Enantioselective synthesis of benzazepinoindoles bearing trifluoromethylated quaternary stereocenters catalyzed by chiral spirocyclic phosphoric acids

Article abstract of DOI:10.1039/c4cc02295e

The first highly enantioselective iso-Pictet-Spengler reaction of C-2-linked o-aminobenzylindoles with trifluoromethyl ketones was developed using chiral spirocyclic phosphoric acids as organocatalysts, which afforded optically active benzazepinoindoles b

Full text of DOI:10.1039/c4cc02295e

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Enantioselective synthesis of benzazepinoindoles
bearing trifluoromethylated quaternary
stereocenters catalyzed by chiral spirocyclic
phosphoric acids†
Cite this: Chem. Commun., 2014,
50, 7538
Received 28th March 2014,
Accepted 22nd May 2014
Xuejian Li, Di Chen, Haorui Gu and Xufeng Lin*
DOI: 10.1039/c4cc02295e
www.rsc.org/chemcomm
The first highly enantioselective iso-Pictet–Spengler reaction of
C-2-linked o-aminobenzylindoles with trifluoromethyl ketones
was developed using chiral spirocyclic phosphoric acids as organo-
catalysts, which afforded optically active benzazepinoindoles
bearing trifluoromethylated quaternary stereocenters.
Chiral trifluoromethylated compounds have shown a great diver-
sity of superior biological properties mainly due to improved
chemical and metabolic stability, lipophilicity, and membrane
permeability of the molecules.1 In particular, some biologically
active molecules with a CF3-containing cyclic quaternary stereo-
center are representative examples, including the HIV reverse
transcriptase inhibitor (Efavirenz),2a the progesterone receptor
antagonist,2b the NK-1 receptor antagonist (CJ-17493),2c and
antimalarial agents (fluoroartemisinin).2d
Thus, great efforts have been devoted to the synthesis of
functionalized molecules with a CF3-containing stereocenter.3 In
particular, the catalytic enantioselective construction of trifluoro-
methylated quaternary stereocenters has recently generated a
tremendous amount of interest.4 In this context, a facile and
flexible asymmetric catalytic cyclization reaction for the con-
struction of cyclic quaternary stereocenters bearing a CF3 moiety
has not been well explored and remains a challenging project in
organic synthesis. To date, only a few examples with a 3, 5 or
6-membered ring have been documented (Scheme 1).5 These
elegant asymmetric cyclization reactions were realized by
transition metal catalysis5a or cooperative catalysis of chiral
N-heterocyclic carbene (NHC) and Lewis acid,5h as well as
organocatalysis with a chiral phase-transfer catalyst (PTC),5b,c
NHC,5d Jørgensen’s catalyst,5e squaramide5f and thiourea.5g In
spite of these notable advances, to the best of our knowledge, no
example has thus far been reported for the catalytic asymmetric
Scheme 1 Approaches to the construction of cyclic quaternary stereo-
centers bearing a CF3 moiety through asymmetric catalytic cyclization
reactions. * = chiral reagent.
cyclization reaction for the synthesis of CF3-containing seven-
membered heterocycles. Therefore, the development of a novel
and elegant method to meet this challenge is highly desirable.
Over the past few years, the catalytic asymmetric Pictet–
Spengler reaction for construction of chiral N-heterocycle
frameworks has attracted enormous attention and witnessed
significant progress.6 However, there is still no general solution
to a catalytic, highly enantioselective Pictet–Spengler reaction
Laboratory of Asymmetric Catalysis and Synthesis, Department of Chemistry,
Zhejiang University, Hangzhou 310027, P. R. China. E-mail: lxfok@zju.edu.cn
† Electronic supplementary information (ESI) available: Experimental procedures
and characterization data for all new compounds. CCDC 953552 (3aa). For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc02295e
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with simple ketone substrates.7 We recently developed a novel
class of chiral spirocyclic phosphoric acids (SPAs), which proved
to be highly efficient in asymmetric catalysis.8,9 Employing such
chiral SPAs we now report the first highly enantioselective
catalytic iso-Pictet–Spengler reaction of C-2-linked o-amino-
benzylindoles with commercial trifluoromethylated ketones to
provide optically active benzazepinoindoles bearing trifluoro-
methylated quaternary stereocenters (Scheme 1D).10
We next turned our attention to assessing the substrate scope
of the reaction. The optimized reaction conditions summarized in
entry 8 in Table 1 were firstly used to study the scope of a series of
commercially available trifluoromethyl ketones. All the reactions
with aromatic trifluoromethyl ketones (2a–2o) provided the
expected products 3aa–3ao in good yields (80–95%) with high
enantiomeric excess (87–499.5%) (Table 2). Product 3aa is a
crystalline compound, and the ee could be easily increased to
99% by one simple recrystallization (Table 2, entry 1). For
substrates 2b–2e bearing a halogen X (X = F, Cl, Br or I) in the
para position of the aromatic ring, the reactions occurred readily
to give the desired products in high yields and excellent enantios-
electivities (Table 2, entries 2–5), and the halogen-substituted
products can participate in subsequent transformations such as
cross-coupling reactions. We found that the best level of stereo-
control was obtained for 4-nitrophenyl trifluoromethyl ketone 2h
which possesses a strong electron-withdrawing group (499.5%
ee, Table 2, entry 8). Meta-substituents on the aryl ring of
trifluoromethyl ketones (2i–2k) had a negligible impact on the
yield and enantioselectivity (Table 2, entries 9–11). Notably,
trifluoroacetophenones (2l–2n), with two additional substituents
on the aromatic ring, could also serve as substrates in this
reaction (Table 2, entries 12–14). Introduction of an electron-
donating group resulted in a slightly lower ee (Table 2, entry 15).
Switching to the trifluoromethyl alkyl ketone (2p) afforded signifi-
cantly diminished enantioselectivity (Table 2, entry 16).
In our initial study, we examined the model reaction between
C-2-linked o-aminobenzylindole (1a) and phenyl trifluoromethyl
ketone (2a) using chiral phosphoric acid catalysts in chloroform
in the presence of powdered 4 Å molecular sieves. The reactions
were run at 35 1C with a catalyst loading of 5 mol% and were
stopped after 2 days. To our delight, the corresponding cycliza-
tion product 3aa was obtained with 92% ee, albeit in a low yield,
when (S)-4a, with bulky 6,60-bis(9-anthracenyl) moieties, was used
as the catalyst (Table 1, entry 1). We next screened chiral SPAs
with various substituents in the 6,60-positions, and found that
(S)-4b, with 6,60-bis(3,5-bis-trifluoromethylphenyl) moieties, gave
the highest yield and excellent enantioselectivity (90% yield and
93% ee, Table 1, entry 2), whereas (S)-4e, with 6,60-bis(4-
nitrophenyl) moieties, exhibited no catalytic activity even under
reflux (Table 1, entry 5). Furthermore, it should be noted that
with a BINOL-based phosphoric acid (PA) catalyst system11 ((R)-5,
with 3,30-bis(3,5-bis-trifluoromethylphenyl) moieties), only 20%
yield was obtained and 84% ee was observed (Table 1, entry 6).
The absolute configuration of the product was coincident with
that of the product formed with the (S)-4b catalyst because (S)-PA
and (S)-SPA are considered to be ‘‘pseudoenantiomers’’ owing to
the difference in the nomenclature. Subsequent optimization
suggested that the solvent remarkably affected the catalytic
activity. The use of 1,2-dichloroethane as a solvent provided both
excellent yield and enantioselectivity (95% yield and 93% ee,
Table 1, entry 8), whereas the reaction hardly occurred in
coordinating solvents, such as CH3CN or THF even at reflux
temperature (Table 1, entries 9 and 10).
Table 2 Scope of the reaction with respect to the trifluoromethyl ketonea
Table 1 Optimization of reaction parametersa
Entry
R
Product
Yieldb (%)
eec (%)
93 (99)d
94
95
96
92
93
93
499.5
96
95
90
94
96
96
87
55
1
2
3
Ph (2a)
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
3aj
3ak
3al
3am
3an
3ao
3ap
95
95
88
92
88
92
84
86
95
87
87
82
80
90
93
93
4-FC6H4 (2b)
4-ClC6H4 (2c)
4-BrC6H4 (2d)
4-IC6H4 (2e)
4-CF3C6H4 (2f)
4-CNC6H4 (2g)
4-NO2C6H4 (2h)
3-FC6H4 (2i)
3-BrC6H4 (2j)
3-CF3C6H4 (2k)
3,5-F2C6H3 (2l)
3,5-Cl2C6H3 (2m)
3,4-F2C6H3 (2n)
4-MeC6H4 (2o)
Benzyl (2p)
4
5e
6
Entry
Catalyst
Solvent
t [h]
Yieldb (%)
eec (%)
7e
8e
9
1
(S)-4a
(S)-4b
(S)-4c
(S)-4d
(S)-4e
(R)-5
(S)-4b
(S)-4b
(S)-4b
(S)-4b
(S)-4b
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CH2Cl2
CH2ClCH2Cl
CH3CN
THF
48
48
48
48
48
48
24
24
48
48
24
35
90
10
50
Trace
20
90
92
93
86
93
—
84
88
93
—
—
89
2
3d
4
10
11e
12e
13
14
15
16
5d
6
7
8
95
9d
10d
11
Trace
Trace
85
a
Toluene
Reaction conditions: 4b (5 mol%, 0.005 mmol), 1 (0.1 mmol), 2
(0.12 mmol), molecular sieves (MS 4 Å, 0.1 g), 1,2-dichloroethane
(0.6 mL), 35 1C, for 24–48 h. b Isolated yields. c Determined by chiral HPLC
analysis. d Data in parentheses were obtained after single recrystallization.
a
Reaction conditions: catalyst (5 mol%, 0.005 mmol), 1a (0.1 mmol),
2a (0.12 mmol), molecular sieves (MS 4 Å, 0.1 g), solvent (0.6 mL), 35 1C.
Isolated yields. Determined by chiral HPLC analysis. Under reflux. e Under reflux.
b
c
d
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Table 3 Scope of the reaction with respect to the o-aminobenzylindole
substratea
Fig. 1 Proposed reaction model.
Entry R1/R2/R3/R4/R5
R
Product Yieldb (%) eec (%)
of the ketoimine intermediate through hydrogen bonding. In
this model, the indole p system attacks the ketoimine moiety
from the Si face, leading to (S)-3 (Fig. 1).13
1
2
3
4
H/Br/H/H/Br (1b) 4-FC6H4 (2b)
H/Br/H/H/Br (1b) 4-ClC6H4 (2c)
H/Br/H/H/Br (1b) 4-BrC6H4 (2d)
Cl/H/H/Cl/H (1c) 4-BrC6H4 (2d)
Cl/H/H/Cl/H (1c) 3,5-Cl2C6H3 (2k) 3ck
H/Me/H/H/Me (1d) Ph (2a)
H/Me/H/H/Me (1d) 4-FC6H4 (2b)
H/Me/H/H/Me (1d) 4-ClC6H4 (2c)
H/Me/H/H/Me (1d) 4-BrC6H4 (2d)
H/H/Bn/H/H (1e) Ph (2a)
3bb
3bc
3bd
3cd
80
88
90
95
87
98
75
86
95
0
88
90
96
90
90
81
84
93
90
—
In summary, we have presented a general, mild, and flexible
method providing access to enantiomerically enriched benza-
zepinoindoles bearing trifluoromethylated quaternary stereo-
centers by utilizing the catalytic asymmetric iso-Pictet–Spengler
reaction. The reaction employs the powerful and fully recycl-
able chiral spirocyclic phosphoric acid catalyst 4b, and involves
a simple scalable experimental procedure without a protecting
group or an activating group. Further exploration of the
potential of our chiral SPAs in asymmetric catalysis is currently
ongoing.
5
6d
7
3da
3db
3dc
3dd
3ea
8d
9d
10
a
Reaction conditions: 4b (5 mol%, 0.005 mmol), 1 (0.1 mmol), 2
(0.12 mmol), molecular sieves (MS 4 Å, 0.1 g), 1,2-dichloroethane
(0.6 mL), reflux for 24–96 h. Isolated yields. Determined by chiral
HPLC analysis. 50 1C.
b
c
d
This work was supported by the National Natural Founda-
tion of China (21272202 and J1210042).
The effects of o-aminobenzylindole substitution were then
evaluated under the optimized conditions (Table 3). Good yields
(75–98%) and high enantioselectivities (81–96% ee) were
achieved with substrates 1 bearing either electron-withdrawing
or electron-donating groups when the reactions were conducted
at elevated temperatures and with a prolonged reaction time,
although these substrates (1b–1d) showed slightly reduced reac-
tivity (Table 3, entries 1–9). When N-benzylindole derivative (1e)
was prepared and treated with phenyl trifluoromethyl ketone
(2a), no desired product was observed even under reflux (Table 3,
entry 10). We suspect that the hydrogen atom on the N atom of
the indole is crucial for the activation of the substrate by SPA 4 in
this iso-Pictet–Spengler reaction (vide infra).
We next carried out a scale-up experiment (3.0 mmol of 1a)
(Scheme 2). This reaction proceeded without compromising the
yield or enantioselectivity, and a gram-scale preparation of 3ad
(1.23 g) was realized with 90% yield and 95% ee as well as 90%
recovery of catalyst 4b. The recovered catalyst was recycled with
negligible loss in reactivity or stereoselectivity.
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