Regioselective ring opening of exo- and endo-3,4-benzylidene acetals of arabinopyranoside derivatives with Lewis acids and reducing agents

Article abstract of DOI:10.1016/j.carres.2011.01.031

Dioxolane type 3,4-benzylidene acetals of benzyl β-l-arabinose either as a mixture or pure exo- and endo-isomers cleavaged with BF₃· OEt₂/Et₃SiH in dichloromethane or acetonitrile regioselectively, provided the 4-O-benzyl-

Full text of DOI:10.1016/j.carres.2011.01.031

Carbohydrate Research 346 (2011) 927–932
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Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Regioselective ring opening of exo- and endo-3,4-benzylidene acetals
of arabinopyranoside derivatives with Lewis acids and reducing agents
⇑
Janjira Rujirawanich, Boonsong Kongkathip , Ngampong Kongkathip
Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Kasetsart University, Chatuchak, Bangkok 10900, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Dioxolane type 3,4-benzylidene acetals of benzyl b-L-arabinose either as a mixture or pure exo- and endo-
Received 18 September 2010
Received in revised form 24 January 2011
Accepted 27 January 2011
Available online 6 March 2011
isomers cleavaged with BF3ꢀOEt2/Et3SiH in dichloromethane or acetonitrile regioselectively, provided the
4-O-benzyl-3-hydroxy derivative. The reaction with TiCl4/Et3SiH or Cu(OTf)2/Et3SiH provided a mixture
of 3- and 4-O-benzyl derivatives whereas with Cu(OTf)2/BH3ꢀTHF gave only hydrolyzed product. The reg-
ioselectivity of the reaction was proved to be directed by the acetyl substitution at C-2. Benzyl substitu-
tion provided a mixture of 3- and 4-O-benzyl derivatives in 1:1 ratio whereas non-substitution yielded
the same mixture in 2:1 ratio.
Keywords:
Regioselective ring opening
Benzylidene acetal
Arabinose
Ó 2011 Elsevier Ltd. All rights reserved.
Lewis acids
BF3ꢀOEt2
1. Introduction
sult of our finding on the reductive opening of 2-O-acetyl-3,4-O-
benzylidene-b-
L-arabinopyranoside and also 2-O-hydroxyl/ben-
Selective protection of different hydroxyl groups in a carbohy-
drate molecule is a key step in the chemical synthesis of complex
carbohydrates and glycosides. The reductive opening of cyclic ben-
zylidene acetals to the corresponding O-benzyl ethers in a regiose-
lective manner is among the methods of choice and has great
utility in synthetic works.1 In the synthetic field of carbohydrate
chemistry such reactions have been applied to 4,6-O-benzylidene
acetal derivatives and high regioselectivity has been often docu-
mented. A number of effective reagents have been utilized such
as NaBH3CN–HCl,2 Et3SiH–trifluoroacetic acid,3 Me3NꢀBH3–AlCl3
in THF,4 Me3NꢀBH3–BF3ꢀOEt2,5 LiAlH4–AlCl3,6, BH3ꢀTHF–Bu2BOTf7
and Et3SiH–Cu(OTf)2 in acetonitrile.8 To the best of our knowledge
only a few studies have been published on the reductive ring
opening of 3,4-O-benzylidene acetal hexapyranosides and the only
reagent used in the reaction was LiAlH4–AlCl3 in THF.6a,6b However
this reagent is too strong, it reduced not only benzylidene acetal
but also other functionalities such as esters and amides. Our
interest focused on the synthesis of a coupling partner of the
disaccharide moiety of an anticancer OSW-19 which contains 2-
zyl-3,4-O-benzylidene-b-
various Lewis acids and reducing agents.
L
-arabinopyranoside derivatives with
2. Results and discussion
In the investigation of the reductive opening of 3,4-benzylidene
acetal, a model system was designed using the 3,4-O-benzylidene
derivative of b-L-arabinopyranoside, varying protecting groups at
C-2. The 2-acetyl derivative was prepared from the corresponding
unprotected syn-1,2-diol 410,11 by the modified method described
by Josephson.10 Thus treatment of 4 with benzaldehyde dimethyl
acetal in the presence of p-toluenesulfonic acid provided an 1:4
mixture (NMR) of benzyl 2-O-acetyl-exo/endo-3,4-O-benzylidene-
b-
zyl-2-O-acetyl-b-
L
-arabinopyranosides (5exo and 5endo)12 in 80% yield. The ben-
L-arabinose 4 was prepared from b-L-arabinose
in four steps involving, that is, benzylation of 1-OH (1),9,11,13 iso-
propylidenation of 3,4-diOH (2),9,11 acetylation of 2-OH (3),9,11
and final removal of isopropylidene as shown in Scheme 1. Com-
pound 5exo and 5endo were separated by column chromatography.
The endo-configuration of the benzylidene ring of 5endo was con-
firmed on the basis of 1H and 13C NMR spectroscopic data; the ben-
zylidene proton resonated at d 5.82 ppm, the signal of the acetalic
carbon atom appeared at 104.39 ppm, whereas the benzylidene
proton of the exo-isomer 5exo was found at d 6.12 ppm and the sig-
nal of the acetalic carbon atom appeared at 102.78 ppm. These val-
ues are in good agreement with the corresponding data reported in
the literature.6f
O-p-methoxybenzoyl-b-D-xylopyranosyl-(1?3)-2-O-acetyl-L-ara-
binopyranoside. The acceptor arabinopyranose derivative with a
free 3-hydroxyl group and 2-O-acetyl group was required for the
synthesis of the OSW-1 disaccharide. In this paper we report a re-
⇑
Corresponding author. Tel.: +662 5625444x2235; fax: +662 5793955.
E-mail address: fscibsk@ku.ac.th (B. Kongkathip).
0008-6215/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2011.01.031

928
J. Rujirawanich et al. / Carbohydrate Research 346 (2011) 927–932
HO
HO
O
O
a
b
O
O
O
H
O
H
O
H
OH
AcO
OH
OBn
OBn
OBn
1
2
3
O
HO
HO
R1
R2
O
O
c
O
d
H
H
AcO
AcO
OBn
R1 = Ph, R2= H
5endo,
OBn
4
5exo,
R
1 = H, R2= Ph
Scheme 1.
In our initial studies the mixture of 5exo and 5endo (1:4)
ing agent Et3SiH is not strong enough. Indeed when Cu(OTf)2 was
used with BH3ꢀTHF in CH3CN, benzyl 4-O-benzyl derivative A5
was the only isomer obtained as a minor product (18.1%) together
with the hydrolyzed product 4 (23.4%) and the major product re-
sulted from the isomerization of the starting compound (entry
10). However when the reaction was carried out in CH2Cl2 or with-
out solvent, a mixture of A5 and B5 was obtained in 5:3 ratio (entry
11, 12).
From the above result, the regioselective ring opening of 5exo
and 5endo may be altered by the presence of 2-OAc functional
group which is capable of coordinating with the BF3 to form the
bidentate complex 8 (see Scheme 2), which allows cleavage of
the C4–O bond to give benzyl 4-O-benzyl isomer A5 as a major
product whereas TiCl4 and Cu(OTf)2 can form complex at both O-
3 and O-4 of the benzylidene ring to permit O-C cleavage to give
a mixture of both benzyl 3-O- and 4-O-benzyl isomers.
obtained directly from the synthesis was employed. TiCl4 in conju-
gation with Et3SiH was found to be effective in the reductive ring
opening of our benzylidene acetal compound at ꢁ78 °C to give
A5 and B5 but regioselectivity was low and some starting material
was recovered (Table 1, entry 1). When BF3ꢀOEt2, a weaker Lewis
acid was used and the amount of Et3SiH was increased, the reac-
tion was completed in 10 min at ꢁ78 °C to give 53.3% of 4-O-ben-
zyl-3-hydroxy derivative (A5) and only trace amount of other
isomer B5 was detected (entry 2). This reagent at 0 °C gave a sim-
ilar yield of A5 but the hydrolyzed product (4) was increased (entry
3). The best result was obtained when the isomeric mixture 5exo/
5endo was treated with BF3ꢀOEt2/Et3SiH in acetonitrile at 0 °C the
benzyl 4-O-benzyl derivative A5 in 69.4% yield, with only trace
amount of the B5 isomer and hydrolysis product 4 (entry 4), while
the pure 5exo or 5endo isomer gave similar selectivity but a signif-
icant amount of hydrolysis product was isolated.
Cu(OTf)2, a catalyst which was used to alter the ring opening of
the 4,6-benzylidene compound8 was also examined in our 3,4-ben-
zylidene compounds. The reaction of the isomeric mixture 5exo
and 5endo with Cu(OTf)2/Et3SiH in CH3CN at 0 °C gave only hydro-
lyzed product, no ring opening product was obtained (entry 7).
Pure 5endo in CH2Cl2 also yielded similar result to that of entry
7, only hydrolyzed product was obtained and trace amount of star-
ing compound (5exo/5endo = 1:1) was detected (entry 8). Increas-
ing the reducing agent (entry 9) provided slight increase of
hydrolyzed product. Since only hydrolyzed products were obtained
when using this reagent system, it may be assumed that the reduc-
To prove our assumption, the free hydroxyl 3,4-benzylidene
acetal compound 66f,14 and the benzyl substituted compound 7
were prepared by the method described by Liptak.6c Treatment
of benzyl b-L-arabinose(1) with a,a-dimethoxytoluene resulted in
a 4:5 mixture of the 6exo and 6endo isomers in good yield. Since
these isomers could not be completely separated by column
chromatography, the mixture was then benzylated and the separa-
tion of the resulting mixture of 7exo and 7endo was successful
(Scheme 3).
We examined the cleavage of 6 and 7 by using BF3ꢀOEt2/Et3SiH
at 0 °C; the results are shown in Table 1. Treatment of 6exo and
6endo with BF3ꢀOEt2/Et3SiH in CH3CN at 0 °C gave 4-O-benzyl ether
Table 1
Reductive cleavage of 3,4-O-benzylidene acetals of benzyl b-L-arabinose derivatives
Entry
Substrate (ratio)
Lewis acid
Reducing
agent
Quantity
(equiv)
Solv.
Temp (°C)/time
Yield (%)
C
A
B
Starting compd
4-O-Bzy type
3-O-Bzy type
Hydrolysis type
(%) (exo:endo)
1
2
3
4
5
6
7
8
5exo/endo (1:4)
5exo/endo (1:4)
5exo/endo (1:4)
5exo/endo (1:4)
5endo
5exo
5exo/endo (1:4)
5endo
TiCl4
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
BH3ꢀTHF
BH3ꢀTHF
BH3ꢀTHF
Et3SiH
Et3SiH
Et3SiH
1.35
12.0
12.0
12.0
12.0
—
2.0
2.0
12.0
5.0
5.0
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
CH3CN
CH3CN
CH3CN
CH2Cl2
CH2Cl2
CH3CN
CH2Cl2
—
ꢁ78/5 h
78/10 min
0/1 h
0/30 min
0/30 min
0/30 min
0/30 min
rt/24 h
41.0
53.3
52.5
69.4
57.9
64.3
—
20.2
Trace
Trace
Trace
Trace
Trace
—
—
—
—
28.7
28.7
9.1
12.8
23.4
—
17.0
—
—
—
—
—
—
Trace (1:1)
—
54.7 (2:1)
—
—
—
—
—
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
5.43
21.6
Trace
31.0
20.5
64.2
48.5
58.4
23.4
—
—
—
9
5endo
rt/24 h
rt/24 h
10
11
12
13
14
15
5exo/endo (1:4)
5exo/endo (1:4)
5exo/endo (1:4)
6exo/endo (4:5)
7endo
18.1
47.6
45.6
20.7
36.3
25.9
rt/1.25 min
rt/24 h
0/10 min
0/30 min
0/30 min
5.0
2.0
2.0
2.0
—
—
29.5
35.5
CH3CN
CH2Cl2
CH2Cl2
7exo

J. Rujirawanich et al. / Carbohydrate Research 346 (2011) 927–932
929
O
H
O
Ph
O
H
O
O
Ph
Ph
O
O
O
O
O
BF3.OEt2
H
H
F3B
O
H
F3B
AcO
O
O
OBn
OBn
OBn
5
8
9
O
OBn
Ph
H
O
O
Et3SiH
H
O
H
HO
BnO
AcO
A5
OBn
OBn
Scheme 2.
OH
O
O
Ph
Ph
H
H
O
O
O
H
OBn
H
H
HO
O
O
HO
HO
BnO
OBn
OBn
6 exo
7 exo
1
6 endo
7 endo
Scheme 3.
(A6) and 3-O-benzyl ether derivative (B6) in 2:1 ratio (entry 13)
whereas the benzyl substitution of both 7endo and 7exo isomers
yielded a mixture of A7 and B715 in 3:1 and 1:1 ratios, respectively,
as well as a considerable amount of hydrolyzed product C7 (entry
14 and 15). These results clearly indicated that the direction of ring
cleavage by BF3ꢀOEt2/Et3SiH is strongly influenced by the acetate
substitution at C-2.
In conclusion, among the Lewis acids tested by us, BF3ꢀOEt2/Et3-
SiH in acetonitrile gave the best result for highly regioselective ring
opening of 3,4-O-benzylidene acetals of arabinoside derivatives
bearing 2-acetoxyl, to furnish the corresponding 4-O-benzyl ethers
with the 3-hydroxy unsubstituted. The reaction condition is mild,
and benzyl or ester protecting groups in the substrates are toler-
ated. Our investigation also revealed that the regioselective ring
opening was not directed by the stereochemistry of the acetalic
center but was altered by an adjacent-acetyl group which could
coordinate with the BF3 to form a bidentate metal complex
intermediate.
and solvents were purchased from the Fluka Co. Ltd as analytical
grade and solvents were purified by general methods before being
used.
3.2. Benzyl 3,4-acetonide-b-L-arabinose (2)
To a solution of 1 (40.0 mg, 0.17 mmol) in DMF (0.8 mL) was
added 2,2-dimethoxypropane (0.024 mL, 0.2 mmol) and p-
TsOHꢀH2O (3.1 mg, 0.02 mmol) at room temperature. The reaction
mixture was stirred at room temperature for 5 h and was
quenched with saturated aqueous sodium bicarbonate. The organic
layer was separated and the water layer was extracted with
methylene chloride three times. The combined organic layer was
washed with brine, dried over anhydrous sodium sulphate, filtered,
and concentrated in vacuo. The residue was purified by flash col-
umn chromatography (EtOAc/hexane, 3:2) to give 2 as a colorless
syrup. (38.0 mg, 82.0%): ½a D20
ꢂ
+272 (c 0.20, CH2Cl2); IR (NaCl) mmax
3437 (O–H), 1219 (C–O), 1079 (C–O) cmꢁ1
;
1H NMR (400 MHz,
CDCl3): d 7.44 (ArH, m, 2H), 7.19 (ArH, m, 3H), 4.49 (CH-1, d,
J = 9.0 Hz, 1H), 4.16 (CH-4, m, 1H), 4.14 (CH2-5, m, 1H), 4.01 (CH-
3, t, J = 6.1 Hz, 1H), 3.68 (CH2-5, m, 1H), 3.58 (CH-2, ddd, J = 3.5,
6.6, 9.0 Hz, 1H), 1.37 (CH3, s, 3H), 1.26 (CH3, s, 3H); 13C NMR
(100 MHz, CDCl3): d 132.5, 132.2, 132.2, 128.8, 128.8, 127.7,
109.9, 87.8, 78.1, 72.6, 71.2, 65.2, 27.7, 25.9.
3. Experimental section
3.1. General methods
Proton nuclear magnetic resonance (1H NMR) spectra and car-
bon nuclear magnetic resonance (13C NMR) spectra were recorded
on a Varian Gemini 300 spectrophotometer. Chemical shifts were
recorded as d values in ppm. Spectra were acquired in CDCl3 unless
otherwise stated. The peak due to residual CHCl3 (7.26 ppm for 1H
and 77.23 ppm for 13C) was used as the internal reference. Cou-
pling constants (J) are given in Hz, and multiplicity is defined as
follows: br = broad, s = singlet, d = doublet, dd = doublet of dou-
blets, dt = doublet of triplet, t = triplet, q = quartet, m = multiplet.
Infrared (IR) spectra were recorded in cmꢁ1 on a Perkin–Elmer
2000 Fourier transform infrared spectrophotometer at the Chemis-
try Department, Faculty of Science, Kasetsart University. Samples
were analyzed as KBr disks. Mass spectra were obtained on an Agi-
lent Technology 1100 series LL/MSD Trap. Melting points (mp)
were determined on a Fisher John apparatus and MEL-TEMP capil-
lary melting point apparatus at the Chemistry Department, Kasets-
art University and are reported uncorrected in °C. All chemicals
3.3. Benzyl 2-O-acetyl-3,4-acetonide-b-L-arabinose (3)
Compound 2 (20.0 mg, 0.07 mmol) was dissolved in methylene
chloride (1 mL). Triethylamine (0.02 mL, 0.11 mmol), acetyl anhy-
dride (0.008 mL, 0.09 mmol), and DMAP (0.2 mg) were added.
The reaction mixture was stirred at room temperature for 2 h then
was quenched with saturated aqueous sodium bicarbonate. The or-
ganic layer was separated and the water layer was extracted with
methylenechloride three times. The combined organic layer was
washed with brine and dried over anhydrous sodium sulphate.
The solvent was removed and the product was purified by flash
column chromatography (EtOAc/hexane, 1:4) to give 3 as pale yel-
low syrup (20.8 mg, 91.0%): ½a D20
ꢂ
+216 (c 0.28, CH2Cl2); IR (NaCl)
mmax 1753 (C@O), 1222 (C–O–C), 1052 (C–O–C) cmꢁ1
;
1H NMR
(400 MHz, CDCl3): d 7.20–7.33 (ArH, m, 5H), 4.94 (CH-1, d,

930
J. Rujirawanich et al. / Carbohydrate Research 346 (2011) 927–932
J = 3.6 Hz, 1H), 4.85 (CH-2, dd, J = 8.9 Hz, 1H), 4.66 (–OCH2Ar, d,
J = 11.8 Hz, 1H), 4.65 (–OCH2Ar, d, J = 12.4 Hz, 1H), 4.46 (–OCH2Ar,
d, J = 11.8 Hz, 1H), 4.44 (–OCH2Ar, d, J = 12.3 Hz, 1H), 3.96–4.02
(CH-3, m, 1H), 3.71 (CH-4, dd, J = 3.5, 2.1 Hz, 1H), 3.75 (CH2-5,
dd, J = 12.6, 1.9 Hz, 1H), 3.68 (CH2-5, dd, J = 12.6, 1.1 Hz, 1H), 2.36
(OH, br d, J = 10.3 Hz, 1H), 2.01 (–OCOCH3, s, 3H); 13C NMR
(100 MHz, CDCl3): d 169.4, 133.9, 131.6, 131.6, 128.8, 128.8,
127.4, 110.3, 85.6, 75.4, 71.9, 71.1, 64.0, 27.5, 26.2, 20.8.
3.6. Benzyl exo/endo-3,4-O-benzylidene-b-
(6exo and 6endo)
L-arabinopyranosides
To a solution of 1 (100 mg, 0.42 mmol) in dry N,N-dimethyl-
formamide (1.5 mL) were added -dimethoxytoluene (0.09 mL,
a,a
0.67 mmol) and para-toluenesulfonic acid (0.8 mg, 0.004 mmol).
The reaction mixture was stirred at room temperature for 24 h. Tri-
ethylamine was added before concentration and the syrup which
was dissolved in EtOAc was extracted with saturated aqueous
NaHCO3, water, dried over anhydrous Na2SO4 and concentrated
in vacuo. The residue was purified by flash column chromatogra-
phy (EtOAc/hexane, 1:4) to afford 6exo and 6endo as a white solid
3.4. Benzyl 2-O-acetyl-b-L-arabinose (4)
A solution of 3 (312 mg, 0.96 mmol) in 70% aqueous acetic acid
(4.7 mL) was stirred at 70 °C for 1 h and then concentrated in va-
cuo, and the traces of acetic acid and water were removed by co-
evaporation with toluene several times. The residue was purified
by flash column chromatography (EtOAc/hexane, 7:3) to afford 4
as a white solid (264 mg, 96%): mp 98–100 °C (hexane/Et2O) (lit.10
in 4:5 ratio (106 mg, 77.2%); ½a D20
ꢂ
+139 (c 0.21, CH2Cl2); IR (NaCl)
mmax 3436 (OH), 1091, 1067, 1022 (C–O–C) cmꢁ1
;
1H NMR
(400 MHz, CDCl3) d 7.48–7.53 (ArH, m, 16H), 7.43–7.44 (ArH, m,
2H), 7.27–7.40 (ArH, m, 2H), 6.16 (CH-6-exo, s, 1H), 5.85 (CH-6-
endo, s, 1H), 4.96 (CH-1-exo, d, J = 3.7 Hz, 1H), 4.91 (CH-1-endo,
d, J = 3.6 Hz, 1H), 4.53 (–OCH2Ar-exo, d, J = 11.80 Hz, 2H), 4.75 (–
OCH2Ar-exo, d, J = 11.8 Hz, 2H), 4.54 (–OCH2Ar-endo, d,
J = 11.8 Hz, 2H), 4.78 (–OCH2Ar-endo, d, J = 11.7 Hz, 2H), 4.41
(CH-2-exo, dd, J = 7.1, 5.6 Hz, 1H), 4.33 (CH-2-endo, dd, J = 6.4,
6.4 Hz, 1H), 4.14–4.16 (CH-3-exo, m, 1H), 4.23–4.25 (CH-3-endo,
m, 1H), 3.89 (CH-4-exo, dd, J = 7.2, 3.7 Hz, 1H), 3.86 (CH-4-endo,
dd, J = 6.2, 3.6 Hz, 1H), 3.93 (CH2-5-exo, dd, J = 13.3, 2.5 Hz, 1H),
4.04 (CH2-5-exo, dd, J = 13.3, 2.4 Hz, 1H), 3.98 (CH2-5-endo, dd,
J = 13.2, 1.0 Hz, 1H), 4.00 (CH2-5-endo, d, J = 13.2, 1.6 Hz, 1H); 13C
NMR (100 MHz, CDCl3) d 136.8, 136.9, 137.0, 138.9, 129.6, 129.4,
129.3, 128.9, 128.8, 128.5, 128.5, 128.5, 128.4, 127.9, 126.6,
125.9, 103.8, 102.8, 97.0, 96.5, 76.9, 75.7, 73.0, 74.7, 69.6, 69.6,
67.9, 69.7, 59.4, 59.8; ESIMS: m/z calcd for C19H20O5Na [M+Na]+
351.1208; found 351.1209.
101 °C); ½a 2D0
ꢂ
+230 (c 0.20, CH2Cl2); IR (NaCl) mmax 3428 (O–H),
1732 (C@O), 1372 (C–H), 1244 (C–O–C), 1059 (C–O–C) cmꢁ1
;
1H
NMR (400 MHz, CDCl3) d 7.20–7.31 (ArH, m, 5H), 4.99 (CH-1, d,
J = 3.7 Hz, 1H), 4.94 (CH-2, dd, J = 3.7, 10.0 Hz, 1H), 4.66 and 4.44
(–OCH2Ar, J = 12.3 Hz, 2H), 4.01 (CH-3, dd, J = 3.5, 10.0 Hz, 1H),
3.93–3.94 (CH-4, m, 1H), 3.85 (CH2-5, dd, J = 1.4, 12.6 Hz, 1H),
3.68 (CH2-5, dd, J = 2.0, 12.6 Hz, 1H), 2.03 (–OCOCH3, s, 3H), 2.95
(–OH, br s, 2H); 13C NMR (100 MHz, CDCl3) d 170.4, 137.1, 128.4,
127.9, 127.6, 95.2, 73.5, 72.8, 72.2, 69.4, 58.8, 20.9; ESIMS: m/z
calcd for C14H18O6Na [M+Na]+ 305.1001; found: 305.1005.
3.5. Benzyl 2-O-acetyl-exo/endo-3,4-O-benzylidene-b-
arabinopyranosides (5exo/5endo)
L-
To a solution of 4 (200 mg, 0.71 mmol) in dry acetonitrile
(2.5 mL) were added -dimethoxytoluene (0.16 mL, 1.13 mmol)
a,a
3.7. Benzyl 2-O-benzyl-(7exo) and endo-3,4-O-benzylidene-b-
arabinopyranosides (7endo)
L-
and para-toluenesulfonic acid (1.4 mg, 0.007 mmol). The reaction
mixture was stirred at room temperature for 6 h. Triethylamine
was added before concentration and the syrup which was dis-
solved in EtOAc was extracted with saturated aqueous NaHCO3,
water, dried over anhydrous Na2SO4, and concentrated in vacuo.
The residue was purified by flash column chromatography
(EtOAc/hexane, 3:17 to afford 5exo as a yellow syrup (42.0 mg,
16%) and 5endo as a yellow syrup (168 mg, 64%) and starting mate-
rial (10.0 mg, 5%).
A mixture of 6exo and 6endo (100 mg, 0.30 mmol) were treated
with benzyl bromide (1.1 mL, 9.14 mmol) in the presence of KOH
(205 mg, 3.65 mmol) for 5 h at 50 °C. The reaction mixture was di-
luted with dichloromethane and filtered, and benzyl bromide was
removed by stream distillation. The oily residue was purified by
flash column chromatography (EtOAc/hexane, 1:9) to afford 7exo
as a yellow syrup (44.4 mg, 35%) and 7endo as a yellow syrup
(54.3 mg, 41%).
Compound 5exo: ½a D20
ꢂ
+177 (c 0.30, CH2Cl2); IR (NaCl)
m
max 1749
(C@O), 1272 (C–O–C), 1089, 1026 (C–O–C) cmꢁ1
;
1H NMR
Compound 7exo: ½a D20
ꢂ
+134 (c 0.45, CH2Cl2); IR (NaCl) mmax 1274
(400 MHz, CDCl3) d 7.41–7.44 (ArH, m, 2H), 7.21–7.38 (ArH, m,
8H), 6.12 (CH-6, s, 1H), 5.01 (CH-1, d, J = 4.0 Hz, 1H), 5.00 (CH-2,
dd, J = 11.8, 3.96 Hz, 1H), 4.60 (CH-3, dd, J = 7.7, 5.3 Hz, 1H), 4.67
(–OCH2Ar, d, J = 12.3 Hz, 1H), 4.44 (–OCH2Ar, d, J = 12.3 Hz, 1H),
4.18 (CH-4, dd, J = 5.6, 2.5 Hz, 1H), 3.92 (CH2-5, dd, J = 13.4,
2.6 Hz, 1H), 4.00 (CH2-5, d, J = 13.5 Hz, 1H), 2.03 (–OCOCH3, s,
3H); 13C NMR (100 MHz, CDCl3) d 170.5, 137.0, 138.7, 129.6,
128.6, 128.5, 128.5, 127.6, 126.8, 126.8, 126.8, 126.1, 126.1,
102.8, 95.1, 73.4, 74.1, 69.5, 69.6, 58.8, 20.9; ESIMS: m/z calcd for
(C–O), 1099, 1045, 1019 (C–O–C) cmꢁ1; 1H NMR (400 MHz, CDCl3)
d
7.18–7.38 (ArH, m, 15H), 5.95 (CH-6, s, 1H), 4.88 (CH-1, d,
J = 3.4 Hz, 1H), 4.71 (–OCH2Ar, d, J = 12.5 Hz, 1H), 4.64 (–OCH2Ar,
d, J = 11.56 Hz, 1H), 4.63 (–OCH2Ar, d, J = 12.2 Hz, 1H), 4.62 (CH-3,
dd, J = 8.0, 5.4 Hz, 1H), 4.48 (–OCH2Ar, d, J = 12.3 Hz, 1H), 4.13
(CH-4, m, 1H), 3.92 (CH2-5, dd, J = 13.4, 1.0 Hz, 1H), 3.88 (CH2-5,
dd, J = 13.3, 2.4 Hz, 1H), 3.62 (CH-2, dd, J = 8.0, 3.5 Hz, 1H). 13C
NMR (100 MHz, CDCl3)
d 139.0, 138.1, 137.1, 129.0, 128.4,
128.4, 128.4, 128.3, 128.3, 128.3, 127.9, 127.8, 127.8,
127.8, 127.7, 126.1, 102.6, 95.8, 76.6, 73.8, 73.4, 72.2, 69.3, 58.9;
C
21H22O6Na [M+Na]+ 393.1314; found: 393.1310.
Compound 5endo: ½a D20
ꢂ
+206 (c 0.30, CH2Cl2); IR (NaCl) mmax
ESIMS: m/z calcd for
C
26H26O5Na [M+Na]+ 441.1678; found
1736 (C@O), 1233 (C–O–C), 1067 (C–O–C) cmꢁ1
;
1H NMR
441.1680.
(400 MHz, CDCl3) d 7.41–7.44 (ArH, m, 2H), 7.21–7.38 (ArH, m,
8H), 5.82 (CH-6, s, 1H), 4.98 (CH-1, d, J = 3.5 Hz, 1H), 4.90 (CH-2,
dd, J = 7.8, 3.5 Hz, 1H), 4.46 (CH-3, dd, J = 7.6, 6.4 Hz, 1H), 4.67 (–
OCH2Ar, d, J = 12.3 Hz, 1H), 4.44 (–OCH2Ar, d, J = 12.3 Hz, 1H),
4.25 (CH-4, dd, J = 6.1, 2.2 Hz, 1H), 4.00 (CH2-5, dd, J = 13.5,
3.0 Hz, 1H), 4.08 (CH2-5, d, J = 13.2 Hz, 1H), 1.99 (–OCOCH3, s,
3H); 13C NMR (100 MHz, CDCl3) d 170.3, 136.6, 137.0, 129.1,
128.5, 128.5, 127.6, 127.6, 126.8, 126.8, 126.8, 126.1, 126.1,
104.4, 95.0, 75.8, 72.6, 72.7, 69.6, 58.7, 20.9; ESIMS: m/z calcd for
Compound 7endo: ½a D20
ꢂ
+170 (c 0.24, CH2Cl2); IR (NaCl) mmax
1272 (C–O), 1090, 1068, 1022 (C–O–C) cmꢁ1 1H NMR (400 MHz,
;
CDCl3) d 7.10–7.37 (ArH, m, 15H), 5.84 (CH-6, s, 1H), 4.77 (CH-1,
d, J = 3.4 Hz, 1H), 4.68 (–OCH2Ar, d, J = 12.3 Hz, 1H), 4.59 (–OCH2Ar,
d, J = 12.3 Hz, 1H), 4.47 (–OCH2Ar, d, J = 12.6 Hz, 1H), 4.46 (–
OCH2Ar, d, J = 12.3 Hz, 1H), 4.44 (CH-3, dd, J = 6.0, 1.9 Hz, 1H),
4.23 (CH-4, ddd, J = 6.2, 2.8, 1.3 Hz, 1H), 4.01 (CH2-5, dd, J = 13.4,
1.2 Hz, 1H), 3.96 (CH2-5, dd, J = 13.3, 3.0 Hz, 1H), 3.52 (CH-2, dd,
J = 7.5, 3.4 Hz, 1H). 13C NMR (100 MHz, CDCl3) d 138.1, 137.5,
137.2, 129.2, 128.4, 128.4, 128.4, 128.3, 128.3, 128.2, 127.9,
C
21H22O6Na [M+Na]+ 393.1314; found 393.1310.

J. Rujirawanich et al. / Carbohydrate Research 346 (2011) 927–932
931
127.9, 127.8, 127.8, 127.6, 127.6, 126.6, 103.8, 95.8, 76.7, 75.9,
75.3, 72.0, 69.3, 58.9; ESIMS: m/z calcd for C26H26O5Na [M+Na]+
441.1678; found 441.1670.
s, 2H), 4.44 (–OCH2Ar, d, J = 12.3 Hz, 1H), 3.95–3.96 (CH-4, m,
1H), 3.88 (CH-3, dd, J = 9.8, 3.5 Hz, 1H), 3.77 (CH2-5, dd, J = 12.6,
1.6 Hz, 1H), 3.70 (CH2-5, dd, J = 12.6, 2.2 Hz, 1H), 2.54 (–OH, s,
1H), 1.98 (–OCOCH3, s, 3H); 13C NMR (100 MHz, CDCl3) d 170.3,
137.3, 137.8, 128.5 ꢃ 2, 128.4 ꢃ 2, 128.0, 127.9, 127.7 ꢃ 2,
127.5 ꢃ 2, 95.9, 75.0, 72.4, 69.4, 70.2, 67.3, 61.8, 20.9; ESIMS: m/z
calcd for C21H24O6Na [M+Na]+ 395.1471; found 395.1471.
3.8. Regioselective reductive ring opening of various
benzylidene acetals
3.8.1. General procedure
Method A: To a solution mixture of 5exo and 5endo (110 mg,
0.3 mmol) in dichloromethane (3.0 mL) was added Et3SiH
(0.07 mL, 0.40 mmol). The reaction mixture was cooled to ꢁ78 °C
and a solution of TiCl4 (0.04 ml, 0.33 mmol) in dichloromethane
(0.3 mL) was slowly added. The reaction was stirred at ꢁ78 °C for
5 h. The reaction mixture was poured into ice and extracted with
EtOAc. The organic layer was washed with saturated NaHCO3 and
brine, dried over anhydrous Na2SO4, and concentrated in vacuo.
The residue was purified by flash column chromatography
(EtOAc/hexane, 3:7).
Method B: To a solution mixture of 5exo and 5endo (60.4 mg,
0.16 mmol) in dichloromethane (3.0 mL) at 0 °C was added Et3SiH
(0.32 mL, 1.96 mmol) and BF3ꢀEt2O (0.04 mL, 0.33 mmol). The reac-
tion was stirred at 0 °C for 1 h. The mixture was diluted with
CH2Cl2, washed with saturated NaHCO3 and brine, dried over anhy-
drous Na2SO4, and concentrated in vacuo. The residue was purified
by flash column chromatography (EtOAc/hexane, 3:7).
Method C: To a solution mixture of 5exo and 5endo (56.0 mg,
0.15 mmol) in acetonitrile (3.0 mL) and Et3SiH (0.05 ml,
0.30 mmol) at 0 °C was added Cu(OTf)2 (5.5 mg, 0.02 mmol). The
reaction was warmed to room temperature and stirred for
30 min. The mixture was diluted with CH2Cl2, washed with
saturated NaHCO3 and brine, dried over anhydrous Na2SO4, and
concentrated in vacuo. The residue was purified by flash column
chromatography (EtOAc/hexane, 1:4).
Method D: A 1 M solution of BH3ꢀTHF complex in tetrahydrofu-
ran (0.23 mL, 0.23 mmol) was added to a solution mixture of 5exo
and 5endo (17.0 mg, 0.05 mmol) at room temperature under nitro-
gen. The mixture was stirred for 10 min, and Cu(OTf)2 (1.7 mg,
0.005 mmol) was added. After stirring for 30 min, the mixture
was cooled down to 0 °C, and the reaction was quenched by
sequential addition of triethylamine and methanol (caution: hydro-
gen gas was evolved). The resulting mixture was concentrated at
reduced pressure followed by coevaporation with methanol. The
residue was purified by flash column chromatography (EtOAc/hex-
ane, 3:7).
3.8.4. Benzyl 4-O-benzyl-b-L-arabinopyranoside (A6)
A white solid; mp 114–116 °C; ½a D20
ꢂ
+165 (c 0.23, CH2Cl2); IR
(NaCl) mmax 3430 (O–H), 1266 (C–O–C), 1140, 1077, 1048, 1025
(C–O–C) cmꢁ1 1H NMR (400 MHz, CDCl3) d 7.22–7.32 (ArH, m,
;
10H), 4.94 (CH-1, d, J = 3.3 Hz, 1H), 4.69 (–OCH2Ar, d, J = 12.0 Hz,
1H), 4.66 (–OCH2Ar, d, J = 12.3 Hz, 1H), 4.62 (–OCH2Ar, d,
J = 11.7 Hz, 1H), 4.51 (–OCH2Ar, d, J = 11.5 Hz, 1H), 3.92–3.94
(CH-3, m, 1H), 3.79–3.80 (CH-4, m, 1H), 3.74 (CH2-5, dd, J = 12.2,
2.4 Hz, 1H), 3.71–3.72 (CH2-5, m, 1H), 3.62 (CH-2, dd, J = 3.5,
9.4 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 137.7, 137.1, 128.6,
128.6, 128.6, 128.6, 128.5, 128.0, 128.0, 127.9, 127.8, 127.8, 98.0,
78.1, 71.7, 70.4, 70.4, 69.8, 59.8; ESIMS: m/z calcd for C19H22O5Na
[M+Na]+ 353.1365; found 353.1367.
3.8.5. Benzyl 3-O-benzyl-b-L-arabinopyranoside (B6)
A white crystal; mp 128–129 °C (hexane–CH2Cl2); ½a D20
ꢂ
+147 (c
0.08, CH2Cl2); IR (NaCl) mmax 3437 (O–H), 1257 (C–O–C), 1137, cm
ꢁ1; 1H NMR (400 MHz, CDCl3) d7.20–7.33 (ArH, m, 10H), 4.89 (CH-
1, d, J = 3.4 Hz, 1H), 4.66 (–OCH2Ar, d, J = 12.2 Hz, 1H), 4.49 (–
OCH2Ar, d, J = 11.7 Hz, 1H), 4.43 (–OCH2Ar, d, J = 11.8 Hz, 1H),
4.42 (–OCH2Ar, d, J = 12.2 Hz, 1H), 4.02 (CH-4, ddd, J = 0.3, 3.2,
9.4 Hz, 1H), 3.94–3.95 (CH-3, m, 1H), 3.80 (CH2-5, dd, J = 12.7,
1.3 Hz, 1H), 3.67 (CH-2, dd, J = 9.7, 3.5 Hz, 1H), 3.66 (CH2-5, dd,
J = 12.6, 1.8 Hz, 1H), 2.46 (–OH, s, 1H), 2.43 (–OH, s, 1H); 13C
NMR (100 MHz, CDCl3) d 137.8, 137.3, 130.0, 128.6, 128.6, 128.4,
128.4, 128.1, 128.1, 128.0, 127.9, 95.4, 76.5, 72.3, 69.2, 69.0, 68.5,
62.0; ESIMS: m/z calcd for C19H22O5Na [M+Na]+ 353.1365; found
353.1363.
3.8.6. Benzyl 2-O-benzyl-4-O-benzyl-b-L-arabinopyranoside
(A7)
A colorless needle; mp 78–79 °C (hexane–Et2O); ½a D20
ꢂ
+128
(c 0.23, CH2Cl2); IR (NaCl) mmax 3453 (O–H), 1344 (C–H), 1260
(C–O–C), 1093, 1048, 1025 (C–O–C) cmꢁ1 1H NMR (400 MHz,
;
CDCl3) d 7.20–7.32 (ArH, m, 15H), 4.86 (CH-1, d, J = 3.4 Hz, 1H),
4.65 (–OCH2Ar, d, J = 11.9 Hz, 1H), 4.64 (–OCH2Ar, d, J = 12.2 Hz,
1H), 4.59 (–OCH2Ar, d, J = 11.9 Hz, 1H), 4.55 (–OCH2Ar, d,
J = 12.1 Hz, 1H), 4.50 (–OCH2Ar, d, J = 11.9 Hz, 1H), 4.42 (–OCH2Ar,
d, J = 12.2 Hz, 1H), 4.00–4.06 (CH-3, m, 1H), 3.7 (CH-4, ddd,
J = 3.6, 1.8, 1.8 Hz, 1H), 3.70 (CH-2, dd, J = 9.6, 3.4 Hz, 1H), 3.67–
3.68 (CH2-5, m, 2H), 2.36 (–OH, br d, J = 6.6 Hz, 1H); 13C NMR
(100 MHz, CDCl3) d 138.3, 138.0, 137.4, 128.0 ꢃ 3, 128.4 ꢃ 4,
128.5 ꢃ 3, 127.8 ꢃ 6, 127.7, 96.2, 76.6, 76.2, 72.8, 71.9, 69.2, 69.0,
59.8; ESIMS: m/z calcd for C26H28O5Na [M+Na]+ 443.1834.; found
443.1834.
3.8.2. Benzyl 2-O-acetyl-4-O-benzyl-b-
L
-arabinopyranoside (A5)
Pale yellow syrup; ½a D20
ꢂ
+218 (c 0.30, CH2Cl2); IR (NaCl) mmax
3462 (OH), 1735 (C@O), 1371 (C–H), 1239 (C–O), 1140, 1057,
1026 (C–O–C) cmꢁ1 1H NMR (400 MHz, CDCl3) d 7.19–7.28 (ArH,
;
m, 10H), 4.99 (CH-2, dd, J = 11.4, 3.6 Hz, 1H), 4.98 (CH-1, d,
J = 8.7 Hz, 1H), 4.66 (–OCH2Ar, d, J = 11.8 Hz, 1H), 4.65 (–OCH2Ar,
d, J = 12.4 Hz, 1H), 4.46 (–OCH2Ar, d, J = 11.8 Hz, 1H), 4.44
(–OCH2Ar, d, J = 12.3 Hz, 1H), 3.96–4.02 (CH-3, m, 1H), 3.71 (CH-
4, dd, J = 3.5, 2.1 Hz, 1H), 3.75 (CH2-5, dd, J = 12.6, 1.9 Hz, 1H),
3.68 (CH2-5, dd, J = 12.6, 1.1 Hz, 1H), 2.36 (–OH, br d, J = 10.1 Hz,
1H), 2.01 (–OCOCH3, s, 3H); 13C NMR (100 MHz, CDCl3) d 170.9,
137.3, 137.5, 128.5 ꢃ 2, 128.4 ꢃ 2, 127.9, 127.8, 127.8 ꢃ 2,
127.6 ꢃ 2, 95.9, 76.7, 76.6, 71.5, 69.4, 67.3, 59.0, 20.9; ESIMS: m/z
calcd for C21H24O6Na [M+Na]+ 395.1471; found 395.1460.
3.8.7. Benzyl 2-O-benzyl-3-O-benzyl-b-L-arabinopyranoside
(B7)
Pale yellow syrup; ½a D20
ꢂ
+132 (c 0.11, CH2Cl2); IR (NaCl) mmax
3467 (O–H), 1342 (C–H), 1261 (C–O–C), 1140, 1098, 1067,
1024 cmꢁ1 1H NMR (400 MHz, CDCl3) d 7.21–7.34 (ArH, m, 15H),
;
4.82 (CH-1, d, J = 3.6 Hz, 1H), 4.75 (–OCH2Ar, d, J = 11.5 Hz, 1H),
4.66 (–OCH2Ar, d, J = 12.3 Hz, 1H), 4.63 (–OCH2Ar, d, J = 12.0 Hz,
1H), 4.62 (–OCH2Ar, d, J = 11.5 Hz, 1H), 4.50 (–OCH2Ar, d,
J = 12.0 Hz, 1H), 4.48 (–OCH2Ar, d, J = 12.3 Hz, 1H), 3.93–4.94
(CH-4, m, 1H), 3.88 (CH-3, dd, J = 9.5, 3.5 Hz, 1H), 3.76 (CH-2, dd,
J = 9.5, 3.5 Hz, 1H), 3.73–3.77 (CH2-5, m, 1H), 3.64 (CH2-5, dd,
J = 12.5, 2.0 Hz, 1H), 2.50 (–OH, s, 1H); 13C NMR (100 MHz, CDCl3)
3.8.3. Benzyl 2-O-acetyl-3-O-benzyl-b-
L
-arabinopyranoside (B5)
Pale yellow syrup; ½a D20
ꢂ
+161 (c 0.40, CH2Cl2); IR (NaCl) mmax
3470 (OH), 1737 (C@O), 1370 (C–H), 1238 (C–O), 1143, 1061,
1023 (C–O–C) cmꢁ1 1H NMR (400 MHz, CDCl3) d 7.20–7.30 (ArH,
;
m, 10H), 5.10 (CH-2, dd, J = 9.9, 3.6 Hz, 1H), 5.00 (CH-1, d,
J = 3.6 Hz, 1H), 4.65 (–OCH2Ar, d, J = 12.3 Hz, 1H), 4.60 (–OCH2Ar,

932
J. Rujirawanich et al. / Carbohydrate Research 346 (2011) 927–932
d 138.4, 138.2, 137.3, 128.5 ꢃ 2, 128.4, 128.3 ꢃ 2, 128.2 ꢃ 2, 127.9,
127.8 ꢃ 4, 127.6 ꢃ 2, 96.3, 77.2, 76.7, 73.1, 72.9, 69.0, 67.7, 61.8;
Supplementary data
ESIMS: m/z calcd for
C
26H28O5Na [M+Na]+ 443.1834; found
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.carres.2011.01.031.
443.1839.
3.8.8. Benzyl 2-O-benzyl-b-L-arabinopyranoside (C7)
References
A white foam; mp 128–130 °C (hexane–CH2Cl2); ½a D20
ꢂ
+176
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(c 0.21, CH2Cl2); IR (NaCl) mmax 3357, 3272 (O–H), 1261 (C–O–C),
1129, 1099, 1066, 1020 cmꢁ1 1H NMR (400 MHz, CDCl3) d 7.20–
;
7.33 (ArH), m, 10H), 4.89 (CH-1, d, J = 3.44 Hz, 1H), 4.66 (–OCH2Ar,
d, J = 12.2 Hz, 1H), 4.49 (–OCH2Ar, d, J = 11.8 Hz, 1H), 4.43
(–OCH2Ar, d, J = 11.8 Hz, 1H), 4.41 (–OCH2Ar, d, J = 12.2 Hz, 1H),
3.99–4.03 (CH-3, m, 1H), 3.93–3.94 (CH-4, m, 1H), 3.79 (CH2-5, d,
J = 12.4 Hz, 1H), 3.67 (CH-2, dd, J = 9.7, 3.5 Hz, 1H), 3.65 (CH2-5,
dd, J = 12.6, 1.8 Hz, 1H), 2.49 (–OH, d, J = 12.4 Hz, 1H), 2.48 (–OH,
d, J = 10.7 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 137.8, 137.3,
128.6 ꢃ 2, 128.4 ꢃ 2, 128.1, 128.0 ꢃ 4, 127.9, 95.4, 76.6, 72.2,
69.2, 68.9, 68.5, 62.1; ESIMS: m/z calcd for C19H22O5Na [M+Na]+
353.1365; found 353.1359.
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13. Fletcher, H. G., Jr. Methods Carbohydr. Chem. 1963, 2, 386.
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J.R. is
a Ph.D. student under the Royal Golden Jubilee
Program, the Thailand Research Fund (TRF). We are grateful for
financial support from the TRF, through the Royal Golden Jubilee
Program. Financial supports from the Center of Excellence for
Innovation in Chemistry (PERCH-CIC), Commission on Higher
Education, Ministry of Education and Kasetsart University Research
and Development Institute (KURDI) are also gratefully
acknowledged.