A facile Er(OTf)3-catalyzed synthesis of 2,3-unsaturated O- and S-glycosides

Article abstract of DOI:10.1016/j.carres.2007.05.034

Er(OTf)₃ is a useful catalyst for the Ferrier rearrangement furnishing high yields of O- and S-glycosides. The transformation has wide applicability, cleaner reaction profiles, mild reaction conditions, and high stereoselectivity and the catalyst, which is also commercially available, can be recovered and reused.

Full text of DOI:10.1016/j.carres.2007.05.034

Carbohydrate Research 342 (2007) 2125–2131
Note
A facile Er(OTf)3-catalyzed synthesis of 2,3-unsaturated
O- and S-glycosides
Antonio Procopio,a,* Renato Dalpozzo,b Antonio De Nino,b Loredana Maiuolo,b
Monica Nardi,b Manuela Oliverioa and Beatrice Russoa
a
` `
Dipartimento Farmacobioiologiche, Universita degli Studi della Magna Græcia Complesso Ninı Barbieri, 88021,
Roccelletta di Borgia (CZ), Italy
Dipartimento di Chimica, Universita della Calabria, Ponte Bucci, 87030, Arcavacata di Rende, (CS), Italy
b
`
Received 10 March 2007; received in revised form 23 May 2007; accepted 30 May 2007
Available online 7 June 2007
Abstract—Er(OTf)3 is a useful catalyst for the Ferrier rearrangement furnishing high yields of O- and S-glycosides. The transfor-
mation has wide applicability, cleaner reaction profiles, mild reaction conditions, and high stereoselectivity and the catalyst, which
is also commercially available, can be recovered and reused.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: O-Glycosylation; S-Glycosylation; Lewis catalyst; Erbium(III); Triflate
2,3-Unsaturated O-glycosides (pseudoglycals) are
important building-blocks in many bioactive molecules.1
Moreover, the 2,3 double bond in the pyran ring may be
easily modified by dihydroxylation, hydrogenation,
epoxidation, and aminohydroxylation to achieve struc-
tural complexity and diversity.1l,2 A direct and straight-
forward method for the synthesis of 2,3-unsaturated
glycosides is the acid-catalyzed allyl rearrangement of
glycals in the presence of alcohols, which is known as
the Ferrier reaction.1b,3 This rearrangement is believed
to involve a cyclic allylic oxocarbonium intermediate
that is formed via displacement of the C-3 substituent
in a glycal, followed by the preferential attack of a
nucleophile from the quasi-equatorial orientation.1a,4
A wide range of acid and oxidizing reagents have
been reported to promote this transformation such as
BF3ÆEt2O,5 SnCl4,6 IDCP,7 DDQ,8 TMSOTf,9 NIS,10
LiBF4,11 trichloroacetimidate,12 montmorillonite K-10,13
FeCl3,14 InCl3,15 InBr3,16 Sc(OTf)3,17 Yb(OTf)3,18
Dy(OTf)3,19 I2,20 CAN,21 BiCl3,22 ZnCl2,23 ZrCl4,24
HClO4 on silica gel,25 and NiCl5.26 All of these methods
offer several advantages, but some of them suffer draw-
backs in terms of yields and reaction time, while others
suffer from the use of harsh reaction conditions, require
non-catalytic amounts of reagents or are of high cost.
Among them, lanthanide triflates have been found to
be quite convenient for the O-glycosylation of glycals
by Ferrier reaction and, as in many other cases, they
can be used in sub-stoichiometric amounts. However,
these useful catalysts also present some limitations; for
example, Dy(OTf)3 is used in an ionic liquid and other
promoters were proposed for glucal substrates only
and/or for only C- or O-, S- or N-glycosylation.17–19
Therefore, the wider acceptability of these reagents has
not been demonstrated so far and a need to develop a
practical method yielding 2,3-dideoxyglycosides still
exists.
In continuation of our interest in exploring the utility
of Er(OTf)3 as a Lewis acid catalyst,27 we utilized glycal
chemistry to achieve diverse building-blocks, developing
a practical low-cost environmentally friendly procedure
for their C- and N-glycosylation.28 We now wish to re-
port a mild and efficient general method for the glycosyl-
ation of tri-O-acetyl-D-glycals (Scheme 1).
*
Corresponding author. Fax: +39 961391270; e-mail: procopio@
unicz.it
0008-6215/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2007.05.034

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A. Procopio et al. / Carbohydrate Research 342 (2007) 2125–2131
AcO
AcO
O
Er(OTf)3
solvent
O
+
AcO
OH
5.0 mol %
AcO
O
AcO
1
a
1a
Scheme 1.
First, we tested the catalytic activity of Er(OTf)3 in
the glycosylation of 3,4,6-tri-O-acetyl-D-glucal 1 at
room temperature in different solvents with 1.5 equiv
of n-butanol. Preliminary results, which are shown in
Table 1, revealed that catalytic activity of Er(OTf)3 is
very poor in non-polar solvents such as CH2Cl2, CH3Cl,
Et2O, and THF (Table 1, entries 1–4), but it is still
unsatisfactory in polar aprotic solvents such as CH3CN
and CH3NO2 (Table 1, entries 5 and 6). As could be pre-
dicted, better results were obtained when the reaction
was performed under dry conditions. Thus, higher yields
and shorter reaction times are reported for the glycosyl-
ation of 1 with n-butanol (Table 1, entries 7–10), but the
best result was obtained in CH3NO2 where 90% of the
product was obtained only after 2 h (Table 1, entry
10). Prolonged reaction times or higher amount of cata-
lyst did not improve the reaction performance (Table 1,
entries 11–13). Er(OTf)3 can be almost quantitatively
recovered and reused several times without significant
loss of activity,28 and yields over 80% were registered
for four recyclings (Table 1, entry 14).
yl-D-glucal with primary, secondary, benzyl, allyl, and
propargyl alcohols proceeds smoothly at room tempera-
ture to afford the corresponding alkyl 2,3-unsaturated
glycosides in high yields. The a-anomer is the major
product (Table 2, entries 1–8), which may arise from
the thermodynamic anomeric effect; product stereo-
chemistry was confirmed from chemical shifts of the
1
anomeric protons in the H NMR spectrum. The reso-
nance for H-1 of the a-glycosides occupies an upfield
position compared to the b-glycosides; moreover, an
NOE was observed between H-1 and H-4.
The generality of the present method was shown by
exposing per-O-acetylated glucal to methyl 2,3,4-tri-O-
methyl-a-D-glucopyranoside and N-Boc-O-tert-butyl-L-
threonine (Table 2, entries 9 and 10). Methyl 6-O-(4,6-
di-O-acetyl-2,3-dideoxy-a-D-erythro-hex-2-enopyranosyl)-
2,3,4-tri-O-methyl-a-D-glucopyranoside 1i was obtained
as anomeric mixture a/b 85/15 in 70% yield, while
glycopeptide precursor 1j was isolated only as the
a-glycoside as already reported by Schmidt and
co-workers.18b
Based on the results reported in Table 1, we decided to
explore the utility of erbium(III) triflate as Lewis acid
catalyst as a general method for the O-glycosylation of
tri-O-acetyl-D-glycals. The glycosylation of tri-O-acet-
The Ferrier rearrangement of the 2,3,6-tri-O-acetyl-D-
glucal with phenols is not as easy a reaction as that with
alcohols and only traces of glycosylated product were
detected when 1 was reacted with 4-methoxyphenol in
dry nitromethane and 5 mol % of Er(OTf)3 (Table 2,
entry 11). However, 35% yield was obtained when the
amount of the catalyst was improved to 10 mol % and
an 80% yield of the product was obtained when the reac-
tion was conducted at 50 °C (Table 2, entries 12 and 13).
Under the same experimental conditions, good results
were observed for other phenols (Table 2, entries 14
and 15).
The Ferrier rearrangement of 3,4,6-tri-O-acetyl-D-
galactal, 2, is not as easy a reaction as that for a glucal
analog and the few synthetically viable methods are
restricted by the use of microwave heating,13b,c,26 or
strongly acidic conditions.25 Thus, we deemed it perti-
nent to test our protocol on this substrate. Treatment
of 2 with 1-octanol in the presence of 5 mol % Er(OTf)3
in dry CH3NO2 resulted in the formation of the Ferrier
adduct in only 15 min (Table 2, entry 16). To extend the
scope of this reaction, we exposed different alcohols to 2
(Table 2, entries 17–19).
Table 1. Attempted O-glycosylation of 3,4,6-tri-O-acetyl-D-glucal 1 in
various solvents
Entry
Solvent
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
CH2Cl2
CHCl3
Et2O
THF
CH3CN
CH3NO2
CH2Cl2 (dry)
Et2O
CH3CN (dry)
CH3NO2 (dry)
CH3NO2 (dry)
CH3NO2 (dry)
CH3NO2 (dry)
CH3NO2 (dry)
Second recycle
Third recycle
Fourth recycle
48
24
48
48
48
24
12
12
12
2
25
18
15
10
35
60
46
25
58
90
88a
81b
90
88
86
82
2
2
6
2
2
2
Finally, the versatility of Er(OTf)3 was furthermore
illustrated by synthesizing thioglycosides from ethane-
thiol and thiophenol (Table 2, entries 20 and 21). In all
a Reaction conducted at 50 °C.
b 10 mol % of catalyst was used.

A. Procopio et al. / Carbohydrate Research 342 (2007) 2125–2131
Table 2. Er(OTf)3 catalyzed synthesis of 2,3-unsaturated glycosides
2127
Entry
Glycal
Substrate
Time (min)
Yielda,b,c (%)
a:bc ratio
AcO
AcO
O
O
O
O
O
O
O
O
CH3(CH2)2CH2-OH
1
120
95
92:8
a
AcO
1
1
1
1
1
1
1
1
AcO
AcO
CH3(CH2)6CH2-OH
2
3
4
5
6
7
8
15
150
20
20
10
10
10
83
85
80
88
75
78
90
100:0
b
AcO
AcO
AcO
CH2=CHCH2-OH
90:10
68:32
77:23
100:0
100:0
c
AcO
AcO
AcO
HC
C-CH2OH
d
AcO
AcO
AcO
CH3CHCH3
e
OH
AcO
AcO
AcO
Br
OH
f
AcO
AcO
AcO
OH
g
AcO
AcO
AcO
PhCH2-OH
100:0
h
AcO
(continued on next page)

2128
A. Procopio et al. / Carbohydrate Research 342 (2007) 2125–2131
Table 2 (continued)
Entry
Glycal
Substrate
Time (min)
Yielda,b,c (%)
a:bc ratio
AcO
AcO
HO
O
O
O
O
O
O
H3CO
H3CO
9
24 h
70
85:15
CH3O
i
OCH3
AcO
1
1
1
1
1
AcO
AcO
NHBoc
HO
10
24
79
100:0
CO2t-Bu
j
AcO
AcO
AcO
H3CO
H3CO
H3CO
OH
OH
OH
11
12
13
3 days
3 days
24 h
—
k
AcO
AcO
AcO
35d
—
k
AcO
AcO
AcO
80e
90:10
k
AcO
77e
78e
80:20
85:15
OH
14
15
1
1
24 h
24 h
l
O2
N
OH
m
AcO
AcO
O
CH3(CH2)6CH2-OH
16
15
85
82:18
b
AcO
2
2
AcO
AcO
O
CH2=CHCH2-OH
17
15
78
88:12
c
AcO

A. Procopio et al. / Carbohydrate Research 342 (2007) 2125–2131
2129
Table 2 (continued)
Entry
Glycal
AcO
Substrate
Time (min)
Yielda,b,c (%)
a:bc ratio
O
CH3CHCH3
AcO
18
20
77
94:6
e
OH
AcO
2
AcO
AcO
O
PhCH2-OH
19
20
90
12
80
85
90
98:2
h
AcO
2
1
AcO
AcO
O
CH3CH2-SH
20
90:10
n
AcO
AcO
AcO
O
SH
21
98:2
o
AcO
1
a All products were identified by comparison of their EI-MS and 1H NMR spectral data with those of authentic compounds and literature reported
data.29
b Isolated yield by flash column chromatography on silica gel is reported.
c The anomeric ratio was determined by 1H NMR.
d 10 mol % catalyst was used.
e 10 mol % catalyst was used at 50 °C.
reactions, the anomeric ratio of the Ferrier product was
1. Experimental
1.1. General methods
found to favor the a-anomer as the major product, as
1
evident from the analysis of H NMR spectrum where
no NOE was registered between H-1 and H-4.
In summary, we describe here a highly efficient and
simple procedure for the synthesis of O-alkyl and aryl
2,3-unsaturated glycosides as a further application of
erbium(III) trifluoromethanesulfonate as a Lewis acid
catalyst. Compared to the efficient protocol that uses
Yb(OTf)3 as a promoter, the present procedure seems
to be better in the glycosylation of alkyl substrates,
but undoubtedly less active toward aromatic acceptors.
The method offers several advantages such as mild reac-
tion conditions, high yields of glycosides, wide range of
applicability, recoverability, and reusability of the cata-
lyst which, keeping in mind the previous reports con-
cerning the synthesis of C- and N-glycosides, makes
Er(OTf)3 a very powerful Lewis acid catalyst in the Fer-
rier rearrangement.
1H and 13C NMR spectra were recorded on a Bruker
WM 300 instrument, at 300 MHz and 75 MHz, respec-
tively, on samples dissolved in CDCl3. Chemical shifts
are given in parts per million (ppm) from tetramethyl-
silane as the internal standard for 1H NMR spectra
(0.0 ppm) and the central line of CDCl3 for 13C NMR
spectra (77.0 ppm). Coupling constants (J) are given in
Hertz.
1.2. Typical procedure
3,4,6-Tri-O-acetyl-D-glucal 1 (200 mg, 0.73 mmol) was
added to 3.0 mL of a nitromethane solution of Er(OTf)3
(22.6 mg, 0.0367 mmol, 5.0 mol %) and stirred at room
temperature for 5 h. Then, n-butanol (0.075 g,

2130
A. Procopio et al. / Carbohydrate Research 342 (2007) 2125–2131
Synthesis; CRC Press: Boca Raton, FL, 1995; (e) Csuk,
R.; Shaade, M.; Krieger, C. Tetrahedron 1996, 52, 6397–
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1.10 mmol, 1.5 equiv) was added and the solution was
stirred at room temperature until a nearly complete con-
version of 1 was achieved (2 h) [TLC CHCl3–MeOH 9:1
v/v]. The organic solution was washed with water. The
crude product, obtained after solvent evaporation of
the dried organic layers, was purified by flash chroma-
tography [CHCl3–MeOH 9:1 v/v] affording 194.2 mg
of 1a (95% yield). The structural identification of 1a
was confirmed by comparison of its EI-MS and 1H
NMR spectral data with those reported in the
literature.29
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hex-2-enopyranoside (1a). Yellowish oil; 1H NMR
(CDCl3): d 5.81 (br s, 2H, H-2, H-3), 5.30–5.25 (dd,
1H, H-4, J = 9.2, 1.1 Hz), 5.02 (br s, 1H, H-1), 4.20–
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1329–1342; 1f: Kelly, D. R.; Picton, M. R. J. Chem. Soc.,
Perkin Trans. 1 2000, 1559–1570; 1i: Ref. 15a; 1a–1e, 1h,
1o, 2b, 2c, 2f, 2i: Ref. 25b; 1j, 1n: Ref. 18.