Cleavage of 4,6- O -benzylidene acetal using sodium hydrogen sulfate monohydrate

Article abstract of DOI:10.1055/s-0033-1338469

The use of protecting groups is an important protocol in carbohydrate synthesis. Among protecting groups, benzylidene acetals are generally more stable than other acetals; therefore, strong conditions are often required for deprotection. We report the deprotection of 4,6-O-benzylidene derivatives using sodium hydrogen sulfate monohydrate under mild conditions. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1338469

PAPER
▌1519
paper
Cleavage of 4,6-O-Benzylidene Acetal Using Sodium Hydrogen Sulfate
Monohydrate
Cleavage of 4,6-O-Benzylidene Acetal
Kyosuke Michigami, Manami Terauchi, Masahiko Hayashi*
Department of Chemistry, Graduate School of Science, Kobe University, Nada, Kobe, 657-8501, Japan
Fax +81(78)8035688; E-mail: mhayashi@kobe-u.ac.jp
Received: 04.03.2013; Accepted after revision: 29.03.2013
actions were carried out at 23–26 °C for one hour. The
amount of sodium hydrogen sulfate monohydrate was op-
timized. (Table 1) and the mixtures were filtered through
Celite to remove undissolved salts. At first, the filtrate
Abstract: The use of protecting groups is an important protocol in
carbohydrate synthesis. Among protecting groups, benzylidene
acetals are generally more stable than other acetals; therefore,
strong conditions are often required for deprotection. We report the
deprotection of 4,6-O-benzylidene derivatives using sodium hydro- was concentrated in vacuo and purified by silica gel col-
gen sulfate monohydrate under mild conditions.
umn chromatography to give the diol product 1b only in
22% yield (entry 1). This low yield was attributed to pro-
duction of starting material 1a in of the presence of acid
in the concentration step. To avoid this reverse reaction,
we added potassium carbonate to the filtrate in order to
neutralize the reaction mixture giving 1b in 87% yield
(entry 2). Addition of three equivalents of sodium hydro-
gen sulfate monohydrate gave 1b in 99% yield (entry 3).
It was also found that sodium hydrogen sulfate monohy-
drate exhibited better performance than dry sodium hy-
drogen sulfate as the catalyst (entry 4). During the course
of the reaction, we observed insoluble sodium hydrogen
sulfate. Therefore, we weighted the amount of dry sodium
hydrogen sulfate from the filtrate after evaporation. Fig-
ure 1 showed sodium hydrogen sulfate monohydrate has
higher solubility than sodium hydrogen sulfate in metha-
nol. The difference in solubility may be the reason why
sodium hydrogen sulfate monohydrate promoted deprot-
ection of benzylidene acetals more efficiently than sodi-
um hydrogen sulfate.
Key words: protecting group, benzylidene acetal, cleavage, sodi-
um hydrogen sulfate monohydrate
There have been increasing interests in protecting groups
in the field of carbohydrate synthesis.1 Acetals are widely
used as protecting groups for diols because of their high
stability under basic conditions. 4,6-O-Benzylidene ace-
tals are frequently employed because they are more stable
than other acetals. In particular, the partial and regioselec-
tive cleavage to benzyl ethers has been well studied, be-
cause of its utility in oligosaccharide synthesis.2 In these
reactions, hydride reagents, such as sodium cyanoborohy-
dride and borane–trimethylamine, have been used. On the
other hand, full deprotection of 4,6-O-benzylidene acetals
leading to 4,6-diols is also an important process in natural
product synthesis. The most common methods for cleav-
ing benzylidene acetals are solvolysis under acidic condi-
tions,3 reduction by hydrogen/palladium on carbon,4 and
Birch reduction.5 For example, Kim and Salmon used
aqueous sulfuric acid for the deprotection of a 4,6-O-ben-
zylidene acetal in the synthesis of halichondrin B.6 Other
methods include the use of silica gel supported sodium
hydrogen sulfate7 and phosphomolybdic acid.8 Alterna-
tive methods using inorganic salts such as sodium hydro-
gen sulfate are also attractive because the catalyst can be
easily removed by extraction or filtration. Sodium hydro-
gen sulfate has moderate acidity, which is sufficient to
cleave benzylidene acetals. However, it is less soluble in
organic solvents. In order to promote solvolysis efficient-
ly, the solubility of sodium hydrogen sulfate must be in-
creased.
Table 1 Optimization of the Reaction Conditions for the Cleavage
of 4,6-O-Benzylidene Acetal 1a
OH
Ph
O
O
promotor (X equiv)
MeOH, 1 h
O
O
HO
BnO
BnO
BnO
BnO
OMe
OMe
1a
1b
Entrya
Promoter
X equiv
Yieldb (%)
1
NaHSO4·H2O
NaHSO4·H2O
NaHSO4·H2O
NaHSO4
1
1
3
3
22
87
99
70
2c
3c
4c
Herein, we report the cleavage of 4,6-O-benzylidene ace-
tals using sodium hydrogen sulfate monohydrate
(NaHSO4·H2O). We found that sodium hydrogen sulfate
monohydrate efficiently promoted the deprotection of
methyl 2,3-di-O-benzyl-4,6-O-benzylidene-α-D-glucopy-
ranoside (1a) to afford the corresponding diol 1b. All re-
a Reaction conditions: promoter was added to a mixture of 1a (0.5
mmol), MeOH (6.0 mL), stirring, 23–26 °C, 1 h.
b Isolated yield after column chromatography (silica gel).
c The filtrate was neutralized with K2CO3.
SYNTHESIS 2013, 45, 1519–1523
Advanced online publication: 24.04.2013
DOI: 10.1055/s-0033-1338469; Art ID: SS-2013-F0180-OP
© Georg Thieme Verlag Stuttgart · New York
Next, we examined the reactivity of other pyranoside de-
rivatives (Table 2). All reactions were carried out using
0.5 mmol of substrates 1a–9a and 1.5 mmol of sodium hy-
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X

1520
K. Michigami et al.
PAPER
drogen sulfate monohydrate. The reaction with methyl were converted into the corresponding diols in 72% and
4,6-O-benzylidene-2,3-di-O-methyl-α-D-glucopyranoside 68% yield (entries 4 and 9). Methyl 4,6-O-benzylidene-
(2a) as a substrate at room temperature provided diol 2b 2,3-di-O-methyl-β-D-glucopyranoside (6a) was converted
in 96% yield (entry 2). It should be noted that product 2b into the corresponding diol without anomerization and
was easily purified by recrystallization from dichloro- methyl 2,3-di-O-benzyl-4,6-O-p-methoxybenzylidene-α-
methane–hexane without silica gel column chromatogra- D-glucopyranoside (8a) showed excellent reactivity (en-
phy. Debenzylidenation of substrate 3a, which contained tries 6 and 8). In the case of compounds 5a and 7a, deacet-
an epoxy moiety, proceeded smoothly without cleavage of ylation and debenzoylation products were obtained in
the epoxide (entry 3). Methyl 4,6-O-benzylidene-2,3-di- small amounts, which led to a decrease in the yields of the
O-tosyl-α-D-glucopyranoside (4a), and methyl 2,3-di-O- desired products (49% and 52%, respectively).
benzyl-4,6-O-benzylidene-α-D-galactopyranoside (9a)
Table 2 Deprotection of 4,6-O-Benzylidene Acetalsa
OH
O
Ph
O
NaHSO4·H2O (3.0 equiv)
solvent, time
O
HO
RO
O
RO
RO
RO
OMe
OMe
Entry
Substrate
Product
Solvent
Time
6 h
Yieldb (%)
OH
Ph
O
O
O
O
HO
BnO
BnO
1
MeOH (6 mL)
99
BnO
BnO
OMe
OMe
1a
1b
OH
O
Ph
O
O
O
HO
MeO
MeO
2
MeOH (3.5 mL)
45 min
96c
74
72
MeO
MeO
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
2a
2b
OH
O
Ph
O
O
O
HO
3
MeOH (2 mL)–CH2Cl2 (1 mL)
MeOH (0.5 mL)–CH2Cl2 (0.5 mL)
1 h
O
O
3a
3b
OH
Ph
O
O
O
O
HO
TsO
TsO
4d
24 h
TsO
TsO
4a
4b
OH
O
Ph
O
O
O
HO
AcO
AcO
5
6
7
MeOH (6 mL)
MeOH (1 mL)
MeOH (1 mL)
1 h
49
71
52
AcO
AcO
5a
5b
OH
O
Ph
O
O
O
HO
MeO
OMe
MeO
OMe
80 min
MeO
MeO
6a
6b
OH
O
Ph
O
O
BzO
O
HO
BzO
BzO
1 h
BzO
OMe
OMe
7a
7b
Synthesis 2013, 45, 1519–1523
© Georg Thieme Verlag Stuttgart · New York

PAPER
Cleavage of 4,6-O-Benzylidene Acetal
1521
Table 2 Deprotection of 4,6-O-Benzylidene Acetalsa (continued)
OH
O
Ph
O
NaHSO4·H2O (3.0 equiv)
solvent, time
O
HO
RO
O
RO
RO
RO
OMe
OMe
Entry
Substrate
Product
Solvent
Time
Yieldb (%)
OH
PMP
O
O
O
O
HO
BnO
BnO
8
MeOH (8 mL)
1 h
98
BnO
BnO
OMe
OMe
8a
1b
Ph
HO
OH
O
O
O
BnO
O
9d
MeOH (0.7 mL)–CHCl3 (1 mL)
23 h
68
BnO
BnO
OMe
BnO
OMe
9b
9a
a Reaction conditions: substrate (0.5 mmol), NaHSO4·H2O (1.5 mmol), solvent, 21–28 °C; the filtrate from the reaction was neutralized with
K2CO3.
b Isolated yield after column chromatography (silica gel) unless otherwise noted.
c Isolated yield after recrystallization.
d Reaction was carried out at 50 °C.
and 13C NMR spectra (400 and 100.6 MHz respectively) were re-
corded on a Jeol JNM-LA 400 instrument using TMS as an internal
standard. FT-IR spectra were recorded using Thermo Scientific, Ni-
colet iS5, iD5 ATR instrument. Mass spectra were measured using
Thermo Quest LCQ DECA plus. Optical rotations were measured
on a Horiba SEPA-300 polarimeter for a soln in a 1-dm cuvette.
TLC was carried out on Merk 25 TLC aluminum sheets silica gel 60
F254
.
Glucopyranosides 1b–9b; General Procedure
A mixture of methyl 4,6-O-benzylidene acetal 1a–9a (0.5 mmol),
NaHSO4·H2O (1.5 mmol) and solvent was stirred for the time indi-
cated in Table 2, followed by filtration through Celite and addition
of K2CO3 (100 mg) to the filtrate for neutralization. After filtration
of K2CO3 through Celite, the filtrate was concentrated and the resi-
due was purified by column chromatography (silica gel) or recrys-
tallization to give the product.
Figure 1 Solubility of sodium hydrogen sulfate monohydrate and
sodium hydrogen sulfate in methanol (6.9 mL) at 23–26 °C
Methyl 2,3-Di-O-benzyl-α-D-glucopyranoside (1b)
White solid; yield: 185 mg (99%); mp 73–75 °C (Lit.8 74–76 °C);
[α]D24 +19.3 (c 1.0, CHCl3) [Lit.8 [α]D30 +18.1 (c 1.5, CHCl3)].
In summary, deprotection of 4,6-O-benzylidene pyranose
derivatives was achieved using sodium hydrogen sulfate
monohydrate under mild conditions. The present method
has advantages compared to previous methods in view of
operational simplicity and mild conditions. It should be
mentioned that commercially available sodium hydrogen
sulfate monohydrate has appropriate acidity in methanol
without destroying other functional groups. Strict adjust-
IR (KBr): 696, 735, 754, 1000, 1023, 1044, 1057, 1119, 3200–3600
cm–1.
1H NMR (400 MHz, CDCl3): δ = 1.97 (br s, 1 H, OH), 2.33 (br s, 1
H, OH), 3.38 (s, 3 H, OMe), 3.4–3.6 (m, 2 H, H2, H4), 3.6–3.7 (m,
1 H, H5), 3.7–3.8 (m, 3 H, H3, H6, H6′), 4.59 (d, J = 3.2 Hz, 1 H,
H1), 4.66 (d, J = 12.0 Hz, 1 H, OCH2Ph), 4.70 (d, J = 12.0 Hz, 1 H,
OCH2Ph), 4.74 (d, J = 12.0 Hz, 1 H, OCH2Ph), 5.03 (d, J = 12.0 Hz,
1 H, OCH2Ph), 7.3–7.4 (m, 10 H, HPh).
ment of acidity is necessary if normal Brønsted acids, 13C NMR (100.6 MHz, CDCl3): δ = 55.2, 62.4, 70.3, 70.6, 73.1,
75.4, 76.7, 79.7, 81.2, 98.1, 127.9, 128.0, 128.1, 128.5, 128.6,
137.9, 138.6.
HRMS (ESI–): m/z [M – H]– calcd for C21H25O6: 373.1651; found:
such as hydrochloric acid and sulfuric acid, are used to
maintain other functional groups.
373.1644.
All reactions were carried out in well-cleaned and oven-dried glass-
Anal. Calcd for C21H26O6: C, 67.36; H, 7.00. Found: C, 66.99; H,
ware with magnetic stirring. All starting materials were obtained
from commercial sources and used without further purification. 1H
6.83.
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2013, 45, 1519–1523

1522
K. Michigami et al.
PAPER
Methyl 2,3-Di-O-methyl-α-D-glucopyranoside (2b)
Methyl 2,3-Di-O-methyl-β-D-glucopyranoside (6b)
32
White solid; yield: 106 mg (96%); mp 78–80 °C (Lit.9 83–85 °C);
[α]D27 +151.5 (c 1.0, H2O) [Lit.9 [α]D23 +146.4 (c 1.0, H2O)].
White solid; yield: 79 mg (71%); mp 42 °C (Lit.13 62–64 °C); [α]D
–41.3 (c 1.0, CHCl3) [Lit.13 [α]D –47.8 (CHCl3)].
IR (KBr): 699, 755, 1072, 3100–3770 cm–1.
IR (KBr): 614, 890, 955, 996, 1025, 1061, 1085, 1103, 3000–3700
cm–1.
1H NMR (400 MHz, CDCl3): δ = 2.07 (br s, 1 H, OH), 2.18 (br s, 1
H, OH), 3.23 (dd, J = 9.2, 3.2 Hz, 1 H, H2), 3.4–3.6 (m, 8 H, 2 OMe,
H3, H4), 3.6–3.7 (m, 4 H, OMe, H5), 3.81 (dd, J = 11.6, 4.0 Hz, 1
H, H6), 3.87 (dd, J = 11.6, 4.0 Hz, 1 H, H6′), 4.86 (d, J = 3.2 Hz, 1
H, H1).
13C NMR (100.6 MHz, CDCl3): δ = 55.3, 55.5, 61.3, 62.4, 76.7,
81.9, 82.7, 97.5,
1H NMR (400 MHz, CDCl3): δ = 2.28 (br s, 1 H, OH), 2.89 (br s, 1
H, OH), 3.00 (dd, J = 9.2, 7.8 Hz, 1 H, H2), 3.14 (dd, J = 9.1, 9.0
Hz, 1 H, H4), 3.3–3.4 (m, 1 H, H5), 3.51 (dd, J = 9.2, 9.1 Hz, 1 H,
H3), 3.56 (s, 3 H, OMe), 3.58 (s, 3 H, OMe), 3.64 (s, 3 H, OMe),
3.7–4.0 (m, 2 H, H6, H6′), 4.25 (d, J = 7.8 Hz, 1 H, H1).
13C NMR (100.6 MHz, CDCl3): δ = 57.2, 60.2, 60.9, 62.5, 70.1,
74.7, 83.6, 95.6, 104.6.
HRMS (ESI–): m/z [M – H]– calcd for C9H17O6: 221.1025; found:
HRMS (ESI–): m/z [M – H]– calcd for C9H17O6: 221.1025; found:
221.1025.
221.1025.
Methyl 2,3-Anhydro-α-D-glucopyranoside (3b)
White solid; yield: 65 mg (74%); mp 109 °C (Lit.10 103–105 °C);
[α]D33 +144.8 (c 0.17, CHCl3).
Methyl 2,3-Di-O-benzoyl-α-D-glucopyranoside (7b)
28
White solid; yield: 105 mg (52%); mp 63–65 °C; [α]D +178.2 (c
1.0, CHCl3) [Lit.14 [α]D19 +165.6 (CHCl3)].
IR (KBr): 616, 750, 793, 840, 894, 911, 964, 981, 1026, 1040, 1051,
1074, 1105, 1306, 1366, 2920, 2964, 3000–3500 cm–1.
IR (KBr): 696, 739, 961, 1028, 1045, 1094, 1194, 1275, 1354, 1453,
1722, 2928, 3000–3500 cm–1.
1H NMR (400 MHz, CDCl3): δ = 3.46 (s, 3 H, OMe), 3.49 (dd,
J = 4.2 Hz, 1 H, H3), 3.57 (dd, J = 3.1, 4.2 Hz, 1 H, H2), 3.6–3.7 (m,
1 H, H5), 3.80 (d, J = 3.9, 11.7 Hz, 1 H, H6), 3.86 (d, J = 3.9, 11.7
Hz, 1 H, H6′), 3.99 (d, J = 8.8 Hz, 1 H, H4), 4.91 (d, J = 3.1 Hz, 1
H, H1).
13C NMR (100.6 MHz, CDCl3): δ = 54.1, 55.5, 55.7, 62.2, 65.9,
69.0, 94.6.
1H NMR (400 MHz, CDCl3): δ = 1.72 (br s, 1 H, OH), 2.16 (br s, 1
H, OH), 3.43 (s, 3 H, OMe), 3.86 (ddd, J = 6.7, 6.7, 3.1 Hz, 1 H,
H5), 3.9–4.0 (m, 3 H, H4, H6, H6′), 5.12 (d, J = 3.9 Hz, 1 H, H1),
5.23 (dd, J = 3.9, 9.5 Hz, 1 H, H2), 5.74 (dd, J = 10.3, 9.5 Hz, 1 H,
H3), 7.3–7.4 (m, 4 H, HPh), 7.5–7.6 (m, 2 H, HPh), 7.9–8.0 (m, 4 H,
HPh).
13C NMR (100.6 MHz, CDCl3): δ = 55.4, 62.1, 69.6, 71.3, 71.5,
74.3, 97.1, 128.4, 129.08, 129.15, 129.87, 129.88, 133.3, 133.5,
166.0, 167.5.
HRMS (ESI–): m/z [M – H]– calcd for C7H11O5: 175.0606; found:
175.0603.
HRMS (ESI–): m/z [M – H]– calcd for C21H21O8: 401.1236; found:
401.1237.
Methyl 2,3-Di-O-tosyl-α-D-glucopyranoside (4b)
29
White solid; yield: 181 mg (72%); mp 55 °C; [α]D +59.6 (c 1.0,
CHCl3) [Lit.11 [α]D +58.5 (c 3.1, CHCl3)].
Methyl 2,3-Di-O-benzyl-α-D-galactopyranoside (9b)
IR (KBr): 564, 664, 732, 811, 830, 907, 972, 995, 1036, 1173, 1190,
1360, 2927, 3000–3600 cm–1.
29
Colorless oil; yield: 128 mg (68%); [α]D +46.9 (c 0.67, CHCl3)
[Lit.15 [α]D30 +43.0 (c 0.74, CHCl3)].
1H NMR (400 MHz, CDCl3): δ = 2.44 (s, 3 H, CH3), 2.46 (s, 3 H,
CH3), 3.27 (s, 3 H, OMe), 3.40 (d, J = 3.3 Hz, 1 H, OH), 3.6–3.7 (m,
1 H, H5), 3.77 (dd, J = 9.2, 9.1 Hz, 1 H, H4), 3.8–3.9 (m, 2 H, H6,
H6′), 4.30 (dd, J = 3.5, 9.4 Hz, 1 H, H2), 4.76 (d, J = 3.5 Hz, 1 H,
H1), 4.85 (dd, J = 9.4, 9.1 Hz, 1 H, H3), 7.29 (d, J = 7.6 Hz, 2 H,
PhCH2), 7.34 (d, J = 7.6 Hz, 2 H, PhCH2), 7.66 (d, J = 8.4 Hz, 2 H,
PhCH2), 7.77 (d, J = 8.4 Hz, 2 H, PhCH2).
13C NMR (100.6 MHz, CDCl3): δ = 21.70, 21.75, 55.5, 61.7, 69.2,
70.7, 75.5, 80.4, 97.0, 128.0, 128.3, 129.81, 129.84, 132.3, 132.9,
145.3, 145.4.
IR (KBr): 686, 706, 991, 1025, 1068, 1094, 1275, 1451, 1720, 2933,
3000–3500 cm–1.
1H NMR (400 MHz, CDCl3): δ = 2.46 (br s, 1 H, OH), 2.78 (br s, 1
H, OH), 3.37 (s, 3 H, OMe), 3.7–3.9 (m, 5 H, H2, H3, H5, H6, H6′),
4.0–4.1 (m, 1 H, H4), 4.67 (d, 1 H, PhCH2), 4.69 (d, J = 11.9 Hz, 1
H, PhCH2), 4.70 (d, J = 3.2 Hz, 1 H, H1), 4.81 (d, J = 11.9 Hz, 2 H,
PhCH2), 7.3–7.4 (m, 10 H, HPh).
13C NMR (100.6 MHz, CDCl3): δ = 55.3, 63.0, 68.8, 69.1, 72.9,
73.5, 75.6, 98.6, 127.8, 127.9, 128.0, 128.4, 128.5, 138.0, 138.2.
HRMS (ESI+): m/z [M + Na]+ calcd for C21H26O10S2Na: 525.0865;
found: 525.0865.
HRMS (ESI–): m/z [M – H]– calcd for C21H25O6: 373.1651; found:
373.1650.
Methyl 2,3-Di-O-acetyl-α-D-glucopyranoside (5b)
29
Colorless oil; yield: 68 mg (49%); [α]D +149.2 (c 0.46, CHCl3)
Acknowledgment
[Lit.12 [α]D19 +127 (CHCl3)].
This work was supported by a Grant-in-Aid for Scientific Research
on Innovative Areas, MEXT, Japan ‘Molecular Activation Directed
toward Straightforward Synthesis’, No. B23350043 from the Mini-
stry of Education, Culture, Sports, Science, and Technology, Japan.
Special Coordination Funds for Promoting Science and Technolo-
gy, Creation of Innovation Centers for Advanced Interdisciplinary
Research Areas (Innovative Bioproduction Kobe), MEXT, Japan is
also acknowledged.
IR (KBr): 604, 755, 918, 1029, 1223, 1370, 1742, 2932, 3000–3500
cm–1.
1H NMR (400 MHz, CDCl3): δ = 1.68 (br s, 1 H, OH), 2.10 (s, 3 H,
CH3COO), 2.11 (s, 3 H, CH3COO), 2.94 (br s, 1 H, OH), 3.41 (s, 3
H, OMe), 3.7–3.8 (m, 2 H, H4, H6), 3.8–3.9 (m, 2 H, H5, H6′), 4.84
(dd, J = 10.3, 3.3 Hz, 2 H, H2), 4.91 (d, J = 3.3 Hz, 1 H, H1), 5.2–
5.3 (m, 1 H, H3).
13C NMR (100.6 MHz, CDCl3): δ = 20.8, 20.9, 55.3, 62.0, 69.9,
70.7, 71.1, 73.4, 96.8, 170.3, 172.0.
HRMS (ESI+): m/z [M + Na]+ calcd for C11H18O8Na: 301.0899;
found: 301.0899.
Supporting Information for this article is available online at
http://www.thieme-connect.de/ejournals/toc/synthesis.
Included
are 1H and 13C NMR spectra for compounds 1b–9b.SnugIiofop
maotrinuSgIopoimnfrtrat
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Synthesis 2013, 45, 1519–1523
© Georg Thieme Verlag Stuttgart · New York

PAPER
Cleavage of 4,6-O-Benzylidene Acetal
1523
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Synthesis 2013, 45, 1519–1523