An aza-Michael addition protocol to fluoroalkylated β-amino acid derivatives and enantiopure trifluoromethylated N-heterocycles

Article abstract of DOI:10.1039/c4gc01357c

The aza-Michael reaction with β-fluoroalkylated acrylates provided the corresponding fluoroalkylated β-amino acid derivatives in up to 99% yield under catalyst- and solvent-free conditions. An enantioenriched β-trifluoromethylated β-amino acid was obtained in good yield through a scale-up diastereoselective aza-Michael addition, which facilitated the installation of enantiopure trifluoromethylated analogues of β-lactam and dihydroquinolin-4-one. This journal is

Full text of DOI:10.1039/c4gc01357c

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An aza-Michael addition protocol to
fluoroalkylated β-amino acid derivatives and
enantiopure trifluoromethylated N-heterocycles†
Cite this: DOI: 10.1039/c4gc01357c
Received 17th July 2014,
Accepted 15th August 2014
Xing Yang,a Zhuo Chen,a Yuan Cai,a Yi-Yong Huang*a and Norio Shibata*b
DOI: 10.1039/c4gc01357c
www.rsc.org/greenchem
The aza-Michael reaction with β-fluoroalkylated acrylates provided
the corresponding fluoroalkylated β-amino acid derivatives in up
to 99% yield under catalyst- and solvent-free conditions. An
enantioenriched β-trifluoromethylated β-amino acid was obtained
in good yield through a scale-up diastereoselective aza-Michael
addition, which facilitated the installation of enantiopure trifluoro-
methylated analogues of β-lactam and dihydroquinolin-4-one.
It has been well demonstrated that the incorporation of the
fluoroalkylated group can effectively enhance the lipophilicity,
electronic interactions, binding selectivity and stability to
metabolic degradation of the parent compounds.1 β-Amino
acids are present in naturally occurring biologically active pep-
tides,2 and have found a wide spectrum of applications in
organic synthesis and medicinal chemistry.3 In view of the syn-
thetic application of fluorinated β-amino acids to fluorinated
β-lactams,4 or key motifs in peptides,5 peptidomimetics6 and
other molecules7 with various important biological activities
(Fig. 1), the development of reliable methodologies for
their preparation has been a topic of great interest in the last
decade.8 In particular, the asymmetric synthesis of optically
pure β-trifluomethylated (CF3) β-amino acids (TFAAs) displays
Fig. 1 Selected bioactive compounds containing fluorinated β-amino
acid scaffolds.
obtain optically pure TFAA derivatives in good yields.10b In the
meantime, Saigo et al. disclosed a highly practical asymmetric
hydride reduction of a seven-membered cyclic enamino-ester
derived from 4,4,4-trifluoro-3-oxobutanoate to form chiral
TFAA.10c In 2013, Grellepois applied the asymmetric Refor-
matsky reaction with chiral α-CF3 N-tert-butanesulfinyl hemi-
aminals to the preparation of TFAA derivatives and further
incorporation into peptides.10f Recently, Shibata, Soloshonok
and co-workers demonstrated a practical approach to enantio-
merically pure TFAAs after hydrolysis and decarboxylation of
the corresponding β-aminomalonates obtained from diastereo-
selective Mannich additions.10g
One of the most simple and powerful tools to construct the
β-amino acid skeleton is the 1,4-addition of amines to unsatu-
rated esters (aza-Michael reaction). Over the past few decades,
tremendous efforts have been devoted to the development of
highly efficient and selective aza-Michael reactions.11 These
reactions are usually accomplished in an organic solvent with
the assistance of an organo- or organometallic catalyst. The
philosophy of green chemistry requires that all atoms are con-
verted into the desired products by minimizing or avoiding the
use of solvents, most ideally, in the absence of any catalyst.
The aza-Michael addition under both catalyst- and solvent-free
conditions remains a challenge, but highly desirable for the
a
challenge for organic chemists. The enantioselective
approach remains relatively rare and difficult to achieve high
enantioselectivity.9 An alternative strategy is based on dia-
stereoselective transformation. All diastereoselective protocols
explored so far to generate TFAAs involved reactions with tri-
fluoromethylated ketones or imines exclusively.10 For instance,
the Fustero group described a diastereoselective addition of
chiral 2-(p-tolylsulfinyl)-benzylic carbanions to trifluoromethyl-
ated imines followed by a desulfuration/oxidation sequence to
aDepartment of Chemistry, School of Chemistry, Chemical Engineering and Life
Science, Wuhan University of Technology, Wuhan 430070, P.R. China.
E-mail: huangyy@whut.edu.cn
bDepartment of Frontier Materials, Nagoya Institute of Technology, Gokiso,
Showa-ku, Nagoya, 466-8555, Japan. E-mail: nozshiba@nitech.ac.jp
†Electronic supplementary information (ESI) available: General experimental
procedures, NMR and HRMS data of all new compounds are included. See DOI:
10.1039/c4gc01357c
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economic and environmentally benign advantage. To the best addition reaction with the trifluoromethylated electrophile
of our knowledge, there is no literature that describes the syn- under catalyst- and solvent-free conditions. Because all acrylate
thesis of TFAAs and their derivatives via aza-Michael addition substrates used herein are in the solid state, all amines
without any catalyst and solvent. In this context, as continuing selected in the aza-Michael protocol are limited to be liquid at
efforts on synthetic fluorine chemistry,12 we report herein the room temperature for guaranteeing a homogeneous reaction
research results on a simple and novel aza-Michael reaction process.
between fluorinated acrylic acid derivatives 1 and amines 2
In the established reaction system, a variety of primary aro-
without any solvent and catalyst. Under the same conditions, matic amines reacted efficiently with 1a to form racemic TFAA
the scale-up synthesis via diastereoselective aza-Michael derivatives 3 (Table 1). Methoxyl-substituted sterically hindered
addition allows for an efficient access to enantiopure TFAA 6, amine 2b resulted in a slight drop in conversion efficiency to
and thus to two enantioenriched trifluoromethylated hetero- furnish 3ab in 95% yield after 32 h (entry 2). To our surprise,
cyclic compounds as well (see Scheme 2).
when one more electron-donating methoxyl group was intro-
Previously, we have shown the versatility of β-trifluoro- duced at the 4-position of 2b, compound 1a was consumed
methylated acrylate 1a in the asymmetric Friedel–Crafts completely within 3 h (entry 3). The examination of meta-sub-
alkylation to install optically pure trifluoromethylated heliotri- stituents showed that the electronic effect played a key role in
dane.12b In order to explore more functions of 1a, we initially the reaction outcomes. Amine 2d featuring an electron-donat-
envisioned
a catalytic asymmetric aza-Michael addition ing group (–Me) worked well (entry 4), whereas electron-with-
between 1a and aniline 2a to obtain a chiral analogue of 3aa drawing groups (–F, –Cl, –Br) yielded the adducts 3ae–3ag with
with the β-CF3 amino acid skeleton. However, even after a diminished levels of efficiency (entries 5–7). Notably, the pres-
thorough screening of several parameters of the model reac- ence of a strong electron-withdrawing group like a CF3 group
tion, including the 3,3′-substituents of chiral BINOL-derived was also tolerated with moderate isolated yield (entry 8).
phosphoric acid catalysts, organic solvents and temperature, Substituting the para-position of aniline, regardless of the
efficient and highly enantioselective transformation to 3aa was electronic and steric nature, generated the target compounds
hardly established. A control experiment revealed that the 3ai–3ak in excellent yields within 3 h (entries 9–11).
mixture of 1a and 2a in the absence of any catalyst in CH2Cl2
Encouraged by these results, the scope of aliphatic amine
at room temperature gave 3aa in 45% yield. But the yield is nucleophiles was next examined. From the point of view of
difficult to be improved by prolonging the reaction time. To nucleophilicity and steric hindrance, benzylamine should have
our delight, neat conditions enabled us to establish a novel a higher activity than aniline in aza-Michael reaction. Treat-
reaction system, which afforded the desired compound 3aa in ment of 1a with 1.1 equivalent of benzylamine led to 3al in
quantitative yield after 22 h (entry 1, Table 1). Most impor- 91% yield and a small amount of an amidolysis product 3al′
tantly, after evaporating the very small excess of aniline, 3aa (eqn (1), Scheme 1). Increasing the amount of benzylamine to
was obtained with a satisfactory purity. 3aa could be further 2.2 equivalents mainly gave rise to 3al′ in 93% yield after 1.5 h
purified by preparative thin layer chromatography (PTLC). It (eqn (2), Scheme 1). Inspired by this result, we focused on the
should be noted that this is the first case of aza-Michael cascade aza-Michael/amidolysis reaction between 1a and a bis-
nucleophile benzylhydrazine 2m.13 As expected, small-ring het-
erocycles pyrazolidinones 3am and 3am′, as core structures in
many pharmacological and biological active molecules,14 were
obtained in 66% and 25% yield, respectively. The less hin-
dered amine part of the hydrazine privileged the 1,4-addition
to afford the major product 3am. In addition, a variety of
N-substituents (–Me, –Et, –Bn and –(CH2)2OH) of secondary
benzylamine were also tested and the corresponding products
3an–3aq were obtained uniformly in excellent yields within a
Table 1 Scope of different aromatic amine nucleophilesa
Entry
Ar
Product
Time (h)
Yieldb (%)
1
2
3
Ph (2a)
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
22
32
3
12
60
56
36
60
3
97
95
99
94
92
88
92
52
99
97
96
o-OMe-Ph (2b)
o,p-(OMe)2-Ph (2c)
m-Me-Ph (2d)
m-F-Ph (2e)
4
5c
6c
7c
8c
9
m-CI-Ph (2f)
m-Br-Ph (2g)
m-CF3-Ph (2h)
p-Et-Ph (2i)
10
11
p-tBu-Ph (2j)
p-F-Ph (2k)
3aj
3ak
1
3
a All reactions were carried out using 1a (0.2 mmol) and 2a–k
(0.22 mmol) at room temperature for the time given. b Isolated yield
after preparative thin layer chromatography. c 0.30 mmol amine was
used.
Scheme 1 Aza-Michael reaction between 1a and 2l or 2m.
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Table 2 Scope of different aliphatic amine nucleophilesa
Entry
RR′NH
Product
Time (h)
Yieldb (%)
1
2
3
4
BnNH2 (21)
3al
1.5
6
4
91
95
95
99
Bn(Me)NH (2n)
Bn(Et)NH (2o)
Bn2NH (2p)
3an
3ao
3ap
1.5
5
3aq
1
92
Fig. 2 Plausible mechanism of the aza-Michael reaction.
6
3ar
2
94
β-substituent may result in different types of β-fluoromethyl-
ated β-amino acid derivatives. The replacement of fluorine
atom(s) with one or two proton(s) reduces the reaction rate
(entries 2–3, Table 3). For comparison, methyl oxazolidinone
1d was also engaged in this reaction with a markedly lower
reactivity (entry 4). Other variations on the β-carbon of 1, such
as –C2F5, –CClF2, and –CBrF2, slightly influences the reaction
speed compared with 1a (entries 5–7). These results indicate
that strong electron-withdrawing action of fluorinated groups,
in particular a CF3 group, plays a crucial role in the transform-
ation. However, such a protocol could not be extended
to β-CF3-β-phenyl disubstituted acrylate 1h, and no desired
adduct 3hk was delivered even after a long reaction time.
The plausible mechanism of this aza-Michael reaction is
illustrated in Fig. 2. Two components of the reaction are orga-
nized through the hydrogen bonding interaction between the
carbonyl oxygen atom and the amine NH proton. Sequentially,
the formed intermediate I undergoes an intramolecular-like
amine conjugate addition to provide enolized intermediate II.
The strong electron-withdrawing nature of the CF3 group in
acrylate 1 plays a vital role in this smooth direct addition
step. Finally, a rapid tautomerization occurs to generate the
aza-Michael adduct 3.
7
8
3as
3at
2
91
99
CyNH2 (2t)
1.5
a All reactions were carried out using 1a (0.2 mmol) and 2k–s
(0.22 mmol) at room temperature for the time given. b Isolated yield
after preparative thin layer chromatography.
Table 3 Scope of various acrylate electrophilesa
Entry
Rf
R
Product
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
CF3 (1a)
H
H
H
H
H
H
H
Ph
3ak
3bk
3ck
3dk
3ek
3fk
3
18
24
44
18
6
96
93
92
72
87
92
87
0
CF2H (1b)
CFH2 (1c)
CH3 (1d)
C2F5 (1e)
CCIF2 (11)
CBrF2 (1g)
CF3 (1h)
The development of novel synthetic methodologies for opti-
cally pure fluorinated β-amino acids is of particular interest in
synthetic organic fluorine chemistry. The present protocol is
3gk
3hk
10
48
a All reactions were carried out using 1 (0.2 mmol) and 2k (0.22 mmol)
at room temperature for the time given. b Isolated yield after difficult to be performed in a highly catalytic enantioselective
preparative thin layer chromatography.
manner due to the fast background reaction mentioned above.
The issue of stereo-outcome of this aza-Michael addition could
be effectively addressed by the use of a chiral oxazolidinone
short time (entries 2–5, Table 2). In the case of two cyclic sec- auxiliary. In a scale-up reaction between the chiral acrylate 4
ondary amines of morpholine 2r and pyrrolidine 2s, compar- (4 mmol) and amine 2k (Scheme 2), the major diastereomer
able results were obtained (entries 6–7). Cyclohexylamine 2t (S,R)-5 was obtained pure in a 68% yield, based on the recov-
also proved to be a perfect nucleophile in this protocol (99% ered starting material (brsm), after chromatographic separ-
yield, entry 8). It should be mentioned that no cleavage of oxa- ation of the diastereomeric mixture (dr = 2.8). An important
zolidin-2-one auxiliary through amidolysis reaction occurred advantage of the present chemistry is that the aza-Michael
under the present condition except with benzylamine 2l and adduct 5 could be easily hydrolyzed into enantioenriched
benzylhydrazine 2m.
TFAA 6 with LiOH-H2O2. With 6 in hand, the access to two
As evident in Table 3, we turned our attention to the scope enantiomerically pure heterocycles with a CF3 group at the
of fluoroalkylated acrylates 1. The electrophiles 1b–h bearing chiral tertiary carbon center was elaborated. Chiral 2-CF3-2,3-
various fluoroalkylated groups at the β-position were syn- dihydro-1H-quinolin-4-one 7, serving as a building block for
thesized by using the same route as for 1a. Modification of the creating trifluoromethlyated analogues of dihydroquinolinone
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Scheme 2 Synthetic utility of chiral β-CF3 β-amino acid 6.
or tetrahydroquinoline type drugs,15 was successfully installed
via a polyphosphoric acid (PPA)-promoted intramolecular
Friedel–Crafts reaction.16 Furthermore, the β-CF3 β-amino
ester 8 derived from 6 was cyclized in the presence of
CH3MgBr to construct enantioenriched trifluoromethylated
β-lactam 9 in 69% yield, which represents the key part of an
ezetimibe analogue as a potent inhibitor of cholesterol absorp-
tion. The absolute stereochemistry of 9 was determined as S by
comparison of the optical rotation with that found in the
literature,4b so the configurations of compounds 6 and 7 were
both assigned as S.
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The features of the present aza-Michael addition are sum-
marized as follows: (1) this study represents the first example
of building TFAA derivatives via aza-Michael addition in a
highly environmentally benign and atom-economic fashion;
(2) fluorinated groups at the β-position of electrophiles are
critical for the efficient transformation; (3) benzylhydrazine as
a bis-nucleophile generated a trifluoromethylated pyrazolidi-
none (5-membered ring) via a novel cascade aza-Michael/
amidolysis cyclization reaction; (4) the reported green process
is a potentially practical and generalized approach. It has been
easily and successfully scaled-up to synthesize enantio-
enriched TFAAs, thereby, giving a rapid access to two structu-
rally diverse chiral trifluoromethylated N-heterocycles (4- and
6-membered ring) in good yields. We will extend this green
strategy to other research involving the expeditious construc-
tion of enantiomerically pure trifluoromethylated heterocycles.
This investigation is underway in our laboratory.
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Acknowledgements
We gratefully thank the financial support of this investigation
by the National Natural Science Foundation of China
(21303128), Scientific Research Foundation for Returned
Scholars, Ministry of Education of China ([2013]1792), and the
Fundamental Research Funds for the Central Universities
(WUT: 2014-Ia-002).
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