Ready transformation of partially unprotected thioglycosides into glycosyl fluorides mediated by NIS/HF-pyridine or ET3N·3HF

Article abstract of DOI:10.1002/ejoc.200800754

The transformation of partially unprotected phenyl 1-thioglycosides into glycosyl fluorides can be conveniently carried out by treatment with NIS in the presence of HF-pyridine. Other sources of halonium ions such as NBS, IDCP, have also been employed. Th

Full text of DOI:10.1002/ejoc.200800754

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200800754
Ready Transformation of Partially Unprotected Thioglycosides into Glycosyl
Fluorides Mediated by NIS/HF–Pyridine or Et3N·3HF
J. Cristóbal López,*[a] Paloma Bernal-Albert,[a] Clara Uriel,[a] and Ana M. Gómez*[a]
Dedicated to Prof. Benito Alcaide on the occasion of his 60th birthday
Keywords: Glycosyl fluorides / Thioglycosides / N-Iodosuccinimide / Carbohydrates / Glycosylation
The transformation of partially unprotected phenyl 1-thiogly-
cosides into glycosyl fluorides can be conveniently carried
out by treatment with NIS in the presence of HF–pyridine.
Other sources of halonium ions such as NBS, IDCP, have also
been employed. The combination NIS/Et3N·3HF, where tri-
ethylamine–tris(hydrogen fluoride) (Et3N·3HF) replaces HF–
pyridine, can also effect this transformation when acid-sensi-
tive substituents are present.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
lent of iodine monofluoride,[17] thus being a source of elec-
trophilic iodonium ion, which activates the anomeric leav-
ing group (e.g. 1 Ǟ 3, Scheme 1), and nucleophilic fluoride
that reacts with the anomeric oxocarbenium ion (e.g. 4 Ǟ
2, Scheme 1). However, our recently reported transforma-
tion of partially unprotected thioglycosides into glycosyl
fluorides[18] (e.g. 1 Ǟ 2, Scheme 1) had demanded the com-
bination of IPy2BF4 with HF–pyridine,[19,20] the latter func-
tioning as an additional source of nucleophilic fluoride
anion intended to avoid self-glycosyl coupling of the inter-
mediate oxocarbenium ion (e.g. 4 Ǟ 5, Scheme 1).
Introduction
Glycosyl coupling, the key event in saccharide synthesis,
requires the controlled assembly of a donor with an ac-
ceptor.[1] Traditionally, glycosylation strategies demand that
all hydroxy groups of the donor must be protected, while
the glycosyl acceptor normally has only one hydroxy group
free.[2] As a consequence, the synthesis of biologically
relevant oligosaccharides[3] incorporates a considerable
number of protection-deprotection steps, thence resulting in
lengthy processes. Minimization in the number of protec-
tion-deprotection steps in a glycosylation strategy has been
accomplished by the use, among others, of rationally de-
signed protecting groups[4] or by regioselective coupling of
polyol glycosyl acceptors.[5] In this context, we have evalu-
ated the influence of the O-2 substituent of the glycosyl
donor in regioselective glycosylations.[6] We and others,
have also explored the possibility of effecting glycosyl coup-
lings where both partners could be partially unprotected.[7,8]
Along this line, we believe that the development of methods
that allow the direct exchange of anomeric leaving groups
between partially unprotected glycosyl donors – or ac-
ceptors – would be useful, since it will avoid additional
steps.[9]
We have described the use of bis(pyridine)iodonium(I)
tetrafluoroborate[10] (IPy2BF4) for the conversion of thio-
glycosides[11] and n-pentenyl glycosides[12] into their semi-
orthogonal[13,14] glycosyl fluoride partners.[15,16] In this Scheme 1. Transformation of partially unprotected thioglycosides
into glycosyl fluorides.
transformation, IPy2BF4 functioned as a synthetic equiva-
Even though IPy2BF4 and HF–pyridine are commer-
cially available reagents, we have sought for more accessible
alternatives, and in this communication we disclose that N-
iodosuccinimide (NIS), N-bromosuccinimide (NBS), or bis-
[a] Instituto de Química Orgánica General, CSIC,
Juan de la Cierva 3, 28006 Madrid, Spain
Fax: +34-915-644853
E-mail: clopez@iqog.csic.es
anago@iqog.csic.es
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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J. C. López, P. Bernal-Albert, C. Uriel, A. M. Gómez
SHORT COMMUNICATION
(collidine)iodonium perchlorate (IDCP),[21] can also be em- Table 1. Preparation of glycosyl fluorides from thioglycosides, by
reaction with a halonium source (NIS and NBS: 1.2 equiv.; IDCP:
2.5 equiv.) in the presence of HF–pyridine in CH2Cl2 at –40 °C.
ployed as halonium sources, whereas triethylamine–tris-
(hydrogen fluoride) (Et3N·3HF)[22] can replace HF–pyr-
idine in the transformation of partially unprotected thiogly-
cosides 1 into glycosyl fluorides 2.[23]
Results and Discussion
For our study we selected phenyl 1-thioglycosides 6–13
as representative examples of polyol systems with different
protecting groups, including acid-sensitive ones, and n-pen-
tenyl glycoside 14. Initially the thioglycosides were treated
with the halonium source in the presence of HF–pyridine
(20 equiv., unless otherwise noted) in CH2Cl2 at –40 °C.
Compounds 6–9 (Table 1) embody examples of primary/
secondary diols, where Ley’s diacetal protecting group[24,25]
imposes rigidity that helps to prevent a competing intramo-
lecular glycosylation process with the 6-OH group, there-
fore minimizing the formation of undesired 1,5-anhydro de-
rivatives. Eventually, glycosyl fluorides 15–17 were obtained
in excellent yields upon treatment of compounds 6–8 with
NIS/HF–pyridine (Table 1, Entries 1, 2, and 4).
Compound 9 furnished only a moderate yield of glycosyl
fluoride 18 (Table 1, Entries 6–8), owing to the formation
of substantial amounts of the 1,5-anhydro derivative. In this
context, in the reaction of compound 10, devoid of Ley’s
acetal protecting group, fluoride 19 was obtained in 37%
yield (Entry 9), because of considerable intramolecular gly-
cosylation leading to the 1,5-anhydro derivative. An attempt
to thwart this undesired result by use of even 40 equiv. of
HF–pyridine was unsuccessful (Entry 10).
Besides Ley’s acetal groups, benzyl and acyl groups were
also compatible with the reaction conditions (Table 1, En-
tries 4, 5, 9–12). The reaction of benzylidene derivative 12 with
NIS/HF–pyridine gave variable amounts of glycosyl fluoride
21 (ca. 45%) (Table 1, Entry 14), the observed by-products re-
sulting from loss of the benzylidene protecting group.
Compound 13, with an acid-labile trityl protecting group,
did not yield the desired glycosyl fluoride 22 but a complex
mixture, which was not analyzed (Table 1, Entry 15).
The use of NBS as the halonium source was also exam-
ined, and normally resulted in reduced yields of glycosyl fluo-
rides (compare Entries 2 with 3, and 4 with 5, and 11 with
12). Bis(collidine)iodonium perchlorate (IDCP) in the pres-
ence of HF–pyridine was also successful in promoting the
fluorination of thioglycoside 9 (Entry 8). The use of iodine in
the presence of HF–pyridine did not cause any transforma-
tion in compound 11, even at room temperature.[26]
[a] Unless otherwise noted, 20 equiv. were used throughout. [b] The
The observed anomeric α/β ratios varied with the halon-
anomeric (α/β) ratio was determined from the integration of the
ium source employed.[27,28] On the other hand, the reaction
anomeric signals in the 1H NMR spectra of the crude reaction
of n-pentenyl glycoside 14 with IDCP in the presence of
HF–pyridine furnished only limited amounts of glycosyl
fluoride 23, a major side reaction being the IF addition
across the terminal double bond.[12b]
In order to extend the applicability of this procedure to
thioglycosides with acid-sensitive functionalities, we turned
our attention to Et3N·3HF. This reagent, non-corrosive in
mixture. [c] 1,5-Anhydro derivative (58% yield) was also isolated.
[d] 1,5-Anhydro derivative (52% yield) was also isolated. [e] No
reaction was observed, even at room temp. [f] The yield for this
transformation was erratic, and compounds resulting from the de-
protection of the benzylidene ring were normally observed. [g] A
complex reaction mixture was obtained under different reaction
conditions. [h] Compounds resulting from addition of IF across the
double bond were also observed (14%).
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Eur. J. Org. Chem. 2008, 5037–5041

Transformation of Thioglycosides into Glycosyl Fluorides
borosilicate glassware, has been used as a source of fluoride chemical shifts within the range δ = 5.63–5.51 ppm, whereas
ion and was selected because of its almost neutral pH.[22] those of the corresponding β-anomers appeared between δ =
Accordingly, our results for the transformation of thiogly- 5.37 and 5.12 ppm.[30] These values were in agreement with
cosides into glycosyl fluorides by means of NIS/Et3N·3HF those found for the 1JC,H coupling constants of the anomeric
are outlined in Table 2. The thioglycosides were treated with carbon atoms of α-25 (δ = 5.51 ppm) and β-25 (δ =
NIS (2.0 equiv.) in the presence of Et3N·3HF in CH2Cl2 at 5.32 ppm), 181.8 and 168.0 Hz, respectively.[31]
–20 °C. These transformations required, in general, higher
temperatures and longer reaction times to proceed, proba-
bly due to the limited activation of the NIS by Et3N·3HF
when compared with the more acidic HF–pyridine. Thiog-
Conclusions
Partially unprotected phenyl 1-thioglycosides can be
transformed into glycosyl fluorides. Primary hydroxy
groups can be present if 3,4-O Ley’s-type diacetal groups
are in the molecule, whereas the method is completely toler-
ant with secondary OH groups. The method involves treat-
ment of the thioglycoside with a halonium ion source in the
presence of either HF–pyridine or Et3N·3HF. Within the
sources for halonium ion examined, IDCP and NIS gave
better results than NBS, whereas iodine did not promote
any transformation. The combination NIS/HF–pyridine is
more potent in provoking the reaction of thioglycosides,
lycosides 6, 7, and 11, which had given excellent yields of
glycosyl fluorides with NIS/HF–pyridine (Table 1, En-
tries 1, 2, 11) also furnished excellent yields with NIS/
Et3N·3HF (Table 2, Entries 1, 2, 4). Benzylidene derivative
12 did not give any noticeable amount of fluoride 21
(Table 2, Entry 5). On the contrary, trityl derivative 13,
which had not endured the treatment with HF–pyridine,
gave acceptable yields of glycosyl fluoride 22 (Table 2, En-
tries 6, 7).
Table 2. Preparation of glycosyl fluorides from thioglycosides by
reaction with NIS (2 equiv.) in the presence of Et3N·3HF in while NIS/Et3N·3HF is more tolerant of acid-sensitive
CH2Cl2.
functionalities. This method represents a more economic al-
Entry[a] Starting
material
Temp.
[°C]
Reaction time [min]/ Glycosyl Yield
ternative to IPy2BF4/HF–pyridine, recently described to ef-
fect this transformation.[18]
Et3N·3HF [equiv.]
fluoride[c]
[%]
1
2
3
4
5
6
7
6
7
9
11
12
13
13
–20
–20
–20 to r.t.
–20
–10
0 to r.t.
r.t.
60/40
40/40
480/120
5/40
120/40
180/40
180/120
15
16
18
20
21
22
22
97
95
37[a]
93
Experimental Section
[b]
–
General: Starting n-pentenyl glycoside and thioglycosides were pre-
pared according to described procedures. The glycosyl fluorides re-
ported in this paper have been described previously[18] with the ex-
57[c]
67[d]
ception of compounds 17 and 25. H and 13C NMR spectra were
1
[a] Unreacted thioglycoside 9 (58%) was also isolated. [b] No ap-
preciable amount of glycosyl fluoride 21 was observed after 2 h. [c]
Unreacted thioglycoside 13 (32%) was also observed (1H NMR).
[d] Unreacted thioglycoside 13 (22%) was also observed (1H
NMR).
recorded with either Varian Inova-300 or Inova-400 spectrometers.
NMR spectra were recorded in CDCl3 solutions with the residual
solvent signal as internal reference. Mass spectra were recorded
with an HP 1100 spectrometer by using the electrospray (ES) chem-
ical ionization method in its positive mode. Optical rotations were
measured with a Perkin–Elmer 241 polarimeter. Elemental analyses
were carried out with a Carlo Erba EA1108 apparatus. Flash col-
Conformationally disarmed[24b,29] glycosyl fluorides 9
and 12 did not react readily with NIS/Et3N·3HF (Table 2,
Entries 3, 5), thus illustrating that this reagent system is a umn chromatography was performed on 230–400 mesh silica gel.
Thin-layer chromatography was conducted on Kieselgel 60 F254
(Merck). Spots were first observed under UV irradiation (254 nm)
and then by charring with a solution of aqueous H2SO4 (20%,
200 mL) in AcOH (800 mL).
less potent source of iodonium ion than NIS/HF–pyridine.
Finally, trityl-protected triol 24 was easily transformed
into glycosyl fluoride 25, as an α/β (6:1) mixture, in 75%
yield (Scheme 2).
General Procedure for the NIS/HF–Pyridine-Mediated Transforma-
tion of Thioglycosides into Glycosyl Fluorides: To a solution of N-
iodosuccinimide (1.2 equiv.) in dry CH2Cl2 under argon cooled to
–40 °C was added HF–pyridine (20 equiv.) followed by a solution
of the thioglycoside in dry CH2Cl2. After all the starting material
had disappeared (TLC), the reaction mixture was quenched with
pyridine and washed with 10% aqueous sodium thiosulfate con-
taining sodium hydrogen carbonate, and water. The organic layer
was dried, concentrated, and the residue purified by flash
chromatography.
Scheme 2. Transformation of triol 24 into glycosyl fluorides α/β-25.
The anomeric (α/β) ratio of the glycosyl fluorides was de-
termined, throughout this work, from the integration of its
1
anomeric signals in the H NMR spectra of the crude reac-
General Procedure for the NIS/Et3N·3HF-Mediated Transformation
of Thioglycosides into Glycosyl Fluorides: To a solution of the thio-
glycoside in dry CH2Cl2 under argon cooled to –20 °C was added
N-iodosuccinimide (2.0 equiv.) followed by Et3N·3HF. After the
tion mixture. The assignment of the α- or β-orientation of
the glycosyl fluorides was based on NMR spectroscopic
data. Thus, α-anomers, in the -gluco (15, 18) and -manno
1
(16, 17, 25) series, displayed H NMR (300 MHz, CDCl3) starting material had disappeared (TLC), the reaction mixture was
Eur. J. Org. Chem. 2008, 5037–5041
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5039

J. C. López, P. Bernal-Albert, C. Uriel, A. M. Gómez
SHORT COMMUNICATION
Uriel, A. Agocs, A. M. Gómez, J. C. López, B. Fraser-Reid,
Org. Lett. 2005, 7, 4899–4902.
quenched with Et3N and washed with 10% aqueous sodium thio-
sulfate containing sodium hydrogen carbonate, and water. The or-
ganic layer was dried, concentrated, and the residue purified by
flash chromatography.
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Selected Data for 6-O-Pivaloyl-3-O,4-O-[(2ЈS,3ЈS)-2Ј,3Ј-dimeth-
oxybutan-2Ј,3Ј-diyl]-α-D
-glucopyranosyl Fluoride (α-17): 1H NMR
(300 MHz, CDCl3): δ = 5.52 (dd, J = 49.0, 1.3 Hz, 1 H), 4.55 (dd,
J = 12.1, 1.9 Hz, 1 H), 4.28–3.96 (m, 5 H), 3.26 (s, 3 H), 3.23 (s, 3
H), 1.31 (s, 3 H), 1.27 (s, 3 H), 1.19 (s, 9 H) ppm. API-ES (positive)
MS: m/z = 398.5 [M + NH4]+, 403.3 [M + Na]+. C17H29FO8
(380.18): calcd. C 53.67, H 7.68; found C 53.43, H 7.56.
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Reaction of Phenyl 6-O-Trityl-1-thio-͗-D-mannopyranose (24) with
NIS/Et3N·3HF: To a solution of the thioglycoside 24 (52 mg,
0.1 mmol) in dry CH2Cl2 (5 mL) under argon cooled to –10 °C was
added N-iodosuccinimide (44.8 mg, 0.2 mmol, 2.0 equiv.) followed
by Et3N·3HF (0.326 mL, 2 mmol). After the starting material had
disappeared (20 min), the reaction mixture was quenched with
Et3N and partially concentrated in the cold to remove most of the
CH2Cl2. The resulting solution was then submitted to filtration
through a short pad of silica gel (CH2Cl2/MeOH, 95:5), and the
purified fractions were resubmitted to flash chromatography
(EtOAc). Pure α-25 (24 mg, 57%) was first eluted, followed by a
mixture of α/β-25 (8 mg, 18%). α-25: [α]D = +9.9 (c = 0.7, CHCl3).
1H NMR (300 MHz, CDCl3): δ = 7.38–7.14 (m, 15 H), 5.51 (dd, J
= 49.4, 1.3 Hz, 1 H), 3.96 (m, 1 H), 3.85–3.63 (m, 3 H), 3.38 (dd,
J = 9.9, 3.8 Hz, 1 H), 3.33 (dd, J = 9.9, 4.6 Hz, 1 H) ppm. 13C
NMR (75 MHz, CDCl3): δ = 143.3 (ϫ3), 128.5 (ϫ6), 128.0 (ϫ6),
127.3 (ϫ3), 107.3 (d, J = 219.4 Hz), 87.4, 72.1 (d, J = 2.5 Hz), 70.6,
69.1, 68.7 (d, J = 38.4 Hz), 63.9 ppm. API-ES (positive) MS: m/z
= 425.5 [M + H]+, 447.7 [M + Na]+. C25H25FO5 (424.17): calcd. C
70.74, H 5.94; found C 70.65, H 5.68.
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Transformation of Thioglycosides into Glycosyl Fluorides
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Received: July 30, 2008
Published Online: September 16, 2008
Eur. J. Org. Chem. 2008, 5037–5041
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