Peptide-catalyzed consecutive 1,6- and 1,4-additions of thiols to α,β,γ,δ-unsaturated aldehydes

Article abstract of DOI:10.1039/c4ob00565a

Regio- and enantioselective addition of thiols to α,β,γ, δ-unsaturated aldehydes was performed with a resin-supported peptide catalyst. It was shown that a 1,4-adduct was generated mainly at the initial stage of the reaction, and this was eventually converted to a thermodynamically stable 1,6- and 1,4-diadduct through retro-addition/addition reactions. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob00565a

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Peptide-catalyzed consecutive 1,6- and
1,4-additions of thiols to α,β,γ,δ-unsaturated
Cite this: Org. Biomol. Chem., 2014,
12, 3581
aldehydes†
Received 14th March 2014,
Accepted 17th April 2014
Kengo Akagawa, Nobuhiro Nishi, Jun Sen and Kazuaki Kudo*
DOI: 10.1039/c4ob00565a
www.rsc.org/obc
Regio- and enantioselective addition of thiols to α,β,γ,δ-unsatu- tuted acetaldehydes as Michael acceptors.10 The same group
rated aldehydes was performed with a resin-supported peptide also used finely designed nucleophiles for attaining the 1,6-
catalyst. It was shown that a 1,4-adduct was generated mainly at addition to linear 2,4-dienals.11 In those examples, however,
the initial stage of the reaction, and this was eventually converted regiochemistry is governed by the intrinsic reactivity of sub-
to a thermodynamically stable 1,6- and 1,4-diadduct through strates, thus the scope of organocatalyzed 1,6-selective reac-
retro-addition/addition reactions.
tions is still limited.12,13 Recently, we have reported the
reaction system aiming for catalyst-controlled 1,6-regioselec-
tivity. With a resin-supported peptide catalyst consisting of
specific secondary structures, regio- and enantioselective
reduction of α,β,γ,δ-unsaturated aldehydes was realized.14–16
Because of the larger size of peptides compared to low-mole-
cular-weight catalysts, the peptide catalysts are expected to
be applied to a wide range of reactions with π-extended
substrates.
The Michael addition of thiols is useful for the synthesis of
sulfur-containing biologically active compounds,17 and some
organocatalytic versions have appeared to date.18 In 2005,
Jørgensen and co-workers reported the asymmetric Michael
addition of thiols to α,β-unsaturated aldehydes with a second-
ary amine catalyst.19 When we tried this type of reaction using
an α,β,γ,δ-unsaturated aldehyde in the presence of an imidazo-
lidinone catalyst,20 it was found that the β,δ-diadduct 3a was
mainly obtained as a mixture of diastereomers (Scheme 1). As
to the enantioselectivity, both diastereomers were nearly
racemic. Product 3a is considered to be formed via consecutive
1,6- and 1,4-additions of two thiol molecules to the substrate
The asymmetric Michael addition of nucleophiles to α,β-un-
saturated carbonyl compounds is one of the most powerful pro-
cedures for the synthesis of chiral molecules. Besides metal-
catalyzed reactions,1 a variety of asymmetric Michael additions
by organocatalysts have been developed in recent years.2,3
Among them, those reactions which proceed through the acti-
vation of carbonyl groups by chiral amine catalysts have been
widely studied and offer a versatile method for utilizing
various nucleophiles.4 In such reactions, the formation of an
iminium-ion intermediate between a catalyst and a substrate
promotes the nucleophilic addition by lowering the LUMO
energy level of the π-conjugated system, which simultaneously
controls the enantioselectivity of the reaction.5
This type of organocatalytic addition has been applied to
the substrates with extended π-systems, e.g. α,β,γ,δ-unsaturated
carbonyl compounds. With such substrates, the conjugate
addition generally takes place in a 1,4-selective manner, which
has been rationalized by calculation; both the π-orbital coeffi-
cient of the LUMO and the partial positive charge at the β-posi-
tion of the iminium-ion intermediate are larger than those at
the δ-position.6 Concerning this, some attempts have been
made to overturn the 1,4-preference of conjugate additions.7,8
Melchiorre and co-workers achieved the 1,6-selective addition
by using 3-alkenylcyclohexenones or 2,4-dienals with a bulky
substituent at the β-position to suppress the 1,4-addition.9
Jørgensen and co-workers employed cycloalkenylidene-substi-
Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku,
Tokyo 153-8505, Japan. E-mail: kkudo@iis.u-tokyo.ac.jp; Fax: +81 3 5452 6357;
Tel: +81 3 5452 6359
†Electronic supplementary information (ESI) available: Experimental details,
spectroscopic data, and copies of 1H and 13C NMR spectra and HPLC traces.
See DOI: 10.1039/c4ob00565a
Scheme 1 Amine-catalyzed conjugate addition of
α,β,γ,δ-unsaturated aldehyde.
a thiol to an
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Table 1 Peptide-catalyzed thiol addition to an aromatic α,β,γ,δ-unsaturated aldehyde
Entry
HX
T (°C)
t (h)
Conversion (%)
1a : 2a : 3a
dr of 3a
ee (%) of 3a
1
2
TFA
TFA
TFA
TFA
TFA
PhCO2H
TCA
RT
RT
0
0
0
0
0
0
24
3
3
3
10
3
83
67
34
40
70
83
29
76
2 : 22 : 76
50 : 7 : 43
53 : 13 : 34
57 : 14 : 29
29 : 39 : 32
18 : 0 : 82
30 : 4 : 66
6 : 3 : 91c
57/43
57/43
60/40
60/40
63/37
57/43
58/42
58/42
29, −14
52, 25
76, 66
70, 59
72, 53
42, 15
62, 40
73, 58
3
4a
5
6
7
3
24
8b
TCA
a The non-supported peptide with a C-terminal amide was used. b Reaction was performed with 4 equiv. of 2-naphthalenethiol in MeOH–H2O
(1 : 2). c The yield of the corresponding alcohol of 3a was 54%.
aldehyde. This is interesting from the viewpoint that, in spite
of using the linear 2,4-dienal, the 1,6-addition seemingly pre-
vails over the intrinsic 1,4-preference of the addition to an
iminium intermediate. To clarify the reaction mechanism and
refine the reaction to an enantioselective version, we set out
investigation for a peptide-catalyzed conjugate addition of
thiols to α,β,γ,δ-unsaturated aldehydes.
Because of the high applicability for various enantio-
selective reactions, the resin-supported peptide 4 was chosen
as a catalyst.21 With the trifluoroacetic acid (TFA) salt of this
Scheme 2 Proposed reaction pathway for generation of 3a.
peptide, the reaction at room temperature for 24 h gave com-
pound 3a as the major product (Table 1, entry 1). Although the
ee values were low, the reaction proceeded in a more enantio-
selective manner than the case with the low-molecular-weight
organocatalyst. When the reaction time was shortened to 3 h
under the same conditions, the distribution of the products
and the enantioselectivity of the diadduct 3a dramatically
changed (Table 1, entry 2). In particular, the ratio of the 1,4-
adduct 1a was considerably higher. This indicates that the 1,4-
addition predominates at the initial stage of the reaction as
reported earlier;6 however, the 1,4-adduct 1a is eventually con-
verted to the thermodynamically most stable diadduct 3a
Scheme 3 Thiol-exchange reaction from the diadduct 3a.
through repetition of retro-addition/addition reactions
(Scheme 2).19,22 The fact that elongating the reaction time
depicted in Scheme 2. To obtain the product 3a in a regio- and
enantioselective manner, it is essential to promote dis-
sociation of the 1,4-adduct 1a while suppressing the retro-
Michael addition from the diadduct 3a. When the reaction was
conducted at 0 °C, higher enantioselectivity was attained
despite low reaction rate and regioselectivity (Table 1, entry 3).
At this temperature, prolonging the reaction could successfully
increase the conversion without significant loss in enantio-
selectivity (Table 1, entry 5). As for the acid component of the
decreased the enantioselectivity of the product 3a suggests the
occurrence of the retro-Michael reaction from the diadduct 3a.
To confirm the retro-Michael/Michael addition process, the
following experiment was conducted. After converting all reac-
tants into the diadduct 3a in the presence of the peptide cata-
lyst, a different thiol was added (Scheme 3). From the analysis
of the resulting mixture after 36 h, it was revealed that thiol-
exchanged products 5 and 6 were generated.23 This demon-
strates that both the 1,4- and 1,6-additions are reversible as
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Table 2 Peptide-catalyzed thiol addition to an aliphatic α,β,γ,δ-unsaturated aldehyde
Entry
HX
T (°C)
t (h)
Conversion (%)
1b : 2b : 3b
dr of 3b
ee (%) of 3b
1a
2
TFA
TFA
PhCO2H
PhCO2H
RT
6
12
12
20
99
12
75
76
0 : 10 : 90
28 : 0 : 82
55 : 0 : 45
22 : 0 : 78c
57/43
55/45
57/43
56/44
18, −6
84, 55
75, 38
75, 44
−10
−15
−15
3
4b
a Reaction was performed in THF–H2O (1 : 2). b Reaction was performed with 4 equiv. of 2-naphthalenethiol. c The yield of the corresponding
alcohol of 3b was 28%.
catalyst, the use of benzoic acid instead of TFA mainly pro- entry 2). In this case, replacing TFA with benzoic acid was
vided the diadduct 3a; however, the enantioselectivity was effective to enhance the reaction (Table 2, entry 3). Increasing
decreased (Table 1, entry 6). Moderate regioselectivity and the amount of the thiol and elongating the reaction time pro-
enantioselectivity were observed with trichloroacetic acid (TCA) vided compound 3b in good regio- and enantioselectivity
(Table 1, entry 7). Further optimization of reaction conditions (Table 2, entry 4). The similar tendency was observed with
such as the amount of the thiol, solvent, and time afforded benzenethiol as a nucleophile. While the reaction with the
compound 3a as the major product with good regio- and benzoic acid salt of peptide 4 for 6 h was accompanied by a
enantioselectivity (Table 1, entry 8).
certain amount of 1,4-adduct 1c (Table 3, entry 1), thermo-
Next, other combinations of substrates were tested for the dynamic convergence into the product 3c occurred after 24 h
peptide-catalyzed thiol addition to an α,β,γ,δ-unsaturated alde- while maintaining good enantioselectivity (Table 3, entry 2).
hyde. With an aliphatic 2,4-dienal instead of the aromatic one, The supported peptide catalyst recovered by filtration after the
the reaction proceeded smoothly to give the diadduct 3b as the reaction could be reused at least three times without a signifi-
major product in a poorly enantioselective manner (Table 2, cant loss in the catalytic activity (Table 3, entries 3 to 5).
entry 1). High regio- and enantioselectivity were attained at a
low temperature, although the reaction was sluggish (Table 2,
Conclusions
Consecutive 1,6- and 1,4-additions of thiols to α,β,γ,δ-unsatu-
rated aldehydes in an enantioselective way were realized with a
resin-supported peptide catalyst. It was demonstrated that
Table 3 Convergence to a β,δ-diadduct by elongating the reaction time
and reuse of the catalyst
apparently good regioselectivity of the reaction was the result
of the reversible nature of the thiol addition. To achieve high
enantioselectivity, suppressing the racemization of the di-
adduct was essential, and the use of the peptide salt under
optimum conditions was effective. Further application of the
peptide catalyst in the reaction with π-extended systems can be
expected.
Reuse of
Entry peptide
Conversion
t (h) (%)
ee (%)
1c : 2c : 3c dr of 3c of 3c
1
2
3
4
5
—
—
6
24
24
99
99
99
99
99
36 : 0 : 64
2 : 0 : 98a
0 : 0 : 100
0 : 0 : 100
2 : 0 : 98
53/47
53/47
53/47
53/47
52/48
79, 42
76, 54
74, 52
78, 56
79, 61
Acknowledgements
1st reuse
2nd reuse 24
3rd reuse 24
This work was partially supported by JSPS KAKENHI (grant
number 23550116), and by MEXT KAKENHI (grant number
24105506).
a The yield of the corresponding alcohol of 3c was 65%.
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the replaced thiol was not determined spectroscopically.
However, from the mechanism of the retro reaction, the
product is assumed to be 5.
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