Synthesis of chiral carbohydrate ionic liquids

Article abstract of DOI:10.1055/s-0028-1087343

Chiral room temperature ionic liquids, containing a carbohydrate moiety linked at the anomeric centre to an N-methylimidazolium group have been synthesised. The ionic liquids were prepared in a concise manner and provided ready access to both the D- and L

Full text of DOI:10.1055/s-0028-1087343

LETTER
2973
Synthesis of Chiral Carbohydrate Ionic Liquids
Synthesis of
C
a
hiral
C
arbo
t
hydra
t
r
Ionic
L
i
iquidsce G. J. Plaza, Bhoomendra A. Bhongade, Gurdial Singh*
Department of Chemistry, University of the West Indies, St. Augustine, Trinidad and Tobago, West Indies
Fax +1(868)6453771; E-mail: gurdial.singh@sta.uwi.edu
Received 2 May 2008
novel chiral solvents. A number of chiral RTIL have been
designed and prepared with the specific goal of influenc-
ing the outcomes of asymmetric reactions.7 However, this
has been realized in a small number of cases and there are
Abstract: Chiral room temperature ionic liquids, containing a car-
bohydrate moiety linked at the anomeric centre to an N-methylimi-
dazolium group have been synthesised. The ionic liquids were
prepared in a concise manner and provided ready access to both the
D- and L-arabino enantiomers. The same strategy enabled the prep- only a few chiral ionic liquids that have found use in syn-
thesis.8 The majority of the existing chiral RTIL are based
aration of D-ribofuranose and D-xylofuranose analogues, in excel-
lent yields.
on the modification of the various cations, namely, pyri-
dinium,9 oxazolium,10 thiazolium,11 and ammonium12 and
Keywords: synthesis, chiral ionic liquid, carbohydrate, N-methyl-
imidazolium
imidazolium species. More recently, a number of groups
have reported RTIL where the imidazolium cation has
been modified to incorporate the chirality.13 The chiral
There has been a major research effort in the area of ionic RTIL based on the imidazole ring system can be regarded
liquids during the last twenty years; with the first example as derivatives of imidazole 1–6 (Figure 1) having counte-
of a Friedel–Crafts acylation being reported in 1986.1 rions commonly found in achiral ionic liquids.
Since then the development and use of room temperature
ionic liquids (RTIL) has emerged as one of the fascinating
–
H
PF6
technologies in science because of their physiochemical
properties and specific applications in bioorganic re-
search. The last decade has seen a substantial growth in
the use of RTIL as environmentally benign2 and useful
solvents in diverse applications such as organic synthesis,
chemical catalysis, biocatalysis, separation technology,
analytical chemistry.3 A feature that adds to the attractive-
ness of the use of RTIL in organic synthesis is the ease of
their separation and recyclability in addition to their im-
measurably low vapor pressure, high stability, range of
viscosities, moderate solvation of organic compounds, as
well as adjustable miscibility and polarity.4
N+
O
N
N+
N
C5H11
Ph
N
(CF3SO2)2N–
Et
1
2
CF3SO3
N+
Br–
CO2Et
N
N
N+
H
Et
OH
H
3
4
–
CO2Me
H
NHBoc
BF4
N+
H
N
O
N
N+
3
Bu
HN
CH2Ph
CO2Me
–
5
Typical RTIL are composed of a large organic cation and
a weakly coordinating inorganic anion.5 The cations being
typically imidazolium, pyridinium, pyrrolidinium, quater-
nary ammonium, or phosphonium ions while the anions
BF4
6
Figure 1
–
–
–
–
–
include Cl–, Br–, ClO4 , AlX4 , BF4 , PF6 , CF3SO3 ,
(CF3SO3)2N– or (CN)2N–. Changing the counterions often Nucleotides having a C-1 imidazolium ring occur widely
results in significant modifications in the properties of and they tend to be highly crystalline structures. For ex-
RTIL such as miscibility with organic solvents and/or wa- ample, a number of syntheses of these compounds pro-
ter, polarity, viscosity, and density. For example, anions ceed via the intermediacy of crystalline cationic
such as Cl– or Br– impart hydrophilic characteristics while structures14 (Scheme 1).
fluorinated anions such as [PF6]– or [BF4]– impart hydro-
phobic properties depending on the nature of the associat-
+
CO2Et
NH2
N
ed cation.3a Employing the appropriate cation and anion,
RTIL with desired physicochemical properties or specific
functions can be achieved.6
N
N
I–
O
O
N
CO2Et
HO
HO
MeI
NH2
The development of chiral RTIL has received increasing
attention over the last few years with the aim of providing
O
O
O
O
7
8
Scheme 1
SYNLETT 2008, No. 19, pp 2973–2976
x
x
.x
x
.2
0
0
8
Advanced online publication: 12.11.2008
DOI: 10.1055/s-0028-1087343; Art ID: S04408ST
© Georg Thieme Verlag Stuttgart · New York

2974
P. G. J. Plaza et al.
LETTER
With these thoughts, we considered the preparation of
chiral ionic liquids based on carbohydrates as these are
rich sources of stereochemistry and chirality and as a con-
sequence have been utilised widely as chiral building
blocks. We reasoned that the incorporation of the imida-
zole nucleus at the anomeric center of monosaccharides
along with the judicious choice of protecting groups
would provide access to a new generation of imidazolium-
based carbohydrate chiral ionic liquids.
O
O
OH
OH
OMe
HO
i
ii
HO
HO
OH
9
10
OH
O
O
OMe
OH
BnO
BnO
iii
iv
BnO
OBn
BnO
OBn
11
12
For our initial approach towards the synthesis of these car-
bohydrate ionic liquids we chose to study the chemistry of
arabinofuranosides based on the fact that both the D- and
L-enantiomers of arabinose are commercially available
and would thus provide an entry to both enantiomeric se-
ries.
N+
O
P
v
O
O
O
N
BnO
14 X = Cl–
15 X = PF6
16 X = BF4
vi
O
X–
BnO
–
–
O
BnO
OBn
13
BnO
OBn
Scheme 2 Reagents and conditions: (i) MeOH, PTSA·H2O, 40 °C,
20 h (98%); (ii), NaH, DMF, 0 °C, 30 min, Bu4NI, BnBr, r.t., 4 h
(95%); (iii) MeCN–H2O–TFA (4:3:1), reflux, 12 h (81%); (iv) pro-
pane-1,3-diyldioxyphosphoryl chloride, CH2Cl2, MeNIm, 0 °C,
(72%); (v) MeNIm·HCl, TMSOTf (cat.), CH2Cl2, –78 °C, (58%); (vi)
NH4PF6 (98%) or NH4BF4 (98%).
2,3,5-Tri-O-benzyl-a,b-D-arabinofuranoside 12 was pre-
pared in three steps from D-arabinose15 (Scheme 2). Sub-
sequent treatment with propane-1,3-diyldioxyphosphoryl
chloride gave the arabinoside 13 consisting predominate-
ly of the a-anomer.16
We next investigated the displacement reaction of phos-
phate 13 with N-methylimidazole as the nucleophile. In
the first instance we were able to obtain the desired ionic
liquid 14 in 15–20% yields using a catalytic amount of tri-
methylsilyl triflate (TMSOTf), an excess of N-methylim-
idazole and potassium chloride. However, reaction of the
crude phosphate 13 with 1.5 equivalents of N-methylimi-
dazole hydrochloride and catalytic TMSOTf gratifyingly
led to ionic liquid 14 in 58% yield after chromatogra-
phy,17,18 furthermore, we were able to conduct this reac-
tion on a 5 g scale with a similar result. The outcome of
this reaction was somewhat unexpected in that we only
obtained the b-anomer, on the basis of its 13C NMR spec-
trum in which the anomeric carbon was found to resonate
at d = 87.3 ppm. In the 1H NMR the anomeric proton ap-
peared at d = 6.46 ppm with a coupling constant of 5.8 Hz
suggestive of a cis relationship to the H-2 proton of the ar-
abinofuranoside. Further support for the assignment as be-
ing b was obtained from COSY, HMQC, and HMBC
experiments. The TGA analysis showed that the ionic liq-
uid was stable up to 180 °C, demonstrating its robustness.
Following this we proceeded to exchange the counterion
to PF6– and BF4– by treatment with ammonium hexafluo-
rophosphate and ammonium tetrafluoroborate, respec-
tively. These two counterions afforded ionic liquids 15
and 16 at room temperature, which were immiscible with
water, soluble in chloroform, dichloromethane, methanol,
and acetonitrile. In the 1H NMR spectrum of ionic liquid
15 the anomeric proton experienced an upfield shift to d =
6.05 ppm while the 31P NMR spectrum showed a septet at
d = –142.9 with a coupling constant of 712.7 Hz.
ponents. Similar chemistry using the L-enantiomer of
arabinose gave the corresponding ionic liquid.
We subsequently turned our attention to the synthesis of
RTIL based on D-ribofuranose (Scheme 3). This proceed-
ed uneventfully and gave the corresponding ionic liquid
19 in good yield. The anomeric proton was found to reso-
nate at d = 6.25 ppm with a coupling constant of 5.8 Hz
suggestive of a cis relationship to the H-2 proton of the ri-
bofuranose ring while the C-2 imidazole proton was ob-
served at d = 9.28 ppm. The inversion of stereochemistry
at the anomeric centre resulting in formation of the cis-
isomer may occur as a result of an SN2 displacement of the
propane-1,3-diyl phosphate group. The chloride counter-
–
ion was exchanged to PF6 using the same protocols that
had been applied to the arabinosides.
O
O
O
O
O
OH
BnO
P
BnO
i
O
BnO
OBn
BnO
OBn
17
18
major anomer (17:1)
N+
ii
N
O
Cl
BnO
BnO
OBn
19
Scheme 3 Reagents and conditions: (i) propane-1,3-diylphos-
phorylchloride, CH2Cl2, MeNIm, 0 °C, (68%); (ii) MeNIm·HCl,
TMSOTf (cat.), CH2Cl2, –78 °C, ( 46%).
In order to confirm that the chloride ion had been ex-
–
changed successfully with PF6 we conducted a 31P NMR
Similarly, synthesis of RTIL based on D-xylofuranose
gave the corresponding ionic liquid 22 in good yield
(Scheme 4). The anomeric proton was found to resonate at
d = 6.30 ppm with a coupling constant of 3.7 Hz while the
C-2 imidazole proton was observed at d = 9.26 ppm.
experiment where we had introduced an equimolar
amount of triphenyl phosphite as an internal standard and
integrated the observed phosphorus resonances. This re-
sulted in a 1:1 ratio for observed signals for the two com-
Synlett 2008, No. 19, 2973–2976 © Thieme Stuttgart · New York

LETTER
Synthesis of Chiral Carbohydrate Ionic Liquids
2975
In summary, we have prepared stable chiral ionic liquids
based on sugar furanose frameworks in an efficient pro-
cess. We envisage that these new chiral RTIL will be use-
ful as new chiral solvents in addition to serve as catalysts
for a range of asymmetric reactions that are being investi-
gated in our laboratories.
O
P
O
O
O
O
OH
BnO
BnO
i
O
BnO
OBn
BnO
OBn
20
21
major anomer
+
N
ii
N
O
Cl
Acknowledgment
BnO
We thank the government of Trinidad and Tobago for financial sup-
port (BAB) and the University of the West Indies for a research
scholarship (PGJP).
BnO
OBn
22
Scheme 4 Reagents and conditions: (i) propane-1,3-diylphos-
phorylchloride, CH2Cl2, MeNIm, 0 °C, (70%); (ii) MeNIm·HCl,
TMSOTf (cat.), CH2Cl2, –78 °C, ( 52%).
References and Notes
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J. Org. Chem. 1986, 51, 480. (b) Welton, T. Chem. Rev.
1999, 99, 2071.
In order to ascertain if any hydrolysis of the D-arabino se-
ries of RTIL 14 had occurred, we rerecorded its 1H NMR
spectrum after six months storage at room temperature.
The NMR data for the synthesised RTIL after six months
of storage at room temperature was exactly comparable to
the data obtained for the newly synthesised ionic liquids.
This experiment established that there were no resonances
due to the formation of the of the hydrolysis product 12.
The possible formation of a-,b-2,3,5-tri-O-benzyl-arabino-
furanosyl chlorides was excluded on the basis that the res-
onances for the anomeric protons for these compounds
occur d = 6.17 and 6.21 ppm,19 and these signals were not
observed in our experiments. Furthermore, the stability of
these ionic liquids was supported by their recovery after
reactions conducted with organozinc reagents, catalytic
hydrogenation, and Grignard reactions. To investigate the
chemical stability of the ionic liquid we undertook a series
of reaction employing methylmagnesium chloride with
benzaldehyde and aryl-substituted benzaldehydes using
THF as the solvent along with one equivalent of the ionic
liquid 15 as co-solvent. These reactions afforded the cor-
responding alcohol in chemical yields >95% and were
complete in 5–10 minutes at –78 °C. Disappointingly no
enantiomeric excess for the alcohols was observed. In or-
der to establish the stability of the heterocyclic ring to-
wards hydrogenolysis, we examined the transfer
hydrogenation of 15 using palladium hydroxide as the cat-
alyst and this resulted in the formation of the primary al-
cohol 23 in 85% yield, where the primary benzyl
protecting group had been removed selectively
(Scheme 5).
(2) (a) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis;
Wiley-VCH: Weinheim, 2003. (b) Wasserscheid, P.; Keim,
W. Angew. Chem. Int. Ed. 2000, 39, 3772. (c) Sheldon,
R. A. Chem. Commun. 2001, 2399. (d) Handy, S. T.;
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Levillain, J.; Cahard, D.; Gaumont, A. C.; Plaqueventa, J.-C.
Tetrahedron: Asymmetry 2003, 14, 3081. (b) Xue, H.;
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Rantwijk, F.; Sheldon, R. A. Chem. Rev. 2007, 107, 2757.
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2007, 40, 107.
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Visser, A. E.; Rogers, R. D. Chem. Commun. 1998, 16,
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1999, 71, 3873. (c) Huddleston, J. G.; Visser, A. E.;
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Chi, Y.-G.; Cai, Y.-Q.; Zhou, Q.-X.; Hu, J.-T. Anal. Chem.
2003, 75, 5870.
(5) Handy, S. T. Curr. Org. Chem. 2005, 9, 959.
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S. Chem. Commun. 2006, 1049.
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Plaquevent, J.-C.; Gaumont, A.-C. Tetrahedron: Asymmetry
2005, 16, 3921.
(8) (a) Earle, M. J.; McCormac, P. B.; Seddon, K. R. Green
Chem. 1999, 1, 23. (b) Wang, Z.; Wang, Q.; Zhang, Y.; Bao,
W. Tetrahedron 2005, 46, 4657. (c) Carda-Broch, S.;
Berthod, A.; Armstrong, D. W. Anal. Bioanal. Chem. 2003,
375, 191.
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J.-C. Tetrahedron Lett. 2005, 46, 1137. (b) Fringuelli, F.;
Pizzo, F.; Tortoioli, S.; Vaccaro, L. J. Org. Chem. 2004, 69,
7745. (c) Tosoni, M.; Laschat, S.; Bato, A. Helv. Chim. Acta
2004, 87, 2742.
+
N
N+
N
N
O
O
i
BnO
HO
–
–
PF6
OBn
PF6
OBn
BnO
BnO
15
23
Scheme 5 Reagents and conditions: (i) Pd(OH)2, cyclohexene,
EtOH, reflux, 7d.
(10) Baudequin, C.; Baudoux, J.; Levillain, J.; Cahard, D.;
Gaumont, A. C.; Plaqueventa, J.-C. Tetrahedron:
Asymmetry 2003, 14, 3081.
Synlett 2008, No. 19, 2973–2976 © Thieme Stuttgart · New York

2976
P. G. J. Plaza et al.
LETTER
(11) Levillian, J.; Dubant, G.; Abrunhosa, I.; Gulea, M.;
Gaumont, A. C. Chem. Commun. 2003, 2914.
78.3, 80.9, 82.1, 87.1, 121.1, 122.1, 127.4–128.4, 134.9,
136.3, 137.0, 137.1 ppm. 19F (376.5 MHz, CDCl3): –151.0
ppm. Glass-transition temperature –36 °C.
(12) Pernak, J.; Feder-Kubis, J. Chem. Eur. J. 2005, 11, 4441.
(13) (a) Génisson, Y.; Lauthde Viguerie, N.; André, C.; Baltas,
M.; Gorrichon, L. Tetrahedron: Asymmetry 2005, 16, 1017.
(b) Ding, J.; Desikan, V.; Han, X.; Xiao, T. L.; Ding, R.;
Jenks, W. S.; Armstrong, D. W. Org. Lett. 2005, 7, 335.
(c) Jodry, J. J.; Mikami, K. Tetrahedron Lett. 2004, 45,
4429. (d) Bao, W.; Wang, Z.; Li, Y. J. Org. Chem. 2003, 68,
591. (e) Guillen, F.; Brégeon, D.; Plaquevent, J.-C.
Tetrahedron Lett. 2006, 47, 1245. (f) Ni, B.; Headley, A. D.
Tetrahedron Lett. 2006, 47, 7331.
Compound 19: [a]D28 +22.3 (c 1.3, CHCl3). 1H NMR (400
MHz, CDCl3): d = 3.46 (1 H, dd, J = 3.2, 10.6 Hz), 3.52
(1 H, dd, J = 3.5, 10.6 Hz), 3.77 (3 H, s), 4.01 (1 H, dd,
J = 1.8, 5.1 Hz), 4.41–4.62 (8 H, m), 6.25 (1 H, d, J = 5.8
Hz), 7.19–7.36 (16 H, m), 7.54 (1 H, s), 9.28 (1 H, s) ppm.
13C NMR (100 MHz, CDCl3): d = 36.0, 69.7, 72.5, 73.3,
73.4, 76.4, 76.9, 77.3, 78.1, 84.7, 87.9, 121.6, 122.1, 127.6,
127.8, 127.9, 128.0, 128.1, 128.3, 128.4, 136.5, 136.8, 137.3
ppm. Glass-transition temperature 16 °C.
(14) Brown, T.; Kadir, K.; Mackenzie, G.; Shaw, G. J. Chem.
Compound 22: 1H NMR (400 MHz, CDCl3): d = 3.67–3.76
(2 H, m), 3.93 (3 H, s), 4.05 (1 H, m), 4.22 (1 H, t, J = 5.3
Hz), 4.35–4.46 (3 H, m), 4.51 (1 H, d, J = 11.9 Hz), 4.59
(1 H, d, J = 11.9 Hz), 4.62–4.69 (2 H, m), 6.30 (1 H, d,
J = 3.7 Hz), 7.08 (1 H, br s), 7.11–7.36 (15 H, m), 7.39 (1 H,
br s), 9.26 (1 H, br s) ppm. 13C (100 MHz, CDCl3): d = 36.3,
68.7, 72.5, 73.3, 75.0, 75.3, 78.0, 82.1, 85.0, 121.1, 122.1,
127.7, 128.5, 135.1, 137.3, 137.7, 137.8, 137.9 ppm.
Compound 23: 1H NMR (400 MHz, CDCl3): d = 3.42 (3 H,
s), 3.63 (1 H, dd, J = 2.1, 11.0 Hz), 3.89 (1 H, dd, J = 2.5,
11.0 Hz), 4.12 (1 H, m), 4.20 (1 H, t, J = 7.7 Hz), 4.38 (1 H,
d, J = 10.5 Hz), 4.47 (1 H, d, J = 10.5 Hz), 4.60 (1 H, d,
J = 11.9 Hz), 4.75 (1 H, dd, J = 5.8, 7.2 Hz), 4.85 (1 H, d,
J = 11.9 Hz), 6.02 (1 H, d, J = 5.7 Hz), 7.06 (1 H, br s), 7.23–
Soc., Perkin Trans. 1 1979, 3107.
(15) (a) Barker, R.; Fletcher, H. G. Jr. J. Org. Chem. 1961, 26,
4605. (b) Austin, P. W.; Hardy, F. E.; Buchanan, J. G.;
Baddiley, J. J. Chem. Soc. 1964, 2128. (c) Finch, P.;
Iskander, G. M.; Siriwardena, A. H. Carbohydr. Res. 1991,
210, 319. (d) Tejima, S.; Fletcher, H. G. Jr. J. Org. Chem.
1963, 28, 2999. (e) Kawana, M.; Kuzuhara, H.; Emoto, S.
Bull. Chem. Soc. Jpn. 1981, 54, 1492.
(16) Yuan, L.; Singh, G. Tetrahedron Lett. 2001, 42, 6615.
(17) Selected Data
Compound 14 (X = Cl): [a]D28 +28 (c 1.1, CHCl3). IR (film):
nmax = 3429, 3143, 3064, 3032, 2923, 2870, 1634, 1578,
1556, 1454, 1364, 1264, 1157, 1090, 1030, 748, 701, 638
cm–1. 1H NMR (400 MHz, CDCl3): d = 3.60 (3 H, s), 3.62
(1 H, dd, J = 2.8, 10.9 Hz), 3.82 (1 H, dd, J = 3.0, 10.9 Hz),
4.13 (1 H, m), 4.23 (1 H, t, J = 6.8 Hz), 4.41 (1 H, d, J = 11.1
Hz), 4.48 (1 H, d, J = 11.1 Hz), 4.53 (1 H, d, J = 11.9 Hz),
4.64 (2 H, d, J = 6.3Hz), 4.57 (1 H, m), 4.69 (1 H, d, J = 11.9
Hz), 6.46 (1 H, d, J = 5.8 Hz), 7.22–7.40 (16 H, m), 7.73
(1 H, m), 9.40 (1 H, s) ppm. 13C NMR (100 MHz, CDCl3):
d = 36.0, 68.0, 72.5, 73.5, 78.2, 81.0, 82.6, 87.3, 121.5,
122.5, 128.1, 127.9, 128.1, 128.2, 128.3, 128.5, 128.6,
135.5, 136.4, 137.1, 137.2 ppm. ESI-MS: m/z calcd
C30H33N2O4: 485.2435; found: 485.2423. Glass-transition
temperature: 18 °C.
7.39 (10 H, m), 7.69 (1 H, br s), 8.96 (1 H, br s) ppm. 13
C
NMR (100 MHz, CDCl3): d = 36.1, 68.0, 72.6, 73.5, 78.1,
80.9, 82.6, 87.3, 121.6, 122.4, 127.7, 128.5, 135.8, 136.5,
137.2, 137.3.
(18) General Procedure
The 2,3,5-tri-O-benzylsugar (1 mmol) was dissolved in dry
CH2Cl2 (10 mL) and cooled to 0 °C under Ar atmosphere.
Propane-1,3-diyldioxyphosphoryl chloride (2 mmol) was
added, followed by 1-methylimidazole (2.5 mmol). The
mixture was allowed to warm up to r.t. and stirred overnight
(16 h). The reaction was then quenched with sat. NaHCO3
(10 mL) and the organic layer washed with H2O (2 × 10 mL)
and dried (Na2SO4). The solvent was then removed in vacuo
to give the crude sugar phosphate, which was re-dissolved in
dry CH2Cl2 (10 mL) under an Ar atmosphere and cooled to
–78 °C. trimethylsilyl triflate (cat.) was added and the
mixture stirred for 2 min. 1-Methylimidazole hydrochloride
(2 mmol) was then added. The reaction mixture was allowed
to warm up to r.t. and stirred until TLC (CHCl3–MeOH,
80:20) showed the reaction had gone to completion (4 h).
The mixture was then diluted with CH2Cl2 (10 mL) and
washed with sat. aq NaHCO3 (2 × 20 mL) and H2O (2 × 20
mL). The organic layer was dried (Na2SO4) and concentrated
in vacuo to give a crude product, that was further purified by
column chromatography (CHCl3–MeOH, 80:20).
–
Compound 15 (X = PF6 ): [a]D28 +14 (c 1.1, CHCl3). 1H
NMR (400 MHz, CDCl3): d = 3.41 (3 H, s), 3.57 (1 H, dd,
J = 3.2, 10.9 Hz), 3.75 (1 H, dd, J = 3.3, 10.8 Hz), 4.09 (1 H,
m), 4.17 (1 H, t, J = 6.7 Hz), 4.37 (1 H, d, J = 10.8 Hz), 4.43–
4.50 (5 H, m), 4.62 (1 H, d, J = 11.8 Hz), 6.05 (1 H, d, J = 5.6
Hz), 7.05 (1 H, t, J = 1.7 Hz), 7.15–7.36 (15 H, m), 7.39
(1 H, t, J = 1.7 Hz), 8.56 (1 H, br s) ppm. 13C NMR (100
MHz, CDCl3): d = 35.9, 68.2, 73.3, 73.4, 73.5, 78.6, 81.3,
82.3, 87.4, 121.3, 122.7, 127.7, 127.9, 128.0, 128.1, 128.2,
128.3, 128.5, 128.6, 134.9, 136.2, 137.1, 137.2 ppm. 31
P
NMR (160 Hz, CDCl3): d = –142.9 (sept., J = 712.7 Hz)
ppm. Glass-transition temperature –23 °C.
Compound 16: [a]D28 +9.5 (c 1.05, CHCl3). 1H NMR (400
MHz, CDCl3): d = 3.48 (3 H, s, NCH3), 3.59 (1 H, dd,
J = 3.2, 10.9 Hz), 3.76 (1 H, dd, J = 3.2, 10.9 Hz, H-5, H-5¢),
4.09 (1 H, m, H-4), 4.18 (1 H, t, J = 6.4 Hz, H-3), 4.38 (1 H,
d, J = 11.0 Hz), 4.45 (1 H, d, J = 11.0 Hz), 4.46–4.53 (4 H,
H-2), 4.61 (1 H, d, J = 11.8 Hz), 6.22 (1 H, d, J = 5.5 Hz),
7.16–7.36 (16 H), 7.53 (1 H, t, J = 1.7 Hz,), 8.75 (1 H, br s)
ppm. 13C (100 MHz, CDCl3): 35.7, 67.9, 72.2, 73.1, 75.0,
Silver nitrate test for ionic liquids with anions other than
chloride, derived via metathesis: The ionic liquid (1 mg) is
dissolved in MeOH–deionized H2O (1:1; 1 mL). The
resulting solution is tested with 0.1 M AgNO3 (2 drops). No
precipitation was observed in BF4 and PF6 ionic liquids.
(19) Cicchillo, R. M.; Norris, P. Carbohydr. Res. 2000, 328, 431.
Synlett 2008, No. 19, 2973–2976 © Thieme Stuttgart · New York