Construction of 3,4-Dihydrocoumarin Derivatives with Adjacent Quaternary and Tertiary Stereocenters: Organocatalytic Asymmetric Michael Addition of 2-Oxochroman-3-carboxylate Esters to trans-β-Nitroolefins

Article abstract of DOI:10.1002/adsc.201600825

An asymmetric Michael addition of 2-oxochroman-3-carboxylate esters to trans-β-nitroolefins is described. This strategy can give direct access to dihydrocoumarin derivatives bearing adjacent quaternary and tertiary stereocenters with up to >99% yield, >20:1 dr, and >99% ee. The adduct was further transformed to a spiro-dihydrocoumarin compound in three steps with good yield. (Figure presented.).

Full text of DOI:10.1002/adsc.201600825

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DOI: 10.1002/adsc.201600825
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Construction of 3,4-Dihydrocoumarin Derivatives with Adjacent
Quaternary and Tertiary Stereocenters: Organocatalytic
Asymmetric Michael Addition of 2-Oxochroman-3-carboxylate
Esters to trans-b-Nitroolefins
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Hui Jin,a Soo Min Cho,a Geum-Sook Hwang,b,* and Do Hyun Ryua,*
a
Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Korea
Fax: (+82)-31-290-5976; e-mail: dhryu@skku.edu
Western Seoul Center, Korea Basic Science Institute, Seoul 120-140, Korea
b
Fax: (+82)-2-6908-6239; e-mail: gshwang@kbsi.re.kr
Received: July 27, 2016; Accepted: September 25, 2016; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600825
b-keto-esters,[7] and monothiomalonates (MTMs).[8]
Abstract: An asymmetric Michael addition of 2-
Barbas III reported the first example of thiourea-
oxochroman-3-carboxylate esters to trans-b-nitro-
olefins is described. This strategy can give direct
access to dihydrocoumarin derivatives bearing ad-
jacent quaternary and tertiary stereocenters with
catalyzed stereoselective Michael additions of oxin-
doles to nitroolefins.[5a] Wennemers used a-substituted
MTMs as nucleophiles to construct acyclic thioesters
with adjacent quaternary and tertiary stereocenters.[8a]
up to >99% yield, >20:1 dr, and >99% ee. The
However, to the best of our knowledge, there is no
adduct was further transformed to a spiro-dihydro-
example of using 2-oxochromans as a nucleophile in
coumarin compound in three steps with good yield.
the Michael addition with nitroolefins. This strategy
can give direct access to 3,4-dihydrocoumarin scaf-
Keywords: 3,4-dihydrocoumarins; quaternary; or-
ganocatalysis; Michael addition; L-proline
folds, which exist in a wide range of natural products
and bioactive compounds.[9]
Recently, we reported asymmetric Michael addi-
tion of dithiomalonates to trans-b-nitroolefins cata-
36 The enantioselective construction of an all-carbon lyzed by L-proline derived tertiary amine bifunctional
37 quaternary stereogenic center is a great synthetic organocatalysts.[10] We envisioned the organocatalytic
38 challenge.[1] In the last decade, substantial efforts have asymmetric Michael addition of 3-substituted 2-oxo-
39 been made in the catalytic asymmetric construction of chroman 1 to nitroolefins 2 as an efficient method for
40 quaternary carbon centers.[2] Among them, the orga- the synthesis of 3,4-dihydrocoumarin derivatives with
41 nocatalytic asymmetric conjugate addition of prochiral adjacent quaternary and tertiary stereocenters
42 trisubstituted nucleophiles to b-substituted Michael (Scheme 1). Herein, we disclose the first examples of
43 acceptors has proven to be a powerful strategy for the thiourea-catalyzed asymmetric additions of 2-oxochro-
44 asymmetric construction of highly functionalized ad- man-3-carboxylate esters 1 to trans-b-nitroolefins 2 in
45 jacent quaternary and tertiary stereocenters.[3] In this excellent yields and stereoselectivities.
46 strategy, it is challenging to control the configurations
47 of the two newly created chiral centers at the same
48 time. When either or both of the two chiral centers
49 are quaternary centers, stereochemistry becomes
50 much harder to control as the Michael reaction is
51 generally sensitive to sterics.[3] Nitroolefins are consid-
Scheme 1. Strategy for asymmetric synthesis of 3,4-dihydro-
coumarin derivatives.
52 ered as ideal Michael acceptors due to their good
53 reactivity and capacity of being easily transformed.[4]
54 Many kinds of nucleophiles have been used in the
55 asymmetric Michael addition with nitroolefins to
56 construct adjacent quaternary and tertiary stereocen-
Initially, asymmetric Michael addition between
57 ters, including oxindoles,[5] benzofuran-2(3H)-ones,[6] ethyl 2-oxochroman-3-carboxylate 1a and trans-nitro-
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styrene 2a was examined in the presence of 10 mol % nitrogen was changed to a methyl group (4b), 3aa was
of quinine derived thiourea catalyst Q-TU (Table 1, obtained at a much better yield (92%) and stereo-
entry 1). When the reaction was carried out at 258C in selectivity (18:1 dr and 99% ee) (Table 1, entry 3).
methyl t-butyl ether (MTBE), the desired product 3aa Upon changing the phenyl group of 4b on the carbon
was obtained in 95% yield, 6:1 dr, and 86% ee in 3 h. bearing the thiourea moiety to a larger 3,5-dimethyl-
Compared with the catalyst Q-TU, L-proline derived phenyl group (4c), no change in yield or stereo-
thiourea 4a showed improved stereoselectivity, but a selectivity was observed (Table 1, entry 4). Comparing
lower yield, affording 3aa in 38% yield, 14:1 dr, and different hydrogen bond donor moieties in the cata-
92% ee under identical conditions (Table 1, entry 2). lyst, the squaramide catalyst 4d yielded a product with
10 When the benzyl group of 4a on the pyrrolidine superior diastereoselectivity, but lower enantioselec-
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tivity (Table 1, entries 3, 5, and 6). The diphenyl-
substituted catalyst 4f with the urea moiety linked to
an achiral carbon showed much lower efficiency than
4e, which reaffirmed the importance of the chiral
center in these catalysts (Table 1, entry 7).[10] Screen-
ing the solvents of toluene, mesitylene, and CH2Cl2
revealed MTBE to be the best solvent (Table 1,
entries 8, 9, and 10). In summary, using 4b as the
catalyst and MTBE as a solvent is identified as the
most suitable condition for this reaction. Lowering the
catalyst loading to 2 mol % did not affect either the
stereoselectivity or yield of the reaction, although a
longer reaction time was required (Table 1, entry 12).
With optimized reaction conditions in hand, a
variety of 2-oxochroman-3-carboxylate esters were
investigated (Scheme 2). Regardless of the electronic
properties of substituents on the aromatic ring of 1,
products 3ba-3fa were obtained in excellent stereo-
selectivities and yields. When the ethyl carboxylate in
1 was changed to a more bulky t-butyl carboxylate, the
adduct 3ga was not produced.
Table 1. Reaction condition optimization for the asymmetric
Michael addition of 1 to 2a.
Entry Cat.
Solvent
Time Yieldb drc
eed
[%]
[%]
1
2
3
4
5
6
7
8
Q-TU MTBE
3 h
3 h
3 h
3 h
3 h
3 h
3 h
3 h
95
38
92
91
85
93
37
85
93
93
96
93
6:1
-86
92
99
99
4a
4b
4c
4d
4e
4f
4b
4b
4b
4b
4b
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
toluene
14:1
18:1
18:1
>20:1 96
10:1
2:1
15:1
12:1
14:1
18:1
18:1
99
60
96
99
85
99
99
9
mesitylene 3 h
10
11e
12f
CH2Cl2
MTBE
MTBE
3 h
2 d
3 d
a) Unless otherwise noted, all reactions were carried out
between 2a (0.15 mmol) with 1a (0.075 mmol) in 1 mL of
solvent at 25oC.
b) Isolated yield of 3aa.
Scheme 2. Asymmetric Michael additions of 2a with
variety of 2-oxochroman-3-carboxylate esters 1.
a
c) Diastereomeric ratios were determined by 1HNMR
spectroscopy of the crude reaction mixture.
d) The ee of 3aa was determined by chiral HPLC analysis.
e) 5 mol % 4b was used.
To further expand the substrate scope, we applied
these catalytic conditions to the reactions between 1a,
f) 2 mol % 4b was used.
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1d, and a range of trans-b-nitroolefins. The processes nitro group of 3ae to an amine followed by intra-
were highly efficient for all the cases with different molecular cyclization provided the lactam 5 at a 91%
substituents on the aromatic nitroolefins, among which yield in two steps.[8c,10] Finally, the spiro-dihydrocou-
nitroolefins with an electron-donating group on the marin compound 6[13] was prepared by intramolecular
aromatic ring afford products with lower stereoselec- transesterification of 5 in the presence of p-toluene-
tivities (Table 2, entries 7 and 9). It is noteworthy that sulfonic acid.
ortho-substituted aromatic nitroolefins furnished the
desired product in excellent stereoselectivities and
yields, but required longer reaction time (Table 2,
10 entries 2 and 6), due to the steric hindrance effect. It
11 was noted that aliphatic nitroalkenes were not effi-
12 cient substrates in the conditions used.[11]
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Table 2. Asymmetric Michael additions of 1a and 1d with a
variety of nitroolefins 2.a
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Entry
1
R1
3
Yieldb drc
eed [%]
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[%]
1
1a phenyl
3aa 92
3ab 96
3ac 89
3ad 99
3ae 90
3af 96
3ag 99
3dh 95
18:1
>20:1 >99
99
2e
3
1a 2-FC6H4
1a 4-CF3C6H4
1a 4-ClC6H4
1a 4-BrC6H4
1a 2-BrC6H4
1a 4-MeC6H4
1d 4-FC6H4
15:1
17:1
18:1
18:1
15:1
20:1
8:1
>99
>99
99
>99
94
>99
96
>99
98
4
5f
6e
7
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10
11
1d 4-MeOC6H4 3di 95
1d 2-thiophenyl 3dj 96
19:1
12:1
1d 2-Naphthyl
3dk 92
a) Unless otherwise noted, all reactions were carried out
between 2 (0.15 mmol) with 1 (0.075 mmol) in 1 mL of
MTBE at 25oC.
Scheme 3. Asymmetric Michael addition of 1a to 2e on a
gram scale and transformations of adduct 3ae.
b) Isolated yield of 3.
c) Diastereomeric ratios were determined by 1HNMR
spectroscopy of the crude reaction mixture.
d) The ee of 3 was determined by chiral HPLC analysis.
e) The reaction was stirred at 25oC for four days.
f) The absolute configuration of 3ae was determined by X-
ray analysis.[12] The absolute configurations of other major
diastereomers were assigned accordingly.
The observed stereochemistry for the asymmetric
Michael addition reaction can be rationalized by the
proposed transition-state model shown in Figure 1.
Deprotonation of the acidic proton from 2-oxochro-
man-3-carboxylate esters 1 by the tertiary amine
To demonstrate the synthetic utility of this method-
49 ology, a gram-scale experiment was carried out
50 (Scheme 3): To a solution of 1a (1 g, 4 mmol, 1 equiv.)
51 and catalyst 4b (37 mg, 0.08 mmol, 2 mol %) in 40 mL
52 of MTBE, nitroolefin 2e (1.37 g, 6 mmol, 1.5 equiv.)
53 was added at room temperature and stirred for four
54 days. The desired product 3ae was obtained with 91%
55 isolated yield, 18:1 dr, and >99% ee. Then, the adduct
56 3ae was further transformed to the spiro-dihydrocou-
57 marin compound 6 in three steps. Reduction of the
Figure 1. Proposed transition-state model for the asymmetric
Michael addition between 1a and 2e catalyzed by 4b.
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Chem. Res. 2015, 48, 740; c) F. Vetica, R. M. de
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group of N-methylpyrrolidine, leads to formation of
an ammonium ion. In route A, nitroolefin 2 is
activated through interaction with the protonated
amino group of 4b, while simultaneously the
carbanion of 1 interacts with the thiourea moiety of
4b through hydrogen bonding to form the ternary
complex 7.[10,14] By contrast, in route B, the carbanion
of 1 coordinates to the protonated amino group of 4b,
while 2 is activated by the thiourea moiety of 4b to
ˇ
´
Coric, O. O. Grygorenko, P. S. J. Kaib, I. Komarov, A.
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10 form the ternary complex 8.[3b,10] With either pathway,
11 the same adduct 3ae was obtained as the major
12 stereoisomer.
13
In conclusion, we have developed a mild, organo-
14 catalytic method to give direct access to dihydrocou-
15 marin derivatives bearing adjacent quaternary and
16 tertiary stereocenters in excellent stereoselectivities
17 and yields. This reaction is not sensitive to air or
18 water, and can be carried out facilely on a gram scale
19 without reducing stereoselectivity or yield. The adduct
20 3ae was further transformed to the more complex
21 spiro-dihydrocoumarin compound 6. Further investi-
22 gations of the substrate scope and synthetic utility of
23 this methodology are in progress.
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[3] For a reviews about organocatalytic asymmetric con-
jugate additions, see: a) Y. Zhang, W. Wang, Catal. Sci.
Technol. 2012, 2, 42. For selected examples of organo-
catalytic asymmetric Michael additions used to con-
struct adjacent quaternary and tertiary stereocenters,
see: b) T. Okino, Y. Hoashi, T. Furukawa, X. Xu, Y.
Takemoto, J. Am. Chem. Soc. 2005, 127, 119; c) G.
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Biomol. Chem. 2006, 4, 2097; e) L. Li, W. Chen, W.
Yang, Y. Pan, H. Liu, C. H. Tan, Z. Jiang, Chem.
Commun. 2012, 48, 5124.
Experimental Section
General procedure for the asymmetric Michael
addition reaction
[4] For a reviews about asymmetric conjugate additions to
nitroolefins, see: O. M. Berner, L. Tedeschi, D. Enders,
Eur. J. Org. Chem. 2002, 1877.
To a solution of the 2-oxochroman-3-carboxylate esters 1
(0.075 mmol, 1.0 equiv.) and catalyst 4b (3.5 mg,
0.0075 mmol, 10 mol %) in MTBE (1 mL), nitroolefin 2
(0.15 mmol, 2 equiv.) was added. Then, the reaction mixture
was stirred for 12 h at room temperature. The solvent was
evaporated and the residue was purified by column chroma-
tography to provide the desired products.
[5] Organocatalytic asymmetric Michael additions with
nitroolefins using oxindoles as nucleophiles: a) T. Bui, S.
Syed, C. F. Barbas III, J. Am. Chem. Soc. 2009, 131,
8758; b) X. Li, B. Zhang, Z. G. Xi, S. Luo, J. P. Cheng,
Adv. Synth. Catal. 2010, 352, 416; c) M. Ding, F. Zhou,
Z. Q. Qian, J. Zhou, Org. Biomol. Chem. 2010, 8, 2912;
d) M. Ding, F. Zhou, Y. L. Liu, C. H. Wang, X. L. Zhao,
J. Zhou, Chem. Sci. 2011, 2, 2035; e) X. Chen, W. Zhu,
W. Qian, E. Feng, Y. Zhou, J. Wang, H. Jiang, Z. Yao,
H. Liu, Adv. Synth. Catal. 2012, 354, 2151.
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Acknowledgements
This work was supported by the National Research Founda-
tion of Korea(NRF) grant funded by the Korea govern-
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[6] X. Li, X. S. Xue, C. Liu, B. Wang, B. X. Tan, J. L. Jin, Y.
Zhang, N. Dong, J. P. Cheng, Org. Biomol. Chem. 2012,
10, 413.
ment(MSIP)
2013R1A1A2073207,
(No.
NRF-2013R1A1A2059838,
No. 2016R1A2B3007119,
No.
No.
2016R1A4A1011451), by the Agency for Defense Develop-
ment through Chemical and Biological Defense Research
Center and by the National Research Council of Science and
Technology (CAP-2012-2-KBSI) and Korea Basic Science
Institute [C36705].
[7] Organocatalytic asymmetric Michael additions with
nitroolefins using b-keto-esters as nucleophiles: a) H.
Li, Y. Wang, L. Tang, F. Wu, X. Liu, C. Guo, B. M.
Foxman, L. Deng, Angew. Chem. Int. Ed. 2005, 44, 105;
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Pedrosa, Adv. Synth. Catal. 2010, 352, 3364; c) Z. Yu, X.
Liu, L. Zhou, L. Lin, X. Feng, Angew. Chem. Int. Ed.
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M. S. Prasad, Org. Biomol. Chem. 2012, 10, 5825.
[8] Organocatalytic asymmetric Michael additions with
nitroolefins using MTMs as nucleophiles: a) Y. Araka-
wa, S. P. Fritz, H. Wennemers, J. Org. Chem. 2014, 79,
3937; b) O. D. Engl, S. P. Fritz, A. Kꢃslin, H. Wen-
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Construction of 3,4-Dihydrocoumarin Derivatives with
Adjacent Quaternary and Tertiary Stereocenters: Orga-
nocatalytic Asymmetric Michael Addition of 2-Oxochro-
man-3-carboxylate Esters to trans-b-Nitroolefins
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H. Jin, S. M. Cho, G.-S. Hwang*, D. H. Ryu*