Cu(OTf)2 as an efficient and dual-purpose catalyst in the regioselective reductive ring opening of benzylidene acetals

Article abstract of DOI:10.1002/anie.200462172

(Chemical Equation Presented) Reducing the choices. Cu(OTf)₂ is an efficient and dual-purpose catalyst for the highly regioselective reductive ring openings of benzylidene acetals with either BH₃ or Me ₂EtSiH to give the corresponding primary and secondary alcohols (see scheme). Isotope studies have confirmed that both modes of ring cleavage proceed by an S_N1 reaction pathway when borane or silane attack the acetal carbon center.

Full text of DOI:10.1002/anie.200462172

Angewandte
Chemie
Synthetic Methods
position, often lack chemoselectivity when substrates contain
base-sensitive functionalities. Alternatively, traditional acid-
promoted reductive cleavage has been reported to open
benzylidene acetals at either the O6[4] or O4 position.[4a,c,f,5]
However, these traditional acids have to be used stoichio-
metrically or in excess and can lead to hydrolysis of the acetal
ring as a major side reaction. Often when these protocols have
been used, a mixture of regioisomers is obtained that can be
difficult to purify by chromatographic techniques. The devel-
opment of new Lewis acids as efficient catalysts for ring
cleavage in a highly selective manner may offer a good
solution (Scheme 1). No Lewis acid catalyzed ring openings of
4,6-O-benzylidene acetals at the O4 position have been
published to date. So far only a reductive cleavage at O6 to
the give corresponding 6-alcohol using borane and with
[V(O)(OTf)2] (Tf = trifluoromethanesulfanyl) as the catalyst
has been reported, and here the required amount of catalyst
was 15 mol%.[6] Herein, we have developed Cu(OTf)2 as an
efficient and dual-purpose catalyst that can be used in
catalytic quantities; it effects the regioselective reductive
ring opening of benzylidene acetals at the O4 or O6 position
by merely altering the reactivity of the reducing agent.
Cu(OTf)2 as an Efficient and Dual-Purpose
Catalyst in the Regioselective Reductive Ring
Opening of Benzylidene Acetals**
Chi-Rung Shie, Zheng-Hao Tzeng, Suvarn S. Kulkarni,
Biing-Jiun Uang, Ching-Yun Hsu, and Shang-
Cheng Hung*
Regioselective ring opening of benzylidene acetals is one of
the major challenges in carbohydrate and natural product
syntheses.[1] Substituted and unsubstituted benzylidene ace-
tals are valuable protecting groups to block 1,3-diols. Aryli-
dene acetals 1 can be opened selectively under appropriate
reaction conditions (Scheme 1) to yield primary 2 (path a) or
Compound 4 was selected for model studies. We examined
the cleavage at O6 by employing various boranes in combi-
nation with Cu(OTf)2 at room temperature; the results are
outlined in Table 1. Initially, treatment of 4 with BH3·THF in
Table 1: Cu(OTf)2-catalyzed regioselective borane-reductive O6-ring
opening of 4,6-O-benzylidene acetal 4 to the corresponding 6-alcohol 5 at
room temperature.
Scheme 1. Lewis acid catalyzed regioselective reductive ring opening of
benzylidene acetals 1 to give primary alcohols 2 (path a) or secondary
alcohols 3 (path b).
secondary alcohols 3 (path b). A number of effective reagents
have been reported for the regioselective ring opening of 4,6-
Entry
x
Borane
Solv.
t [h]
Yield [%]
[2]
5
6
O-benzylidene acetals in hexopyranosides. Of these, AlH3
and iBu2AlH,[3] which are commonly used to cleave at the O6
1
2
3
4
5
6
7
8
15
15
10
5
BH3·THF[a]
BH3·THF
BH3·THF
BH3·THF
BH3·THF
BH3·Me2S[b]
BH3·Me3N
9-BBN[c]
CH2Cl2
–
–
–
–
–
0.75
0.75
1.5
2.5
27
10
25
27
94
92
93
95
70
78
0
0
0
0
0
0
3
40
0
[*] C.-R. Shie, Z.-H. Tzeng, Prof. Dr. B.-J. Uang, Prof. Dr. S.-C. Hung
Department of Chemistry
1
5
5
5
National Tsing Hua University
Hsinchu 300 (Taiwan)
Fax: (+886)3-571-1082
E-mail: schung@chem.sinica.edu.tw
CH2Cl2
–
40
[a] 1m solution in THF. [b] 2m solution in THF. [c] 0.5m solution in THF,
9-BBN=9-borabicyclo[3.3.1]nonane.
Dr. S. S. Kulkarni, Prof. Dr. S.-C. Hung
Institute of Chemistry & Genomics Research Center
Academia Sinica
128 Academia Road, Section 2
Taipei 115 (Taiwan)
the presence of 15 mol% of catalyst in CH2Cl2 rapidly
furnished the expected ring-opened product 5[7] in excellent
yield (entry 1, 45 min, 94%). Exclusion of CH2Cl2 gave
similar results (entry 2, 92%). Lowering the concentration
of catalyst to 10 mol% and 5 mol% (entries 3 and 4) led to
similar selectivity and yields, while decreasing it to 1 mol%
extended the reaction time and resulted in a drop in yield
(entry 5).
Dr. C.-Y. Hsu
Department of Chemical Engineering
Cheng-Shiu University
840 Cheng-Ching Road
Kaohsiung County 833 (Taiwan)
[**] This work was supported by the National Science Council of Taiwan
(NSC 92-2113M-001-028 and NSC 92-2113M-001-061).
We then tested various borane reagents in tandem with
5 mol% of Cu(OTf)2 to study the effect of ligation and bulk
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200462172
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1665

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on the regiochemical outcome of the reactions. In entry 6, use
tected d-glucose derivative 7 successfully furnished the
expected 4-benzyl-protected product 8 (entry 1, 4.5 h, 87%),
while the 2,3-dibenzoyl-protected compound 9 gave the
corresponding 6-alcohol 10[4a] in 53% yield after 23 h
(entry 2). When 15 mol% of Cu(OTf)2 was used, the latter
transformation was carried out over a short period (5 h),
affording compound 10 in a high yield (entry 3, 91%). The
electron-withdrawing groups in the substrates 7 and 9 made
their reaction much slower than that of the 2,3-dibenzyl
analogue 4 (0.75 h). The reaction rate is closely associated
with the nucleophilicity of the oxygen atom at the C6 position
and the Lewis acid catalyst. In the LiAlH4–AlCl3 system, the
congestion of the protecting group at O3 in d-glucopyrano-
sides plays an important role, and the presence of a bulkier
substituent at C3 has been found to favor a higher proportion
of O6-opened product.[2b] However, no such steric depen-
dence was observed in our system, and only O4-benzyl ethers
were obtained in high yields irrespective of their nature (H,
Bn, or Bz).[4b] Similarly, the methyl b-pyranoside 11 (entry 4),
b-thioglycoside 13 (entry 5), d-mannose-derived acetal 23
(entry 14), and non-sugar substrate 25 (entry 15) underwent a
high-yielding facile ring fission to provide 6-OH derivatives
12[8] (92%), 14 (93%), 24[9] (84%), and 26[10] (90%),
respectively. In the d-glucosamine series, the b-benzoyl 3-
benzyl-protected 15 led to the desired product 16 in 82%
yield (entry 6). Its structure was determined by single-crystal
X-ray structure analysis.[11] The 3-benzoyl analogue 17,
although sluggish to react under the optimized conditions
(entry 7, 63%), did furnish the expected compound 18 rapidly
and in excellent yield (90%) when 15 mol% of Cu(OTf)2 was
used (entry 8). When the a-form 3-benzyl 19 (entry 9) and 3-
benzoyl 21 (entry 12) were employed, the expected ring-
opened products 20 and 22 were obtained in 55 and 56%
yields, respectively. Increasing the catalyst concentration to
15 mol% and the reaction temperature to 08C improved the
yield remarkably in case of the former (entry 11, 86%),
whereas a substantial amount of the 4,6-diol (30%) from
hydrolysis was present in the latter (entry 13, 57%).
With success in the Cu(OTf)2-catalyzed borane-induced
reductive O6-ring opening of benzylidene acetals, we then
explored the catalytic properties of Cu(OTf)2 for silane-
induced reductive cleavage at the O4 position, including the
effects of the solvent, silane agent, and catalyst concentration
(Table 3). The catalyst was added at 08C, and the reaction
mixture was gradually warmed up to room temperature.
When 1 mol% of Cu(OTf)2 was used together with triethyl-
silane in CH2Cl2 (entry 1), the reaction took 15 h to provide
the secondary alcohol 6[5b] (62%) as the only regioisomer. A
smaller reducing agent, Me2EtSiH, offered a marginally
improved yield of 6 in a much shorter reaction time
(entry 2, 9 h, 65%). Employment of a more polar solvent
like nitromethane speeded up the reaction of 4 with Et3SiH
(entry 3, 1 h) and Me2EtSiH (entry 4, 1 h), which afforded 6 as
the sole product in 60% and 68% yields, respectively.
Although no other regioisomer was detected, hydrolysis of
compound 4 to the corresponding 4,6-diol seemed to become
a dominant factor limiting the yield. Less polar solvents, for
example, THF and toluene, gave disappointing results. Never-
theless, reduction of 4 in acetonitrile using Et3SiH (entry 5,
of the BH3·Me2S complex with 4 afforded product 5 (78%)
along with a minor 4-alcohol 6 (3%). Interestingly, the mode
of regioselection was markedly shifted when BH3·NMe3 was
used as the reductant (entry 7); compound 6 was formed in
40% yield as the sole product, and some starting material was
recovered (55%). A bulkier reagent, 9-borabicyclo[3.3.1]no-
nane (9-BBN), was sluggish to react (entry 8), and overnight
stirring was needed to furnish 5 in a modest yield (40%)
together with the hydrolyzed 4,6-diol (40%) and starting
material (15%).
We then proceeded to investigate the compatibility of
various substrates under these optimized conditions (1m
BH3·THF in THF, 5 mol% Cu(OTf)2, without additional
solvent, room temperature; Table 2). The 2-benzoyl-pro-
Table 2: Reductive ring opening of various benzylidene acetals at the O6
position using BH3·THF and x mol% of Cu(OTf)2 as the catalyst.
Entry Acetal
x
t [h] Product
Yield [%]
[mol%]
1
2
3
7: R=H
9: R=Bz
9
5
5
15
4.5 8: R=H
23 10: R=Bz
87
53
91
5
10
4
5
11: R=Bz, R1 =OMe
13: R=Bn, R1 =STol
5
5
4
12: R=Bz,
R1 =OMe
92
93
3.5 14: R=Bn,
R1 =STol
6
7
8
15: R=Bn
17: R=Bz
17
5
5
15
3
16: R=Bn
82
63
90
21 18: R=Bz
4.5 18
9
19: R=Bn
19
19
21: R=Bz
21
5
6.5 20: R=Bn
3.5 20
14 20
55
67
86
56
57
10
11
12
13
15
15[a]
5
5
9
22: R=Bz
22
15[a]
14
15
5
5
3.5
1.5
84
90
[a] The reaction was conducted in an ice bath. Bz=Benzoyl, Bn= benzyl,
OMe=methoxy, Tol=tolyl.
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Table 3: Cu(OTf)2-catalyzed reductive ring opening of compound 4 in
various solvents with silanes to give the corresponding 4-alcohol 6.
Table 4: Regioselective reductive ring opening of various benzylidene
acetals in acetonitrile employing Me2EtSiH and with 1 mol% of Cu(OTf)2
as the catalyst.
x mol % CuðOTfÞ2, 2 equiv silane
4
5 + 6
t [h]
ƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ!
solvent, 0 oC!RT
Entry
Acetal
t [h]
Product
Yield [%]
Entry
x
Silane
Solv.
Yield [%]
5
6
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
0.5
5
10
Et3SiH
Me2EtSiH
Et3SiH
Me2EtSiH
Et3SiH
Me2EtSiH
Me2EtSiH
Me2EtSiH
Me2EtSiH
CH2Cl2
CH2Cl2
CH3NO2
CH3NO2
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
15
9
1
1
1
0.5
4
0.5
0.5
0
0
0
0
7
0
3
3
2
62
65
60
68
76
84
75
80
82
1
2
7
9
1
1
27: R=H
28: R=Bz
71
85
3
4
5
11
13
17
0.5
1.5
0.5
74
79
87
1 h) readily furnished the expected compound 6 (76%) along
with the minor isomer 5 (7%), while Me2EtSiH led to 6 in
84% yield (entry 6, 0.5 h), exclusively. In these cases the
hydrolyzed product was recovered in up to 10% yield. In
entry 7, when the catalyst concentration was halved, the
transformation took place gradually (4 h) to afford compound
6 in 75% yield. In contrast, increasing the concentration of
catalyst to 5 mol% or 10 mol% (entries 8 and 9) did not
improve the yields of 6 (80–82%).
6
7
19
21
1
0.5
32: R=Bn
33: R=Bz
80
83
With these optimized reaction conditions (1 mol%
Cu(OTf)2, Me2EtSiH, CH3CN, 08C!room temperature),
we examined a number of a- and b-hexopyranosides bearing
different protecting groups to check the generality of this
protocol (Table 4). In entries 1–8, reactions of the d-glucose-
derived benzoates 7 and 9; b-glucopyranoside 11 and b-
thioglycoside 13; d-glucosamine-derived acetals 17, 19, and
21; and d-mannopyranosyl sugar 23[12] led to O4-opened
products 27–34 in 71, 85, 74, 79, 87, 80, 83, and 70% yield,
respectively. These experiments revealed that the electron-
withdrawing group at the O3 position does not affect the
reactivity of substrates, in contrast to the observations of the
reductive O6-opening reactions with borane. In the case of
the non-carbohydrate compound 25 (entry 9), regioisomeric
benzyl ethers 26 (22%) and 35 (24%) were generated along
with 1,3-dibenzyl ether 36 (36%) as the major product.
To examine the reaction pathway in greater depth, we
performed two experiments using deuterated reducing agents
(Scheme 2). Reductive ring opening of 4 with BD3·THF
furnished primary alcohols 39 and 40 in unequal proportions
(5:1 ratio), as judged from the signals of the O4-benzylic
protons in the 1H NMR spectrum of the mixture with those of
compound 5 (see the Supporting Information). The Cu(OTf)2
catalyst may first coordinate with the more accessible
O6 atom and lead to a zwitterionic species 37, which can be
reduced by the reactive borane reagent. The reaction
essentially follows the SN1 pathway, and the stereochemical
bias is perhaps offered by the chirality at C4, which is
reflected in the observed product ratio. On the other hand,
the ring fission of 4 with Et3SiD generated a 1:1 diastereo-
meric mixture of secondary alcohols 41 and 42. The hindered
O4-benzyl cation of the intermediate 37 cannot be reduced as
the silane reagent is bulky and less reactive than borane and
8
9
23
25
1
2
70
26: R1 =H, R2 =Bn
35: R1 =Bn, R2 =H
36: R1 =R2 =Bn
22
24
36
Scheme 2. The treatment of 4 with deuterated reducing agents.
Bn=Benzyl.
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[10] E. L. Eliel, L. Clawson, D. E. Knox, J. Org. Chem. 1985, 50,
2707 – 2711.
[11] CCDC162432 (16) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
an equilibrium is soon established between the O6- and O4-
coordinated complexes, which leads to another zwitterionic
species 38. The silane agent can approach the intermediate 38
at the well-exposed O6-benzyl cation from either side to
generate equal amounts of diastereomers.
In conclusion, we have successfully developed Cu(OTf)2
as an excellent dual-purpose catalyst for highly regioselective
reductive ring opening of various benzylidene acetals with
BH3 and Me2EtSiH to furnish the corresponding primary and
secondary alcohols, respectively. The reaction conditions are
mild, and various protecting groups in the substrates are
tolerated. The isotope studies provide the first experimental
evidence that neither O6- nor O4-cleavage of the benzylidene
ring proceeds through the SN2 reaction pathway when borane
or triethylsilane attacks the acetal carbon center.
[12] R. Madiyalakan, M. S. Chowdhary, S. S. Rana, K. L. Matta,
Carbohydr. Res. 1986, 152, 183 – 194.
Received: October 1, 2004
Published online: January 31, 2005
Keywords: carbohydrates · copper · homogeneous catalysis ·
.
Lewis acids · regioselectivity
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