Chiral Phosphoric Acid Catalyzed Asymmetric Ugi Reaction by Dynamic Kinetic Resolution of the Primary Multicomponent Adduct

Article abstract of DOI:10.1002/anie.201600751

Reaction of isonitriles with 3-(arylamino)isobenzofuran-1(3H)-ones in the presence of a catalytic amount of an octahydro (R)-binol-derived chiral phosphoric acid afforded 3-oxo-2-arylisoindoline-1-carboxamides in high yields with good to high enantioselec

Full text of DOI:10.1002/anie.201600751

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International Edition: DOI: 10.1002/anie.201600751
German Edition: DOI: 10.1002/ange.201600751
Synthetic Methods
Chiral Phosphoric Acid Catalyzed Asymmetric Ugi Reaction by
Dynamic Kinetic Resolution of the Primary Multicomponent Adduct
Yun Zhang, Yu-Fei Ao, Zhi-Tang Huang, De-Xian Wang,* Mei-Xiang Wang,* and Jieping Zhu*
Abstract: Reaction of isonitriles with 3-(arylamino)isobenzo-
furan-1(3H)-ones in the presence of a catalytic amount of an
octahydro (R)-binol-derived chiral phosphoric acid afforded
3-oxo-2-arylisoindoline-1-carboxamides in high yields with
good to high enantioselectivities. An enantioselective Ugi four-
center three-component reaction of 2-formylbenzoic acids,
anilines, and isonitriles was subsequently developed for the
synthesis of the same heterocycle. Mechanistic studies indicate
that the enantioselectivity results from the dynamic kinetic
Our group[11] and that of Maruoka[12] have independently
reported chiral phosphoric acid (CPA) and chiral dicarboxylic
acid catalyzed Ugi-type three-component reactions in which
the transient nitrilium intermediate was trapped by an
internal carboxamide function (Scheme 1a,b). Very recently,
À
resolution of the primary Ugi adduct, rather than from the C C
bond-forming process. The resulting heterocycle products are
of significant medicinal importance.
T
he Ugi 4CR converts an aldehyde, an amine, an acid, and
an isonitrile into an a-acetamidoamide at room temperature
under catalyst (promoter)-free conditions and is therefore
one of the rare chemical transformations that proceeds in
a truly mix-and-go manner.[1] The reaction generates one
À
=
À
C C, one C O, and two C N bonds with water as the only by-
product. While the prototypical Ugi 4CR provides a linear
peptide-like adduct, many heterocycles[2] and even macro-
cycles[3] are now readily accessible by modification of this
versatile and powerful reaction. Since one stereogenic center
is generated in this transformation, the ability to control the
stereochemical outcome would expand significantly its syn-
thetic utilities. However, the development of a catalytic,
enantioselective Ugi reaction remains an unsolved problem in
spite of the progress recorded in the field of asymmetric
Passerini reactions,[4–9] a closely related three-component
condensation of an aldehyde, carboxylic acid, and isonitrile.[10]
Scheme 1. Enantioselective Ugi-type multicomponent reaction.
M.S.=molecular sieves.
Wulff and co-workers described a chiral BOROX-catalyzed
reaction of aldehydes, secondary amines, and isonitriles for
the synthesis of enantioenriched a-aminoacetamides (Scheme
1c).[13] List and Pan have also examined the CPA (TRIP)-
catalyzed reaction of benzaldehyde, 4-methoxyaniline, and
tert-butylisonitrile, however, the three-component adduct was
obtained in low yield and ee value.[14] In all these truncated
Ugi reactions, the carboxylic acid is notably absent. Indeed, its
presence renders the development of an enantioselective
process extremely difficult since carboxylic acid is not only
a reactant, but also a catalyst for the classic Ugi reaction. We
report herein a novel CPA-catalyzed enantioselective reac-
tion between isonitriles (3) and 3-(arylamino)isobenzofuran-
1(3H)-ones (7), as well as the first examples of the Ugi four-
center three-component reaction of 2-formylbenzoic acids
(1), anilines (2), and isonitriles (3) for the synthesis of
enantioenriched 3-oxoisoindoline-1-carboxamides (4).[15] We
also document that the observed enantioselectivity results
from the dynamic kinetic resolution (DKR) of the primary
[*] Y. Zhang, Dr. Y.-F. Ao, Prof. Dr. Z.-T. Huang, Prof. Dr. D.-X. Wang
Beijing National Laboratory for Molecular Sciences
CAS Key Laboratory of Molecular Recognition and Function
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (China)
E-mail: dxwang@iccas.ac.cn
Prof. Dr. M.-X. Wang
Key Laboratory of Bioorganic Phosphorous and
Chemical Biology (Ministry of Education), Tsinghua University
Beijing 100184 (China)
E-mail: wangmx@tsinghua.edu.cn
Prof. Dr. J. Zhu
Laboratory of Synthesis and Natural Products
Institute of Chemical Sciences and Engineering
Ecole Polytechnique FØdØrale de Lausanne
EPFL-SB-ISIC-LSPN, BCH 5304, 1015 Lausanne (Switzerland)
E-mail: jieping.zhu@epfl.ch
Homepage: http://lspn.epfl.ch
À
Ugi adduct, rather than from the C C bond-forming step. The
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201600751.
heterocycle 4 is known for its analgesic properties[16] and has
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been found, as a key structural motif, in many bioactive
(50%) was obtained in EtOAc (entry 4). More rewarding
natural products [for example, (S)-(+)-lennoxamine (5)][17]
and medicinally relevant compounds, such as (R)-pazina-
clone, a sedative and anxiolytic drug.[18]
results were obtained when the reaction was carried out in
chlorinated solvents, with DCE affording the best results
(entry 9). A slightly higher ee value was observed when
performing the reaction in toluene. However, there was
a solubility issue with this solvent and DCE was chosen for
further optimization. Adding molecular sieves (M.S.)
improved significantly the enantioselectivity of the reaction
(entries 12–16), with 3 Š M.S. (10 mg per 0.2 mmol of
substrate) being optimum (entry 15). The reverse molar
ratio (3a/7a = 1.2:1) increased the yield of the product
without affecting the enantioselectivity (entry 16). Finally,
both decreasing and increasing the catalyst loading had
a negative effect on the ee value of the product (entries 17 and
18), whereas a similar result was obtained when the reaction
was carried out at À208C.
As a prelude to the development of enantioselective Ugi
four-center three-component reaction, we began our studies
by examining the two-component reaction between the
isonitrile 3a and 3-(phenylamino) isobenzofuran-1(3H)-one
(7a: see Table 1). The latter is easily synthesized from 2-
formylbenzoic acid (1a) and aniline (2a). The reaction itself
was unknown, but we assumed that 7a could serve as a latent
iminium species under mild acidic conditions. Therefore, the
reaction was expected to follow the Ugi pathway involving
nucleophilic addition of the isonitrile to the iminium species
as a key step. After initial screening of different (R)-binol-
derived CPAs,[19,20] the phosphoric acid 8 stood out as the
catalyst of choice in terms of both the reactivity and
selectivity (see the Supporting Information for a list of
CPAs screened). The results of a survey of reaction conditions
using 8 as the catalyst are summarized in Table 1. As
expected, the racemic 4a was isolated in 89% yield when
the reaction was performed in MeOH, one of the preferred
solvents for the Ugi 4CR (entry 1). Other polar solvents
capable of forming hydrogen bonds with CPA were equally
inefficient (entries 2–5), although a reasonable ee value
With optimal reaction conditions in hand [molar ratio 3a/
7a = 1.2:1, 8 (0.1 equiv) and 3 Š M.S., DCE (c = 0.1m), 08C],
the generality of the reaction was next examined (Scheme 2).
Anilines with different electronic properties participated in
this reaction. Electron-poor anilines afforded the 3-oxoisoin-
doline-1-carboxamides 4 in excellent yields and ee values
(4b–f). However, 7 bearing an electron-rich aniline afforded
the desired product 4g with a reduced ee value. The com-
pounds 4h and 4i with 1-naphthyl and 2-naphthyl substitu-
ents, respectively, were similarly prepared in high yields and
ee values. Both electron-withdrawing and electron-donating
groups on the aromatic ring of 7 were also well tolerated (4k–
Table 1: Two-component version: Optimization of the reaction condi-
tions.[a]
Entry Solv
M.S. (mg) T [8C] t [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16[f]
17[g]
18[h]
19
MeOH
MeCN
THF
EtOAc
MeNO2
CH2Cl2
CHCl3
CCl4
DCE
toluene
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
–
–
–
–
–
–
–
–
–
20
20
20
20
20
20
20
20
20
20
0
0
0
0
0
0
0
0
1.5
1.5
48
20
3
1.5
6.5
24
3
36
6
3
2
89
77
trace
68
96
79
74
75
97
65
95
98
90
95
93
99
81
87
95
0
12
n.d.[d]
50
26
67
52
59
73
75
79
86
80
84
87
87
69
84
87
–
–
3 Š (20)[e]
4 Š (20)[e]
3 Š (40)
3 Š (10)
3 Š (10)
3 Š (10)
3 Š (10)
3 Š (10)
0.4
3
1
52
0.5
À20
7
[a] Reaction conditions: 7a (0.24 mmol), 3a (0.2 mmol), 8 (0.02 mmol,
0.1 equiv), solvent (2.0 mL). [b] Yield of isolated product. [c] Determined
by HPLC analysis using a chiral stationary phase. [d] n.d.=not
determined. [e] Used without preactivation. [f] Used 7a (0.2 mmol), 3a
(0.24 mmol). [g] Used 0.05 equiv of 8. [h] Used 0.2 equiv of 8. DCE=1,2-
dichloroethane, THF=tetrahydrofuran.
Scheme 2. Scope of the catalytic enantioselective synthesis of isoindo-
linones. [a] 7 (0.2 mmol), 3a (0.24 mmol), 8 (0.02 mol, 0.1 equiv), DCE
(c=0.1m), 3 Š M.S. (10.0 mg), 08C. [b] Reaction was performed at
À108C. [c] With 0.15 equiv of 8. [d] With 0.2 equiv of 8. [e] With
0.3 equiv of 8. [f] With 0.4 equiv of 8.
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n). With regard to the isonitrile, tertiary, secondary, and
primary alkyl isonitriles, including a-isocyanoacetate, were all
tolerated to afford the corresponding isoindolinones (4o–s).
The absolute configuration of 4j was determined by X-ray
crystallographic analysis[21] and that of other isoindolinones
was assigned accordingly since all these compounds have very
similar CD spectra.
With the aforementioned results in hand, the develop-
ment of a catalytic enantioselective Ugi four-center three-
component reaction of 1, 2, and 3 was straightforward. As
illustrated in Scheme 3, yields of the three-component
Scheme 4. Possible reaction pathway. The role of CPA in these steps
was not illustrated for the sake of clarity.
To gain insight on the reaction mechanism, control
experiments were performed (Scheme 5). Submitting the
isocoumarine 12a, prepared following literature proce-
dure,[23a] to our standard reaction conditions afforded the
isoindolinone 4o (Scheme 5a) in 95% yield with 88% ee, and
is a perfect match with the result of the direct CPA 8-
catalyzed condensation between 3b and 7a. Reaction of the
C3-deuterated isobenzofuranone [D]-7b, synthesized by
condensation between 2-(formyl-D)benzoic acid ([D]-1a)[24]
and 4-chloroaniline (2b), with tBuNC (3a) under our
standard reaction conditions afforded 4b with 76% loss of
deuterium. Once again, the ee value of 4b (92%; Scheme 5b)
matched that obtained from the reaction of 7b and 3a. The
results of these two control experiments clearly indicate that
the imine–enamine equilibrium occurs (Scheme 4, steps c, d)
and that the overall transformation involves a DKR of 12
which is formed in situ. This mechanistic interpretation is also
in accord with our experimental observation that electron-
rich aniline, hence increased rate of Mumm rearrangement of
11 (step b, Scheme 4), afforded product in low enantioselec-
tivity (see 4g, Scheme 2). That the deuterium is not com-
pletely exchanged is understandable and may reflect the
kinetic isotopic effect in the prototropic tautomerization step.
In summary, we have developed a novel chiral phosphoric
acid catalyzed enantioselective reaction between isonitriles
(3) and 3-(arylamino)isobenzofuran-1(3H)-ones (7) to afford
Scheme 3. Enantioselective four-center three-component reaction. [a] 1
(0.1 mmol), 2 (0.1 mmol), 3 (0.12 mmol), 8 (0.03 mol, 0.3 equiv), DCE
(c=0.1m), 3 Š MS (5.0 mg), 08C. [b] With 0.4 equiv of 8.
reaction were generally excellent considering that four
chemical bonds were generated in this operationally simple
process. The enantioselectivity was slightly decreased and
a higher catalyst loading was needed relative to the two-
component version. To the best of our knowledge, this
represents the first example of an enantioselective Ugi
reaction in which a carboxylic acid was incorporated as one
of the participating functional groups.[22]
Possible reaction pathways accounting for the formation
of 4 from 1, 2, and 3 are depicted in Scheme 4. Condensation
of 1 with 2 affords the iminium salt 9. Intermolecular
nucleophilic addition of the divalent carbon atom of 3 to 9
can provide the nitrilium 10 (step a, Scheme 4), which can be
trapped by the tethered carboxylate to provide 11. Mumm
rearrangement of 11 via the bridged intermediate 13 can
furnish the isoindolinone 4. However, 11 is known to undergo
facile isomerization to the isocoumarine 12 by prototropic
tautomerization.[23] While this equilibrium is transparent in
a racemic process, it would have an important consequence in
our catalytic enantioselective version. In light of the observed
enantioselectivity, two scenarios could be advanced: a) the
Mumm rearrangement (step b) is much faster than the imine–
enamine isomerization (step c/d; kb @ kc, kd). In this case, the
À
C C bond-forming step (step a) leading to 10 would deter-
mine the absolute configuration of the final adduct; b) imine–
enamine isomerization (step c/d) is much faster than the
Mumm rearrangement (step b; kc, kd @ kb). In this case, the
DKR of 12 would be responsible for the observed enantio-
selectivity.
Scheme 5. Control experiments.
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À
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Acknowledgments
Financial supports from National Natural Science Foundation
of China (21320102002, 21502202) and the EPFL (Switzer-
land) are gratefully acknowledged.
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Keywords: asymmetric synthesis · kinetic resolution ·
multicomponent reaction · organocatalysis ·
reaction mechanisms
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Received: January 22, 2016
Published online: March 21, 2016
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