Catalytic asymmetric hydrolysis: Asymmetric hydrolytic protonation of enol esters catalyzed by phase-transfer catalysts

Article abstract of DOI:10.1002/chem.201100833

Like an enzyme: Asymmetric hydrolysis of enol esters is accomplished by chiral phase-transfer catalysts under biphasic base hydrolysis conditions. Stoichiometric reactions support the generation of a well-organized chiral ammonium hydroxide species (Q⁺OH^-). Copyright

Full text of DOI:10.1002/chem.201100833

DOI: 10.1002/chem.201100833
Catalytic Asymmetric Hydrolysis: Asymmetric Hydrolytic Protonation of
Enol Esters Catalyzed by Phase-Transfer Catalysts
Eiji Yamamoto, Ayano Nagai, Akiyuki Hamasaki, and Makoto Tokunaga*[a]
Hydrolase-catalyzed stereoselective ester hydrolysis is one
of the most important and fundamental reactions in fine
chemical productions.[1] However, the reaction with artificial
catalysts[2] has still not been attained even though enzymatic
reactions have many drawbacks.[3] In this context, we have
investigated the development and expansion of the substrate
scope of transition-metal-catalyzed asymmetric hydrolysis
and alcoholysis, but the asymmetric hydrolysis of unactivat-
ed carboxylic esters remains undeveloped.[4] The difficulties
of ester hydrolysis probably arise from the low reactivity
with acid catalysts. In contrast, base hydrolysis of esters pro-
ceeds smoothly with water under homogeneous conditions
even at low temperature. Accordingly, asymmetric hydroly-
sis of esters is expected to be achieved by using chiral
phase-transfer catalysts (PTC) to catalytically generate a
chiral ammonium hydroxide salt (Q+OHÀ) although asym-
metric reactions with Q+OHÀ as a chiral hydroxide nucleo-
phile have yet to be reported.[5] Based on our investigation,
we envisioned hydrolytic asymmetric protonation of enol
esters, which is one of the important hydrolase-catalyzed
asymmetric hydrolysis reactions and also has been hitherto
conducted only by hydrolases[1c,6,7] (Scheme 1). Herein, we
report the first synthetically useful asymmetric hydrolysis of
enol esters with artificial phase-transfer catalysts. In the pre-
liminary studies, several kinds of esters such as aryl esters,
b-lactones, and activated a,a-disubstituted esters were found
to be hydrolyzed by phase-transfer catalytic conditions.
In particular, enol esters derived from 2-substituted ke-
tones gave a-tertiary alkyl ketones with good enantioselec-
tivities. Therefore, further investigations of the reaction
were performed. A simple acetyl enolate derived from 2-
propylcyclohexanone (2a) were sluggish and the enantiose-
lectivity gradually decreased during the reaction (with an
enantiomer ratio (e.r.) of up to 79:21 with 2a; see the Sup-
porting Information). A faster reaction was observed with a
chloroacetyl enolate (2c). In this case, the reaction proceed-
ed and gave the desired product with a moderate e.r. (30%
yield, 85:15 e.r., Table 1, entry 1). The addition of small
amount of alcohol had a good effect on the yield and enan-
tioselectivity; the best results were achieved with 2-chloroe-
thanol (Table 1, entries 2–4; for details, see the Supporting
Information).
Next, the optimum solvent system was surveyed and mesi-
tylene/CHCl3 (1:2) was found to be the best system (Table 1,
entry 5). Reducing the amount of alcohol to 0.5 equiv
showed no effect on the enantioselectivity (Table 1, entry 5
vs. entry 6). Although roles of the alcohol are still obscure,
promoting mass transfer of PTC between phases seems
plausible since addition of a surfactant (Triton X-100,
5 mol%) as an alternative of 2-chloroethanol also gave a
similar selectivity (Table 1, entry 11). In an investigation of
catalysts, catalyst 1a, which has anthracenylmethyl group
developed by Lygo et al.[8] and Corey et al.[9], was found to
be the best performance in the asymmetric hydrolysis of
enol esters; the functional group at the 9-position was
shown to have a significant effect on the enantioselectivities
(Table 1, entries 5 and 7–10). Catalyst loading of 1a can be
lowered to 2 mol% with a slight loss of enantioselectivity.
This reaction was also carried out on a gram scale and gave
virtually the same result (Table 1, entry 12). Under the con-
ditions of further lower catalyst loadings (0.5 and
0.1 mol%), good yields (99 and 78%) and slightly lower
enantiomeric ratios (90:10 and 87:13) were obtained
(Table 1, entries 13 and 14). In these entries, concentrated
conditions (substrate concentration of 14%) and lower
amounts of additive alcohols (10 mol%) were employed. It
is noteworthy that the reported enzymatic reaction with an
enol acetate of 2-propylcyclohexanone catalyzed by a yeast,
Pichia miso IAM 4682, could not be tolerated under concen-
trated conditions (substrate conc=0.2%, 80% yield, 9:91
Scheme 1. Working hypothesis of catalytic asymmetric hydrolytic proto-
nation of enol esters.
[a] E. Yamamoto, A. Nagai, Dr. A. Hamasaki, Prof. Dr. M. Tokunaga
Department of Chemistry, Kyushu University
Fukuoka 812-8581 (Japan)
Fax : (+81)92-642-7528
E-mail: mtok@chem.kyushu-univ.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100833.
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Chem. Eur. J. 2011, 17, 7178 – 7182

COMMUNICATION
Table 1. Asymmetric hydrolysis of enol esters catalyzed by PTC.
tyl)cyclohexanone (3k) with an
e.r. of 89:11. Further catalyst
screening disclosed that the re-
action with catalyst 1 f gave 3k
in excellent yield and with a
higher e.r. (Scheme 2, 96%,
92:8 e.r.). The desired lactone
was obtained from 3k by a
Baeyer–Villiger oxidation in
90% yield in the literature.[11]
Entry
cat. [(mol%)]
t
Additive
ACHTUNGTRENN[UNG (equiv)]
Solvent
Yield[a]
[%]
e.r.
A previous asymmetric synthe-
sis of (R)-3k was performed
through 6 steps with rather ex-
pensive and explosive reagents.
Our method is advantageous in
terms of a shorter synthetic
[h]
1[b]
1a (10)
1a (10)
1a (10)
1a (10)
1a (10)
1a (10)
1b (10)
1c (10)
1d (10)
1e (10)
1a (10)
1a (2)
1
1
1
1
1
12
1
1
1
1
1
24
24
48
none
ethanol (1.0)
2,2,2-trifluoro
CHCl3
CHCl3
CHCl3
CHCl3
30
66
73
67
64
98
28
57
87
75
65
96
99
78
85:15
87:13
89:11
90:10
92:8
2[b]
3[b]
ethanol (1.0)
4[b]
2-chloroethanol (1.0)
2-chloroethanol (1.0)
2-chloroethanol (0.5)
2-chloroethanol (1.0)
2-chloroethanol (1.0)
2-chloroethanol (1.0)
2-chloroethanol (1.0)
Triton-X100 (0.05)
5[b]
mesitylene:CHCl3 (1:2)
mesitylene:CHCl3 (1:2)
mesitylene:CHCl3 (1:2)
mesitylene:CHCl3 (1:2)
mesitylene:CHCl3 (1:2)
mesitylene:CHCl3 (1:2)
mesitylene:CHCl3 (1:2)
mesitylene:CHCl3 (1:2)
mesitylene:CHCl3 (1:2)
mesitylene:CHCl3 (1:2)
6[c]
92:8
route,
a simpler and safer
7[b]
48:52
89:11
88:12
66:34
91:9
91:9
90:10
87:13
method, and better total yield
from a commercially available
inexpensive starting material.
With respect to the structure
of the catalyst, there is a possi-
bility that the hydroxy group
of catalyst 1a may react with
primary alkyl halides such as
2-chloroethanol, substrate, or
chloroacetic acid to give other
catalytic species. Indeed, the
alkylation of hydroxyl group of
8[b]
9[b]
10[b]
11[c]
12[b,d]
13[e]
14[f]
2-chloroethanol (0.5)
2-chloroethanol (0.1)
2-chloroethanol (0.1)
1a (0.5)
1a (0.1)
[a] GC yield. [b] 0.1 mmol scale, organic solvents (400 mL), 50% aq KOH (200 mL). [c] 0.1 mmol scale, organic
solvents (400 mL), 50% aq KOH (100 mL). [d] 5 mmol scale (approx 1 g), organic solvents (20 mL), 50% aq
KOH (5 mL). [e] 1 mmol scale, organic solvents (1 mL), 50% aq KOH (0.5 mL). [f] 2 mmol scale, organic sol-
vents (2 mL), 50% aq KOH (1 mL).
e.r.; substrate conc=1.0%, 84% yield, 16:84 e.r.).[6c] In addi-
tion, the reactions with 2–0.1 mol% of catalyst loadings are
rather low and rare in the field of asymmetric organocataly-
sis. With the optimized catalyst in hand, the substrate scope
of the reaction was investigated using a series of enol esters
derived from 2-substituted cyclic ketones. The substrates de-
rived from cyclohexanones and cycloheptanones having
simple alkyl groups were efficiently transformed into the
corresponding 2-substituted cyclic ketones in excellent
yields and with high enantioselectivities (Table 2). When
comparing the 10 and 2 mol% conditions for three sub-
strates, the former conditions gave a slightly better e.r. in
every case (Table 2, entries 2–
the catalyst was already reported in the field of the PTC cat-
alyzed asymmetric alkylation.[12] Thus, the mass balance of
the reaction was confirmed and it was revealed that 99% of
the product ketone, 99% of chloroacetic acid, and 88% of
2-chloroethanol existed in the reaction mixture (for details,
see the Supporting Information). Alkylation of the catalyst
is not likely because the other alcohols (that do not act as
alkylating agents) and the surfactant brought a similar effect
to 2-choroethanol although the alkylation cannot be exclud-
ed completely. Therefore, it is reasonable that the structure
of the cation part of the catalyst remains intact. A stoichio-
metric reaction was carried out in the presence of the in situ
7), therefore the 10 mol% con-
ditions were used for the other
substrates. Among them, an
e.r. of 95:5 was observed with
3 substrates. As an application
of this reaction, we conducted
a formal total synthesis of (R)-
10-methyl-6-undecanolide (6),
which is a biologically active
natural product isolated from a
marine streptomycete (isolate
B6007, Scheme 2).[10] In case of
the substrate 2k, the asymmet-
ric hydrolysis with catalyst 1a
afforded (R)-2-(4-methylpen-
Scheme 2. Application for the synthesis of a biologically active cyclic lactone. a) Reaction conditions; catalyst
1 f (10 mol%), 2-chloroethanol (0.5 equiv), and CHCl3/mesitylene (2:1) at À408C for 13h. BV=Baeyer–
Villiger.
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7179

M. Tokunaga et al.
Table 2. Substrate scope for the asymmetric hydrolysis of enol ester.[a]
chiometric reactions using the
chiral ammonium 2,2,2- tri-
fluoroethoxide
(Q+
CF3CH2OÀ) and Q+OHÀ in
the presence of water were
performed. As a result, the re-
action with Q+OHÀ gave the
product with a higher e.r. than
Entry
1
Substrate
Product
Yield[a]
[%]
e.r.
AHCTUNGTRENN(GUN config)
that
with
Q+CF3CH2OÀ
(Scheme 3, 77:23 vs. 69:31).
These results indicate that Q+
OHÀ is the active species of
2b
2c
3b
3c
98[b]
87:13 (S)
asymmetric
hydrolysis.
To
2
3
96[c,d]
92:8 (S)
91:9 (S)
obtain the insights of the struc-
ture of the active species, we
performed NMR experiments
of Q+CF3CH2OÀ as an ana-
logue of Q+OHÀ.
96[d,e,f]
4
5
99[d]
92:8 (+)
92:8 (+)
96[d,e]
An interionic NOE between
2d
2e
2 f
2g
2h
2i
3d
3e
3 f
3g
3h
3i
the
2,2,2-trifluoroethanoxide
protons and the quinoline
proton located proximal to the
9-hydroxyl group was observed
(For detail, see the Supporting
6
7
94[d]
87:13 (R)
86:14 (R)
93[d,e]
Information).
Additionally,
given all the obtained NOEs,
the alkoxide anion is likely lo-
cated near hydroxy group of
the ammonium cation. Further-
more, it was reported that the
8
91[d]
91[d]
97[d]
96[d]
99[d]
95:5[g] (+)
95:5[g] (+)
95:5[h] (+)
94:6 (R)
À
BH4 anion of the N-9-anthra-
cenylmethyl
9
cinchonidinium
tetrahydroborate salt prefers to
be located near the 9-hydroxy
group.[14] These facts suggest
that OHÀ is located near the 9-
hydroxy group of the ammoni-
um cation. In terms of the re-
action mechanism, this asym-
metric hydrolysis is composed
of hydrolysis followed by pro-
tonation. Although the first hy-
drolysis step can affect the
enantioselectivity of next pro-
tonation step, the result of stoi-
chiometric reaction of the am-
monium alkoxide indicated
10
11
12
2j
3j
89:11 (R)
[a] Reactions were carried out on 0.1 mmol scale with 10 mol% of the catalyst unless otherwise noted. [b] GC
yield. [c] 1 mmol scale. [d] Yield of the isolated product. [e] 24 h, 2 mol% of the catalyst was used. [f] 5 mmol
scale. [g] Enantiomer ratios of products were analyzed by 1H NMR spectroscopy after ketones were reduced
and esterified to the corresponding Mosher’s esters. [h] The enantiomer ratio of product was analyzed by
chiral GC after ketones were reduced to the corresponding alcohol.
that
the
enantioselectivity
mainly originated from the
protonation step. Additionally,
the stoichiometric reactions
suggested that the contact ion
generated Q+OHÀ ion pair[13] to assess the involvement of
Q+OHÀ as an active species in the reaction, which afforded
the desired product in 92% yield and with an e.r. of 89:11
(Scheme 3). The absolute structure of the product coincided
with the product of the catalytic reaction. Additionally, stoi-
pair of the unstable enolate and the ammonium cation was
generated in the organic phase after the hydrolysis of the
substrate. It is different from the PTC-catalyzed asymmetric
alkylation which generates contact ion pair of a stable eno-
late and an ammonium cation by the anion exchange with
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Chem. Eur. J. 2011, 17, 7178 – 7182

Asymmetric Hydrolytic Protonation of Enol Esters
COMMUNICATION
In conclusion, we have ach-
ieved the first non-biomimetic
asymmetric hydrolysis of esters
catalyzed by artificial molecu-
lar catalysts with high catalytic
efficiency in buffer-free condi-
tions. Enzymatic reactions
follow Michaelis–Menten ki-
netics because of the tight sub-
strate binding characteristic of
enzymes, which means the re-
action rate enhancement by
raising the concentration of
substrates is highly limited. On
the other hand, artificial cata-
lysts often show collision-type
Scheme 3. In situ generated Q+ORÀ as a stoichiometric reagent of asymmetric hydrolysis.
metal enolates.[15] The mecha-
nism of enantioface differenti-
ating protonation may be simi-
lar to those of PTC-catalyzed
asymmetric aldol reactions.[5a]
From the structural insights of
active species, the enantiose-
lective protonation of an eno-
late likely occurs near the 9-
hydroxy group of the catalysts.
Scheme 5. Kinetic resolution of an acetate ester with a binaphthyl backbone.
We speculate that the hydroxy
group of the catalyst interacts
with reaction intermediates
through hydrogen bonding (Scheme 4). In this reaction, it is
still obscure whether a well-organized chiral hydroxide spe-
cies was generated or not because enantioselective step of
asymmetric hydrolysis of the enol ester is not the nucleo-
philic attack of hydroxide but in the protonation of the eno-
late. However, the fact that asymmetric induction was also
observed in the case of hydrolytic kinetic resolution of an
acetate ester derived from 2,2’-dihydroxybinaphthyl is indi-
rect evidence for the generation of a well-organized chiral
hydroxide (Scheme 5, krel =4.1).
kinetics. Therefore, reactions catalyzed by artificial molecu-
lar catalysts are free from that limitation. Non-enzymatic
catalytic reactions have a considerable potential to replace
industrially used enzymatic reactions. The well-organized
chiral ammonium hydroxide species will give researchers nu-
merous opportunities to develop new asymmetric reactions
with water, which have been performed only by hydrolases
in the past. Further mechanistic study, extension of the reac-
tion scope, and development of more efficient catalytic sys-
tems are currently underway in our laboratory.
Experimental Section
Typical Procedures of asymmetric hydrolysis of enol esters (Table 2,
entry 3): N-9-anthracenylmethyl cinchonidinium chloride (61 mg,
0.1 mmol, 2 mol%) was added to the CHCl3/mesitylene (2:1, 20 mL) so-
lution followed by the addition of 2-chloroethanol (67 mL, 0.5 mmol,
0.1 equiv) and 50% aq KOH (5 mL). Then, the mixture was stirred for
10 min at À408C followed by the addition of enol ester 2c (1.08 g,
5 mmol, 1 equiv). The reaction mixture was stirred for 24 h at À408C.
Then, the reaction mixture was immediately passed through a thin pad of
silica gel and the resultant crude product was purified by silica gel
column chromatography to give (R)-3c (673 mg, 4.80 mmol, 96%, 91:9
e.r.).
Scheme 4. A possible reaction mechanism for the asymmetric hydrolysis
of enol esters.
Chem. Eur. J. 2011, 17, 7178 – 7182
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7181

M. Tokunaga et al.
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Acknowledgements
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This study was financially supported by a Grant-in-Aid for Global-COE
program, “Science for Future Molecular Systems” and Scientific Re-
search (A) (No. 20245014).
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Keywords: alkaloids · asymmetric protonation · enol esters ·
hydrolysis · phase-transfer catalysts
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Received: March 18, 2011
Published online: May 12, 2011
7182
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 7178 – 7182