Synthesis of polysubstituted β-amino cyclohexane carboxylic acids via Diels-Alder reaction using Ni(II)-complex stabilized β-alanine derived dienes

Article abstract of DOI:10.1007/s00726-012-1404-x

This paper describes the design and synthesis of a new class of β-alanine derived dienes stabilized by Ni(II)-complex. Preliminary study of their Diels-Alder cycloaddition reactions with several types of dienophiles demonstrates their significant synthetic potential for the preparation of various polyfunctional β-aminocyclohexane carboxylic acids.

Full text of DOI:10.1007/s00726-012-1404-x

Amino Acids (2013) 44:791–796
DOI 10.1007/s00726-012-1404-x
ORIGINAL ARTICLE
Synthesis of polysubstituted b-amino cyclohexane carboxylic acids
via Diels–Alder reaction using Ni(II)-complex stabilized b-alanine
derived dienes
•
Xiao Ding Hengshuai Wang Jiang Wang Sinan Wang Daizong Lin
•
•
•
•
•
•
•
•
•
´
Li Lv Yu Zhou Xiaomin Luo Hualiang Jiang Jose Luis Acen˜a
•
Vadim A. Soloshonok Hong Liu
Received: 17 April 2012 / Accepted: 14 September 2012 / Published online: 30 September 2012
Ó Springer-Verlag Wien 2012
Abstract This paper describes the design and synthesis
of a new class of b-alanine derived dienes stabilized by
Ni(II)-complex. Preliminary study of their Diels–Alder
cycloaddition reactions with several types of dienophiles
demonstrates their significant synthetic potential for the
preparation of various polyfunctional b-aminocyclohexane
carboxylic acids.
between acyclic and cyclic compounds (Diels and Alder
1928). Although numerous variants on this reaction have
been studied, the potential synthetic wealth of this
approach for preparation of six-membered cyclic com-
pounds has still not been completely explored (Takao et al.
2005). In particular, application of Diels–Alder reactions
for the preparation of b-amino cyclohexane carboxylic
acids is one of the least studied areas (Juaristi and Solos-
honok 2005). Current established methods usually rely on
indirect introduction of the desired amino group via nitro
(Pitacco et al. 1977) or carboxylic (Furuta et al. 1987)
functionalities on the starting dienophile. Besides increas-
ing synthetic steps, these approaches required operationally
inconvenient high-pressure reduction or Curtius protocols
(Pitacco et al. 1977; Furuta et al. 1987). One may agree that
methods using dienes/dienophiles directly containing pro-
tected amino group would be methodologically more
straightforward and therefore practically attractive. How-
ever, this approach presents several synthetic challenges
associated, most notably, with low stabilities of the
required amino-containing starting compounds (Juaristi
and Soloshonok 2005). To the best of our knowledge, there
are only two reports in the literature representing this direct
approach. As shown in Scheme 1, b-amino acid 3 can be
prepared by a Lewis acid-catalyzed cycloaddition between
amino-containing diene 2 and acrylic acid derivative 1
(Wipf and Wang 2000). Alternatively, dienophile 4,
derived from glycine, can react at room temperature with
activated dienes (Danishefsky’s diene or cyclopentadiene)
5 to afford a particular type of a-/b-amino acids 6 in a
5-step procedure (Bunuel et al. 1996a, b). Remarkably, this
type of Diels–Alder reactions still remain underdeveloped
and presents an intellectually challenging line of research.
Furthermore, besides methodological novelty, this area of
research has additional aspects of general synthetic
Keywords Diels–Alder reaction ꢀ Nickel (II) ꢀ b-Alanine ꢀ
b-Aminocyclohexane carboxylic acids
Introduction
The Diels–Alder reaction is one of the most magnificent
discoveries in organic chemistry by bridging the realms
X. Ding and H. Wang contributed equally to this work.
Electronic supplementary material The online version of this
article (doi:10.1007/s00726-012-1404-x) contains supplementary
material, which is available to authorized users.
X. Ding ꢀ H. Wang ꢀ J. Wang ꢀ S. Wang ꢀ D. Lin ꢀ L. Lv ꢀ
Y. Zhou ꢀ X. Luo ꢀ H. Jiang ꢀ H. Liu (&)
State Key Laboratory of Drug Research, Shanghai Institute
of Materia Medica, Shanghai Institutes for Biological Sciences,
Chinese Academy of Sciences, 555 Zu Chong Zhi Road,
Shanghai 201203, People’s Republic of China
e-mail: hliu@mail.shcnc.ac.cn
J. L. Acen˜a ꢀ V. A. Soloshonok
IKERBASQUE, Basque Foundation for Science,
48011 Bilbao, Spain
V. A. Soloshonok
University of the Basque Country UPV/EHU,
San Sebastian, Spain
123

792
X. Ding et al.
Literature approaches
CbzHN
O
HOOC
R
NH2
COOH
R
NH2
R'O
O
O
+
O
COOH
N
+
R
N
1
O
R
Ph
5
4
6
2
3
This work
R
L
L
Ni
NH2
O
N
Ph
+
HOOC
R1
* R3
*
R3
R4
O
*
R4
*
R2
R1
R2
7
8
9
Scheme 1 Methodologically different approaches for preparation of b-amino cyclohexane carboxylic acids via Diels–Alder reaction
importance (Chola and Masesane 2008; Csende et al. 2008;
Songis et al. 2008), considering a notable role of b-amino
cyclohexane carboxylic acid motif in the chemistry of
natural products (Oki et al. 1989; Iwamoto et al. 1990) and
b-peptides (Epand et al. 2004; Hayen et al. 2004).
Ni(II)-complexes of glycine of type 10 (Scheme 2)
(Deng et al. 2007; Ueki et al. 2003a, b; Soloshonok et al.
2005a, b; Ellis et al. 2005, 2006) proved to be of quite
general application for preparation of various tailor-made
(Soloshonok et al. 1999a, b) a-amino acids (Soloshonok
2002; Soloshonok et al. 2000a; Yamada et al. 2006).
Homologation of glycine moiety in 10 can be conducted
with high chemical yields and stereoselectivity via alkyl-
ation (Soloshonok et al. 1992, 2001a; Ellis et al. 2003a;
Wang et al. 2011), Michael (Soloshonok et al. 1999c,
2000b, 2005a), Mannich (Soloshonok et al. 1997; Wang
et al. 2008) and aldol (Soloshonok et al. 1995, 1996a, b)
addition reactions. Of particular importance is that the
homologation of 10 can be performed under operationally
convenient conditions (Ellis et al. 2003b; Soloshonok et al.
2004, 2006), allowing synthesis of target a-amino acids on
large scales (Qiu et al. 2000; Soloshonok et al. 2001b; Tang
et al. 2000).
Scheme 2 Ni(II)-complexes of glycine (10), b-alanine (11) and its
a,b-unsaturated a-bromo derivative 12
a-aryl-substituted b-amino acids via two-step alkylation-
reduction sequence (Ding et al. 2009). Continuing our work
on synthetic applications of b-alanine derived complexes
11 and 12, we found that the bromine atom in 12 can be
easily substituted with boronic acids containing unsaturated
moieties by Suzuki coupling reaction giving rise to new
compoundsof type 13 containing1,3-dienesystem(Table 1).
Using the reported conditions, the reaction proceeded satis-
factorily when reacting 12 with various boronic acids in the
presence of Pd(PPh3)4 and Na2CO3 in toluene and water
under microwave irradiation (Ding et al. 2009). As shown in
Table 1, a variety of acyclic and cyclic boronic acids can
participate in the reaction to afford the corresponding prod-
ucts 13 and 16a–g in moderate to excellent yields (73–93 %,
entries 1–8). While a relatively lower yield (76 %, entry 8)
was obtained in the case of tert-butyl related boronic acid
presumbably due to a steric effect.
Results and discussion
Ni(II)-complex 13 (R1 = Me, R2 = H) (Table 2), con-
taining a structurally simple diene residue, and dienophile
N-ethylmaleimide 14 were used as the model substrate to
study the possibility to conduct the corresponding Diels–
Alder cycloaddition reactions. We found that the formation
of target product 15 takes place upon heating of the starting
compounds in toluene with a yield of 60 % (Table 2, entry
1). Higher temperatures and longer reaction time favored
the formation of cycloaddition product 15 allowing its
On the other hand, the chemistry and synthetic potential of
the corresponding complex 11, derived from b-alanine, still
remains virtually unexplored. Our initial results in this area
convincingly demonstrate that compounds of this type hold
significant promise for practical preparations of various
b-amino acids (Ding et al. 2009). In particular, we have
recently developed a convenient procedure for the prepa-
ration of the corresponding a-bromo-a,b-unsaturated ana-
log 12, and demonstrated its application for preparation of
123

Synthesis of polysubstituted b-amino cyclohexane carboxylic acids
793
Table 1 Synthesis of complexes 13 and 16a–g containing b-alanine
and 1,3-diene fragments
Table 3 Diels–Alder reactions of the dienophile N-ethylmaleimide
with different dienes
Entry
Dienes
R
Products
Yield (%)
Entry
R1
R2
Dienes
Yield (%)
1
2
3
4
5
6
7
16a
16b
16c
16d
16e
16f
H
17a
17b
17c
17d
17e
17f
93
90
88
85
90
82
76
1
2
3
4
5
6
7
8
CH3
H
H
13
82
73
84
91
93
90
87
76
CH3
H
16a
16b
16c
16d
16e
16f
16g
Propyl
H
CH3
Hexyl
H
Propyl
Hexyl
Cyclopropyl
Cyclohexyl
C(CH3)3
Cyclopropyl
Cyclohexyl
C(CH3)3
H
H
16g
17g
H
Reactions were conducted using 0.1 mmol of dienes 16a–g and
0.3 mmol of dienophile 14 in 2 mL of toluene at 120 °C for 24 h
H
Reactions were run with 1.31 mmol of 12, 3.9 mmol of boric acids,
3.9 mmol Na2CO3 in 5 mL of toluene and 2 mL of water with
0.014 mmol of Pd(PPh3)4 at 70 °C under microwave irradiation
diverse groups. The results summarized in Table 3 dem-
onstrated that the reaction under study has rather general
application. Substrates 16a–g bearing various alkyl and
cycloalkyl groups gave the target products 17a–g in
excellent yields (76–93 %, entries 1–7). Acyclic and cyclic
substituents on the starting dienes 16b–g showed no dra-
matic influence on the yields. Remarkably, this cycload-
dition reaction was also tolerated in the presence of
sterically bulky tert-butyl group, though the reaction rate
was noticeably slower and resulted in lower isolated yield
of the corresponding product 17g (76 %, entry 7).
Table 2 Diels–Alder reaction of the nickel(II) complex 13 with N-
ethylmaleimide
Another aspect of the synthetic generality of this Diels–
Alder reaction was explored by investigating the cycload-
ditions between dienes of type 13 and various dienophiles
18. The results were shown in Table 4. First, we examined
the effects of the N-substituent of maleimide on the Diels–
Alder reaction using derivatives 18a and 18b bearing
methyl and phenyl groups, respectively. Expectedly, the
reaction of N-methylmaleimide 18a with Ni(II)-complex
13 gave the cycloaddition product 19a with 92 % yield
(entry 1). While a relatively lower yield was obtained when
the dienophile was phenyl-substituted maleimide presum-
ably because of the steric effect (72 %, entry 2). These
results suggested that the cyclic dienophiles of type 14
showed good reactivities and provide reliable access to the
corresponding b-amino acids derivatives. However, only
trace amount of the product 19c (entry 3) was obtained
when benzoquinone was used as the dienophile. We then
decided to concentrate our further study on acyclic deriv-
ative 18d–h containing electronically activating as well as
deactivating functional groups. Cycloadditions of diene 13
with dienophiles 18d and 18e, bearing electron-
Entry
Temperature (°C)
Time (h)
Yield (%)
1
2
3
4
80
120
120
120
12
18
24
24
60
85
92
45a
Reactions were conducted using 0.1 mmol of diene 13 and 0.3 mmol
of dienophiles in 2 mL of toluene
a
1.0 equiv of BF3 was added as the Lewis acid
preparation in 92 % yield (Table 2, entry 3). Attempt to
further improve the reaction outcome by catalysis with
Lewis acid BF3 was less successful and only 45 % yield of
the product 15 was obtained. This is mainly because the
Ni(II)-complex is unstable under acidic conditions result-
ing in the decomposition and the recovery of the starting
material PBP (Table 2, entry 4).
With these promising results in hand, we examined
further the generality of Diels–Alder reactions using a
series of Ni(II)-complexes 16a–g containing structurally
123

794
X. Ding et al.
Table 4 Diels–Alder reactions of the diene 13 with various dienophiles
Entry
1
Dienophiles
R3
R4
R5
Products
Yield (%)
92
18a
19a
N
O
O
2
3
18b
18c
19b
19c
72
Ph
N
O
O
O
O
Tracea
4
5
6
7
8
18d
18e
18f
H
COOCH3
COOCH3
19d
19e
19f
58
61b
Tracea
H
CN
H
CN
H
COOEt
CH3
OEt
18g
18h
CH3
H
CH3
H
19g
19h
0
0
Reactions were run with 0.1 mmol of dienes 13 and 0.3 mmol of dienophiles 18 in 2 mL toluene at 120 °C for 24 h
a
Detected by LC/MS
b
Not isolated yield. Detected by LC/MS. The product was not stable during the workup
withdrawing substituents on both sides of the double bond,
occurred with relatively low yields of 58 % (entry 4) and
61 % (entry 5), respectively. These results clearly sug-
gested that this approach can be optimized for the efficient
preparation of b-amino acid derivatives of type 19c–e. In
sharp contrast, the reaction of dienophile 18f, bearing only
one activating substituent (entry 6) proceeded very slowly
Scheme 3 Disassembling of nickel(II) complex 17c to release target
b-amino acid 20 and recovery of the ligand 21
and only trace amount of the product was detected by
LC/MS. No product was observed when the dienophiles
were alkyl- or electron-donating substituted (Table 4,
entries 7 and 8). These results clearly pointed to obvious
limitation of this approach in terms of generality of the
dienophile type.
Conclusion
In conclusion, we have successfully developed a new type
of Ni(II) complex stabilized b-alanine derived dienes and
outlined their reactivity with different types of dienophiles.
The results presented in this work convincingly suggested
that the designed dienes have a significant synthetic
potential for highly diastereospecific preparation of poly-
functional b-amino-cyclohexanecarboxylic acids via
Diels–Alder reaction. Further work on asymmetric version
of this approach is currently under study and will be
reported in a due course.
Taking into account that the cycloaddition products 15,
17 and 19 are structurally new type of Ni(II) complexes, we
needed to demonstrate that they can be reliably disassem-
bled to release the target b-amino acid. To this end, we
treated product 17c under the standard disassembly con-
ditions by heating suspension of 17c in 1 N HCl/MeOH.
Decomposition of complex 17c took place quite in a usual
manner allowing isolation of the target b-amino acid 20 in
90 % yield along with quantitative recovery of ligand 21,
which can be recycled to prepare starting b-alanine derived
Ni(II) complex 11 (Scheme 3).
Acknowledgments We gratefully acknowledge financial support
from National Basic Research Program of China (Grants
2009CB940903 and 2009CB918502), the National Natural Science
123

Synthesis of polysubstituted b-amino cyclohexane carboxylic acids
795
Foundation of China (Grants 20721003 and 81025017), National S&T
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