Umpolung of o-Hydroxyaryl Azomethine Ylides: Entry to Functionalized ?-Aminobutyric Acid under Phosphine Catalysis

Article abstract of DOI:10.1021/acs.orglett.8b02297

A phosphine-catalyzed reaction between o-hydroxyaryl azomethine ylides and MBH carbonates provides access to highly functionalized ?-aminobutyric acid derivatives in moderate to good yields. Mechanistically, the reaction involves a phosphine-catalyzed tandem S_N2′/2-aza-Cope rearrangement/intramolecular addition process.

Full text of DOI:10.1021/acs.orglett.8b02297

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Umpolung of o‑Hydroxyaryl Azomethine Ylides: Entry to
Functionalized γ‑Aminobutyric Acid under Phosphine Catalysis
Qingqing Chen, Yishu Bao, Xiuqin Yang, Zonghao Dai, Fulai Yang, and Qingfa Zhou*
State Key Laboratory of Natural Medicines, Department of Organic Chemistry, China Pharmaceutical University, Nanjing, 210009,
China
S
* Supporting Information
ABSTRACT: A phosphine-catalyzed reaction between o-
hydroxyaryl azomethine ylides and MBH carbonates provides
access to highly functionalized γ-aminobutyric acid derivatives
in moderate to good yields. Mechanistically, the reaction
involves a phosphine-catalyzed tandem SN2′/2-aza-Cope
rearrangement/intramolecular addition process.
γ-Aminobutyric acid (GABA) is a unique substructure present in
the central nervous system (CNS) of mammals exhibiting many
important bioactivities.1 For example, GABA in humans plays a
central part in the regulation of muscle tone, and it has been one
important target for drug discovery.2 Compounds containing
this structural motif are also widely present in natural products,
and many of them exhibit a wide range of biological activities:3 γ-
amino-β-hydroxybutyric acid (I), which has been identified as a
key fragment of the microsclerodermins, exhibits antitumor and
antifungal activity;4 gabapentin (II), which was developed by
Pfizer, has been used as a drug for the treatment of neuropathic
pain;5 Hemiasterlin (III), which is extracted from the marine
sponge Hemiasterella minor, exhibits a potent in vitro cytotoxin
and antimitotic agent (see Figure 1).6 Because of the prevalence
ylides derived from α-amino acids as a nucleophile is very scarce,
although they have proven to be versatile synthetic inter-
mediates to synthesize diverse pyrrolidines11 by coupling with
various dipolarophiles under metal catalysis or organcatalysis
conditions and to synthesize α,α-disubstituted α-amino acids12
under various conditions (see Scheme 1). Umpolung of
Scheme 1. Reaction Patterns of Azomethine Ylide Derived
from α-Amino Acids
Figure 1. Related compounds containing the γ-aminobutyric acid
scaffold.
of γ-aminobutyric acids in medicinal chemistry, it is highly
demanded to develop novel synthetic methodologies that enable
an avenue to new molecular architectures that contain this
important pharmacophore.
azomethine ylides may provide alternative addition/cyclo-
addition patterns to synthesize chemical entry with structural
diversity, which has neither been previously accessible nor
required. Very recently, an intramolecular hydrogen bond in the
azomethine ylide has shown a huge impact on both reactivity
and reaction site of the substrates.13 Inspired by the above work
and our continual interest in phosphine catalysis,14 we
attempted the reaction of o-hydroxyaryl azomethine ylides
with MBH carbonates in the presence of phosphine catalyst.
Unexpectedly, a functionalized γ-aminobutyric acid was
Recently, imine umpolung reactions have emerged as a
powerful synthetic strategy for constructing structural amino
compounds.7 For example, many γ-amino aldehydes/ketones
have been constructed via umpolung β-addition of ketimines to
enals or enones, using the phase-transfer catalysts reported by
Deng’s group.8 A series of functionalized α-quaternary amines
could be effectively synthesized through phosphine-catalyzed
umpolung γ-addition of ketimines to allenoates by Zhang’s
group9 and diverse γ-aminobutyric acids containing a trifluor-
omethyl group could also be obtained by phosphine-catalyzed
umpolung addition of trifluoromethyl ketimines to Morita−
Baylis−Hillman (MBH) carbonates.10 In contrast with
umpolung of imines, inverting the polarization of azomethine
Received: July 21, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02297
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
obtained, which not only shows a new reaction pattern but also
exploits the potential of azomethine ylides derived from α-
amino acids. Herein, we report a novel phosphine-catalyzed and
highly regioselective umpolung addition of o-hydroxyaryl
azomethine ylides to MBH carbonates.15
After establishing the optimized reaction conditions for the
synthesis of functional γ-aminobutyric acid derivatives, several o-
hydroxyaryl azomethine ylides were first synthesized and applied
to couple with MBH carbonate 2a, and the results are outlined in
Scheme 2. As can be seen from Scheme 2, o-hydroxyaryl
In the beginning, o-hydroxyaryl azomethine 1a and MBH
carbonate 2a were selected for the initial reaction using 20
mol % triphenylphosphine as a catalyst in CH2Cl2 at room
temperature (see Table 1, entry 1). A regioselective SN2′
Scheme 2. Substrate Scope for Umpolung Reactions of o-
Hydroxyaryl Azomethine with MBH Carbonates
a b
,
a
Table 1. Survey on Conditions for Formation of 3a
b
Yield (%)
catalyst
(mol %)
oil bath temperature
entry
solvent
(°C)
3a
4a
5a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
CH2Cl2
CH2Cl2
DCE
CHCl3
CH3CN Ph3P (20)
Ph3P (20)
Ph3P (20)
Ph3P (20)
Ph3P (20)
25
40
80
60
80
150
70
110
0
75
40
46
76
61
83
75
69
DMF
THF
Ph3P (20)
Ph3P (20)
Ph3P (20)
Ph3P (20)
Ph3P (20)
Ph3P (20)
Ph3P (20)
Ph2PEt (20)
PhPEt2 (20)
Bu3P (20)
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
20
57
60
80
20
16
29
57
52
150
200
150
150
150
65
67
58
27
30
a
The reaction was performed with o-hydroxyaryl azomethine (1a)
(0.1 mmol) and MBH carbonate (2a) (0.12 mmol) in a sealed tube
b
for 6 h. Isolated yields based on 1.
product 5a with a terminal CC bond was obtained. We
conjectured that product 5a could undergo a 2-aza-Cope
rearrangement under higher-temperature conditions,16 so we
performed the reaction at 40 °C in CH2Cl2. To our
disappointment, product 5a and 4a were obtained with
comparative yields and no 2-aza-Cope rearrangement product
was found. Various solvents were examined carefully (see Table
1, entries 3−8). Product 4a was selectively generated in
dichloroethane (DCE), CHCl3, acetonitrile, dimethylforma-
mide (DMF), and tetrahydrofuran (THF). Remarkably, when
toluene was used, a novel product 3a was isolated, despite the
fact that the yield remained to be raised to a higher level. The
structure of the product 3a was confirmed through X-ray
crystallographic analysis. The temperature was then examined
using toluene as a solvent. No reaction was found when the
temperature was reduced to 0 °C. However, the product 5a was
obtained with a yield of 80% when the reaction was performed at
60 °C. Pleasingly, a good yield of product 3a was obtained when
the reaction was performed at 150 °C in a sealed tube. A higher
temperature did not give a better yield of 3a. No better result was
obtained when a more nucleophilic phosphine instead of PPh3
was employed, such as Ph2PEt, PhPEt2, or Bu3P. Thus, the
optimal conditions for the formation of γ-aminobutyric acid
derivatives were established.
a
The reaction was performed with o-hydroxyaryl azomethine 1 (0.1
mmol) and MBH carbonates 2 (0.12 mmol) in a sealed tube under
b
150 °C for 6 h. Isolated yields based on 1.
azomethine ylides containing different electrical groups on the
benzene ring worked well with MBH carbonate 2a, giving the
target products γ-aminobutyric acid derivatives in moderate to
good yields. For example, for the substrates with a methyl or
methoxyl group attached on the benzene ring, the correspond-
ing γ-aminobutyric acid derivatives 3b and 3d were obtained in
yields of 64% and 73%, respectively. Various halogen
substituents (F, Cl, Br) could still give the corresponding γ-
aminobutyric acid derivatives in moderate to good yields under
the reaction conditions, which offers a potential handle for the
subsequent coupling operation. In addition, dihalogen sub-
stituents also afforded the corresponding products. Note that
naphthyl-containing substrates also are compatible, efficiently
giving γ-aminobutyric acid 3i in a yield of 63%. We next
B
DOI: 10.1021/acs.orglett.8b02297
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
investigated the scope of MBH carbonates. As shown in Scheme
2, both electron-withdrawing and electron-donating groups at
the para position of the benzene ring were tolerated and gave the
corresponding γ-aminobutyric acid derivatives in good yields.
For example, for substrates with a methyl or methoxyl group
attached on the benzene ring, the corresponding γ-aminobutyric
acid derivatives 3k and 3l were obtained in yields of 57% and
56%, respectively. Substrates with an electron-withdrawing
group, such as a nitrile group, on the benzene ring also gave the
target product 3p in a yield of 55%. However, for the substrate
with a stronger electron-withdrawing group, such as methyl 2-
[((tert-butoxycarbonyl)oxy) (4-nitrophenyl)methyl] acrylate, a
novel azetidine derivative 7 was found (see Scheme 3, eq 1).
Scheme 5. Plausible Mechanisms
Scheme 3. Diverse Reaction of 1a with MBH Carbonates
MBH carbonate to produce the phosphonium intermediate A
with concurrent release of CO2 and tert-butoxide. The tert-
butoxide then deprotonates o-hydroxy aromatic aldimine 1a and
generates intermediates B. The hydroxy group located at the 2-
position in 1a might stabilize azomethine ylide B via an
intramolecular hydrogen bond. The azomethine ylide B might
happen a nucleophilic attack on the β-carbon of MBH
carbonate, which led to the formation of allylation product 4a
(Scheme 5, path a). On the other hand, azomethine ylide B
undergoes a SN2′ with phosphonium intermediate A to give γ-
allylation product 5. Product 5 follows a facile 2-aza-Cope
rearrangement to deliver the γ-aminobutyric acid C, which
might then undergo an intramolecular addition pathway to form
product 3 ultimately. Because of the effect of nitro group at the
para position of the benzene ring, compound 7 could be formed
via an intramolecular anti-Michael process (Scheme 5, path b).
In summary, we have developed an unprecedented
phosphine-catalyzed and highly regioselective domino process
of o-hydroxyaryl azomethine with MBH carbonates to highly
functionalized γ-aminobutyric acid derivatives. This catalytic
process provides access to γ-aminobutyric acid derivatives in
moderate to good yields. This transformation is a rare example
of umpolung of azomethine ylides derived from α-amino acids.
Pleasingly, the benzene ring can be replaced by a naphthalene
ring or a heteroaromatic ring to provide corresponding γ-
aminobutyric acid derivatives 3q and 3r in yields of 66% and
64%, respectively. We also tested the feasibility of this reaction
using alkyl MBH carbonate; for example, methyl 2-[((tert-
butoxycarbonyl)oxy)methyl] acrylate, under the same reaction
conditions, gave the SN2′ product 9 (Scheme 3, eq 2).
To understand these novel processes, diethyl (E)-2-
(benzylideneamino) malonate was synthesized and treated
with MBH carbonate 2a under the aforementioned conditions
(see Scheme 4). The reaction was complicated and no major
Scheme 4. Control Experiments
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b02297.
Full experimental procedures and copies of NMR spectra
(PDF)
Accession Codes
CCDC 1857200 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, U.K.; fax: +44 1223 336033.
product was formed. This implied that the hydroxy group
located at the 2-position of azomethine ylide might play an
important role for the present process.13 Product 5a could be
effectively transformed to 3a when 5a was treated in toluene at
150 °C in a sealed tube. However, no product was formed when
4a treated in toluene at 150 °C in a sealed tube. These suggested
that steric effect of phenyl group in 4a was responsible for the
failure of 2-aza-Cope rearrangement (see Scheme 4).
AUTHOR INFORMATION
Corresponding Author
■
On the basis of these results and the previous work,17 we
proposed a reasonable mechanism for present domino
processes18 (see Scheme 5). Initially, phosphine reacts with
*E-mail addresses: zhouqingfa@cpu.edu.cn, zqf@mail.ustc.edu.
cn.
C
DOI: 10.1021/acs.orglett.8b02297
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
ORCID
Letter
139, 11184. (q) Xu, X.; Liu, J. Y.; Chen, D. J.; Timmons, C.; Li, G. G.
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Fulai Yang: 0000-0002-1136-0867
Qingfa Zhou: 0000-0001-6360-2285
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (Grant Nos. 21102179 and
21572271), the Qing Lan Project of Jiangsu Province, the
National Found for Fostering Talents of Basic Science (Grant
No. J1030830).
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DOI: 10.1021/acs.orglett.8b02297
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