Facile and regioselective preparation of partly O-benzylated d-glucopyranose acetates via acid-mediated simultaneous debenzylation-acetolysis

Article abstract of DOI:10.1016/j.carres.2006.05.018

Fully O-benzylated methyl α-d-glucopyranoside shows a steady order in stepwise debenzylation when it is treated with sulfuric acid in acetic anhydride. Based on the order of debenzylation, regioselective preparations of 2,3,4-tri-, 2,3-, 2,4-, 3,4-di-, an

Full text of DOI:10.1016/j.carres.2006.05.018

Carbohydrate Research 341 (2006) 2219–2223
Facile and regioselective preparation of partly O-benzylated
D-glucopyranose acetates via acid-mediated simultaneous
debenzylation–acetolysis
Yang Cao, Yasunori Okada and Hidetoshi Yamada*
School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda 669-1337, Japan
Received 22 April 2006; received in revised form 22 May 2006; accepted 23 May 2006
Available online 27 June 2006
Abstract—Fully O-benzylated methyl a-D-glucopyranoside shows a steady order in stepwise debenzylation when it is treated with
sulfuric acid in acetic anhydride. Based on the order of debenzylation, regioselective preparations of 2,3,4-tri-, 2,3-, 2,4-, 3,4-di-, and
2-O-benzyl-D-glucopyranose acetates were facilitated in greater than 80% yields. The key points of the preparative reactions were the
control of the acid strength and choice of suitable substrates.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Selective protection; Debenzylation; Acetolysis; Glucopyranose
1. Introduction
important part of the preparation of partly benzylated
sugars.8
The benzyl group is one of the most useful protecting
groups in synthetic organic chemistry. Its protecting
ability has been verified in numerous syntheses, and a
variety of methods for its installation and removal have
been developed.1 Benzyl-protected sugars are also signi-
ficant for the syntheses of oligosaccharides and natural
products.2
In our previous paper, we revealed that the benzyl
groups of methyl 2,3,4,6-tetra-O-benzyl-a-D-glucopyran-
oside (1) were cleaved in the order of 6-O-Bn > 3-O-
Bn > 4-O-Bn > 2-O-Bn under acid-mediated conditions
(Scheme 1).9 Thus, when 1 is acetolyzed in acetic anhy-
dride containing 1% (v/v) sulfuric acid, the anomeric
Although full benzylation of carbohydrate hydroxy
groups is generally effortless,3 regioselective partial benz-
ylation requires ingenious strategies. For example, the
following methods have been developed, which include
the regioselective reductive ring-opening of the 4,6- or
1,2-O-benzylidene group,4 metal complex (e.g., tin and
copper) mediated benzylation,5,6 and indirect methods
that include the regioselective introduction of other pro-
tecting group(s), followed by benzyl protection of the
remaining hydroxy group(s).7 In addition, the regio-
selective debenzylation of fully benzylated sugars is an
OBn
OAc
O
6
O
BnO
BnO
BnO
BnO
1
BnO
BnO OAc
OMe
1
2
OAc
O
BnO
AcO
3
BnO OAc
3
OAc
O
OAc
O
4
AcO
AcO
AcO
AcO
BnO OAc
AcO OAc
4
5
Scheme 1. The debenzylation–acetolysis sequence of 1.9 Reagents and
*
Corresponding author. Tel.: +81 79 565 8342; fax: +81 79 565
conditions: Ac2O, 1% (v/v) H2SO4, rt.
9077; e-mail: hidetosh@kwansei.ac.jp
0008-6215/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2006.05.018

2220
Y. Cao et al. / Carbohydrate Research 341 (2006) 2219–2223
methyl, and 6-O-benzyl groups are first replaced by acet-
yl groups, and then the 3-, 4-, and 2-O-benzyl groups are
also replaced in this order to afford the 2,3,4-tri-O-ben-
zyl 2, the 2,4-di-O-benzyl 3, the 2-O-benzyl 4, and then
eventually the glucose pentaacetate 5.
However, the reaction rate of this debenzylation is too
fast to stop at any specific stage to obtain only one de-
sired product. In our laboratory, mixtures were always
produced and the separation process was troublesome,
while products 2, 3, and 4 are the synthetically useful
intermediates that have been used as building blocks
for more complex compounds and natural products.10
By controlling the reaction speed, therefore, this order
of debenzylation should be applicable for the prepara-
tion of the partly benzylated sugars.
TsOH. Although the sequence (Scheme 1) showed that
dibenzylated 3 is a subsequent product of 2, we could
not determine optimum conditions that would produce
3 as the sole product starting from 1. Therefore, we
changed the starting material to 6 (Scheme 3), which
can be prepared from methyl a-D-glucopyranoside,
sodium hydride, and benzyl chloride without solvents
(benzyl chloride assumes the role of the solvent) in
62% yield along with 1.11 Starting from 6, the TsOH-
mediated acetolysis conditions afforded 3 in 94% yield
(Scheme 2).8c The reaction should start at room temper-
ature with 0.3 equiv of TsOH to acetylate the free
hydroxy group, then 1.2 equiv of TsOH was further
added, and the reaction mixture was heated at 70 ꢁC
for 2 h to complete the replacement of the 6-O-benzyl
group.
The suitable choice of substrates with control of
the acid strength facilitated the preparation of other
partly benzylated glucose derivatives, the 2,3-di-O-benz-
yl 7 and the 2-O-benzyl 4 (Scheme 4). Thus, starting
from the 4,6-O-benzylidene substrate 8,12 7 was pre-
pared in 95% yield when 2 equiv of TsOH were used
in acetic anhydride at 70 ꢁC for 3 h. In addition, more
vigorous reaction conditions afforded the 2-O-benzyl
product 4 according to the order of debenzylation.
Thus, the use of sulfuric acid instead of TsOH
completed this selective debenzylation in 85% yield.
Both reactions should be started at room temperature
for first removing the O-benzylidene group.
To control this stepwise reaction, we adjusted the
strength of the used acid. We disclose here that the suit-
able choice of both substrates and the strength of the
acid facilitated the synthesis of five kinds of partly benz-
ylated D-glucopyranose acetates in greater than 80%
yield.
2. Results and discussion
First, we optimized the reaction conditions to produce 2
from 1 using p-toluenesulfonic acid (TsOH, Scheme 2),
which is a weaker acid than sulfuric acid. When 1 was
treated with TsOH in acetic anhydride, no distinct reac-
tion occurred at room temperature in 5 h. However, at
an elevated temperature, the anomeric methyl and the
6-O-benzyl groups began to be replaced to afford 2.
When the amount of TsOH was less than 1 equiv, the
reaction was not completed within 15 h. On the other
hand, the excessive use of the acid induced a further
acetolysis. As a result, the suitable quantity of TsOH
was found to be 1.3 equiv, and the reaction was comple-
ted in 2 h at 70 ꢁC. Under these conditions, the isolated
yield of 2 was 92%,12 and thus the use of the proper
amount of TsOH and choice of the appropriate reaction
temperature would allow controlling such simultaneous
debenzylation–acetolysis sequences. Similarly, the use of
(1S)-camphor-10-sulfuric acid in acetic anhydride in-
stead of TsOH also afforded 2 in 92% yield. However,
we chose to use TsOH for our later investigation due
to its lower cost.
The selection of a 1,2-acetylidene compound 9 as the
substrate facilitated the preparation of the 3,4-di-O-benz-
ylated 10 (Scheme 5).13 This protocol required three
steps, but all operations could be completed in one
TsOH (0.3 equiv)
Ac2O, rt, 2 h
OBn
O
OAc
O
BnO
HO
BnO
AcO
then addition of
TsOH (1.2 equiv)
70 °C, 2 h
BnO
BnO OAc
OMe
6
3, 94%
Scheme 3. Preparation of 3.
OAc
O
TsOH (0.5 equiv)
Ac2O, rt, 2 h
AcO
BnO
Next, the preparation of 2,4-di-O-benzyl 3 was carried
out by the debenzylation–acetolysis sequence using
then addition of
TsOH (1.5 equiv)
70 °C, 2 h
BnO OAc
Ph
O
O
7, 95%
O
BnO
BnO
OMe
OAc
O
H2SO4 (1.5 equiv)
Ac2O, rt, 2 h
OBn
O
OAc
O
6
8
TsOH (1.3 equiv)
AcO
AcO
BnO
BnO
BnO
BnO
1
then addition of
H2SO4 (0.5 equiv)
40 °C, 5 h
Ac2O, 70 °C, 2 h
BnO OAc
BnO
BnO OAc
OMe
4, 85%
1
2, 92%
Scheme 2. Preparation of 2.
Scheme 4. Preparation of 7 and 4 from 8.

Y. Cao et al. / Carbohydrate Research 341 (2006) 2219–2223
2221
chromatography (precoated Silica Gel 60, F-254). After
each reaction was quenched, the reaction mixture was
successively extracted twice with EtOAc, and washed
with satd aq NaHCO3 and brine, dried over MgSO4,
then concentrated under reduced pressure. The resulting
residue was purified by column chromatography using
Silica Gel 60 (70–230 mesh, 30 g) unless otherwise sta-
ted. The eluent for each chromatography is shown in
each section. The a/b ratios of the products were deter-
1. TsOH, Ac2O
2. TsOH, MeOH
OBn
O
OAc
O
4
BnO
BnO
BnO
BnO
3. Ac2O, pyridine
3
O
AcO OAc
O
9
10, 82%
TsOH (2.5 equiv)
Ac2O, 60 °C, 3 h
Ac2O, pyridine
rt, 4 h
OAc
OAc
O
O
1
BnO
BnO
BnO
1
mined by the integral of the H-1 peak in the H NMR
BnO
AcO
AcO OH
+
spectra, and part of the mixtures was separated by
high-performance liquid chromatography (HPLC) for
characterization. HPLC was performed using a normal
phase column (4.6 · 250 mm) with detection by UV at
254 nm.
O
OAc
11a
12a
TsOH, MeOH
rt, 30 min
+
OAc
OAc
O
O
BnO
BnO
BnO
2
BnO
O
OAc
HO OAc
All new compounds were determined to be >95% pure
by HPLC or 1H NMR spectroscopy. The melting points
are uncorrected. The infrared (IR) spectra are reported
in wavenumbers (cmꢀ1). The high-resolution mass spec-
tra (HRMS) were recorded using electrospray-ioniza-
tion (ESI) and are reported in units of mass to charge.
The nuclear magnetic resonance (NMR) spectra were
recorded in CDCl3 at 400 and 100 MHz for the 1H
and 13C NMRs, respectively. The 13C NMR chemical
shifts are reported relative to the central line of the trip-
let for CDCl3 at 77.0 ppm. For the notations regarding
the NMR data, refer to the general methods and mate-
rials section in our previous report.15
11b
12b
OAc
Scheme 5. Preparation of 10 and its stepwise diagram.
pot. A stepwise description of the preparation is as fol-
lows. The acetolysis of 9 first produced both the 1- and
2-O-(10-acetoxy)ethyl ethers 11a and 11b.14 After re-
moval of the acetic anhydride, the addition of methanol
liberated the hydroxy groups at the 1- or 2-position to
give a mixture of 12a and 12b. Then the removal of
methanol, followed by the addition of acetic anhydride
and pyridine, afforded 10 in 82% overall yield.
4.2. 1,6-Di-O-acetyl-2,3,4-tri-O-benzyl-a- and b-D-gluco-
pyranose (2)
3. Conclusions
To a solution of methyl 2,3,4,6-tetra-O-benzyl-a-D-
glucopyranoside (1)4 (390 mg, 0.717 mmol) in Ac2O
(4 mL) was added TsOHÆH2O (170 mg, 0.895 mmol),
and the mixture was stirred for 2 h at 70 ꢁC. The reac-
tion mixture was then poured into ice water and sub-
jected to the routine workup–purification sequence
with 10:1 hexane–EtOAc as the eluent for chromato-
graphy to afford an anomeric mixture of 2 (347 mg,
92%, a:b = 1:0.3) as a colorless syrup. The NMR
spectra of the compound were identical to those
reported.9,16
Control of the acid strength made the simultaneous de-
benzylation–acetolysis sequence (Scheme 1) possible for
the practical preparation of partly benzylated D-glucose
derivatives. The combination of the order of debenzyl-
ation and choice of suitable substrates facilitated the
synthesis of five kinds of partly O-benzylated D-gluco-
pyranose acetates, that are the 2,3,4-tri-, 2,3-, 2,4-, 3,4-
di-, and 2-O-benzyl compounds, in greater than 80%
yields. Because the reagents used for these transforma-
tions are very popular, and the procedures are simple,
the debenzylation–acetolysis techniques should be a
solution to preparing the partly benzylated derivatives
of sugars that can be the building blocks, not only for
more complex sugar compounds, but also for optically
active natural products.
4.3. 1,3,6-Tri-O-acetyl-2,4-di-O-benzyl-a- and b-D-gluco-
pyranose (3)
To a solution of methyl 2,4,6-tri-O-benzyl-a-D-gluco-
pyranoside (6)11 (310 mg, 0.668 mmol) in Ac2O (4 mL)
was added TsOHÆH2O (38 mg, 0.20 mmol), and the mix-
ture was stirred for 2 h at room temperature. Then TsOHÆ
H2O was further added (152 mg, 0.791 mmol), and
the mixture was stirred for 2 h at 70 ꢁC. The reaction
mixture was then poured into ice water and subjected
to the routine workup–purification sequence with 8:1
hexane–EtOAc as the eluent for chromatography to
4. Experimental
4.1. General methods
The sulfuric acid solution in Ac2O was prepared just
prior to use. The reactions were checked by thin-layer

2222
Y. Cao et al. / Carbohydrate Research 341 (2006) 2219–2223
afford an anomeric mixture of 3 (307 mg, 94%,
a:b = 1:0.3) as a colorless syrup. The NMR spectra of
the compound were identical to those reported.9,17
4.6. 1,2,6-Tri-O-acetyl-3,4-di-O-benzyl-a- and b-D-gluco-
pyranose (10)
To a solution of 3,4,6-tri-O-benzyl-1,2-O-ethylidene-a-
D-glucopyranose (9)13 (250 mg, 0.525 mmol) in Ac2O
(4 mL) was added TsOHÆH2O (250 mg, 1.31 mmol).
The mixture was stirred for 3 h at 60 ꢁC, and the
Ac2O was removed under vacuum. After cooling the
residue in an ice-water bath, cold MeOH (15 mL) was
added dropwise, and the solution was stirred at room
temperature for 30 min. After evaporation of MeOH
from the mixture, Ac2O (1 mL) and pyridine (1.5 mL)
were added. The mixture was stirred at room tempera-
ture for 4 h. Ac2O and pyridine were then removed
under vacuum, and 1 N HCl was added. The mixture
was subjected to the routine workup–purification sequence
with 8:1 hexane–EtOAc as the eluent for chromato-
graphy to afford an anomeric mixture of 10 (211 mg,
82%, a:b = 1:0.56) as a colorless syrup. HPLC condi-
tions: 6.5:1 hexane–EtOAc, 3.0 mL/min, tR 11.3 min
for the a isomer (10a), tR 12.8 min for the b isomer
4.4. 1,4,6-Tri-O-acetyl-2,3-di-O-benzyl-a- and b-D-gluco-
pyranose (7)
To a solution of methyl 2,3-di-O-benzyl-4,6-O-benzyl-
idene-a-D-glucopyranoside (8)12 (268 mg, 0.580 mmol)
in Ac2O (4 mL) was added TsOHÆH2O (55 mg,
0.29 mmol), and the mixture was stirred for 2 h at room
temperature. Then, TsOHÆH2O was further added
(165 mg, 0.868 mmol), and the mixture was stirred for
2 h at 70 ꢁC. The reaction mixture was then poured into
ice water and subjected to the routine workup–purifica-
tion sequence with 8:1 hexane–EtOAc as the eluent for
chromatography to afford an anomeric mixture of 9
(269 mg, 95%, a:b = 1:0.3) as a colorless syrup. HPLC
conditions: 6.5:1 hexane–EtOAc, 3.0 mL/min; tR
14.6 min for the a isomer (7a), tR 15.7 min for the b iso-
1
mer (7b). The H NMR spectra of 7a were identical to
26
24
those reported.18 Data for 7b: colorless syrup; ½aꢁD
(10b). Data for 10a: colorless syrup; ½aꢁD +79.7 (c
ꢀ3.3 (c 0.52, CHCl3); IR (thin film) mmax (cmꢀ1): 3032,
0.35, CHCl3); IR (thin film) mmax (cmꢀ1): 3032, 2922,
1747, 1454, 1369, 1238, 1051, 739, 700; 1H NMR d:
7.29–7.17 (m, 10H, Ar–H), 6.17 (d, JH-1–H-2 3.6 Hz,
1H, H-1), 4.97 (dd, JH-1–H-2 3.6 Hz, JH-2–H-3 10.0 Hz,
1H, H-2), 4.79 (d, J 10.7 Hz, 1H, Bn), 4.77 (d, J
11.2 Hz, 1H, Bn), 4.70 (d, J 11.2 Hz, 1H, Bn), 4.52 (d,
J 10.7 Hz, 1H, Bn), 4.23 (dd, JH-5–H-6a 2.4 Hz, JH-6a–H-6b
12.0 Hz, 1H, H-6a), 4.18 (dd, JH-5–H-6b 3.6 Hz, JH-6a–H-6b
12.0 Hz, 1H, H-6b), 3.94 (dd, JH-2–H-3 10.0 Hz, JH-3–H-4
9.0 Hz, 1H, H-3), 3.89 (ddd, JH-4–H-5 10.0 Hz, JH-5–H-6a
2.4 Hz, JH-5–H-6b 3.6 Hz, 1H, H-5), 3.58 (dd, JH-3–H-4
9.0 Hz, JH-4–H-5 10.0 Hz, 1H, H-4), 2.05 (s, 3H, Ac),
1.97 (s, 3H, Ac), 1.90 (s, 3H, Ac); 13C NMR d: 170.6
(s), 169.9 (s), 168.9 (s), 138.1 (s), 137.3 (s), 128.6 (d),
128.5 (d), 128.2 (d), 128.1 (d), 127.8 (d), 127.5 (d), 89.6
(d), 79.9 (d), 76.6 (d), 75.5 (t), 75.3 (t), 71.7 (d), 71.3
(d), 62.3 (t), 20.9 (q), 20.8 (q), 20.6 (q); HRESIMS
(m/z) [M+Na+]: calcd for C26H30O9, 509.1788; found,
1
2920, 1748, 1454, 1368, 1222, 1055, 748, 700; H NMR
d: 7.29–7.17 (m, 10H, Ar–H), 5.56 (d, JH-1–H-2 7.6 Hz,
1H, H-1), 5.00 (dd, JH-3–H-4 9.3 Hz, JH-4–H-5 9.5 Hz,
1H, H-4), 4.75 (d, J 11.5 Hz, 1H, Bn), 4.71 (J 11.2 Hz,
1H, Bn), 4.67 (J 11.2 Hz, 1H, Bn), 4.57 (d, J 11.5 Hz,
1H, Bn), 4.17 (dd, JH-5–H-6a 4.8 Hz, JH-6a–H-6b 12.4 Hz,
1H, H-6a), 3.98 (dd, JH-5–H-6b 2.4 Hz, JH-6a–H-6b
12.4 Hz, 1H, H-6b), 3.62 (dd, JH-2–H-3 9.0 Hz, JH-3–H-4
9.3 Hz, 1H, H-3), 3.60 (m, 1H, H-5), 3.57 (dd, JH-1–H-2
7.6 Hz, JH-2–H-3 9.0 Hz, 1H, H-2), 2.00 (s, 6H, Ac),
1.86 (s, 3H, Ac); 13C NMR d: 170.8 (s), 169.5 (s),
169.1 (s), 137.9 (s), 137.8 (s), 128.5 (d), 128.4 (d), 127.9
(d), 127.9 (d), 127.8 (d), 93.7 (d), 81.9 (d), 80.8 (d),
75.3 (t), 75.2 (t), 72.8 (d), 69.1 (d), 61.9 (t), 21.0 (q),
20.7 (q), 20.7 (q); HRESIMS (m/z) [M+Na+]: calcd
for C26H30O9, 509.1788; found 509.1805.
23
4.5. 1,3,4,6-Tetra-O-acetyl-2-O-benzyl-a- and b-D-gluco-
pyranose (4)
509.1770. Data for 10b: colorless syrup; ½aꢁD +30.0 (c
0.22, CHCl3); IR (thin film) mmax (cmꢀ1): 3032, 2916,
1757, 1454, 1367, 1232, 1059, 752, 700; 1H NMR
d: 7.28–7.18 (m, 10H, Ar–H), 5.55 (d, JH-1–H-2 8.3 Hz,
1H, H-1), 5.03 (dd, JH-1–H-2 8.3 Hz, JH-2–H-3 9.0 Hz,
1H, H-2), 4.75 (d, J 11.0 Hz, 1H, Bn), 4.74 (d, J
11.5 Hz, 1H, Bn), 4.62 (d, J 11.5 Hz, 1H, Bn), 4.50 (d,
J 11.0 Hz, 1H, Bn), 4.24 (d, JH-6a–H-6b 12.0 Hz, 1H,
H-6a), 4.15 (dd, JH-5–H-6b 3.0 Hz, JH-6a–H-6b 12.0 Hz,
1H, H-6b), 3.67 (dd, JH-2–H-3 9.0 Hz, JH-3–H-4 9.0 Hz,
1H, H-3), 3.69–3.58 (m, 2H, H-4 and H-5), 2.01 (s,
3H, Ac), 1.96 (s, 3H, Ac), 1.87 (s, 3H, Ac); 13C NMR
d: 170.6 (s), 169.4 (s), 169.2 (s), 137.7 (s), 137.2
(s), 128.5 (d), 128.5 (d), 128.1 (d), 128.1 (d), 127.9
(d), 127.8 (d), 91.9 (d), 82.8 (d), 76.9 (d), 75.3 (t), 75.0
(t), 73.8 (d), 71.9 (d), 62.4 (t), 20.8 (q), 20.8 (q), 20.7
To a solution of 812 (304 mg, 0.658 mmol) in Ac2O
(3 mL) was dropwise added 2% H2SO4 in Ac2O (v/v)
(2.65 mL, 1.0 mmol as H2SO4), and the mixture was
stirred for 1 h at room temperature. Then further 2%
H2SO4 solution was dropwise added (0.90 mL, 0.34
mmol as H2SO4), and the mixture was stirred for 5 h
at 40 ꢁC. The reaction mixture was then poured into
ice water and subjected to the routine workup–purifica-
tion sequence with 6:1 hexane–EtOAc as the eluent for
chromatography to afford an anomeric mixture of 4
(244 mg, 85%, a:b = 1:0.25) as a colorless syrup. The
NMR spectra of the compound were identical to those
reported.9,19

Y. Cao et al. / Carbohydrate Research 341 (2006) 2219–2223
2223
(q); HRESIMS (m/z) [M+Na+]: calcd for C26H30O9,
509.1788; found, 509.1771.
6. (a) Eby, R.; Schuerch, C. Carbohydr. Res. 1982, 100, C41–
C43; (b) Eby, R.; Webster, K. T.; Schuerch, C. Carbohydr.
Res. 1984, 129, 111–120.
7. (a) Kuster, J. M.; Dyong, I. Liebigs Ann. Chem. 1975,
¨
2179–2189; (b) Koto, S.; Morishima, N.; Yoshida, T. Bull.
Chem. Soc. Jpn. 1983, 56, 1171–1175.
Acknowledgments
8. (a) Shah, R. N.; Baptista, J.; Perdomo, G. R.; Carver, J.
P.; Krepinsky, J. J. J. Carbohydr. Chem. 1987, 6, 645–660;
(b) Eby, R.; Sondheimer, S. J.; Schuerch, C. Carbohydr.
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This work was partly supported by a Grant-in-Aid
for Scientific Research (16510170) from JSPS and for
Scientific Research on Priority Areas (17035086) from
MEXT.
Rucker, E. Liebigs Ann. Chem. 1989, 423–428; (d) Zhang,
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G. T.; Guo, Z. W.; Hui, Y. Z. Synth. Commun. 1997, 27,
1907–1917; (e) Yang, G.; Ding, X.; Kong, F. Tetrahedron
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