Direct Ferrier rearrangement on unactivated glycals catalyzed by indium(III) chloride

Article abstract of DOI:10.1016/j.tetlet.2009.04.084

Anhydrous InCl₃ has been shown to efficiently catalyze the Ferrier rearrangement by a direct allylic substitution of the hydroxyl group at C-3 position of glycals to afford the corresponding 2,3-unsaturated glycosides in high yields at ambient

Full text of DOI:10.1016/j.tetlet.2009.04.084

Tetrahedron Letters 50 (2009) 3970–3973
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Tetrahedron Letters
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Direct Ferrier rearrangement on unactivated glycals catalyzed
by indium(III) chloride
*
Paramathevar Nagaraj, Namakkal G. Ramesh
Department of Chemistry, Indian Institute of Technology-Delhi, Hauz Khas, New Delhi 110 016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Anhydrous InCl3 has been shown to efficiently catalyze the Ferrier rearrangement by a direct allylic
substitution of the hydroxyl group at C-3 position of glycals to afford the corresponding 2,3-unsaturated
glycosides in high yields at ambient temperature. This methodology obviates the need for protecting and/
or activating the C-3 hydroxyl group of glycals. The reaction works in equal ease with both 4,6-di-O-ben-
Received 9 March 2009
Revised 20 April 2009
Accepted 22 April 2009
Available online 3 May 2009
zyl-D-glucal and 4,6-di-O-benzyl-D-galactal. The mildness of InCl3 makes this approach compatible for
glycosyl acceptors with acid labile groups. The generality of the reaction has been demonstrated with
a diversity of alcohols, phenols, and sugar nucleophiles.
Keywords:
Glycosylation
Lewis acid
Ó 2009 Elsevier Ltd. All rights reserved.
Indium(III) chloride
Glycals
Disaccharides
Ferrier rearrangement
17
Ferrier rearrangement is one among the few name reactions
which continues to offer widespread applications in organic syn-
thesis for over four decades.1 In his pioneering work, Ferrier
IDCP,14 Et2Zn–Pd(OAc)2/DTTBP,13 and AuCl3 are often required
to further activate the leaving group at the C-3 position to realize
the allylic rearrangement. Except for a few scattered reports,14,16
examples of a direct Ferrier rearrangement on glycals possessing
a free hydroxyl group at the C-3 position are uncommon. Even in
such cases, low yield of the resulting glycosides14 or use of a very
strong acid (such as trifluoroacetic acid) in large excess limits their
synthetic applications, especially toward acid labile glycosyl accep-
tors/donors in the latter case.16 A couple of examples on Ferrier-
type rearrangement under Mitsunobu conditions though available,
are limited to the use of carboxylic acids19 and phenols20 as glyco-
syl acceptors, and are not suitable for alcohols. Development of an
efficient method for direct substitution of alcohols is still a chal-
lenging goal in organic chemistry given the poor leaving ability
of the hydroxyl group. As a part of our research program aimed
at the use of InCl3 as a mild catalyst in carbohydrate chemistry,
we recently reported a stereoselective synthesis of unsaturated
glycosides via InCl3-catalyzed 1,3-alkoxy migration in glycal
ethers.21 A recent report of Baba et al.22 on direct allylic substitu-
tion of alcohols with carbon nucleophiles using InCl3 at high
temperatures prompted us to explore the catalytic behavior of
InCl3 toward glycals possessing a free hydroxyl group at C-3
position. In this Letter, we report a successful outcome of Ferrier
reported that tri-O-acetyl-
D
-glucal 1 on exposure to BF3ÁOEt2 in
the presence of an alcohol afforded 2,3-unsaturated glycosides 2
as an anomeric mixture (Scheme 1).2 Since then, the rearrange-
ment has become synthetically the most useful transformation in
glycal chemistry, as the resulting 2,3-unsaturated glycosides have
served as versatile chiral intermediates in the synthesis of antibiot-
ics,3 natural products,4 glycopeptides,5 nucleosides,6 oligosaccha-
rides,1f,7 uronic acids,8 and modified carbohydrates.9 A wide
range of catalysts have been employed in the Ferrier rearrange-
ment.10 The synthetic importance of this glycosylation reaction is
clearly evident from the vast number of publications on this to-
pic.10 Literature reports that tri-O-acetyl-
D-glucal has been the
most common choice of the glycosyl donor due to its ready avail-
ability, although the rearrangement has been studied on other gly-
cals too.11 However, successful Ferrier rearrangement has been
carried out only with glycals possessing a good leaving group at
the C-3 position such as tricholoracetamidate,12 tert-butyloxycar-
bonyl ester,13 n-pentenoyl ester,14 benzoyl ester,15 carbonate,16
propargyl ether,17 and benzyl ether.10b,11a,18 Thus, an additional
step of appropriately protecting the C-3 hydroxyl group of glycals
is an essential pre-requisite in many examples reported so far. In
addition, expensive catalysts such as Pd(PhCN)2Cl2/DTTBP,12a
rearrangement of di-O-benzyl-D-glucal as well as di-O-benzyl-D-
galactal with a variety of alcohols as well as phenols in presence
of just 5 mol % of InCl3 as a catalyst. The reaction proceeds at ambi-
ent temperature (30 °C) affording products in high yields (68–92%),
* Corresponding author. Tel.: +91 11 26596584; fax: +91 11 26582037.
E-mail address: ramesh@chemistry.iitd.ac.in (N.G. Ramesh).
is stereoselective yielding predominantly
a-anomers, and is
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.04.084

P. Nagaraj, N. G. Ramesh / Tetrahedron Letters 50 (2009) 3970–3973
3971
prove to be of exceptional synthetic value. Thus, when we treated
4,6,-di-O-benzyl-
-galactal25 with methanol in presence of 5 mol %
of InCl3 at 30 °C, we were surprised to notice that the expected 2,3-
unsaturated glycoside 10 was formed in a very high yield of 91%.
OAc
OAc
R-OH
D
O
O
AcO
AcO
AcO
BF3.OEt2
benzene
OR
R = alkyl, aryl etc.
2
1
Better anomeric diastereoselectivity (
case as compared to glycoside 4 obtained from 4,6-di-O-benzyl-
-glucal deserves mention. Inspired with the initial success, the
a:b = 95:5) observed in this
D
Scheme 1. Lewis acid-catalyzed Ferrier rearrangement of tri-O-acetyl-D-glucal.
reaction was tested with a variety of alcohols (Table 2, entries 2–
6). Gratifyingly, in all the cases, Ferrier rearrangement occurred
smoothly affording the corresponding 2,3-unsaturated glycosides
in very high yields and high a-selectivity. Noteworthy is that most
of these reactions are much faster than their glucal counterparts
compatible with acid-labile glycosyl acceptors. The interest of this
methodology relies on the extremely mild conditions required
even with a free hydroxyl group at the C-3 position.
Initially, Ferrier rearrangement was investigated on 4,6-di-O-
benzyl-D
-glucal 323 with methanol in presence of 5 mol % of InCl3
(Tables 1 and 2, entries 2–5). As in the previous case, sugar nucle-
ophiles such as 1,2:3,4-di-O-isopropylidene-
D-galactopyranose and
at ambient temperature (30 °C). The progress of the reaction was
monitored by TLC which indicated a gradual disappearance of
the starting material and the appearance of a non-polar product.
Complete consumption of the starting material was noticed after
14 h. Upon purification and careful analysis, the product was in-
deed identified as methyl 4,6-di-O-benzyl-2,3-dideoxy-hex-2-eno-
1,2:5,6-di-O-isopropylidene- -glucofuranose also reacted with
D
ease to afford the disaccharides in high yields (Table 2, entries 4
and 5). Further, formation of 2-deoxy galactosides in these exam-
ples was hardly observed even in the 1H NMR spectra of the crude
reaction mixtures. This circumvents the existing difficulty in the
synthesis of 2,3-unsaturated galactosides and makes this method-
ology more attractive and reliable. The success of this reaction with
phenols (Table 2, entries 7–9) further demonstrates its generality
and applicability to aryl nucleophiles. Notably, an earlier report
on a Mitsunobu approach for the synthesis of 17 resulted in an
inseparable mixture of compounds.20 Further, while Mitsunobu
pyranoside 4, obtained as an anomeric mixture (a:b = 4:1) in an
overall isolated yield of 87%.
A higher catalyst load while reducing the reaction time, did not
bring about any significant change in the yield or anomeric selec-
tivity. Encouraged by the initial results, the reaction was then car-
ried out with benzyl and allyl alcohols, which also underwent
InCl3-catalyzed Ferrier rearrangement smoothly to afford the cor-
responding 2,3-unsaturated glycosides 5 and 6, respectively, in
high yields (Table 1, entries 2 and 3). Synthetically more rewarding
is the success of the reaction with sugar nucleophiles possessing
acid-labile acetal functionalities (Table 1, entries 4 and 5). The
disaccharides 7 and 8 were obtained in good yields with the acetal
groups remaining intact. Notably, in all these examples the ano-
reaction of 4,6-di-O-benzyl-D
-galactal 9 with carboxylic acids19b
or phenols20 as nucleophiles predominantly or exclusively afforded
products arising out of a direct SN2 reaction, exclusive Ferrier rear-
rangement of 9 reported here is complimentary. Given the litera-
ture background that 2,3-unsaturated aryl glycosides serve as
useful precursors for palladium-catalyzed stereoselective C-glyco-
sylation reactions,26 compounds 16–18 reported here would prove
to be of a synthetic value.
meric ratio (
mer being the major isomer.
Having successfully accomplished the direct InCl3-catalyzed
Ferrier rearrangement of 4,6-di-O-benzyl- -glucal with a few alco-
hols, we next focused our attention with a greater emphasis on a
similar reaction with 4,6-di-O-benzyl- -galactal.
a:b = 4:1) remained almost the same with the a-ano-
In conclusion, we have demonstrated that InCl3, though consid-
ered as a mild Lewis acid, is an excellent catalyst for the Ferrier rear-
rangement of unactivated glycals. The reaction condition is mild
and the reaction requires only 5 mol % of InCl3.27 The reaction is
compatible with acid-labile functional groups, is stereoselective,
and is general toward a wide range of oxygenated nucleophiles.
Our results significantly highlight that protection and/or prior acti-
vation of C-3 hydroxyl group of glycals is not an essential criterion
to realize a successful Ferrier rearrangement. Surpassing result is
the facile synthesis of a wide array of 2,3-unsaturated galactosides,
which are otherwise difficult to obtain. We believe that the meth-
odology reported here is synthetically quite attractive and would
spur on further interests toward the synthesis of complex
glycosides.
D
D
It is well documented in the literature that Ferrier rearrange-
ment of protected galactals is rather difficult and not as straight
forward as for protected glucals due to the competing formation
of 2-deoxy glycosides.16,24 Only a very few successful methods
are available for the Ferrier rearrangement of galactals even with
a good leaving group at the C-3 position such as acetyl, n-pentenoyl
ester, and trichloroacetamidate.12,14 Given this background, we
considered that a successful Ferrier rearrangement on galactal
especially with a free hydroxyl group at the C-3 position would
Table 1
InCl3-catalyzed Ferrier rearrangement of 4,6-di-O-benzyl-D-glucal
OBn
OBn
R-OH (1.2 equiv.)
InCl3 (5 mol%)
O
O
BnO
BnO
HO
CH2Cl2
30 0C
OR
4-8
3
Entry
R
Product
Time (h)
Yielda (%)
Ratio (a
/b)b
1
2
3
4
5
Methyl
Benzyl
Allyl
4c
5c
6c
7c
8c
14
10
13
9
87
84
92
75
68
81:19 (90:10)
83:17 (96:4)
83:17 (85:15)
83:17 (100:0)
80:20 (90:10)
1,2:3,4-Di-O-isopropylidene-
1,2:5,6-Di-O-isopropylidene-
D
-galactopyranosyl
-glucofuranosyl
D
8
a
Isolated yield after column chromatography.
b
Anomeric ratios were obtained from 1H NMR spectra of the crude products; values in parantheses refer to the anomeric ratios of the products obtained after column
chromatography.
c
Spectral data are consistent with literature values (Refs. 10b,13,18a,21).

3972
P. Nagaraj, N. G. Ramesh / Tetrahedron Letters 50 (2009) 3970–3973
Table 2
InCl3-catalyzed Ferrier rearrangement of 4,6-di-O-benzyl-D-galactal
BnO
OBn
BnO
OBn
R-OH (1.2 equiv.)
InCl3 (5 mol%)
O
O
HO
CH2Cl2
30 0C
OR
9
10-18
Entry
R
Product
Time (h)
Yielda (%)
Ratio (a
:b)b
1
2
3
4
5
6
7
8
9
Methyl
Benzyl
Allyl
10
11c
12
13
14
15c
16
17
18
26
2
8
5
7
14
7
8
91
90
86
82
68
88
90
89
90
95:5 (100:0)
92:8 (100:0)
92:8 (100:0)
84:16 (89:11)
82:18 (91:9)
92:8 (100:0)
91:9 (96:4)
91:9 (93:7)
93:7 (100:0)
1,2:3,4-Di-O-isopropylidene-
1,2:5,6-Di-O-isopropylidene-
Cyclohexyl
Phenyl
p-Methylphenyl
p-Methoxyphenyl
D
-galactopyranosyl
-glucofuranosyl
D
5
a
Isolated yield after column chromatography.
b
Anomeric ratios were obtained from 1H NMR spectra of the crude products; values in parantheses refer to the anomeric ratios of the products obtained after column
chromatography.
c
Spectral data are consistent with the literature values (Refs. 16,21).
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Acknowledgments
We are grateful to the Council of Scientific and Industrial Re-
search, New Delhi, India for financial support. P.N. thanks CSIR, In-
dia and IIT Delhi for a research fellowship.
Supplementary data
Supplementary data (spectral data of compounds 4–8, 10–18
(pp. S1–S13) and copies of 1H and 13C NMR spectra (pp. S14–
S41)) associated with this article can be found, in the online ver-
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P. Nagaraj, N. G. Ramesh / Tetrahedron Letters 50 (2009) 3970–3973
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3973
25. For the synthesis of 4,6-di-O-benzyl-
D
three-necked round-bottomed flask (flame-dried and cooled under argon).
Nucleophile (1.2 mmol) was added and after 5 min, 5 mol % of anhydrous InCl3
was added and the reaction mixture was stirred under argon atmosphere for
specific time mentioned in Tables 1 and 2. The reaction mixture was then
quenched with water and extracted with chloroform (3 Â 50 mL). The
combined organic layer was washed with water, dried over anhydrous
sodium sulfate, filtered, and concentrated. The product was purified by
column chromatography with hexane/ethyl acetate as an eluent to obtain
the corresponding 2,3-unsaturated glycosides (4–8, 10–18).
27. General procedure: Glycal 3 or 9 (1 mmol) was dissolved in dry CH2Cl2 (10 mL)
and dried over activated molecular sieves (4 Å) overnight in order to remove
any moisture if present. The solution was then transferred by syringe to a dry