Bromodimethylsulfonium bromide catalyzed synthesis of methyl 2-dexoy-4,6-o-benzylidene galactopyranoside from galactal and the rapid route to 2,3- and 2,6-dideoxygalactopyranoses

Article abstract of DOI:10.1002/cjoc.201180487

4,6-O-Benzylidenation of D-galactal with PhCH(OCH₃)₂ catalyzed by bromodimethylsulfonium bromide leads to methyl 2-dexoy-4,6-O- benzylidene galactopyranoside efficiently, which serves as a key intermediate to the ready preparation of

Full text of DOI:10.1002/cjoc.201180487

FULL PAPER
DOI: 10.1002/cjoc.201180487
Bromodimethylsulfonium Bromide Catalyzed Synthesis of
Methyl 2-Dexoy-4,6-O-benzylidene Galactopyranoside from
Galactal and the Rapid Route to 2,3- and
2,6-Dideoxygalactopyranoses
Ding, Ninga(丁宁)
Chun, Yuexingb(淳月兴)
Zhang, Weia(张伟)
Li, Yingxia*,a(李英霞)
a Department of Medicinal Chemistry, Fudan University, 826 Zhangheng Road, Shanghai 201203, China
b School of Medicine and Pharmacy, Ocean University of China, Qingdao, Shandong 266003, China
4,6-O-Benzylidenation of D-galactal with PhCH(OCH3)2 catalyzed by bromodimethylsulfonium bromide leads
to methyl 2-dexoy-4,6-O-benzylidene galactopyranoside efficiently, which serves as a key intermediate to the ready
preparation of 2,3- and 2,6-dideoxy galactopyranosides.
Keywords deoxysugars, bromodimethylsulfonium bromide, synthetic methods, galactal, carbohydrate
Introduction
formed at any later point in a synthetic sequence, allows
selective access to either the 4- or 6-hydroxyl group as
desired. However, direct 4,6-O-benzylidenation of gly-
cal is messy and very low yielding due to the acid sensi-
tivity of the enol ether functionality.[9]
As part of our continuing interest in studying the
structure-activity relationships of deoxysugars as anti-
cancer agents, we present here a concise and practical
protocol for the preparation of 2,3- and 2,6-dideoxy
galactopyranoses from D-galactal.
Deoxysugars, monosaccharides in which one or
more hydroxyl groups is/are replaced with hydrogen
atoms, are an important class of carbohydrates that occur
widely in natural products.[1-3] For example, 2-deoxy-D-
ribose is present in DNA as the skeletal sugar compo-
nent. 2-Keto-3-deoxy-D-manno-octulosonic acid (KDO)
is an essential component of lipopolysaccharides in
Gram-negative bacteria.[4] 2,6-Dideoxy-hexoses are fre-
quently present in antibiotics and anti-cancer agents
such as anthracyclines, angucyclines, aureolic acid anti-
biotics, cardiac glycoside, enediynesm macrolides, and
pluramycins.[3] In this context, the ready synthesis of
such sugars has the potential of being used for the crea-
tion of vaccine candidates, antibiotics and other small
molecule drugs.
So far a variety of approaches for the synthesis of de-
oxysugars have been developed either through multistep
transformations of relatively economical common sug-
ars[3,5] or alternatively, through non-carbohydrate precur-
sors.[6] However, many of these methods either em-
ployed expensive reagents or performed extensive pro-
tecting group manipulations.
Results and Discussion
In a routine experiment, 4,6-O-benzylidenation of
D-galactal 1 with benzaldehyde dimethylacetal in ace-
tonitrile at room temperature catalyzed by 10-cam-
phorsulfonic acid (CSA), which is a commonly used
protonic acid catalyst, formed very messy products (Ta-
ble 1, Entry 1). To our surprise, further experiment
showed that when the stronger acid p-toluenesulfonic
acid monohydrate (p-TsOH•H2O) was employed as the
catalyst instead of CSA under the above condition,
methyl 2-dexoy-4,6-O-benzylidene galactopyranoside 2
was provided as a major product (55%, Table 1, Entry
2). This result was interesting since an early report[10]
described the reaction of D-glucal in benzaldehyde di-
methylacetal with p-TsOH•H2O as catalyst furnished
2,3-unsaturated Ferrier rearranged product (methyl
4,6-O-benzylidene-2,3-dideoxy-α-D-erythro-hex-2-
enopyranoside) in 90% yield. Because that compound 2
is an extremely useful building block for the synthesis
of deoxysugars, a more efficient synthesis of compound
Glycals are extremely useful carbohydrate derivatives,
finding uses in oligosaccharide synthesis and for many
other synthetic purposes as chiral building blocks.[7]
Selective 4,6-O-protection of a glycal is particularly
useful since it allows manipulation of the remaining
3-hydroxyl group. Moreover, 4,6-O-benzylidene protec-
tion is particularly desirable since regioselective cleav-
age of the benzylidene group,[8] which may be per-
*
E-mail: liyx417@fudan.edu.cn; Tel./Fax: 0086-021-51980120
Received April 29, 2011; accepted September 5, 2011.
Chin. J. Chem. 2012, 30, 409—412
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
409

Ding et al.
FULL PAPER
2 would be valuable. Thus the next effort is to improve
the yielding of compound 2.
products (Table 1, Entries 4, 5, 6).
Scheme 1 The formation of 2 from 1 catalyzed by BDMS
Table 1 Synthesis of methyl 2-dexoy-4,6-O-benzylidene ga-
lactosylpyranose 2a
Entry
Reagent
Catalyst
CSA
Time Yield/% (α∶β)
1
2
3
4
5
6
PhCH(OCH3)2
—
messy products
55 (5∶2)
PhCH(OCH3)2 p-TsOH•H2O 30 min
PhCH(OCH3)2
PhCHO
BDMS
CSA
10 min
—
87 (3∶1)
messy products
messy products
messy products
PhCHO
p-TsOH•H2O
BDMS
—
With compound 2 in hand, deoxysugars were readily
prepared. The α/β isomers of 2 can be easily separated
by silica gel column chromatography. To facilitate the
characterization and purification of the intermediates
and products, 2α was used as the key intermediate.
Treatment of compound 2α (1 equiv.) by NBS (1.25
equiv.), AIBN (0.05 equiv.) and barium carbonate (0.6
equiv.) in CCl4 at 70 ℃ (Hanessian-Hullar reaction)[13]
afforded compound 4 in 68% yield. The 4-O-benzoyl
and 6-bromo groups were then removed by LiAlH4 in
THF simultaneously to provide 2,3-dideoxygalacto-
sylpyranoside (5) in 83% yield. The preparation of
PhCHO
—
a
The catalyst (0.1 equiv.) was added to a stirred solution of
D-galactal 1 (1.0 equiv.) and PhCH(OCH3)2 (1.5 equiv.) or
PhCHO (1.5 equiv.) in dry acetonitrile at room temperature. The
reaction was monitored by TLC. After the full conversion of 1,
Et3N was added to quench the reaction. The reaction mixture was
then concentrated and purified by silica gel column chromato-
graphy.
Since Meerwein’s discovery of bromodimethylsulfo-
nium bromide (BDMS), it has gained considerable in-
terest in the field of organic chemistry, due to its easy
handling and low cost, as well as its varied applications
both as a catalyst and as an effective reagent.[11] Recently,
Khan and co-workers demonstrated the usefulness of
BDMS as a mild catalyst for O,O-isopropylidenation of
free sugars.[12] They claimed that BDMS may generate
dry HBr in the medium upon reaction with the sugars,
and the in situ-generated HBr catalyzed the isopro-
pylidenation reaction of sugars into the corresponding
O,O-isopropylidene derivatives. Inspired by this report,
we investigated the effect of BDMS as the catalyst on
the benzylidenation of D-galactal. To our delight, BDMS
drove the reaction of galactal 1 and PhCH(OCH3)2 to
provide compound 2 more efficiently than p-TsOH•H2O.
As shown in Table 1, in this case not only the yield of
compound 2 was improved but also the reaction time
was shortened (Entry 3).
2,3-dideoxygalactosylpyranoside
8
was achieved
through a radical deoxygenation procedure (Barton-
McCombie reaction)[14] to remove the 3-OH on 2 (71%
over two steps), followed by the cleavage of the
4,6-O-benzylidene group through hydrogenation in the
presence of Pd(OH)2 (92%).
Scheme 2 Synthesis of α-methyl 2,6- and 2,3-dideoxygalacto-
sylpyranoside (5 and 8)
We propose the formation of compound 2 was
through a two-step conversion. The first step is 4,6-O-
benzylidenation of 1 catalyzed by the in situ-generated
HBr, which formed intermediate 3 and released two
molecules of methanol. In the presence of HBr catalyst,
intermediate 3 was unstable due to the acid sensitivity
of the enol ether functionality but was trapped by a
methanol molecule rapidly to form the stable product 2
(Scheme 1). Further experiments confirmed the impor-
tance of the presence of methanol molecules, as em-
ploying benzaldehyde instead of benzaldehyde di-
methylacetal in the above reaction provided messy
Reagents and conditions: (i) 2α (1 equiv.), NBS (1.25 equiv.),
AIBN (0.05 equiv.), BaCO3 (0.6 equiv.), CCl4, 70 ℃, 1 h, 68%; (ii)
LiAlH4 (5 equiv.), THF, 1 h, 83%; (iii) PhOC＝SCl, DMAP, CH2Cl2,
12 h, 96%; (iv) Bu3SnH (1.6 equiv.), AIBN (0.2 equiv.), toluene, 80
℃, 2 h, 74%; (v) Pd(OH)2, H2, CH3OH, 30 ℃, 92%
410
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© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2012, 30, 409—412

Synthesis of Methyl 2-Dexoy-4,6-O-benzylidene Galactopyranoside
Conclusions
CCl4 (40 mL) was treated with freshly recrystallized
N-bromosuccinimide (601.5 mg, 3.4 mmol) and BaCO3
(266.4 mg, 1.4 mmol). The solution was deoxygenated
by sparging with argon for 1 h and then AIBN (18.5 mg,
0.1 mmol) was added. The reaction mixture was heated
at 65 ℃ for 1.5 h. After the mixture was cooled to
room temperature and filtered, it was dried (Na2SO4),
concentrated, and purified by column chromatography
on silica gel to give 4 (529.7 mg, 68%) as a white foamy
In conclusion, BDMS was discovered to be an effi-
cient catalyst to drive the 4,6-O-benzylidenation of
D-galactal with PhCH(OCH3)2 to afford methyl
2-dexoy-4,6-O-benzylidene galactopyranoside without
Ferrier rearrangement, which serves as a key intermedi-
ate for ready preparation of 2,3- and 2,3-dideoxygalac-
tosylpyranosides.
1
solid. H NMR (CDCl3, 400 MHz) δ: 8.09 (dd, J＝8.3,
Experimental
1.2 Hz, 2H, ArH), 7.61 (t, J＝7.5 Hz, 1H, ArH), 7.48
(dd, J＝7.9, 7.5 Hz, 2H, ArH), 5.55 (d, J＝3.2 Hz, 1H,
H-4), 4.99 (d, J＝2.0 Hz, 1H, H-1), 4.38—4.35 (m, 1H);
4.18—4.16 (m, 1H), 3.46—3.44 (m, 5H, OCH3, H-6ab),
2.04—2.02 (m, 2H, H-2ab); 13C NMR (CDCl3, 100
MHz) δ: 167.0, 133.6, 129.8, 128.6, 99.0, 71.0, 69.5,
65.1, 55.2, 33.1, 30.5; ESI-MS m/z: 367.1 (M＋Na),
711.1 (M2＋Na).
General methods
All commercial reagents and solvents were used as
received without further purification unless specified.
Reaction solvents were distilled from CaH2 for di-
chloromethane and from sodium metal and benzophe-
none for tetrahydrofuran. Flash column chromatography
was performed on silica gel (200—300 mesh, Qingdao,
China). 1H NMR and 13C NMR spectra were taken on a
Bruker Avance 400 MHz spectrometer with tetrame-
thylsilane (TMS) as an internal standard at room tem-
perature. Mass spectra were obtained on a Waters
Q-TOF micro mass spectrometer (Waters).
Synthesis of methyl 2,6-di-deoxy-α-D-galactopyrano-
side (5)
A solution of methyl 4-O-benzoyl-6-bromo-6-deoxy-
2-deoxy-α-D-galactopyranoside (4) (140.0 mg, 0.4
mmol) in THF (5 mL) was treated with LiAlH4 (77 mg,
2.0 mmol). The reaction mixture was stirred at room
temperature for 12 h. EtOAc (20 mL) was added drop-
wise to quench the reaction. The reaction mixture was
filtered through a pad of Celite and concentrated in
vacuo. The residue was purified by column chromato-
graphy on silica gel to give 5 (54.6 mg, 83%) as a yel-
Synthesis of methyl 4,6-O-benzylidene-2-dexoy-D-
galactopyranoside (2)
Compound 1 (146.1 mg, 1.0 mmol) was dissolved in
dry acetone (10 mL), then benzaldehyde dimethyl acetal
(228.3 mg, 1.5 mmol) and BDMS (22.0 mg, 0.1 mmol)
were added. After stirring at room temperature for 10
min, the reaction mixture was neutralized by addition of
solid K2CO3 and then evaporated to dryness. The silica
gel column chromatography of the residue furnished 2
1
low oil. H NMR (CDCl3, 400 MHz) δ: 4.78 (d, J＝3.5
Hz, 1H, H-1), 3.98—4.01 (m, 1H, H-3), 3.92 (dd, J＝
12.9, 6.7 Hz, 1H, H-5), 3.63 (s, 1H, H-4), 3.33 (s, 3H,
OCH3), 2.0—2.2 (bm, 2H, OH), 1.92 (dd, J＝13.3, 5.7
Hz, 1H, H-2a), 1.78 (ddd, J＝12.9, 12.9, 3.5 Hz, 1H,
H-2b), 1.29 (d, J＝6.7 Hz, 3H, CH3); 13C NMR (CDCl3,
100 MHz) δ: 98.6, 71.2, 65.8, 65.4, 54.9, 32.7, 16.7;
ESI-MS m/z: 185.2 (M＋Na, 100), 347.2 (M2＋Na, 85).
1
(231.5 mg, 87%) as a foamy solid. Compound 2α: H
NMR (CDCl3, 400 MHz) δ: 7.50—7.48 (m, 2H, ArH),
7.40—7.38 (m, 3H, ArH), 5.61 (s, 1H, PhCH), 4.95 (s,
1H, H-1), 4.30 (dd, J＝12.3, 1.2 Hz, 1H, H-6a), 4.13—
4.07 (m, 3H, H-4, H-3, H-6b), 3.65 (s, 1H, H-5), 3.36 (s,
3H, OCH3), 2.23 (d, J＝10.5 Hz, 1H, 3-OH), 1.98 (m,
2H, H-2); 13C NMR (CDCl3, 100 MHz) δ: 137.8, 129.1,
128.2, 126.3, 101.1 (PhCH), 99.4 (C-1), 74.8 (C-4),
70.0 (C-6), 64.4 (C-3), 62.5 (C-5), 55.0 (OCH3), 33.7
(C-2); ESI-MS m/z (%): 289.1 (M＋Na, 65), 555.2 (M2
Synthesis of methyl 4,6-O-benzylidene-2-dexoy-3-O-
phenylthiocarbonate-α-D-galactopyranoside (6)
Phenyl chlorothionoformate (103.6 mg, 0.6 mmol)
and DMAP (147.0 mg, 1.2 mmol) were added sequen-
tially to a flame-dried round-bottom flask containing a
solution of compound 2 (160.0 mg, 0.6 mmol) in anhy-
drous dichloromethane (25 mL). The reaction solution
was stirred at room temperature for 2 h and then con-
centrated under reduced pressure. The residue was puri-
fied by column chromatography on silica gel to give 6
(390.0 mg, 96%) as a white solid. 1H NMR (CDCl3, 400
MHz) δ: 7.50—7.08 (m, 10H, ArH), 5.80—5.78 (m, 1H,
H-3), 5.65 (s, 1H, PhCH), 5.06 (d, J＝2.0 Hz, 1H, H-1),
4.58 (d, J＝2.7 Hz, 1H, H-4), 4.32 (d, J＝12.5 Hz, 1H,
H-6a), 4.13 (d, J＝12.5 Hz, 1H, H-6b), 3.74 (s, 1H,
H-5), 3.40 (s, 3H, OCH3), 2.46 (ddd, J＝12.1, 12.1, 3.5
Hz, 1H, H-2a), 2.18 (dd, J＝12.5, 5.1 Hz, 1H, H-2b);
ESI-MS m/z (%): 425.2 (M＋Na, 100), 827.3 (M2＋Na,
75).
1
＋Na, 55). Compound 2β: H NMR (CDCl3, 400 MHz)
δ: 7.52—7.50 (m, 2H, ArH), 7.37—7.36 (m, 3H, ArH),
5.60 (s, 1H, PhCH), 4.39 (d, J＝8.3 Hz, 1H, H-1), 4.37
(d, J＝10.1 Hz, 1H, H-6a), 4.11 (dd, J＝12.3, 1.6 Hz,
1H, H-6b), 3.83—3.81 (m, 1H, H-3), 3.53 (s, 3H,
OCH3), 3.38 (s, 1H, H-5), 2.13—1.82 (m, 2H, H-2); 13C
NMR (CDCl3, 100 MHz) δ: 129.2, 128.1, 126.5, 101.3
(PhCH), 101.2 (C-1), 73.8 (C-4), 69.6 (C-6), 68.2 (C-3),
66.8 (C-5), 56.6 (OCH3), 35.6 (C-2); ESI-MS m/z (%):
289.1 (M＋Na, 70), 555.2 (M2＋Na, 55).
Synthesis of methyl 4-O-benzoyl-6-bromo-6-deoxy-2-
deoxy-α-D-galactopyranoside (4)
A solution of methyl 4,6-O-benzylidene-2-dexoy-α-
D-galactosylpyranoside (2α) (600.0 mg, 2.3 mmol) in
Chin. J. Chem. 2012, 30, 409—412
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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411

Ding et al.
FULL PAPER
Synthesis of methyl 4,6-O-benzylidene-2,3-di-deoxy-
α-D-galactopyranoside (7)
Fudan University.
To a solution of compound 6 (50.0 mg, 0.12 mmol)
in toluene (10 mL) was added tributyltin hydride (70.0
mg, 0.24 mmol) and AIBN (6.0 mg, 0.04 mmol), and
the mixture was heated at reflux for 3 h. Solvent was
then removed under reduced pressure, and the resulting
residue was purified by silica gel flash chromatography
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To a solution of compound 7 (20.0 mg, 0.08 mmol)
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Acknowledgement
This work is sponsored by National Natural Science
Foundation of China (No. 21002014) and the Startup
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412
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© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2012, 30, 409—412