One-pot acetalation-acetylation of sugar derivatives employing perchloric acid immobilised on silica

Article abstract of DOI:10.1016/j.carres.2005.02.012

Perchloric acid immobilised on silica gel has been used as an efficient promoter for per-O-acetylation, and acetalation and subsequent O-acetylation of glycosides and thioglycosides in one-pot using stoichiometric reagents.

Full text of DOI:10.1016/j.carres.2005.02.012

Carbohydrate
RESEARCH
Carbohydrate Research 340 (2005) 1075–1080
One-pot acetalation–acetylation of sugar derivatives
employing perchloric acid immobilised on silica
Balaram Mukhopadhyay,a,* David A. Russella and Robert A. Fielda,b,
*
aCentre for Carbohydrate Chemistry, School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich NR4 7TJ, UK
bDepartment of Biological Chemistry, John Innes Centre, Norwich NR4 7UH, UK
Received 11 January 2005; received in revised form 17 February 2005; accepted 18 February 2005
Abstract—Perchloric acid immobilised on silica gel has been used as an efficient promoter for per-O-acetylation, and acetalation and
subsequent O-acetylation of glycosides and thioglycosides in one-pot using stoichiometric reagents.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Perchloric acid on silica; Acetal–ketal; Acetylation
1. Introduction
access to these important sugar building blocks using
stoichiometric reagents would be useful. Noting recent
reports of the use of perchloric acid supported on silica
gel as an effective promoter for acetylation of simple
alcohols16a and sugars16b and the Ferrier rearrangement
of glycals,17 and the related conversion of carbonyl com-
pounds to dithioacetals by polyphosphoric acid sup-
ported on silica gel,18 we were encouraged to
investigate HClO4–silica in carbohydrate chemistry.
We now report a facile, one-pot reaction sequence from
unprotected sugar glycosides, using stoichiometric
reagents, with minimal work-up and purification, that
provides straightforward access to per-O-acetylated
sugar acetals or ketals.
Designing synthetic strategies that minimise waste by
using stoichiometric reagents and catalytic promoters
is central to the principle of atom economy and green
chemistry.1 Our current efforts have been aimed at meet-
ing the demand for various protected sugar building
blocks in a fashion that minimises reaction steps,
work-up and chromatographic purification.2 Selective
protection of 1,2-cis-diols through formation of acetal
or ketal groups is one of the most frequently used reac-
tions in carbohydrate chemistry.3 Generally this reac-
tion is carried out with an aldehyde or ketone in
presence of a Lewis acid promoter.4 To facilitate the
reaction, use of the respective dimethyl acetals5 or ke-
tals,6 or enol-ethers7 are also well known in this area.
The most frequently used catalysts includes sulfuric
acid,8 formic acid,9 CuSO4,10 ZnCl2, 11 p-toluenesulfonic
acid,12 camphorsulfonic acid13 and iodine.14 Acetalation
under basic condition with dibromotoluene15 in the
presence of pyridine has also been used. Most of these
reactions use large excess of reagents, which makes
extensive work-up and chromatographic purification al-
most inevitable. A practical synthetic strategy providing
2. Results and discussion
As reported for the acetylation of simple alcohols16a and
sugars,16b we found treatment of methyl a-D-glucopyr-
anoside (1) with stoichiometric acetic anhydride and
HClO4–silica leads to per-O-acetylated derivative (2) in
excellent yield (Scheme 1). Free reducing sugars and
thioglycosides also undergo per-O-acetylation in very
good to excellent yield. Beyond these obvious applica-
tions of immobilised perchloric acid, treatment of
methyl a-D-glucopyranoside (1) with 1 mol equiv of
benzaldehyde dimethylacetal in dry acetonitrile (2 mL
per mmol of sugar) in the presence of HClO4–silica
*
Corresponding authors. Tel.: +44 01603 592321; fax: +44 01603
592003; e-mail addresses: a.mukhopadhyay@uea.ac.uk; r.a.field@
uea.ac.uk
0008-6215/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2005.02.012

1076
B. Mukhopadhyay et al. / Carbohydrate Research 340 (2005) 1075–1080
2 mol of acetic anhydride to form volatile methyl acetate
OAc
as a byproduct. Similarly methyl b-D-galactopyranoside
(5) led to the corresponding galactose derivative (6) (Ta-
ble 1). This suggests that HClO4–silica is compatible
with the pseudo cis-decalin system (6) as well as the
more robust trans-decalin equivalent (benzylidenated
glucoside 4). Thioglycosides of hexopyranoses also gave
similar results with this reagent system (Table 1). When
methyl a-D-mannopyranoside (7) was treated with equi-
molar benzaldehyde dimethyl acetal, TLC showed a
mixture of compounds containing mono- and di-benzyl-
idene derivatives and starting material in a ca. 1:2:1 ra-
tio. After addition of another equivalent of reagent, the
known bis-benzylidene derivative (8) was obtained in
93% yield. When DMF was used as a solvent instead
of acetonitrile, treatment with 1.1 equiv of a,a-dime-
thoxy toluene resulted 4,6-O-benzylidene derivative (9)
in 73% yield, which was transformed into methyl 2,3-
di-O-acetyl-4,6-O-benzylidene-a-D-mannopyranoside (10)
on treatment with acetic anhydride (Scheme 2).
After successful completion of sequential benzylidene
acetal formation and acetylation in one-pot, we focused
our attention on isopropylidene ketal formation using a
similar strategy (Table 2). Thus, methyl a-L-rhamnopyr-
anoside 19 (1 mmol) was treated with 2,2-dimethoxy
propane (1 mmol) and HClO4 on silica (30 mg) in ace-
tone (2 mL), after 30 min when TLC showed complete
conversion of the starting material to a faster running
O
AcO
AcO
Ac2O
OH
HClO4 on silica
AcO
OMe
2
O
HO
HO
O
PhCH(OMe)2
Ph
HO
OMe
O
O
RO
HClO4 on silica
1
RO
OMe
3 R = H
Ac2O
4 R= Ac
Scheme 1.
(50 mg) leads to complete conversion of the starting
material to the corresponding benzylidene acetal deriva-
tive (3) within 30 min. The resulting product, after filtra-
tion and concentration proved to be >98% pure, as
judged by NMRspectroscopy and mass spectrometry
analysis. After complete formation of benzylidene acetal
(3), as judged by TLC, 4 mol equiv of acetic anhydride
was added to the mixture. Within 30 min at ambient
temperature methyl 2,3-di-O-acetyl-4,6-O-benzylidene-
a-D-glucopyranoside 419 was obtained as essentially a
single compound (Scheme 1) in 91% isolated yield. Use
of 4 mol equiv of acetic anhydride is necessary due to
the fact that 1 mol of benzaldehyde dimethylacetal liber-
ates 2 mol of methanol upon formation of the sugar
benzylidene acetal. The methanol produced reacts with
Table 1. One-pot benzylidenation–acetylation with HClO4 supported on silica gel
Starting material
Product
Time (min)
Isolated yield (%)
Ph
O
O
Methyl b-D-galactopyranoside (5)
60
86
O
OMe
AcO
AcO
6
O
Ph
O
O
OC8H17
NHAc
AcO
Octyl 2-acetamido-2-deoxy-b-D-glucopyranoside (11)
Methyl 1-thio-b-D-glucopyranoside (13)
90
60
60
88
94
83
12
O
Ph
O
O
SMe
AcO
AcO
Ph
14
O
O
Ethyl 1-thio-b-D-galactopyranoside (15)
O
SEt
AcO
AcO
16
OAc
O
Ph
O
O
AcO
Ethyl 1-thio-a-D-mannopyranoside (17)
80
72
18
SEt

B. Mukhopadhyay et al. / Carbohydrate Research 340 (2005) 1075–1080
1077
rhamnopyranoside 2020 was obtained in 92% yield
(Table 2). Similarly, methyl glycosides and thioglyco-
sides of D-galactose, L-arabinose and D-mannose are
converted to the corresponding isopropylidene ketal
derivatives (22–26) without difficulty (Table 2). In a sep-
arate experiment, after formation of the 2,3-O-isopropyl-
idene derivative from thiotolyl rhamnoside (25), chloro-
acetic anhydride was added instead of acetic anhydride
to form the corresponding chloroacetyl derivative (27)
in 88% isolated yield. Benzyl a-L-arabinopyranoside
and benzyl b-D-galactopyranoside also gave satisfactory
results for acetalation–acetylation without cleavage of
the anomeric benzyl group.
O
O
O
Ph
O
PhCH(OMe)2
CH3CN
HClO4 on silica
O
HO
OH
O
OMe
Ph
HO
8
HO
PhCH(OMe)2
DMF
OR
O
O
OMe
Ph
7
O
HClO4 on silica
RO
OMe
R = H
9
Ac2O
10 R = Ac
Scheme 2.
component (presumably the corresponding isopropylid-
ene ketal), acetic anhydride (3 mmol) was added and the
mixture was stirred for another 30 min. After filtration
through Celite and evaporation of the solvent, the
expected methyl 4-O-acetyl-2,3-O-isopropylidene-a-L-
In conclusion, HClO4–silica is a useful reagent for for-
mation of acetals/ketals coupled with per-O-acetylation
giving a range of variously protected sugar building
blocks. The ease of application of this reagent, the use
of stoichiometric reagents, and minimal work-up and
Table 2. One-pot isopropylidenation–acetylation with HClO4 supported on silica gel
Starting material
Product
Time (min)
60
Isolated yield (%)
OMe
Me
O
Methyl a-L-rhamnopyranoside (19)
92
AcO
O
O
20
OAc
O
O
SMe
Methyl-1-thio-b-D-galactopyranoside (21)
p-Tolyl 1-thio-a-L-arabinopyranoside (23)
p-Tolyl 1-thio a-L-rhamnopyranoside (25)
60
45
50
87
87
92
O
AcO
22
O
O
STol
O
AcO
24
STol
Me
AcO
O
O
O
26
STol
Me
RO
O
O
p-Tolyl 1-thio a-L-rhamnopyranoside (25)
Benzyl -L-arabinopyranoside (28)
Benzyl b-D-galactopyranoside (30)
60
45
60
88
84
80
O
R = ClCH2CO
27
O
O
O
AcO
OBn
29
OAc
O
O
OBn
O
AcO
31

1078
B. Mukhopadhyay et al. / Carbohydrate Research 340 (2005) 1075–1080
purification serves to illustrate the versatility of this
practical reagent system.
derivatives 2 mmol) in acetonitrile (2 mL), HClO4–silica
(50 mg) was added and the mixture was stirred at room
temperature until TLC (1:1 hexane–ethyl acetate)
showed complete conversion. Ac2O (4 mmol) was added
to the mixture and stirring was continued until TLC
showed complete reaction. The mixture was then neutra-
3. Experimental
1
3.1. General methods
lised with Et3N and filtered through Celite. H and 13C
NMRspectroscopy of the crude products showed them
to be pure, without the need for further purification.
Methyl glycosides of glucose, galactose and mannose
were purchased from Aldrich Chemical Company.
Compounds 11,21 13,22 15,23 17,24 19,25 21,26 23,27 2828
and 3029 were prepared according to the literature pro-
cedures. Compound 21 has been prepared in a similar
manner to that reported for the corresponding thio-
methyl glycoside.30 1H and 13C NMRspectra were re-
corded on a Varian Unity Plus spectrometer at 400
and 100 MHz, respectively, using Me4Si as an internal
standard. Optical rotations were measured at the Na
D-line with a Perkin Elmer 141 polarimeter. Com-
3.5. General procedure for formation of isopropylidene
acetals
To a solution of the starting material (1 mmol) and 2,2-
dimethoxypropane (2 mmol for hexopyranoses or
1 mmol for deoxy-hexopyranoses and pentopyranoses)
in acetone (2 mL), HClO4–silica (50 mg) was added
and the mixture was stirred at room temperature until
TLC (1:1 hexane–ethyl acetate) showed complete con-
version. Ac2O (4 mmol for hexopyranoses or 3 mmol
for deoxy-hexopyranoses and pentopyranoses) was
added and the mixture was stirred until TLC showed
complete conversion. The product was then isolated as
outlined above. No further purification was needed as
1H and 13C NMRspectroscopy confirmed the purity
of the compounds.
1
pounds 4,19 6,31 10,32 16,33 20,20 2234 and 3135 gave H
and 13C NMRdata and optical rotations as reported
in the literature. Experimental data for the other prod-
ucts are provided below.
3.2. Preparation of HClO4 supported on silica
Immobilised perchloric acid on silica was prepared
essentially as described previously,16 except it was dried
for 2 h at 110 °C instead of 6 h. HClO4 (0.3 mmol, as a
70% aqueous solution) was added to a slurry of silica gel
(5 g, 200 mesh) in Et2O (15 mL) and the solvent was re-
moved under reduced pressure. The resulting powder
was kept at 110 °C for 2 h and used directly in reactions.
WARNING: Although there is no specific report
about the danger of perchloric acid, care should be
taken when handling this potentially explosive material.
All reactions have been done in an efficient fumehood
and special caution was taken during the drying of the
HClO4–silica.
3.6. Octyl 2-acetamido-3-O-acetyl-4,6-O-benzylidene-2-
deoxy-b-D-glucopyranoside (12)
25
D
1
½aꢀ +49.0 (c 1.0, CHCl3); H NMR(CDCl ): dH, 7.43–
3
7.32 (m, 5H, ArH), 5.76 (d, 1H, J2,NH 9.6 Hz,
NHCOCH3), 5.49 (s, 1H, CHPh), 5.25 (t, 1H, J2,3, J3,4
10.0 Hz, H-3), 4.45 (d, 1H, J1,2 8.4 Hz, H-1), 4.31 (dd,
1H, J5,6 4.8 Hz, J6a,6b 10.4 Hz, H-6a), 4.05 (m, 1H, H-
2), 3.80–3.73 (m, 2H, H-6b, OCH2), 3.69 (t, 1H, J3,4
,
J4,5 H-4), 3.52 (m, 1H, OCH2), 3.36 (m, 1H, H-5), 2.07
(s, 3H, COCH3), 1.95 (s, 3H, NHCOCH3), 1.51 (m,
2H, octyl-CH2), 1.25 (m, 10H, octyl-CH2), 0.87 (t, 3H,
octyl-CH3); 13C NMR(CDCl 3): dC, 171.2 (COCH3),
170.2 (NHCOCH3), 137.1, 129.2, 128.3, 126.2 (ArC),
102.1 (CHPh), 101.4 (C-1), 78.7, 72.0, 70.1, 68.7, 66.4,
54.6, 31.7, 29.4, 29.2, 29.1, 25.7, 23.2, 22.5 (NHCOCH3),
20.8 (COCH3), 13.9; HRMS m/z calcd for
[C25H38NO7]H+: 464.2643. Found: 464.2643.
3.3. General procedure for per-O-acetylation
To a mixture of the sugar (1 mmol) and Ac2O, HClO4–
silica (50 mg) was added and the mixture was stirred at
room temperature. When the reaction was complete as
judged by TLC, the mixture was neutralised with
Et3N, diluted with CH2Cl2 (10 mL) and filtered through
Celite. The filtrate was evaporated and the residue was
analysed by 1H and 13C NMRspectroscopy and by
mass spectrometry.
3.7. Methyl 2,3-di-O-acetyl-4,6-O-benzylidene-1-thio-b-
D-glucopyranoside (14)
25
½aꢀ ꢁ68.0 (c 1.0, CHCl3); 1H NMR(CDCl 3): dH, 7.44–
D
7.34 (m, 5H, ArH), 5.51 (s, 1H, CHPh), 5.36 (t, 1H, J1,2
,
3.4. General procedure for formation of benzylidene
acetals
J2,3 9.3 Hz, H-2), 5.09 (t, 1H, J2,3, J3,4 9.3 Hz, H-3), 4.49
(d, 1H, J1,2 H-1), 4.40 (dd, 1H, J5,6 4.8 Hz, J6a,6b
10.5 Hz, H-6a), 3.78 (dd, 1H, H-6b), 3.69 (t, 1H, J3,4
H-4), 3.62 (m, 1H, H-5), 2.19 (s, 3H, S–CH3), 2.09 (s,
3H, COCH3), 2.06 (s, 3H, COCH3); 13C NMR(CDCl 3):
To a solution of the starting material (1 mmol) and
benzaldehyde dimethylacetal (1 mmol, for mannose

B. Mukhopadhyay et al. / Carbohydrate Research 340 (2005) 1075–1080
1079
dC, 170.1, 169.8 (2COCH3), 136.8, 128.3, 126.2, 125.3
(ArC), 101.5 (CHPh), 83.6 (C-1), 78.4, 72.7, 70.7, 69.8,
68.4, 20.6, 20.5 (2COCH3), 11.4 (S–CH3); HRMS m/z
(dd, 1H, J3,4 8.0 Hz, J4,5 10.0 Hz, H-4), 4.35 (d, 1H,
J2,3 5.2 Hz, H-2), 4.26–4.20 (m, 2H, H-3, H-5), 4.10 (s,
2H, COCH2Cl), 2.32 (s, 3H, SC6H4CH3), 1.56, 1.34
(2s, 6H, isopropylidene CH3), 1.13 (d, 1H, J5,6 6.0 Hz,
C–CH3); 13C NMR(CDCl 3): dC, 166.7 (COCH3),
138.2, 132.6, 130.0, 129.1 (ArC), 110.2 [C(CH3)2], 83.9
(C-1), 76.7, 76.6, 75.1, 64.9, 40.7 (COCH2Cl), 27.5,
+
calcd for [C18H26NO7S]NH4
400.1423.
:
400.1425. Found:
3.8. Ethyl 2,3-di-O-acetyl-4,6-O-benzylidene-1-thio-a-D-
mannopyranoside (18)
26.3 (isopropylidene CH3), 20.9 (S–C6H4–CH3), 16.6
+
(C–CH3); HRMS m/z calcd for [C18H27NClO5S]NH4
404.1293. Found: 404.1296.
:
25
D
½aꢀ +25.0 (c 1.1, CHCl3); 1H NMR(CDCl ): dH, 7.50–
3
7.35 (m, 5H, ArH), 5.60 (s, 1H, CHPh), 5.46 (d, 1H, J2,3
3.3 Hz, H-2), 5.36 (dd, 1H, J2,3, J3,4 10.5 Hz, H-3), 4.38–
4.24 (m, 2H, H-6a, H-6b), 4.11 (t, 1H, J3,4, J4,5 10.5 Hz,
H-4), 3.89 (m, 1H, H-5), 2.64 (m, 2H, S–CH2–CH3),
2.18 (s, 3H, COCH3), 2.01 (s, 3H, COCH3), 1.30 (t,
3.12. Benzyl 2-O-acetyl-3,4-O-isopropylidene-b-L-arabi-
nopyranoside (29)
25
D
½aꢀ ꢁ43.0 (c 1.1, CHCl3); 1H NMR(CDCl ): dH, 7.34–
3
3H, S–CH2–CH3); 13C NMR(CDCl ): dC, 169.8, 169.7
3
7.25 (m, 5H, ArH), 4.97 (d, 1H, J1,2 3.3 Hz, H-1), 4.90
(dd, 1H, J1,2, J2,3 8.1 Hz, H-2), 4.71, 4.48 (2d, 2H, J
12.3 Hz, CH2Ph), 4.36 (dd, 1H, J2,3, J3,4 5.7 Hz, H-3),
4.24 (m, 1H, H-4), 4.00 (m, 2H, H-5a, H-5b), 2.06 (s,
3H, COCH3), 1.52, 1.34 (2s, 6H, isopropylidene CH3);
(2COCH3), 137.0, 128.2, 126.1, 125.2 (ArC), 101.8
(CHPh), 83.0 (C-1), 76.6, 71.5, 68.4, 68.3, 64.3, 25.1
(S–CH2–CH3), 20.5, 20.4 (2COCH3), 14.5 (S–CH2–
+
CH3); HRMS m/z calcd for [C19H28NO7S]NH4
414.1581. Found: 414.1580.
:
13C NMR(CDCl ): dC, 170.5 (COCH3), 137.2, 128.5,
3
127.9, 127.6 (ArC), 109.3 [C(CH3)2], 95.2 (C-1), 73.4,
72.8, 72.1, 69.4, 58.7, 27.8, 26.2 (isopropylidene CH3),
3.9. p-Tolyl 2-O-acetyl-3,4-O-isopropylidene-1-thio-a-L-
arabinopyranoside (24)
+
20.7 (COCH3); HRMS m/z calcd for [C17H26NO6]NH4
340.1755. Found: 340.1756.
:
25
D
1
½aꢀ ꢁ21.0 (c 1.2, CHCl3); H NMR(CDCl ): dH, 7.35,
3
7.07 (2d, 4H, J 8.4 Hz, ArAB), 5.09 (dd, 1H, J1,2 7.6 Hz,
J2,3 6.4 Hz, H-2), 4.72 (d, 1H, J1,2 H-1), 4.27–4.20 (m,
2H, H-3, H-5a), 4.14 (m, 1H, H-4), 3.74 (m, 1H, H-
5b), 2.29 (s, 3H, S–C6H4–CH3), 2.10 (s, 3H, COCH3),
1.53, 1.33 (2s, 6H, isopropylidene-CH3); 13C NMR
(CDCl3): dC, 169.5 (COCH3), 137.8, 132.4, 130.1,
129.6 (ArC), 110.3 [C(CH3)2], 86.1 (C-1), 75.5, 72.0,
71.1, 64.0, 27.5, 26.1 (isopropylidene CH3), 20.9 (S–
C6H4–CH3), 20.8 (COCH3); HRMS m/z calcd for
[C17H26NO5S]NH4+: 356.1526. Found: 356.1529.
Acknowledgements
We thank Dr. Alan H. Haines for valuable discussions
during the course of this work. This work was supported
by EPSRC, UK. We gratefully acknowledge the EPSRC
Mass Spectrometry Service Centre, University of Wales,
Swansea, for invaluable support.
Supplementary data
3.10. p-Tolyl 4-O-acetyl-2,3-O-isopropylidene-1-thio-a-L-
rhamnopyranoside (26)
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.carres.
2005.02.012.
25
D
½aꢀ ꢁ153.0 (c 0.9, CHCl3); 1H NMR(CDCl 3): dH, 7.35,
7.12 (2d, 4H, J 8.0 Hz, ArAB), 5.68 (s, 1H, H-1), 4.92 (t,
1H, J3,4, J4,5 9.0 Hz, H-4), 4.34 (d, 1H, J2,3 5.4 Hz, H-2),
4.23–4.16 (m, 2H, H-3, H-5), 2.32 (s, 3H, S–C6H4–CH3),
2.10 (s, 3H, COCH3), 1.11 (d, 3H, J5,6 6.0 Hz, H-6); 13
C
References
NMR(CDCl 3): dC, 170.14 (COCH3), 138.0, 132.5, 129.9,
129.4 (ArC), 109.9 [C(CH3)2], 83.9 (C-1), 76.6, 75.4, 74.5,
65.3, 27.5, 26.4 (isopropylidene CH3), 20.9 (S–C6H4–
CH3), 20.8 (COCH3), 16.6 (C–CH3); HRMS m/z calcd
for [C18H28NO5S]NH4+: 370.1683. Found: 370.1683.
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