieu, F.; Bennett, C.; Bill, D. R.; Clayton, S.; LaFlamme, F.;
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3401–3411.
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1287–1289.
(Neat) mmax 2990, 1357, 1224, 1177, 1085, 1001, 966 cmꢁ1
; HRMS: m/z
calculated for C10H17FNaO6S: 307.0628. Found: 307.0645.
(c) 2,3-Di-O-acetyl-4,6-
68%; colorless solid; mp 82 °C; [
CDCl3) 7.51–7.48 (m, 2H, Ar-H), 7.39–7.35 (m, 3H, Ar-H), 5.89 (dd, J1-
O
-benzylidene-
a
-
D
-galactopyranosyl fluoride (8a) Yield
]
+175.4 (c 1, CHCl3); 1H NMR (400 MHz,
D
a
d
F = 54 Hz, J1-2 = 2.56, 1H, H-1), 5.53 (s, 1H, ꢁCHPh), 5.43–5.31 (m, 2H, H-2 and
H-3), 4.53 (d, J4-3 = 2.9 Hz, 1H, H-4), 4.32 (dd, J6a–6b = 12.7 Hz, J5-6 = 1.64 Hz, 1H,
H-6a), 4.06 (dd, J6a-6b = 12.7 Hz, J5–6 = 1.64 Hz, 1H, H-6b), 4.00 (br s, 1H, H-5),
2.11, 2.10 (2 ꢂ s, 6H, 2 ꢂ ꢁCOCH3); 19F NMR (376 MHz, CDCl3) d ꢁ149.63 (dd,
J1,F = 54 Hz, J2,F = 22 Hz); 13C NMR (100 MHz, CDCl3) d 170.55, 170.07, 137.22,
129.18, 128.25, 126.16, 106.36, 104.11, 100.90, 73.24, 68.62, 67.96, 67.51,
67.27, 64.44, 64.42, 20.89, 20.66; R (Neat) mmax 2917, 2847, 1743, 1372, 1221,
1163, 1076, 1050, 1020, 991 cmꢁ1; HRMS: m/z calculated for C17H19FNaO7:
377.1013. Found: 377.1018.
(d) 4,6-Di-O-acetyl-2,3-di-O-benzyl-a-D-glucopyranosyl fluoride (9a).
43. Yield 59%; colorless syrup; [
a
]
D ꢁ10.5 (c 1, CHCl3); 1H NMR (400 MHz, CDCl3) d
7.36–7.26 (m, 10H, Ar-H), 5.51 (dd, J1,F = 52 Hz, J1,2 = 2.64 Hz, 1H, H-1), 5.06 (pt,
J = 9.8 Hz, 1H, H-4) 4.87, 4.66 (2 ꢂ d, J = 11.5 Hz, 2H, ꢁOCH2Ph), 4.80, 4.69
(2 = d, J = 11.8, 2H, ꢁOCH2Ph), 4.22 (dd, J5,6 = 3.8 Hz, J6a,6b = 12.6 Hz, 1H, H-6a);
4.06–4.01 (m, 2H, H-5 and 6b), 3.93 (pt, J = 9.48 Hz, 1H, H-3), 3.61 (ddd,
J2,F = 25 Hz, J1,2 = 2.64 Hz, J2,3 = 9.5 Hz, 1H, H-2), 2.06, 1.92 (2 ꢂ s, 6H, ꢁCOCH3);
19F NMR (376 MHz, CDCl3) d ꢁ150.07 (dd, J1,F = 52 Hz, J2,F = 25 Hz); 13C NMR
(100 MHz, CDCl3) d 170.61, 169.49, 138.09, 137.40, 128.61, 128.44, 128.19,
128.03, 128.85, 128.80, 127.85, 127.80, 106.34, 104.07, 79.21, 78.96, 78.45,
17. Yin, J.; Zarkowsky, D. S.; Thomas, D. W.; Zhao, M. M.; Huffman, M. A. Org. Lett.
2004, 6, 1465–1468.
18. Umemoto, T.; Singh, R. P.; Xu, Y.; Saito, N. J. Am. Chem. Soc. 2010, 132, 18199–
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9
9
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75.47, 73.71, 70.18, 68.60, 61.67; IR (Neat) mmax 2915, 1745, 1365, 1237, 1114,
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1043, 913 cmꢁ1; HRMS: m/z calculated for C24H27FNaO7: 469.1639. Found:
469.1642.