Reductive ring-opening reaction of 1,2-O-benzylidene and 1,2-O-p-methoxybenzylidene-α-D-glucopyranose using diisobutyl aluminum hydride

Article abstract of DOI:10.1081/CAR-200030087

Regioselectivity in the reductive ring-opening reaction of 3,4,6-tri-O-benzyl-1,2-O-benzylidene and 3,4,6-tri-O-benzyl-1,2-O-p- methoxybenzylidene-α-D-glucopyranose using diisobutyl aluminum hydride (DIBAH) was examined. The ratio of the 1-O- and 2-O-p-me

Full text of DOI:10.1081/CAR-200030087

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Reductive Ring‐Opening Reaction
of 1,2‐O‐Benzylidene and
1,2‐O‐p‐Methoxybenzylidene‐α‐D‐glucopyran
Using Diisobutyl Aluminum Hydride
Katsuhiko Suzuki a , Hisato Nonaka a & Masanori Yamaura a
a Department of Environmental Science, Faculty of Science and
Engineering , Iwaki Meisei University , 5‐5‐1 Iino, Chuohdai,
Iwaki‐shi, Fukushima, 970‐8551, Japan
Published online: 17 Aug 2006.
To cite this article: Katsuhiko Suzuki , Hisato Nonaka & Masanori Yamaura (2004) Reductive
Ring‐Opening Reaction of 1,2‐O‐Benzylidene and 1,2‐O‐p‐Methoxybenzylidene‐α‐D‐glucopyranose
Using Diisobutyl Aluminum Hydride, Journal of Carbohydrate Chemistry, 23:4, 253-259, DOI: 10.1081/
CAR-200030087
To link to this article: http://dx.doi.org/10.1081/CAR-200030087
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JOURNAL OF CARBOHYDRATE CHEMISTRY
Vol. 23, No. 4, pp. 253–259, 2004
Reductive Ring-Opening Reaction of 1,2-O-
Benzylidene and 1,2-O-p-Methoxybenzylidene-a-D-
glucopyranose Using Diisobutyl Aluminum Hydride
*
Katsuhiko Suzuki, Hisato Nonaka, and Masanori Yamaura
Department of Environmental Science, Faculty of Science and Engineering,
Iwaki Meisei University, Iwaki-shi, Fukushima, Japan
CONTENTS
ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
II. RESULTS AND DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . 254
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258
ABSTRACT
Regioselectivity in the reductive ring-opening reaction of 3,4,6-tri-O-benzyl-1,2-O-
benzylidene and 3,4,6-tri-O-benzyl-1,2-O-p-methoxybenzylidene-a-D-glucopyranose
using diisobutyl aluminum hydride (DIBAH) was examined. The ratio of the
*
Correspondence: Masanori Yamaura, Department of Environmental Science, Faculty of Science
and Engineering, Iwaki Meisei University, 5-5-1 Iino, Chuohdai, Iwaki-shi, Fukushima 970-8551,
Japan; Fax: þ81-246-29-0577; E-mail: yamaura@iwakimu.ac.jp.
253
DOI: 10.1081/CAR-200030087
0732-8303 (Print); 1532-2327 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

254
Suzuki, Nonaka, and Yamaura
1-O- and 2-O-p-methoxybenzyl ethers, which were generated from endo-type
1,2-O-p-methoxybenzylidene, was variable by the change of solvent.
Key Words: Benzylidene; Acetal; Reduction; Ring-opening reaction; Protecting
group.
INTRODUCTION
Benzylidene acetal, which can fix the ring conformation and react oxidatively or
reductively, is a useful protective group in carbohydrate chemistry.[1,2] 4,6-O-Benzylidene
aldohexopyranoses have been the most popular precursors, and numerous studies on the
ring-opening reaction are known.[3] In the reaction of benzylidene acetal, the regioselec-
tivity is dependent on a steric and/or an electronic factor.[4] However, the reaction of the
1,2-O-benzylidene derivatives linking the anomeric position is not yet well known. Very
recently, an improved and practical synthesis of 1,2-O-benzylidene and 1,2-O-p-methoxy-
benzylidene hexopyranoses was reported, and a study on the regioselectivity in the reduc-
tive ring-opening reaction of 1,2-O-benzylidene and 1,2-O-p-methoxybenzylidene sugars
was started.[5,6] The benzylidene sugar can generate two regiospecific benzyl ethers by the
reductive ring-opening reactions.[7–16] However, the nature of the 1,2-O-benzylidene hexo-
pyranoses, which differs from that of the 4,6-O-benzylidene hexopyranoses, is interesting.
The ratio of the 1-O- and 2-O-benzyl ethers, which were obtained from the reductive
ring-opening reaction of 1,2-O-benzylidene-a-D-glucopyranose, was variable by the
conditions. This result prompted us to investigate reaction conditions for controlling
the direction of this ring-opening (Fig. 1).
In this communication, we would like to report the reductive ring-opening reaction of
1,2-O-benzylidene and 1,2-O-p-methoxybenzylidene-a-D-glucopyranose using diisobutyl
aluminum hydride (DIBAH).
RESULTS AND DISCUSSION
The acetal-bond cleavage of the 4,6-O-benzylidene and 4,6-O-p-methoxybenzylidene
derivatives by the reaction with DIBAH is known,[17] and we applied it to 1,2-O-benzy-
lidene and 1,2-O-p-methoxybenzylidene derivatives. In the reduction of benzylidene
acetal, it has been known that the stereochemistry of the benzylidene position influences
Figure 1. Reductive ring-opening reaction of 1,2-O-benzylidene-a-D-glucopyranose.

Reductive Ring-Opening Reaction
255
Scheme 1. Reagents and conditions: (a) NaBH4, KI, MeCN; (b) NaOMe, MeOH; (c) BnBr,
NaH, DMF.
the direction of ring-opening.[7,11] To compare the regioselectivity, endo- and exo-isomers
of 1,2-O-benzylidene and 1,2-O-p-methoxybenzylidene-a-D-glucopyranoses (endo-7,
exo-7, endo-8, and exo-8) were prepared as shown in Sch. 1.
1,2-O-Benzylidene-a-D-glucopyranose 3 and 1,2-O-p-methoxybenzylidene-a-D-
glucopyranose 4 were synthesized from the corresponding 2-O-benzoyl-glucopyranosyl
bromide
1 or 2-O-p-methoxybenzoyl-glucopyranosyl bromide 2 by reductive
cyclization.[5] Although the endo- and exo-isomers of 3 and 4 or 3,4,6-hydroxy-1,2-O-ben-
zylidene 5 and 3,4,6-hydroxy-1,2-O-p-methoxybenzylidene 6 were difficult to separate,
the corresponding 3,4,6-tri-O-benzyl derivatives were separable smoothly into the
diastereomeric components by silica gel column chromatography. The structures of
1
endo-7, exo-7, endo-8, and exo-8 were confirmed by H NMR.a The coupling constants
aSelected spectral data for 3,4,6-tri-O-benzyl-1,2-O-(R)-benzylidene-a-D-glucopyranose endo-7:
[a]D þ 48.48 (c 1.0, CH2Cl2); 1H-NMR (CDCl3) d 7.54–7.16 (m, 20H, aromatic), 5.87 (s, 1H, ben-
zylidene), 5.78 (d, 1H, J1,2 ¼ 5.5 Hz, H-1), 4.72, 4.62, 4.59, 4.58, 4.48, 4.39 (each d, 6H, J ¼ 11.4–
12.1 Hz, benzyl), 4.30 (dd, 1H, J2,3 ¼ 3.7 Hz, H-2), 4.04 (dt, 1H, J4,5 ¼ 9.5, J5,6a ¼ J5,6b ¼ 3.3 Hz,
H-5), 3.99 (dd, 1H, J3,4 ¼ 4.8 Hz, H-3), 3.77 (dd, 1H, H-4), 3.67 (d, 2H, H-6a and 6b); 3,4,6-tri-O-
benzyl-1,2-O-(S)-p-benzylidene-a-D-glucopyranose exo-7: [a]D þ 42.88 (c 1.0, CH2Cl2); m.p.: 53–
548C (crystallized from EtOH); 1H-NMR (CDCl3) 7.46–7.26 (m, 20H, aromatic), 6.25 (s, 1H, ben-
zylidene), 5.80 (d, 1H, J1,2 ¼ 4.8 Hz, H-1), 4.79, 4.73, 4.66, 4.60, 4.52, 4.48 (each d, 6H, J ¼ 10.1–
12.5 Hz, benzyl), 4.36 (dd, 1H, J2,3 ¼ 4.8 Hz, H-2), 3.97 (dd, 1H, J3,4 ¼ 6.2 Hz, H-3), 3.93 (dt, 1H,

256
Suzuki, Nonaka, and Yamaura
of J1,2, J2,3, and J3,4 of both isomers were in good agreement with the known tri-O-acetate
derivatives, respectively.[18] A NOE was observed between H-2 and the methine proton of
the benzylidene group in the endo-isomers (endo-7 and endo-8). On the other hand, the
exo-isomers (exo-7 and exo-8) showed a NOE between H-3 and the methine proton of
the benzylidene group.
Isolated endo-7, exo-7, endo-8 and exo-8 were used in the reaction with DIBAH.
Treatment of endo-7, exo-7, endo-8, or exo-8 with DIBAH gave the corresponding
ether, 2-O-Bn 9[19] (or 2-O-Bn 10) and 1-O-Bn 11 or 2-O-pMBn 12 and 1-O-pMBn 13
(Fig. 2). Results of the reductive ring-opening reaction are summarized in Table 1.
The ratio of 2-O-Bn 9 and 1-O-Bn 11, which were generated from endo-type
1,2-O-benzylidene endo-7, was variable by the conditions. The change in regioselectivity
by the solvent, temperature and reaction time was observed. In the reaction using CH2Cl2
J4,5 ¼ 9.5, J5,6a ¼ J5,6b ¼ 2.9 Hz, H-5), 3.75 (dd, 1H, H-4), 3.71 (d, 2H, H-6a and 6b); 3,4,6-tri-O-
benzyl-1,2-O-(R)-p-methoxybenzylidene-a-D-glucopyranose endo-8: [a]D þ 60.88 (c 1.0, CH2Cl2);
1
m.p.: 76–778C (crystallized from EtOH); H-NMR (CDCl3) d 7.45 (d, 2H, J ¼ 8.8 Hz, aromatic),
7.35–7.17 (m, 15H, aromatic), 6.86 (d, 2H, J ¼ 8.8 Hz, aromatic), 5.83 (s, 1H, benzylidene), 5.76
(d, 1H, J1,2 ¼ 5.1 Hz, H-1), 4.71, 4.62, 4.59, 4.58, 4.49, 4.39 (each d, 6H, J ¼ 11.4–12.5 Hz,
benzyl), 4.28 (dd, 1H, J2,3 ¼ 4.4 Hz, H-2), 4.06 (dt, 1H, J4,5 ¼ 9.5, J5,6a ¼ J5,6b ¼ 2.9 Hz, H-5),
4.00 (dd, 1H, J3,4 ¼ 4.4 Hz, H-3), 3.78 (s, 3H, OMe), 3.76 (dd, 1H, H-4), 3.68 (d, 2H, H-6a and
6b); 3,4,6-tri-O-benzyl-1,2-O-(S)-p-methoxybenzylidene-a-D-glucopyranose exo-8: [a]D þ 38.68
1
(c 1.0, CH2Cl2); m.p.: 73–748C (crystallized from EtOH); H-NMR (CDCl3) 7.38–7.20 (m, 17H,
aromatic), 6.91 (d, 2H, J ¼ 8.8 Hz, aromatic), 6.20 (s, 1H, benzylidene), 5.79 (d, 1H,
J1,2 ¼ 5.1 Hz, H-1), 4.80, 4.74, 4.67, 4.60, 4.51, 4.49 (each d, 6H, J ¼ 10.3–12.1 Hz, benzyl),
4.37 (dd, 1H, J2,3 ¼ 5.1 Hz, H-2), 3.95 (dd, 1H, J3,4 ¼ 6.2 Hz, H-3), 3.91 (dt, 1H, J4,5 ¼ 9.5,
J5,6a ¼ J5,6b ¼ 2.9 Hz, H-5), 3.81 (s, 3H, OMe), 3.75 (dd, 1H, H-4), 3.71 (d, 2H, H-6a and 6b);
2,3,4,6-tetra-O-benzyl-a-D-glucitol 10: [a]D þ 15.48 (c 1.0, CH2Cl2); 1H-NMR (CDCl3) 7.41–
7.21 (m, 20H, aromatic), 4.71, 4.67, 4.65, 4.62, 4.58, 4.55, 4.54, 4.50 (each d, 8H, J ¼ 11.4–12.8
Hz, benzyl), 4.06–4.00 (m, 1H, H-5), 3.89 (dd, 1H, J2,3 ¼ 6.6, J3,4 ¼ 3.7 Hz, H-3), 3.82–3.53
(m, 6H, H-1a, 1b, 2, 4, 6a and 6b), 2.95 (d, 1H, J5,
¼ 5.1 Hz, OH), 2.10 (t, 1H,
OH
J1a,OH ¼ 6.4 Hz, OH); benzyl 3,4,6-tri-O-benzyl-a-D-glucopyranoside 11: [a]D þ 90.68 (c 1.0,
CH2Cl2); 1H-NMR (CDCl3) 7.39–7.12 (m, 20H, aromatic), 5.03 (d, 1H, J1,2 ¼ 3.7 Hz, H-1),
4.93, 4.83, 4.82, 4.75, 4.68, 4.54, 4.51, 4.48 (each d, 8H, J ¼ 9.9–12.8 Hz, benzyl), 3.82 (ddd,
1H, J4,5 ¼ 9.9, J5,6a ¼ 3.3, J5,6b ¼ 1.8 Hz, H-5), 3.79 (dd, 1H, J2,3 ¼ 9.9 Hz, H-2), 3.74 (dd, 1H,
J3,4 ¼ 8.8 Hz, H-3), 3.74 (dd, 1H, J6a,6b ¼ 10.6 Hz, H-6a), 3.66 (dd, 1H, H-4), 3.62 (dd, 1H,
H-6b), 2.13 (d, 1H, J2,OH ¼ 8.4 Hz, OH); 3,4,6-tri-O-benzyl- 2-O-p-methoxybenzyl-D-glucopyra-
nose 12: [a]D þ 40.08 (c 0.6, CH2Cl2); mp: 122–1238C (crystallized from EtOH); 1H-NMR
(CDCl3) d 7.33–7.20, 7.14–7.10 and 6.83–6.79 (m, 19H, aromatic), 5.16 (br s, 0.8H, H-1a),
4.94–4.44 (m, 8.2H, benzyl and H-1b), 4.36 (d, 0.2H, J1,OH ¼ 5.5 Hz, OH), 4.03 (m, 0.8H,
H-5a), 3.97 (t, 0.8H, J2,3 ¼ J3,4 ¼ 9.2 Hz, H-3a), 3.74 (s, 2.4H, OMea), 3.74 (s, 0.6H, OMeb),
3.68 (d, 0.8H, J1,OH ¼ 2.6 Hz, OH), 3.66 (dd, 0.8H, J5,6a ¼ 4.0, J6a,6b ¼ 9.9 Hz, H-6aa), 3.66–
3.46 (m, 1.8H, H-3b, 4b, 5b, 6ab, 6ba and 6bb), 3.38 (t, 0.2H, J1,2 ¼ J2,3 ¼ 8.3 Hz, H-2b); p-meth-
oxybenzyl 3,4,6-tri-O-benzyl-a-D-glucopyranoside 13: [a]D þ 86.68 (c 1.0, CH2Cl2); 1H-NMR
(CDCl3) 7.37–7.21 (m, 15H, aromatic), 7.15–7.12 (m, 2H, aromatic), 6.87–6.85 (d, 2H,
J ¼ 8.4 Hz, aromatic), 4.99 (d, 1H, J1,2 ¼ 3.3 Hz, H-1), 4.92, 4.81, 4.81, 4.67, 4.64, 4.51, 4.48,
4.47 (each d, 8H, J ¼ 10.6–12.5 Hz, benzyl), 3.82 (ddd, 1H, J4,5 ¼ 9.9, J5,6a ¼ 3.3,
J5,6b ¼ 1.5 Hz, H-5), 3.77 (s, 3H, OMe), 3.79–3.75 (m, 2H, H-2 and 3), 3.74 (dd, 1H,
J6a,6b ¼ 10.6 Hz, H-6a), 3.64 (dd, 1H, J3,4 ¼ 8.1 Hz, H-4), 3.63 (dd, 1H, H-6b).

Reductive Ring-Opening Reaction
257
Figure 2. Products of the ring-opening reactions.
or toluene as a solvent, the reaction smoothly proceeded to give 2-O-Bn 9 and 1-O-Bn 11
with the former preference (entry 1, 2). On the other hand, this reaction did not occur in the
THF solution, and endo-7 was recovered quantitatively (entry-3). It seems that DIBAH
lost the reactivity to contact with O1 or O2 of endo-7 by the Lewis basicity of THF.
When the reaction was started at 278 to 2208C to control the reaction site, a reverse
of the regioselectivity was observed (entry 4, 5). Under these conditions, the hemiacetal
moiety of the product (2-O-Bn 9) was reduced to the corresponding alditol (2-O-Bn 10)
because of the long reaction time. In this case, 2-O-Bn 10, which must be transformed
from the O1-aluminum complex via O5-aluminum complex, appeared within the time
the starting compound endo-7 remained. This result suggested that two types of the alumi-
num complex with the oxygen atoms, O1 and O2, of endo-7 are estimated to exist as an
Table 1. Reductive ring-opening reactions of 1,2-O-benzylidene or 1,2-O-p-methoxybenzylidene-
a-D-glucopyranose derivatives (endo-7, endo-8, exo-7, and exo-8) with DIBAH.
Temperature
(8C)
Entry
Substrate
Time
Solvent
Product (Yield %)a
1
2
endo-7
endo-7
endo-7
endo-7
endo-7
endo-7
endo-8
endo-8
endo-8
endo-8
exo-7
0
0
5 min
15 min
5 hr
CH2Cl2
Toluene
THF
9 (15)
9 (30)
—
11 (85)
11 (70)
—
3
250 to reflux
220 to 0
278 to 20
0 to 20
250
250
278
250 to 210
0
4
3 hr
5 hr
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Toluene
CH2Cl2
THF
10 (75)
10 (89)
10 (72)
12 (23)
12 (23)
12 (18)
12 (88)
9 (89)
11 (25)
11 (11)
11 (28)
13 (77)
13 (77)
13 (82)
13 (12)
11 (11)
—
5
6b
7
3 hr
5 min
15 min
15 min
5 hr
8
9
10
11
12
13
14
5 min
15 min
15 min
5 hr
CH2Cl2
CH2Cl2
CH2Cl2
THF
exo-7
exo-8
exo-8
278 to 20
278
250 to 210
10 (100)
12 (100)
12 (100)
—
—
Note: These reaction were performed under the argon, atmosphere. Except for entry 6, the 1.0 M
solution in toluene (10 eq.) was poured into the substrate solution (ca. 0.1 mmol/mL).
aThe mixture of the products were recovered quantitatively and not isolated. The ratio of the products
were determined by 1H-NMR.
bThe 1.0 M solution in toluene (3 eq.) was poured into the substrate solution (ca. 0.01 mmol/mL).

258
Suzuki, Nonaka, and Yamaura
Scheme 2. An estimated equilibrium between the two aluminum complex of endo-7.
equilibrium state. Thus, the ratio of 1-O-Bn 11 and 2-O-Bn 10 may indicate that of
the O2- and O1-aluminum complex. But, under the condition requiring long time to complete
the ring-opening reaction, the O1-aluminum complex is consumed by generation of the
corresponding alditol derivative (2-O-Bn 10), and then is afforded by the equilibrium. As
a result, the ratio of 1-O-Bn 11 to 2-O-Bn 10 decreased relatively (Sch. 2). This phenomenon
was also observed in the reaction in the diluted solution (entry 6). The same tendency of the
regioselectivity was observed in the reactions of endo-8. The reductive ring-opening reaction
of endo-8 in CH2Cl2 or toluene proceeded smoothly to give 2-O-pMBn 12 and 1-O-pMBn 13
with the former preference (entry 7–9). Based on the higher reactivity of endo-8 than that of
endo-7, this reaction occurred even in the THF solution (entry 10). Surprisingly, the ratio of
2-O-pMBn 12 and 1-O-pMBn 13 is dramatically inversed by the use of THF as a solvent.
This result implies that the solvation of DIBAH is not advantageous for the formation of
the O1-aluminum complex. At present, our research did not identify the factor controlling
this reaction. But, the strict change of the reaction site with DIBAH is a matter of interest.
Reaction of the exo-type substrates, exo-7 and exo-8, gave the corresponding 2-O-Bn 9
or 2-O-pMBn 12 exclusively (entry 11–14). Interestingly, change of the solvent did not
influence the regioselectivity in the reaction of the exo-isomer.
In summary, the procedure described herein offers a new option for the preparation of
1-O- or 2-O-benzyl type sugars, which are useful synthons for oligo-saccharide synthesis.
To discuss the difference between endo- and exo-type substrates in the reaction with
DIBAH, more examination, such as from the viewpoint of the stereoelectronic effect,
including conformational studies, is currently under way.
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