Enantioselective palladium-catalyzed ring-opening reaction of azabenzonorbornadienes with methyl 2-iodobenzoate: An efficient access to cis-dihydrobenzo[c]phenanthridinones

Article abstract of DOI:10.1002/adsc.201300452

The palladium-catalyzed asymmetric ringopening (ARO) reaction of azabenzonorbornadienes with iodobenzoate derivatives was realized by using chiral spirophosphine ligands with zinc powder as a reducing reagent. Various enantiomerically enriched cis-dihydrobenzo[c]phenanthridinones, the core structure of numerous chiral natural products, were prepared by the ARO reaction and tandem amidation from easily obtained starting materials with favorable enantioselectivity (up to 86% ee) under mild reaction conditions. The present palladium-catalyzed ARO reaction avoids the direct use of organometallic reagents, enables tandem cyclization, and thus improves synthetic efficiency. The formal total synthesis of a chiral natural product, (+)-chelidonine, was accomplished with the ARO reaction as a key step, which demonstrates the potential synthetic applications of the present methodology.

Full text of DOI:10.1002/adsc.201300452

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DOI: 10.1002/adsc.201300452
Enantioselective Palladium-Catalyzed Ring-Opening Reaction
of Azabenzonorbornadienes with Methyl 2-Iodobenzoate: An
Efficient Access to cis-Dihydrobenzo[c]phenanthridinones
Kang-Lun Huang,+a Cui Guo,+a Li-Jie Cheng,a Long-Guan Xie,a Qi-Lin Zhou,a
Xiao-Hua Xu,a,* and Shou-Fei Zhua,*
a
State Key Laboratory and Institute of Elemento-Organic Chemistry, Department of Chemistry, Nankai University,
Tianjin 300071, Peopleꢀs Republic of China
Fax : (+86)-22-2350-4228; e-mail: xiaohuaxu@nankai.edu.cn or sfzhu@nankai.edu.cn
+
These authors contributed equally to this work.
Received: May 23, 2013; Revised: August 6, 2013; Published online: October 9, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300452.
Abstract: The palladium-catalyzed asymmetric ring-
opening (ARO) reaction of azabenzonorborna-
dienes with iodobenzoate derivatives was realized
by using chiral spirophosphine ligands with zinc
powder as a reducing reagent. Various enantiomeri-
cally enriched cis-dihydrobenzo[c]phenanthridin-
studied and good to excellent enantioselectivities
have been obtained for diverse substrates.[2] However,
although the ring-opening reactions of azabicyclic al-
kenes with various carbon nucleophiles provide effi-
cient methods for biologically important nitrogen-
containing compounds, the asymmetric versions of
these reactions are rarely reported.[3] To date, only
few ARO of azabicyclic alkenes with organozinc re-
agents,[3a–c] organoboronic acids,[3d,e] and silylacety-
ACHTUNGTRENNUNGones, the core structure of numerous chiral natural
products, were prepared by the ARO reaction and
tandem amidation from easily obtained starting ma-
terials with favorable enantioselectivity (up to 86%
ee) under mild reaction conditions. The present pal-
ladium-catalyzed ARO reaction avoids the direct
use of organometallic reagents, enables tandem cy-
AHCTUNGTRENNUNG
lenes[3f,g] as carbon nucleophiles have been reported.
The direct utilization of easily accessible organic hal-
ides instead of organometallic reagents for better
functional group tolerance in the non-asymmetric
ring-opening reactions of azabicyclic alkenes has been
realized.[4] As well, the concise synthesis of benzo[c]-
phenanthridines through palladium-catalyzed ring-
opening coupling of azabicyclic alkenes with o-iodo-
benzoates followed by tandem cyclization has been
achieved.[5] Although the reaction creates a potentially
excellent method for the synthesis of chiral amines
with multiple chiral centers, the transition metal-cata-
lyzed ARO reaction of azabicyclic alkenes with or-
ganic halides remains unknown. Herein, we report for
the first time the ARO reactions of azabicyclic al-
kenes with iodobenzoate derivatives or phenyl triflate
catalyzed by palladium complexes of chiral spiro
phosphine ligands. Through tandem cyclization, vari-
ACHTUNGTRENNUNGclization, and thus improves synthetic efficiency.
The formal total synthesis of a chiral natural prod-
uct, (+)-chelidonine, was accomplished with the
ARO reaction as a key step, which demonstrates
the potential synthetic applications of the present
methodology.
Keywords: asymmetric ring-opening reaction; chiral
spiro ligands; cis-dihydrophenanthridinones; palla-
dium catalysis
Transition metal-catalyzed asymmetric ring-opening ous chiral cis-dihydrobenzo[c]phenanthridinones,
(ARO) reactions of oxa- and azabicyclic alkenes pro- which are the core of a series of chiral natural prod-
vide efficient methods to access versatile chiral alco- ucts (Scheme 1),[6] have been prepared from easily
hols or amines with multiple chiral centers from available starting materials with good enantioselectiv-
easily accessible starting materials.[1] As such, organic ity (up to 86% ee).
synthesis using this reaction has attracted intense re-
We have evaluated the palladium-catalyzed ARO
search interest. Over the last few decades, the ARO reaction of azabicyclic alkene 1a and commercially
reactions of oxabicyclic alkenes have been extensively available methyl 2-iodobenzoate 2a with zinc powder
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Scheme 1. New strategy for the synthesis of chiral cis-dihydrobenzo[c]phenanthridinones via ARO reaction and selected nat-
ural products with cis-dihydro benzo[c]phenanthridinone as a core structure.
as a reducing agent, Et3N as a base, and ZnCl2 as eties of ligand 5 were inferior (entries 3–7). Electron-
a Lewis acid additive (Table 1). Chiral palladium cata- rich phosphine (R)-6a[8] gave the best yield and enan-
lysts were generated in situ from PdACTHNUTRGNE(UNG MeCN)2Cl2 and tioselectivity (82% yield, 72% ee; entry 8). Additional
chiral ligands. A number of chiral monodentate phos- substituents on the P-phenyl of ligand 6 exhibited
phorus ligands (4–6) developed in our laboratory[7] negative impacts on both yield and enantioselectivity
were evaluated (entries 1–10). The tested ligands pro- (entries 9 and 10). Two widely used bidentate ligands,
duced the desired cis-dihydrobenzo[c]phenanthridi- namely 7 and 8, were also tested in this reaction.
none 3a with moderate to good yields. Phosphite (R)- Both produced extremely low enantioselectivity or
4 exhibited moderate yield with low enantioselectivity yield (entries 11 and 12).
(entry 1). Phosphoramidite (R)-5a generated a good
yield and 60% ee (entry 2). Compared with those of using ligand (R)-6a (Table 2). The tested Pd(II) salts,
5a, the results obtained after changing the amine moi- such as Pd(MeCN)2Cl2, PdCl2, Pd(OAc)2, and
Other parameters of this reaction were optimized
A
ACHTUNGTRENNUNG
Table 2. Palladium-catalyzed ARO reaction of azabenzonorbornadienes with methyl 2-iodobenzoate: optimization of reac-
tion conditions.[a]
Entry
[Pd]
Pd(MeCN)2Cl2
Base
Additive
Solvent
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
G
Et3N
Et3N
Et3N
Et3N
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnI2
I2
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
MeCN
acetone
EtOAc
82
80
72
63
72
72
63
68
PdCl2
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
N
[b]
[b]
Et3N
-
-
DIPEA
DABCO
n-BuNH2
pyrrolidine
Et3N
Et3N
Et3N
Et3N
Et3N
35
81
47
60
[b]
[b]
–
–
–
–
[b]
[b]
9
10
11
12
13
14
15
73
85
70
23
36
51
81
81
80
82
80
81
ICl
I2
I2
I2
Et3N
[a]
Reaction conditions: 1a/2a/[Pd]/(R)-6a/Zn/additive/base=0.5:0.75:0.01:0.021:5:0.5:0.25 (mmol), in 4 mL of solvent at
608C, reaction time=24 h; analysis is identical to that shown in Table 1.
No desired product.
[b]
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Enantioselective Palladium-Catalyzed Ring-Opening Reaction of Azabenzonorbornadienes
Table 1. Palladium-catalyzed ARO reaction of azabenzonor-
PdACTHUNTRGENNG(U acac)2, were observed to be suitable precursors for
the ARO reaction and produced almost identical
yields and enantioselectivities (entries 1–4). By con-
bornadienes with methyl 2-iodobenzoate: ligand evalua-
ACHTUNGTRENNUNG
tion.[a]
trast, the Pd(0) precursor PdACHTUNRGTNEUNG(dba)2 failed to develop
the desired product under the standard reaction con-
ditions (entry 5). Various bases were also evaluated in
this reaction (entries 6–9). Besides Et3N, other tertiary
amines,
(DIPEA)
including
and
N,N-diisopropylethylamine
1,4-diazabicyclo[2.2.2]octane
ACHTUNGTRENNUNG
(DABCO), promoted the ARO reaction but with
lower yields and enantioselectivities (entries 6 and 7).
Secondary amines, such as n-BuNH2 and pyrrolidine
did not produce the desired product (entries 8 and 9).
When ZnI2 was used as additive instead of ZnCl2, the
enantioselectivity improved significantly to 81% ee
(entry 10). Addition of I2 or ICl, which can in situ
generate ZnI2 through reaction with zinc powder, pro-
duced an enantioselectivity that was essentially identi-
cal to that obtained with ZnI2 (entries 11 and 12).
When I2 was used as an additive, a better yield was
obtained (entry 11). Other solvents, including acetoni-
trile, acetone, and ethyl acetate, essentially yielded
the same level of enantioselectivity as that produced
by THF but with lower yields (entries 13–15). On the
other hand, no desired product was obtained when
N,N-dimethylformamide, dimethyl sulfoxide, chloro-
form, or toluene was used as a solvent (data not
shown).
The protecting groups of substrate 1 were evaluated
in the ARO reaction (Table 3). The nature of the pro-
tecting groups strongly influenced the yield and enan-
tioselectivity of the reaction. Azabenzonorborna-
dienes with carbamates (1a–1g) or benzoyl (1h) as
protecting groups exhibited good yields and enantio-
Entry
Ligand
Yield [%][b]
ee [%][c]
À
selectivity (entries 1–8). Among these, 1c (N
COOCHMe2) demonstrated the best results (88%
yield, 86% ee; entry 3). Azabenzonorbornadienes
with different sulfonyl protecting groups (1i–1k) were
also tested, but none produced the desired product
(entries 9–11).
1
2
3
4
5
6
7
8
(R)-4
(R)-5a
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
(R)-5d
(R)-5e
(R)-5f
(R)-6a
(S)-6b
(R)-6c
(S)-7
46
81
38
52
62
61
67
82
38
62
72
17
28
60
18
67
60
45
56
72
47
46
7
Under our optimized reaction conditions, various
substituted methyl 2-iodobenzoates (2a–2k) were
tested in the palladium-catalyzed ARO reaction with
azabenzonorbornadiene 1c (Table 4). In most cases,
good enantioselectivities were obtained. In general, 5-
substituted substrates 2 exhibited good yields (70–
82%) and enantioselectivities (79–85% ee; entries 4,
6, and 9–11). Substrate 2b with a 3-fluoro and sub-
strate 2g with a 4-trifluoromethyl substituent yielded
products with significantly decreased enantioselectivi-
ty (entries 2 and 7). When the model reaction was
scaled up to 4 mmol scale, the desired product 3a was
obtained with 64% yield and 86% ee. The optical
purity of 3a could be improved to 96% ee through re-
crystallization form ether with 53% yield.
9
10
11[d]
12[d]
(S)-8
31
[a]
Reaction conditions: 1a/2a/PdACTHNUTRGNEUNG(MeCN)2Cl2/ligand/Zn/
ZnCl2/Et3N=0.5:0.75:0.01:0.021:5:0.5:0.25 (mmol), in
4 mL of THF at 608C, reaction time=24 h.
Isolated yields.
Determined by chiral HPLC with a Chiralcel OD-H
column.
[b]
[c]
[d]
Using 2 mol% of ligand.
Similar to 2-iodobenzoates, methyl 2-(trifluorome-
thylsulfonyloxy)benzoate (2l) is a suitable substrate
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Table 3. Palladium-catalyzed ARO reaction of azabenzonor-
bornadienes with methyl 2-iodobenzoate: evaluation of pro-
tecting groups.[a]
Table 4. Palladium-catalyzed asymmetric ring opening of 1c
with substituted methyl 2-iodobenzoates.[a]
Entry
X
Substrate Products Yield [%] ee [%]
1
2
3
4
5
6
7
8
H
3-F
4-F
5-F
4-Cl
5-Cl
4-CF3
3-Me
5-Me
5-OMe 2j
5-OCF3 2k
2a
2b
2c
2d
2e
2f
2g
2h
2i
3a
3b
3c
3d
3e
3f
3g
3h
3i
88
47
52
80
67
70
64
88
75
82
78
86
15
78
85
76
79
45
79
82
81
80
Entry
Azabicycle
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
85
78
88
68
59
70
63
63
81
75
86
82
81
85
86
70
9
10
11
3j
3k
[a]
Reaction conditions and analysis are identical to those in
Table 2, entry 11.
[b]
[b]
9
10
11
–
–
–
–
[b]
[b]
1j
1k
[b]
[b]
82% ee. Thereby, the core structure of (+)-chelido-
nine which features a fused ring system with two con-
junctive chiral centers was constructed in one step.
After reduction of cis-dihydrobenzo[c]phenanthridin-
–
–
[a]
Reaction conditions and analysis are identical to those in
Table 2, entry 11.
No desired product.
[b]
AHCTUNGTRENNUNG
for
the
palladium-catalyzed
ARO
(Scheme 2). Under standard reaction conditions, the pound 10 was improved to 94% ee during the purifi-
desired cis-dihydrobenzo[c]phenanthridinone 3a was cation. According to the literature,[3e] the total synthe-
obtained with 82% yield and 86% ee.
sis of (+)-chelidonine could be achieved beginning
The formal synthesis of (+)-chelidonine was accom- with intermediate 10 through three steps with 63%
plished with the palladium-catalyzed ARO reaction overall yield. The absolute configuration of 3l was as-
as a key step, which further demonstrates the poten- signed as (13R,14S) by comparing its optical rotation
tial synthetic applications of the present reaction with that of the same compound reported by Lau-
(Scheme 3). The palladium-catalyzed ARO reaction tens.[3e]
of asymmetric ring opening with azabenzonorborna-
In conclusion, the palladium-catalyzed ARO of aza-
diene 1l and methyl 2-iodobenzoate 2m gave cis-dihy- benzonorbornadienes with iodobenzoate derivatives
drobenzo[c]phenanthridinone 3l with 88% yield and was realized. This reaction is efficient and provides
Scheme 2. Palladium-catalyzed ARO reaction of azabenzonorbornadiene with methyl 2-(trifluoromethylsulfonyloxy)ben-
zoate.
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Enantioselective Palladium-Catalyzed Ring-Opening Reaction of Azabenzonorbornadienes
Scheme 3. Formal synthesis of (+)-chelidonine.
a practical access to optically active cis-dihydroben- 1H, Ar-H), 7.36–7.24 (m, 4H, Ar-H), 7.16 (d, J=7.2 Hz,
1H, Ar-H), 6.58 (dd, J=9.6, 2.7 Hz, 1H, CH), 5.89–5.67 (m,
2H, CH & NH), 4.87 (d, J=5.7 Hz, 1H, CH), 3.87–3.84 (m,
1H, CH); 13C NMR (100 MHz, CDCl3): d=165.03, 139.57,
132.96, 132.64, 131.91, 129.25, 128.58, 128.31, 127.95, 127.61,
127.58, 127.25, 127.08, 127.06, 52.18, 38.63. IR: n=767, 1091,
1398, 1465, 1658 cmÀ1; HR-MS: m/z=248.1074, calcd. for
C17H13NO [M+H]+: 248.1070; ee=86%; HPLC (Chiralpack
OD-H, isopropyl alcohol/hexane=15:85, flow rate
0.9 mLminÀ1, l=220 nm): tR =13.6 min (minor), tR =
20.3 min (major)]; [a]D25: +2.7 (c 1.0 in chloroform).
zo[c]phenanthridinones in good yields and enantiose-
lectivities under mild reaction conditions. The elec-
tron-rich chiral spirophosphine ligand (R)-6a and I2
additive were key factors in attaining good reactivity
and enantioselectivity. The new methodology uses
easily available organic halides instead of organome-
tallic reagents and remarkably improves the efficiency
of the construction of fused ring systems with multiple
chiral centers. The formal total synthesis of a chiral
natural product (+)-chelidonine was accomplished
with the ARO reaction as a key step. Further investi-
gations on the detailed reaction mechanism and test-
ing of the biological and pharmaceutical activities of
hexahydrobenzo[c]phenanthridine alkaloids with cis-
fused B/C rings are in progress in our laboratory.
Acknowledgements
Generous financial supports from the National Natural Sci-
ence Foundation, the National Basic Research Program of
China (2011CB808600), National Key Technology R&D Pro-
gram (No2011BAE06B05), and State Key Laboratory of Ele-
mento-Organic Chemistry are gratefully acknowledged.
Experimental Section
Typical Procedure for the Palladium-Catalyzed ARO
Reaction
References
To
a flame-dried round-bottom flask, PdACTHNUTGNRUEGN(MeCN)2Cl2
(2.6 mg, 0.01 mmol, 2 mol%), (R)-6a (7.4 mg, 0.021 mmol,
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atmosphere and stirred for 1 hour at room temperature.
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