Asymmetric isomerization of ω-hydroxy-α,β-unsaturated thioesters into β-mercaptolactones by a bifunctional aminothiourea catalyst

Article abstract of DOI:10.1021/ol500637x

We present a novel methodology for the asymmetric synthesis of β-mercaptolactones via isomerization of ω-hydroxy-α,β- unsaturated thioesters by means of a bifunctional aminothiourea catalyst. The catalyst interacts with the substrate through the cooperati

Full text of DOI:10.1021/ol500637x

Letter
pubs.acs.org/OrgLett
Asymmetric Isomerization of ω‑Hydroxy-α,β-Unsaturated Thioesters
into β‑Mercaptolactones by a Bifunctional Aminothiourea Catalyst
Yukihiro Fukata,‡ Takaaki Okamura,‡ Keisuke Asano,* and Seijiro Matsubara*
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyotodaigaku-Katsura, Nishikyo, Kyoto
615-8510, Japan
S
* Supporting Information
ABSTRACT: We present a novel methodology for the asym-
metric synthesis of β-mercaptolactones via isomerization of ω-
hydroxy-α,β-unsaturated thioesters by means of a bifunctional
aminothiourea catalyst. The catalyst interacts with the substrate
through the cooperative action of both a covalent bond at the
amino group and noncovalent bonding at the thiourea group. The
potential for an enantiodivergent synthesis could also be
demonstrated by carrying out the reaction in a different solvent
system.
rganocatalysis has seen tremendous progress through the
exploration of various catalytic functional groups and the
hydroxy-α,β-unsaturated thioesters A by means of a bifunc-
tional aminothiourea catalyst10 to afford a cyclic ester bearing a
mercapto group at the β-position (Scheme 1). Such products
O
exploitation of a variety of molecular interactions. In the design
of organocatalytic reactions, it is important to utilize covalent
and noncovalent interactions thoughtfully. Examples of this are
rife in biosynthesis, in which enzymes employ both interactions
situationally: for example, polyketide synthase fixes substrates
such as acetyl-CoA through a covalent interaction at the
catalytic site,1 while noncovalent interactions including hydro-
gen bonding also play crucial roles in a range of enzymatic
processes.2
Scheme 1. Reaction Design via Organocatalytic
Isomerization of ω-Hydroxy-α,β-Unsaturated Thioesters
Asymmetric sulfa-Michael addition is one of the most
powerful methods for the synthesis of chiral sulfur-containing
compounds,3 which are found in various organisms and play
important roles in biological and medicinal chemistry.4
However, asymmetric hetero-Michael additions to α,β-unsatu-
rated esters generally suffer from low enantioselectivity in
contrast to reactions of α,β-unsaturated ketones,5,6 and only a
limited number of successful asymmetric sulfa-Michael
additions starting from esters have been reported.7 In light of
these previous studies, the activation of simple esters through
only a noncovalent interaction such as hydrogen bonding
seems to be insufficient for reasonable asymmetric induction.
Thus, modified α,β-unsaturated carboxylic acid derivatives have
been employed in order to furnish specifically strong
interactions to attain the effective transmission of chiral
information:8 α,β-unsaturated acyl oxazolidinones,8a−g acyl
pyrazoles,8h imides,8i and thioamides8j have been demonstrated
as useful substrates for asymmetric sulfa-Michael additions
mediated by chiral hydrogen-bond donor catalysts or Lewis
acid catalysts.
have been catalytically synthesized with only slight enantiose-
lectivity before.5b,7b Since carboxylic acid derivatives allow for
an addition−elimination reaction by a nucleophilic catalyst,
their treatment with a tertiary amine catalyst may lead to the
generation of an ion pair intermediate B, which includes a
thiolate anion and an acylammonium ion, in which the catalyst
is fixed through a covalent bond to offer an efficient chiral
environment. In addition, the cationic intermediate is
considered to be more reactive as a Michael acceptor than
the substrate A11 and can undergo the stereoselective sulfa-
In this context, we attempted to explore an alternative
catalytic pathway to realize a rigid catalytic interaction in a
direct access to the desired esters. As it is a powerful synthetic
strategy to design an organocatalytic isomerization of
appropriate substrates,9 we designed an isomerization of ω-
Received: March 3, 2014
Published: March 31, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
Michael addition of the thiolate interacting with the thiourea in
the catalyst.12 As the Michael addition removes the rigidity of
the (E)-olefinic moiety, subsequent cyclization may be
facilitated to form the desired cyclic ester and regenerate the
catalyst.
Scheme 2. Reactions of 1a in the Presence of Thiols
On the basis of this reaction design, a study was initiated by
treating γ-hydroxy-α,β-unsaturated thioester 1a with amino-
thiourea catalyst 3a. As expected, the desired lactone 2a was
obtained in high yield with moderate enantioselectivity (Table
1, entry 1). Catalysts 3b and 3c, which bear electron-
Table 1. Isomerization of γ-Hydroxy-α,β-Unsaturated
a
Thioester 1a by Aminothiourea Catalyst
b
entry
catalyst
concn (M)
time (h)
yield (%)
ee (%)
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3c
3c
3c
0.5
24
95
97
99
92
<1
98
83
72
40
59
66
3
reaction proceeded to completion in 24 h even at 0.05 M,
whereas a much longer time was required in the absence of 4a
(Table 1, entry 7). Furthermore, the reaction starting from 1a
in the presence of a less-substituted thiol exclusively afforded
the product that incorporated the mercapto group derived from
the external thiol (Scheme 2, eq 2). Probably due to its
bulkiness, the thiolate anion generated initially could be readily
exchanged with the external thiol during the conversion from B
to C shown in Scheme 1.
0.5
24
0.5
24
0.5
24
0.5
24
−
0.1
96
74
76
74
0.05
0.03
144
336
a
Reactions were run using 1a (0.15 mmol) and the catalyst (0.015
mmol) in CH2Cl2. Isolated yields.
b
We explored the substrate scope for the reaction at 25 °C
and 0.05 M (Table 2). In several instances, the reactions were
carried out both by a simple isomerization (method A) and by a
reaction starting from the bulky thiol ester 1a in the presence of
the corresponding external thiol (method B); method B always
afforded a higher reaction rate and higher enantioselectivity
(Table 2, entries 1−6, 15, and 16). Notably, in method B for
the reactions using 2b−2j, the simple isomerization product 2a
was not generated (Table 2, entries 4 and 6−13). Reactions
using electron-rich thiols afforded the corresponding products
in higher enantioselectivity than in other cases (Table 2, entries
6, 7, and 9), although only an ortho-substituent decreased the
enantiomeric excess (Table 2, entry 8). Electron-deficient thiols
exhibited efficient reactivity and moderate to good enantiose-
lectivity (Table 2, entries 10 and 11). Thiols bearing p-
bromophenyl and 2-naphthyl groups were also tolerated and
resulted in excellent yields with good enantioselectivity (Table
2, entries 12 and 13). A highly bulky thiol attained high
enantioselectivity, but the yield was moderate due to the
generation of byproduct 2a through the competing simple
isomerization (Table 2, entry 14). An aliphatic thiol ester or
external thiol could also be applied in this process, albeit with
moderate yield and selectivity, and byproduct 2a was also
obtained via method B in this case (Table 2, entries 15 and 16).
In addition, this reaction could also be applied to the
synthesis of a δ-valerolactone (Scheme 3). The isomerization of
δ-hydroxy-α,β-unsaturated thioester 5a through method A
afforded six-membered lactone 6a in comparable enantiose-
lectivity. Although the reaction was much slower than the
formation of five-membered lactones, it was improved by the
use of method B, and the enantioselectivity was also better in
this case. The absolute configuration of 2b was determined by
comparing the optical rotation with the literature value7b (see
SI for details), and the configurations for all other examples
were assigned analogously.
withdrawing groups on the thiourea moiety, attained higher
enantioselectivity (Table 1, entries 2 and 3). Catalyst 3d
afforded the product in high yield, but the enantioselectivity
was only slight (Table 1, entry 4). In addition, catalyst 3e,
which has no nucleophilic nitrogen atom, proved to be inactive
for this transformation (Table 1, entry 5). These results imply
that the bifunctionality consisting of an anion receptor and a
nucleophilic amino group is crucial for catalysis of the
reaction.13 The concentration of the reaction mixture also
had a large effect on the enantioselectivity (Table 1, entries 3
and 6−8).
Using catalyst 3c, we investigated the effects of the reaction
temperature; the enantioselectivity improved as the temper-
ature increased (see Table S3 in the Supporting Information
(SI) for details). The Eyring plot resulted in a straight line with
a negative slope (see Figure S1a in the SI). This indicates that a
single mechanism is maintained over the range of temperatures
investigated and that this reaction has a manner of asymmetric
induction in which the differential entropy of activation
(ΔΔS‡) is the determinant factor.14−16
In addition, when the reaction was performed in the presence
of 2,6-dimethylbenzenethiol (4a) at 25 °C, the enantioselec-
tivity grew slightly higher (Scheme 2, eq 1). It is also notable
that the addition of 4a dramatically accelerated the reaction: the
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Letter
a
Table 2. Substrate Scope
Scheme 4. Reactions Using 3c in CH3CN/H2O
To gain more information regarding the reaction mechanism,
the sulfa-Michael addition of 4a to 2-furanone (7) was carried
out using 3c as a catalyst in CH2Cl2; the absolute configuration
of the obtained product was opposite to that of the product of
the isomerization under the corresponding conditions (Scheme
5). This fact rules out the possibility that the isomerization of
b
entry
R (2)
method
yield (%)
83 (6 d)
ee (%)
1
2,6-(CH3)2C6H3 (2a)
A
B
A
B
A
B
B
B
B
B
B
B
B
B
A
B
76
78
77
84
69
89
86
76
90
66
77
76
85
88
33
52
2
93 (24 h)
95 (24 h)
99 (6 h)
99 (4 d)
97 (6 h)
99 (12 h)
92 (12 h)
94 (6 h)
94 (6 h)
99 (6 h)
99 (6 h)
98 (6 h)
50 (24 h)
68 (10 d)
86 (36 h)
Scheme 5. Sulfa-Michael Addition to 2-Furanone
3
Ph (2b)
4
5
4-CH3OC6H4 (2c)
6
7
3-CH3OC6H4 (2d)
2-CH3OC6H4 (2e)
3,4-(CH3O)2C6H4 (2f)
4-CF3C6H4 (2g)
4-ClC6H4 (2h)
8
9
10
11
12
13
4-BrC6H4 (2i)
1a takes place mainly via the formation of 7 followed by the
sulfa-Michael addition of 4a mediated by the aminothiourea
catalyst. In addition, the HRMS analysis of a solution in which
1a was subjected to a stoichiometric amount of 3c detected a
molecular ion peak corresponding to the acylammonium cation
involved in B (found: m/z 498.1658; calcd: [M]+, 498.1644;
see Scheme S2 in the SI for details). Although a precise
understanding of the mechanism requires additional studies,
these and other experimental findings contain no contradiction
to the reaction pathway we proposed in Scheme 1.
2-naphthyl (2j)
c
14
2,4,6-(i-Pr)3C6H4 (2k)
Bn (2l)
15
de
,
16
a
Method A: reactions were run using 1 (0.15 mmol) and 3c (0.015
mmol) in CH2Cl2 (0.3 mL). Method B: reactions were run using 1a
(0.15 mmol), 4 (0.15 mmol), and 3c (0.015 mmol) in CH2Cl2 (0.3
b
c
mL). Isolated yields. 2a was also obtained in 37% yield with 80% ee.
d
e
Reaction was run using 3 equiv of 4l (0.45 mmol). 2a was also
obtained in 14% yield with 80% ee.
In summary, we have presented a novel method for the
asymmetric synthesis of β-mercaptolactones via isomerization
of ω-hydroxy-α,β-unsaturated thioesters mediated by a bifunc-
tional aminothiourea catalyst. To our delight, the sulfur-
containing cyclic esters were obtained in high yield with
reasonably high enantioselectivity owing to a catalysis that
exploited a covalent interaction cooperatively with anion
binding; this is the first example of bifunctional catalysis
utilizing the activation of electrophiles via α,β-unsaturated
acylammonium intermediates and an interaction with anionic
nucleophiles through a thiourea, thereby realizing a novel type
of reaction. In addition, the potential of the system as an
enantiodivergent synthetic method was also demonstrated by
the choice of a specific solvent system such as CH3CN/H2O.
Furthermore, the process in which the thiolate generated
initially is exchanged with an external thiol suggests the high
applicability of this reaction to various nucleophiles. Studies
aiming at further applications of this synthetic methodology are
currently underway in our laboratory, and the results will be
reported in due course.
Scheme 3. Synthesis of δ-Valerolactone
Moreover, in order to demonstrate the potential of this
reaction for an enantiodivergent synthesis using a single chiral
catalyst, we investigated other solvent systems as an external
factor to influence the enthalpy and entropy terms, as such
observations have also been reported previously (see Table S2
and Figure S2 in the SI for details).16,17 Preliminary studies
showed that a cosolvent system consisting of acetonitrile and
water afforded the opposite enantiomer ent-2a both in the
absence and in the presence of 4a, albeit with modest
enantiomeric excess (Scheme 4).
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures including spectroscopic and analytical
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: asano.keisuke.5w@kyoto-u.ac.jp.
*E-mail: matsubara.seijiro.2e@kyoto-u.ac.jp.
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported financially by the Japanese Ministry of
Education, Culture, Sports, Science and Technology.
■
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