Ultrasound-assisted selective deprotection of terminal acetonides catalyzed by silica-supported boron trifluoride

Article abstract of DOI:10.1080/07328303.2012.762980

An efficient and convenient method for the selective cleavage of terminal acetonides is described. Treatment of terminal acetonides in the presence of a wide range of functional groups with silica-supported boron trifluoride as a catalyst furnished the corresponding diols in 82-95% yield under ultrasound irradiation conditions. The acid-labile p-methoxybenzyl group as a protecting group remained intact under the conditions employed to the present deprotection condition. Copyright Taylor & Francis Group, LLC.

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Ultrasound-Assisted Selective
Deprotection of Terminal Acetonides
Catalyzed by Silica-Supported Boron
Trifluoride
Junlong Xiong a , Shiqiang Yan a , Ning Ding a , Wei Zhang a & Yingxia
Li a
a School of Pharmacy , Fudan University , Shanghai , China
Published online: 24 May 2013.
To cite this article: Junlong Xiong , Shiqiang Yan , Ning Ding , Wei Zhang & Yingxia Li
(2013) Ultrasound-Assisted Selective Deprotection of Terminal Acetonides Catalyzed by
Silica-Supported Boron Trifluoride, Journal of Carbohydrate Chemistry, 32:3, 184-192, DOI:
10.1080/07328303.2012.762980
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Journal of Carbohydrate Chemistry, 32:184–192, 2013
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Copyright Taylor & Francis Group, LLC
ISSN: 0732-8303 print / 1532-2327 online
DOI: 10.1080/07328303.2012.762980
Ultrasound-Assisted Selective
Deprotection of Terminal
Acetonides Catalyzed
by Silica-Supported
Boron Trifluoride
Junlong Xiong, Shiqiang Yan, Ning Ding, Wei Zhang,
and Yingxia Li
School of Pharmacy, Fudan University, Shanghai, China
An efficient and convenient method for the selective cleavage of terminal acetonides is
described. Treatment of terminal acetonides in the presence of a wide range of func-
tional groups with silica-supported boron trifluoride as a catalyst furnished the corre-
sponding diols in 82–95% yield under ultrasound irradiation conditions. The acid-labile
p-methoxybenzyl group as a protecting group remained intact under the conditions em-
ployed to the present deprotection condition.
Keywords Carbohydrate; Acetonide; Ultrasound; Silica-supported; Boron trifluoride
INTRODUCTION
Selective protection and deprotection of hydroxyl groups are the key to
the success of oligosaccharide synthesis.[1] It is well known that the ace-
tonide group is one of the most utilized moieties to protect both terminal
and internal 1,2- and 1,3-diols in carbohydrate and nucleoside chemistry.[2]
As a result, a variety of catalysts have been employed for the deprotec-
tion of terminal acetonides, including protonic acids such as HCl,[3a] HBr,[3b]
HOAc,[3c] H2SO4,[3d] and TFA[3e] and Lewis acid–based reagents such as
(Zn(NO3)2.6H2O,[4a] CeCl3.7H2O(COOH)2,[4b] VCl3,[4c] BiCl3,[4d] La(NO3)3,[4e]
and In(OTf)3.[4f] Nevertheless, many of these procedures suffer from disadvan-
tages such as too strongly acidic conditions,[3d,3e] expensive metals used,[4c–e]
Received October 29, 2012; accepted December 27, 2012.
Address correspondence to Yingxia Li, School of Pharmacy, Fudan University, Shanghai
201203, China. E-mail: liyx417@fudan.edu.cn
184

Deprotection of Terminal Acetonides
185
long reaction times,[4e] and high reaction temperatures.[4f] Additionally, the
protonic acids or Lewis acid–based reagents are used in homogeneous solu-
tions, making the removal of these catalysts a problem. Alternatively, sup-
ported reagents including FeCl3.6H2O on silica,[5a] H2SO4 on silica,[5b] HClO4
on silica,[5c] and NaHSO4 on silica[5d] have been employed for this transforma-
tion. Though easily removed as they might be, they are endowed with some
drawbacks, including lower yields,[5a] long reaction times,[5c] and incompatibil-
ity with some other protecting groups.[5d]
Boron trifluoride has been widely used as a Lewis acid catalyst in many
organic reactions.[6a–d] The silica-supported boron trifluoride (BF3-SiO2) is a
bench-top reagent, which is inexpensive, eco-friendly, and reusable. It is effi-
cient to promote many acid-catalyzed organic reactions.[7] Ultrasound activa-
tion has been emerging as a powerful technique to enhance reaction rates of a
variety of chemical transformations.[8] In particular, the beneficial effects of ul-
trasonic irradiation play an increasingly important role in chemical processes,
especially in the cases where classical methods require drastic conditions or
prolonged reaction times.[9]
Along this line, herein we disclose an efficient and facile method for the
selective deprotection of terminal acetonides with BF3-SiO2 in methanol under
ultrasound irradiation (Sch. 1). To the best of our knowledge, the ultrasound-
assisted deprotection of terminal acetonides has not yet been reported in the
literature.
Scheme 1: Selective hydrolysis of the terminal O-isopropylidene with BF3-SiO2
RESULTS AND DISCUSSION
As a model reaction, we treated 1,2:5,6-di-O-isopropylidene-α-D-glucofuranose
1a (entry 1, Table 1) with 5 mol% of BF3-SiO2 as a catalyst in the absence or
presence of ultrasound irradiation at room temperature in methanol, respec-
tively. In the case of absence of ultrasound irradiation, the reaction mixture
was stirred for 3 h to give product 2a in a moderate yield of 78%, while the
application of ultrasound irradiation resulted in the dramatic decrease of re-
action time (0.5 h), maintaining an excellent yield of 85%. It should be noted
that only the product with terminal acetonide removed was detected under the
above conditions. Encouraged by this result, we applied a set of carbohydrate

J. Xiong et al.
186
a
Table 1: Selective cleavage of terminal acetonides using BF3-SiO2
Time (min)
Stirring )))c Stirring )))
Yield (%)b
Entry
Substrate
Product
1
2
1a R = H
1b R = Bz
2a R = H[10a]
2b R = Bz[4d]
180
180
30
30
78
76
85
89
3
4
1f R = Bz
2f R = Bz[4a]
2g R = Bn[4e]
150
180
30
30
82
81
90
92
1g R = Bn
aThe structures of the products were established by 1H NMR data.
bIsolated yields.
c))) under ultrasound irradiation.
substrates to the condition to investigate the ultrasonic effect. The results are
listed in Table 1. When substrates 1b, 1f, and 1g were exposed to the ul-
trasound irradiation, yields of the corresponding products 2b, 2f, and 2g in-
creased from 76%, 82%, and 81% to 89%, 90%, and 92%, respectively, as com-
pared to reaction without ultrasound irradiation. In addition, in all of the cases
tested here, the reaction time was dramatically shortened.
Reactions under heating condition without ultrasound irradiation were
also carried out for comparison. We treated compound 1a and 1b with 5 mol%
BF3-SiO2 as catalyst in methanol at 50◦C, and the reaction mixtures were
stirred for 1.5 h to give corresponding products 2a and 2b in yields of 79%
and 80%, respectively. Apparently, heating could shorten the reaction time to
a certain extent, but the effect was not comparable to ultrasonication.
To explore the scope of the applicability of this deprotection methodol-
ogy, substrates with terminal acetonides and a wide range of other func-
tional groups were investigated under the above ultrasound irradiation
condition (Table 2). The substrate 3-O-benzoyl-1,2:5,6-di-O-isopropylidene-α-
D-glucofuranose (1c) furnished the corresponding diol 2c in an isolated yield
of 87% within 35 min (entry 3). A diacetonide derivative of D-glucose 1d that
contained an allyl ether linkage produced the corresponding diol product 2d

Deprotection of Terminal Acetonides
187
Table 2: Selective cleavage of terminal acetonides using BF3-SiO2 under ultrasound
irradiationa
Entry
Substrate
Product
Time (min) Yield (%)b
1
2
3
4
5
1a R = H
1b R = Bz
1c R = Bn
1d R = All
1e R = Me
2a R = H[10a]
2b R = Bz[4d]
2c R = Bn[4d]
2d R = All[10b]
2e R = Me[10c]
30
30
35
35
30
85
89
87
92
95
6
7
8
9
1f R = Bz
1g R = Bn
1h R = All
1i R = Me
2f R = Bz[4a]
2g R = Bn[4e]
2h R = All[11]
2i R = Me[4d]
30
30
30
30
90
92
88
89
10
11
1j R = MOM
1k R = PMB
2j R = MOM
2k R = PMB
30
30
82
88
12
1l
2l2a
40
85
aThe structures of the products were established by their 1H NMR data.
bIsolated yields.
in an excellent yield of 92% within 35 min (entry 4), and 3-O-methyl-1,2:
5,6-di-O-isopropylidene-α-D-glucofuranose (1e) afforded the corresponding diol
2e in a yield of 95% (entry 5) within 30 min. Similarly, a series of D-mannose
derivatives containing Bz, Bn, All, and Me groups also furnished the expected
corresponding diols in good to excellent yields (entries 6–9). To our delight, the
diacetonide derivative of D-allose possessing the acid-labile p-methoxybenzyl

J. Xiong et al.
188
(PMB) group (1k) underwent a clean deprotection reaction to produce the cor-
responding diol 2k in a good yield, under which condition the PMB group
was unaffected (entry 11). In the case of D-xylofuranose derivative, reaction
of 1,2:3,5-di-O-isopropylidene-α-D-xylofuranose (1l) under the established con-
dition produced the corresponding diol (2l) in a good yield of 85% (entry
12). Apparently, all the experiments were performed in relatively short time
(30–40 min) and in good yields.
In summary, we have utilized BF3-SiO2 as an excellent catalyst for the se-
lective deprotection of terminal acetonides under ultrasound irradiation condi-
tions. The use of ultrasound irradiation not only speeded up the reaction pro-
cess but also improved the reaction yields. Furthermore, acid-sensitive groups
were found to be stable under this reaction condition. In addition, the use of
solid-supported Lewis acid offers substantial advantages with respect to sim-
plifying the reaction and workup procedures. We expected this methodology to
find applications in oligosaccharide synthesis.
EXPERIMENTAL
General Methods
1H NMR spectra were recorded with a Bruker DPX400 spectrometer in
CDCl3 solutions. Internal references: TMS (δ 0.00 ppm for 1H), CDCl3 (δ
77.00 ppm for 13C). Thin-layer chromatography (TLC) was performed on sil-
ica gel HF with detection by charring with 5% (v/v) H2SO4 in CH3OH or by
UV detection. Column chromatography was conducted by elution of a column
of silica gel (200–300 mesh) with EtOAc/petroleum ether (bp. 60–90◦C) as the
eluent. Solutions were concentrated at a temperature <60◦C under diminished
pressure.
The ultrasound-assisted reactions were carried out in a KUDOSꢀR .
SK5200H Ultrasonic Bath Cleaner, with a frequency of 53 kHz. The ultrasonic
cleaner had a power consumption of 200 W (305 × 250 × 285 mm) with a liquid-
holding capacity of 10 L. The reactions were carried out in a round-bottomed
flask of 25-mL capacity suspended at the center of the cleaning bath, 5 cm
below the surface of the liquid. The reaction flask was located in the cleaner,
where the surface of reactants is slightly lower than the level of the water. The
reaction temperature was controlled by addition or removal of water from an
ultrasonic bath.
Preparation of BF3-SiO2 Reagent System[12]
Five milliliters of methanol containing 0.6 g (4.2 mmol) of BF3.OEt2 and 0.4 g of
unpreheated silica gel was stirred for 1 h at rt. The slurry was dried slowly on

Deprotection of Terminal Acetonides
189
a rotary evaporator at 40◦C. The obtained solid was dried at ambient temper-
ature for 2 h. The BF3-SiO2 reagent system could be stored in a dry container
(the drying agent is dry silica particles) for at least 3 months.
General Procedure for the Deprotection of Terminal Acetonides
Catalyzed by BF3-SiO2 Under Ultrasound Irradiation
To a solution of acetonides of sugar derivatives (1 mmol) in CH3OH
(10 mL), BF3-SiO2 (5 mmol%) was added and the heterogeneous mixture was
agitated in an ultrasonic cleaner at rt for the required time. After complete
conversion, the mixture was filtered and washed with CH3OH (5 mL). The
combined filtrate was concentrated under vacuum and the residue was puri-
fied by column chromatography to obtain the pure product.
1
The products were characterized by H NMR, and the spectroscopic data
were identical with the data reported in the literature. Spectral data for new
compounds, which were not reported earlier, are presented below.
1, 2-O-Isopropylidene-3-O-methoxymethyl-α-D-allofuranose (2i)
25
1
Viscous liquid, [α]D −49.6 (c 1.00, CHCl3); H NMR (CDCl3, 400 MHz) δ:
5.77 (d, J = 3.6 Hz, 1H), 4.74 (d, J = 4.0 Hz, 1H), 4.70–4.65 (m, 2H), 4.24–4.13
(m, 3H), 4.06 (d, J = 3.0 Hz, 1H), 3.43 (s, 3H), 1.58 (s, 3H), 1.36 (s, 3H); 13C
NMR (100 MHz, CDCl3) δ: 111.51, 104.67, 96.61, 82.89, 80.77, 79.37, 68.23,
63.82, 55.67, 26.25, 25.77; HRMS (ESI) calcd for C11H20NaO7 (M + Na)+
287.1101, found 287.1102.
1, 2-O-Isopropylidene-3-O-p-methoxybenzyl-α-D-allofuranose
(2j)
Viscous liquid, [α]2D5 −51.51 (c 1.06, CHCl3); 1H NMR (CDCl3, 400 MHz) δ:
7.30 (d, J = 8.8 Hz, 2H), 6.89 (d, J = 8.8 Hz, 2H), 5.75 (d, J = 3.6 Hz, 1H), 4.72
(d, J = 11.2 Hz, 1H), 4.60 (t, J = 3.6 Hz, 1H), 4.50 (d, J = 11.2 Hz, 1H), 4.09
(dd, J = 9.2, 3.6 Hz, 1H), 3.99 (m, 1H), 3.91 (dd, J = 8.8, 4.4 Hz, 1H), 3.80 (s,
3H), 3.65–3.69 (m, 2H), 2.70 (br s, 1H), 2.69 (br s, 1H), 1.59 (s, 3H), 1.36 (s, 3H);
13C NMR (100 MHz, CDCl3) δ: 159.09, 129.18, 128.79, 113.62, 111.33, 104.68,
81.71, 81.07, 79.39, 71.36, 68.78, 63.84, 54.85, 26.26, 25.76; HRMS (ESI) calcd
for C17H24NaO7 (M + Na)+ 363.1414, found 363.1411.
ACKNOWLEDGMENTS
This work was supported by the National Nature Science Foundation of China
(No. 210022014 and 81072525).

J. Xiong et al.
190
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