Synthesis of 2,3-unsaturated glycosides via metal-free Ferrier reaction

Article abstract of DOI:10.1016/j.tet.2008.09.005

Hexafluoroisopropanol (HFIP) is explored as an effective medium for the synthesis of 2,3-unsaturated glycosides through allylic rearrangement of 3,4,6-tri-O-acetyl glucal. This metal-free, exclusively solvent-promoted Ferrier glycosylation, affords products in good to excellent yields and with good α-selectivity.

Full text of DOI:10.1016/j.tet.2008.09.005

Tetrahedron 64 (2008) 10497–10500
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of 2,3-unsaturated glycosides via metal-free Ferrier reaction
*
`
Kavita De, Julien Legros, Benoit Crousse , Daniele Bonnet-Delpon
Laboratoire BioCIS-CNRS, Faculte´ de Pharmacie, Univ Paris Sud, rue JB Cle´ment, F-92296 Chatenay-Malabry, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 July 2008
Received in revised form 2 September 2008
Accepted 3 September 2008
Available online 11 September 2008
Hexafluoroisopropanol (HFIP) is explored as an effective medium for the synthesis of 2,3-unsaturated
glycosides through allylic rearrangement of 3,4,6-tri-O-acetyl glucal. This metal-free, exclusively solvent-
promoted Ferrier glycosylation, affords products in good to excellent yields and with good
a-selectivity.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Fluorine
Hexafluoropropan-2-ol
Sugar
Glycals
1. Introduction
2. Results and discussion
2,3-Unsaturated-O-glycosides are useful chiral intermediates in
the synthesis of a variety of important compounds: nucleosides,1
antibiotics2 and several biologically active natural products.3 One
elegant way to synthesize these molecules goes through an acid-
catalyzed Ferrier reaction between a glycal and an alcohol as nu-
cleophile.4 After the pioneer work of Ferrier and Prasad who used
BF3$Et2O,4c numerous examples involving either Lewis or Brønsted
acids have been described.5 In order to obtain greener and safer
processes, avoiding heavy metals is a current target for synthetic
chemists. In this line, we6 and others7 have demonstrated the
usefulness of fluorous alcohols as reaction solvent: various useful
organic transformations can be conducted in trifluoroethanol (TFE)
or hexafluoroisopropanol (HFIP) under mild conditions, without
using any external promoter. Reactions are generally selective and
without effluents, allowing thus a facile isolation of the product and
a recovery of the solvent by distillation. In some rare cases, fluo-
roalcohols can be involved in the reaction;8,9 in particular we re-
cently reported the facile self-promoted addition of HFIP onto the
3,4,6-tri-O-acetyl glucal through Ferrier reaction, by simple heating
of the substrate in the fluoro alcohol.9 We now found out that
performing the same reaction, still in HFIP, but also in presence of
another alcohol, led to the selective addition of the latter onto the
acetyl glucal to yield the corresponding Ferrier product.
According to our previous work on the competitive addition of
alcohols on enol ethers in HFIP,9 it seemed reasonable to envision
that using a more nucleophilic alcohol ROH in this reaction should
create a competition with HFIP, and the fluorinated solvent should
thus just act as a promoter to facilitate the addition of ROH. For this
purpose, the reaction was first assessed with ethanol as nucleo-
philic partner by adding various amounts to a solution of 3,4,6-tri-
O-acetyl glucal 1 HFIP, followed by reflux heating (Table 1).
When 1 was treated with 1 equiv of ethanol in HFIP (25 equiv)
for 12 h at reflux, the Ferrier reaction took place with quantitative
conversion to afford the HFIP addition product 2a as the major
product, along with some traces of the ethanol product 2b (2a/2b,
90:10). However, when 10 equiv of ethanol was used, the expected
Table 1
EtOH versus HFIP addition onto 1
AcO
AcO
AcO
AcO
AcO
O
HFIP
O
+
EtOH
reflux, 12 h
>98% conversion
2a [R = (CF3)2CH)] + 2b [R = Et]
OR
1
Entry
EtOH (equiv)
2a/2b
1
2
3
4
0
1
3
100:0a
90:10
50:50
10:90b
10
a
* Corresponding author. Fax: þ33 1 46 83 57 40.
E-mail address: benoit.crousse@u-psud.fr (B. Crousse).
Compound 2a was isolated in 95% yield.
Compound 2b was isolated in 88% yield.
b
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.09.005

10498
K. De et al. / Tetrahedron 64 (2008) 10497–10500
Table 2
were obtained in excellent yields (82–96%), as a mixture of
anomers with being favoured ( , 60:40 to 82:18). As expected,
the ratios were in the same range as those reported in the lit-
a and
Glycosylation of 3,4,6-tri-O-acetyl glucal with various alcohols and phenolsa
b
a
a/b
a/
bb
a/b
Entry Alcohol
Product
Yield (%)
erature.5 The poor nucleophile p-nitrophenol also reacted, but the
product 2j was isolated in 65% yield, along with 30% of the adduct
2a (entry 9). Similarly, with trifluoroethanol the reaction afforded
a mixture of the corresponding product 2k in moderate yield (60%),
here also accompanied with 2a (entry 10). However, surprisingly
TFE was also able to promote its self addition: by dissolving glucal 1
in neat TFE (25 equiv), product 2k was afforded after 12 h at 60 ꢀC
in much better yields (84%, entry 10). This self-promoted addition
of HFIP and TFE offers a new clean preparation of fluoroacetals in
carbohydrate series.10 Finally, we were pleased to find that the
multifunctionalized N-protected serine could also react with glucal
1 in HFIP through its hydroxyl moiety to afford 2l (71% yield, entry
11).
AcO
AcO
O
1
2
3
4
EtOH
88
67:33
70:30
70:30
82:18
2b
OEt
AcO
AcO
O
MeOH
93
96
95
2c
OMe
AcO
AcO
O
(CH3)2CHOH
Allyl alcohol
2d
O
AcO
AcO
O
2e
O
AcO
AcO
O
However, the question about the precise nature of the reaction
mechanism in HFIP arises. To make this clear, we studied the case of
the reaction of triacetyl glucal 1 with ethanol (Scheme 1).
Due to hydrogen bonding properties of HFIP,11 it is reasonable
to assume that the latter assists the leaving of the acetyl moiety,
leading to the oxonium 3, the widely accepted intermediate of
the reaction.12 Then, two pathways can be envisioned from 3 to
product 2b: either ethanol adds directly onto 3 [path (a)], or it
goes through the product intermediate 2a [path (b)]. In order to
determine, which of these two paths is followed, we used the
5
Propargyl alcohol
82
80:20
2f
O
AcO
AcO
O
6
7
PhCH2OH
PhOH
83
92
60:40
80:20
2g
2h
OBn
AcO
AcO
O
OPh
AcO
AcO
O
OMe
NO2
8
9
4-MeO–C6H4OH
4-NO2–C6H4OH
95
80:20
85:15
2i
HFIP adduct 2a (a/b
, 75:25)9 and put it under the conditions
described in Table 1 (EtOH, 10 equiv; HFIP, 25 equiv). After 12 h
at 60 ꢀC, no product 2b was obtained and the starting material
2a remained unchanged. Consequently, it can be deduced that
O
AcO
AcO
O
65c
2j
O
the conversion of the glucal
1 into the addition products
AcO
AcO
reported in Table 1 involves 3 as sole intermediate. Thus, HFIP
may only act as an assisting agent for the leaving of the acetate
moiety from 1.
In conclusion, we have developed an efficient methodology for
the synthesis of 2,3-unsaturated-O-glycosides via Ferrier rear-
rangement. The reaction is performed in HFIP as solvent and re-
quires no use of any acid or metal promoter.
O
60c
84d
70:30
55:45
10
11
CF3CH2OH
2k
O
CF3
AcO
AcO
O
L-CbzNH–Ser–OMe
71
85:15
2l
CO2Me
O
CbzHN
The reactions were performed at reflux of HFIP (25 equiv) for 12 h with 10 equiv
of the alcohol.
a
3. Experimental section
b
The ratio was determined on the basis of the integration of the anomeric protons
in the 1H NMR spectrum.
3.1. Materials and methods
c
Compound 2a was also obtained in notable amounts (ca. 30%).
The reaction was performed in TFE as single solvent at 60 ꢀC.
d
Hexafluoroisopropanol (HFIP) was kindly provided by Central
Glass Co. Ltd. and 1,1,1-trifluoroethanol was purchased from Flu-
orochem. 3,4,6-tri-O-Acetyl glucal was bought from Sigma
Aldrich. Melting points were recorded on a Stuart SMP10 appa-
ratus. IR spectra were recorded on a Bruker Vector 22 FTIR. NMR
spectra were recorded on Brucker AC 200 and 300 instruments, in
CDCl3. Chemical shifts are given in parts per million (ppm) from
TMS as internal standard for 1H and 13C NMR, and from CFCl3 for
19F NMR. The optical rotations were measured on a PolAAr 3
polarimeter.
product 2b was obtained in 88% yield, as a mixture of
a and
b
anomers (70:30). It is worth noting that the presence of HFIP is
absolutely required: as expected, when the reaction was performed
in ethanol only, the substrate 1 did not react.
Encouraged by these results, this methodology was extended to
other alcohols such as alkyl, allyl, propargyl and benzyl alcohols, as
well as phenols (Table 2, entries 1–9). For all these reagents, the
reaction proceeded smoothly under the same conditions. Products
path (a)
2b
AcO
AcO
AcO
AcO
EtOH
O
O
HFIP
reflux
O
AcO
AcO
1
3
O
O
path (b)
CF3
H
O
O
2a
CF3
F3C
CF3
Scheme 1.

K. De et al. / Tetrahedron 64 (2008) 10497–10500
10499
3.2. General procedure for the synthesis of 2,3-unsaturated-
O-glycosides in HFIP
2.40 (t, J¼1.8 Hz, 1H, –ChCH), 3.99–4.06 (m, 1H, H-5), 4.15 (dd,
J¼5.1, 12 Hz, 2H, Ha-6, Hb-6), 4.25 (d, J¼2.4 Hz, 2H, Ha-10, Hb-10),
5.18 (br s, 1H, H-1), 5.27 (dd, J¼9.6,1.5 Hz, 1H, H-4), 5.75–5.88 (m,
To a stirred mixture of 3,4,6-tri-O-acetyl-
D-glucal (0.184 mmol,
2H, H-2, H-3); 13C NMR (75 MHz, CDCl3):
d 20.8, 21.0, 55.1, 62.8,
50 mg) in HFIP (0.5 mL) was added the alcohol (10 equiv) and the
reaction mixture was heated at reflux (60 ꢀC). After 12 h stirring,
the reaction was complete (TLC monitoring). Then HFIP was
evaporated under vacuum and the product was purified by column
chromatography using cyclohexane/ethyl acetate (cyclohexane/
AcOEt 90:10) as eluent.
65.2, 67.2, 74.9, 79.1, 92.8,127.3, 129.8,170.2,170.8; ESI m/z (rel int.):
291.2 [MþNa]þ (100).
3.2.7. Benzyl 4,6-di-O-acetyl-2,3-dideoxy-a-D-erythro-hex-2-
enopyranoside (2g)13
Colourless oil; [
a
]
150 (c 0.8, MeOH); 1H NMR (300 MHz,
D
CDCl3): 2.01 (s, 3H, –OCOCH3), 2.03 (s, 3H, –OCOCH3), 4.00–4.24
d
3.2.1. Hexafluoroisopropyl 4,6-di-O-acetyl-2,3-dideoxy-
a
-
D
-
(m, 3H, H-5, Ha-6, Hb-6), 4.53 (d, J¼12 Hz, 1H, –O–HCH–Ph), 4.74
(d, J¼12 Hz, 1H, –O–HCH–Ph), 5.06 (br s, 1H, H-1), 5.27 (dd, J¼9.6,
1.2 Hz, 1H, H-4), 5.79–5.85 (m, 2H, H-2, H-3), 7.27–7.30 (m, 5H, Ph);
erythro-hex-2-enopyranoside (2a)9
White oil; [
2.07 (s, 3H, –CO–CH3), 2.09 (s, 3H, –CO–CH3), 3.98–4.06 (m, 1H, H-
a]
D 208.2 (c 0.8, MeOH); 1H NMR (200 MHz, CDCl3):
d
13C NMR (75 MHz, CDCl3):
d 20.9, 21.0, 62.9, 65.3, 67.1, 67.3, 93.6,
5), 4.15 (d, J¼4 Hz, 2H, H-6), 4.51 (sept, J¼6 Hz), 5.22 (br s, 1H, H-1),
127.8, 127.9, 128.0, 128.5, 129.4, 138.2, 170.3, 170.9; ESI m/z (rel int.):
5.29 (dd, J¼9.6, 1.6 Hz, 1H, H-4), 5.79 (ddd, J¼2, 2.4, 10.2 Hz, 1H,
344.3 [MþNa]þ (100).
H-2), 5.95–5.99 (d, J¼10.4 Hz); 13C NMR (75 MHz, CDCl3):
d 20.7,
20.9, 62.2, 64.6, 68.1, 72 (sept, J¼33 Hz), 121.2 (q, J¼283 Hz), 121.7
3.2.8. Phenyl 4,6-di-O-acetyl-2,3-dideoxy-
a
-
D
-erythro-hex-2-
White solid, mp 50 ꢀC (lit. 47–48 ꢀC); [
a] 102 (c 0.65, MeOH);
D
(q, J¼284 Hz), 125, 131.5, 170.1, 170.6; 19F NMR (188 MHz, CDCl3):
enopyranoside (2h)13
d
ꢁ74.2 (m, 3F), ꢁ74.0 (m, 3F). Anal. calcd for C13H14F6O6: C 41.06, H
3.71; found: C 41.30, H 3.85.
1H NMR (300 MHz, CDCl3):
d 1.91 (s, 3H, –OCOCH3), 2.04 (s, 3H,
–OCOCH3), 4.05–4.25 (m, 3H, H-5, Ha-6, Hb-6), 5.31 (d, J¼9 Hz, 1H,
3.2.2. Ethyl 4,6-di-O-acetyl-2,3-dideoxy-
a
-
D
-erythro-hex-2-
H-4), 5.63 (br s, 1H, H-1), 5.94–5.98 (m, 2H, H-2, H-3), 6.94–7.05 (m,
enopyranoside (2b)13
3H, Ph), 7.19–7.26 (m, 2H, Ph); 13C NMR (75 MHz, CDCl3):
d 20.7,
White oil; [
a
]
120 (c 1.2, MeOH); 1H NMR (300 MHz, CDCl3):
21.0, 62.6, 65.0, 67.8, 92.9,117.0,122.5,127.1,129.5,130.1,157.1,170.3,
D
d
1.25 (t, J¼7.2 Hz, 3H, –O–CH2–CH3), 2.11 (s, 3H, –CO–CH3), 2.12 (s,
170.8; ESI m/z (rel int.): 329.3 [MþNa]þ (100).
3H, –CO–CH3), 3.58–3.63 (m, 1H, –O–HCH–CH3), 3.83–3.88 (m, 1H,
–O–HCH–CH3), 4.12–4.31 (m, 3H, H-5, Ha-6, Hb-6), 5.07 (s,1H, H-1),
5.34 (dd, J¼1.2, 9.6 Hz, 1H, H-4), 5.84–5.98 (m, 2H, H-2, H-3); 13C
3.2.9. p-Methoxyphenyl 4,6-di-O-acetyl-2,3-dideoxy-a-D-erythro-
hex-2-enopyranoside (2i)13
NMR (75 MHz, CDCl3):
d
15.3, 20.7, 20.9, 63.0, 64.3, 65.3, 66.8, 94.5,
White solid, mp 69 ꢀC (lit. 69–70 ꢀC); [
a
]
D 104 (c 1.4, MeOH); 1H
NMR (300 MHz, CDCl3): d 1.95 (s, 3H, –OCOCH3), 2.04 (s, 3H,
128.0, 129.0, 170.3, 170.7; ESI m/z (rel int.): 281.2 [MþNa]þ (100).
–OCOCH3), 3.69 (s, 3H, –OCH3), 4.07–4.21 (m, 3H, H-5, Ha-6, Hb-6),
5.29 (d, J¼9.0 Hz, 1H, H-4), 5.49 (br s, 1H, H-1), 5.93 (br s, 2H, H-2,
H-3), 6.75 (dd, J¼9.0 Hz, 2H, Ph), 6.97 (dd, J¼9 Hz, 2H, Ph); 13C NMR
3.2.3. Methyl 4,6-di-O-acetyl-2,3-dideoxy-a-D-erythro-hex-2-
enopyranoside (2c)14
Colourless oil; [
a
]D 124 (c 1.2, MeOH); 1H NMR (300 MHz, CDCl3):
(75 MHz, CDCl3): d 20.8, 21.0, 55.7, 62.8, 65.2, 67.7, 94.1, 114.6, 118.7,
d
2.02 (s, 3H, –OCOCH3), 2.04 (s, 3H, –OCOCH3), 3.39 (s, 3H, OCH3),
127.3, 130.0, 151.2, 155.3, 170.3, 170.8; ESI m/z (rel int.): 359.1
3.96–4.04 (m, 1H, H-5), 4.13–4.19 (m, 2H, H-6a, H-6b), 4.86 (br s,1H,
H-1), 5.25 (dd, J¼9.0, 1.57 Hz, 1H, H-4), 5.77–5.91 (m, 2H, H-2, H-3);
13C NMR (75 MHz, CDCl3): 20.8, 21.0, 56.0, 63.0, 65.3, 66.9, 95.5,
127.7, 129.3, 170.3, 170.8; ESI m/z (rel int.): 267.1 [MþNa]þ (100).
[MþNa]þ (100).
3.2.10. p-Nitrophenyl 4,6-di-O-acetyl-2,3-dideoxy-a-D-erythro-
hex-2-enopyranoside (2j)13
White solid, mp 90 ꢀC (lit. 94–95 ꢀC); [
a]
D 170 (c 0.6, MeOH); 1H
NMR (300 MHz, CDCl3): d 1.89 (s, 3H, –OCOCH3), 2.05 (s, 3H,
3.2.4. Isopropyl 4,6-di-O-acetyl-2,3-dideoxy-
a
-D-erythro-hex-2-
enopyranoside (2d)15
–OCOCH3), 4.05–4.23 (m, 3H, H-5, Ha-6, Hb-6), 5.34 (dd, J¼9.6,
1.2 Hz, 1H, H-4), 5.74 (br s, 1H, H-1), 5.93 (dd, J¼1.8, 10.2 Hz, 1H,
H-3), 6.04 (d, J¼10.2 Hz, 1H, H-2), 7.11 (d, J¼9.0 Hz, 2H, Ph), 8.15 (d,
Colourless oil; [a]
D 97 (c 0.7, MeOH); 1H NMR (300 MHz, CDCl3):
1.11 (d, J¼6.3 Hz, 3H, –O–CH(CH3)CH3), 1.18 (d, J¼6.3 Hz, 3H, –O–
d
CH(CH3)CH3), 2.01 (s, 3H, –OCOCH3), 2.02 (s, 3H, –OCOCH3), 3.87
(hept, J¼6.2 Hz, 1H, –O–CH(CH3)CH3), 4.01–4.19 (m, 3H, H-5, Ha-6,
Hb-6), 5.06 (br s, 1H, H-1), 5.23 (dd, J¼9.6, 1.5 Hz, 1H, H-4), 5.70–
J¼9.0 Hz, 2H, Ph); 13C NMR (75 MHz, CDCl3):
d 20.6, 20.9, 62.34,
64.7, 68.4, 92.6, 116.6, 125.7, 125.8, 131.2, 142.6, 161.8, 170.1, 170.5;
ESI m/z (rel int.): 374.2 [MþNa]þ (100).
5.87 (m, 2H, H-2, H-3); 13C NMR (75 MHz, CDCl3):
d 20.8, 21.0,
22.00, 23.5, 63.2, 65.4, 66.8, 70.8, 92.9, 128.5, 128.8, 170.3, 170.8; ESI
3.2.11. Trifluoroethyl 4,6-di-O-acetyl-2,3-dideoxy-a-D-erythro-
m/z (rel int.): 295.2 [MþNa]þ (100).
hex-2-enopyranoside (2k)
Colourless oil; [
d 2.09 (s, 3H), 2.1 (s, 3H), 3.93–4.13 (m, 3H, H-5, OCH2), 4.21–4.23
a]
D 79 (c 0.8, MeOH); 1H NMR (300 MHz, CDCl3):
3.2.5. Allyl 4,6-di-O-acetyl-2,3-dideoxy-
a
-
D
-erythro-hex-2-
enopyranoside (2e)13
(m, 2H, H-6), 5.11 (br s, 1H, H-1), 5.33 (dd, J¼9.6, 1.5 Hz, 1H, H-4),
White oil; [a]
91 (c 0.45, MeOH); 1H NMR (300 MHz, CDCl3):
5.83–5.88 (m, 1H, H-2), 5.96 (d, J¼10.2 Hz, 1H, H-3); 13C NMR
D
d
2.01 (s, 3H, –OCOCH3), 2.03 (s, 3H, –OCOCH3), 3.97–4.25 (m, 5H,
(75 MHz, CDCl3):
d
20.6, 20.9, 62.6, 65.0 (t, J¼34 Hz), 67.5, 94.5,
H-5, Ha-6, Hb-6, Ha-10, Hb-10), 5.01 (br s, 1H, H-1), 5.1–5.27 (m, 3H,
121.9, 123.7 (q, J¼276 Hz), 126.2, 130.4, 170.2, 170.7; 19F NMR
Ha-30, Hb-30, H-4), 5.78–5.94 (m, 3H, H-2, H-3, H-20); 13C NMR
(188 MHz, CDCl3):
d
ꢁ74.64 (t, J¼9 Hz); ESI m/z (rel int.): 335.2
(75 MHz, CDCl3):
d
20.8, 21.0, 63.0, 65.3, 67.0, 69.3, 93.7, 117.6, 127.8,
[MþNa]þ (100).
129.3, 134.1, 170.3, 170.8; ESI m/z (rel int.): 293.2 [MþNa]þ (100).
3.2.12. 2-(N-Benzyloxycarbonylamino)-3-methoxy-
3.2.6. Prop-2-ynyl 4,6-di-O-acetyl-2,3-dideoxy-
a
-
D
-erythro-hex-2-
carbonylethyl 4,6-di-O-acetyl-2,3-dideoxy-a-D-erythro-hex-2-
enopyranoside (2f)14
enopyranoside (2l)13
White solid, mp 59 ꢀC; [
a]
110 (c 0.8, MeOH); 1H NMR
Colourless oil; [
d 2.04 (s, 3H, –OCOCH3), 2.07 (s, 3H, –OCOCH3), 3.76 (s, 3H, –OCH3),
a]
D 46 (c 1.5, MeOH); 1H NMR (200 MHz, CDCl3):
D
(300 MHz, CDCl3):
d
2.02 (s, 3H, –OCOCH3), 2.04 (s, 3H, –OCOCH3),

10500
K. De et al. / Tetrahedron 64 (2008) 10497–10500
6. For reviews on fluorous alcohols as solvents, see: (a) Be´gue´, J.-P.; Bonnet-
3.96–4.05 (m, 3H, H-5, Ha-6, Hb-6), 4.18–4.20 (m, 2H,
bCH2), 4.51–
´
´
Delpon, D.; Crousse, B. Synlett 2004, 18; (b) Begue, J.-P.; Bonnet-Delpon, D.;
Crousse, B. In Handbook of Fluorous Chemistry; Gladysz, J. A., Curran, D. P.,
Horvath, I. T., Eds.; Wiley-VCH: Weinheim, 2004; p 341; For some repre-
sentative examples of the use of fluorous alcohols in our group, see: (c)
Legros, J.; Crousse, B.; Oure´vitch, M.; Bonnet-Delpon, D. Synlett 2006, 1899;
4.58 (m, 1H, CH), 4.98 (br s, 1H, H-1), 5.13 (br s, 2H, PhCH2), 5.26
a
(dd, J¼10.2 Hz, 1H, H-4), 5.73 (ddd, J¼2.2, 4.2, 8.8 Hz, 1H, H-2), 5.87
(dd, J¼0.4, 9.8 Hz, 1H, H-3), 7.35 (br s, 5H, Ph); 13C NMR (75 MHz,
CDCl3): 20.7, 21.0, 52.7, 54.5, 62.8, 65.00, 67.1, 67.4, 69.5, 95.1, 127.01,
128.2, 128.6, 129.6, 136.2, 156.00, 170.3, 170.5, 170.8; ESI m/z (rel
int.): 488.3 [MþNa] (100).
´
´
(d) Ravikumar, K. S.; Kesavan, V.; Crousse, B.; Bonnet-Delpon, D.; Begue, J.-P.
Organic Synthesis; Wiley: New York, NY, 2003; Coll. Vol. 80, 184; (e)
p
´
´
Spanedda, M. V.; Hoang, V. D.; Crousse, B.; Bonnet-Delpon, D.; Begue, J.-P.
Tetrahedron Lett. 2003, 44, 217; (f) Di Salvo, A.; Spanedda, M. V.; Oure´vitch,
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Acknowledgements
7. For a review on the use of fluorinated alcohols, as cosolvents and additives, see:
Shuklov, I. A.; Dubrovina, N. V.; Bo¨rner, A. Synthesis 2007, 2925.
Central Co. Ltd. is gratefully acknowledged for kind gift of HFIP.
We thank the Indo-French program CEFIPRA/IFCPAR for financial
support and for the PhD fellowship of K.D.
´
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