AgOTf-catalyzed sequential synthesis of 4-isoquinolones: Via oxidative ring opening of aziridines and aza-Michael addition

Article abstract of DOI:10.1039/c7ob02167d

An efficient AgOTf-catalyzed sequential reaction involving the oxidative ring-opening of aziridines by DMSO and aza-Michael addition has been developed. A series of 2,3-dihydro-4(1H)-isoquinolones were afforded in moderate to good yields by the formation of one new CO bond and one new C-N bond. The features of this sequential reaction include high bonding efficiency, use of a catalytic amount of catalysts, a broad substrate scope and mild conditions. This methodology provides a good choice for constructing the libraries of 2,3-dihydro-4(1H)-isoquinolones.

Full text of DOI:10.1039/c7ob02167d

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
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AgOTf-Catalyzed Sequential Synthesis of 4-Isoquinolone via
Oxidative Ring Opening of Aziridines and Aza-Michael Addition
Siyang Xing,* Hong Cui, Nan Gu, Ying Li, Kui Wang,* Dawei Tian,* Jiajing Qin and Qiaoyang Liu
Received 00th January 20xx,
Accepted 00th January 20xx
An efficient AgOTf-catalyzed sequential reaction involving oxidative ring-opening of aziridines by DMSO and aza-Michael
addition has been developed. A series of 2,3-dihydro-4(1H)-isoquinolones were afforded in moderate to good yields by the
formation of one new C=O bond and one new C-N bond. The features of this sequential reaction include high bonding
efficiency, use of catalytic amount of catalysts, broad substrate scope and mild conditions. The methodology provide a
good choice for constructing the libraries of 2,3-dihydro-4(1H)-isoquinolones.
DOI: 10.1039/x0xx00000x
www.rsc.org/
OMe
H
H
Introduction
N
2,3-Dihydro-4(1H)-isoquinolones1 are an important class of
nitrogen-containing cyclic compounds which are widely
employed as organic synthetic intermediates of high
significance in the synthesis of biologically active compounds
containing the tetrahydroisoquinoline moieties. For example,
they have been used to access several important biologically
active compounds such as boehmeriasin A2, 11-
aminobenzoquinolizidine3, cyanoamine-cribrostatin analogue4
and 11-hydrooxyerythratidine5 (Scheme 1). Besides, they are
found in several biologically active compounds such as 11-
saxicolaline A6a and 11-oxoerysodine 6b. Typical strategies have
been developed for the assembly of 2,3-dihydro-4(1H)-
isoquinolones, including Claisen condensations7, Friedel–Crafts
acylations8 and lithiation-acylation reactions9 (Scheme 2).
However, most of these existing strategies suffered from
limited substrate scope, harsh reaction conditions or use of
stoichiometric catalysts. Efficient and general approaches
allowing more diversified syntheses of these valuable
N
N
OMe
OMe
11-aminobenzoquinolizidines
boehmeriasin A
O
OMe
OH
Me
BnO
MeO
OAc
H
H
NH
Me
N
N
MeO
Me
N
MeO
H
MeO
OH
OMe
CN
OBn
cyanoamine-cribrostatin analogue 11-hydroxyerythratidine
Scheme 1 Application examples of 2,3-dihydro-4(1H)-isoquinolones.
OR
O
Claisen
CO2R
Condensation
O
N
R
CO2R
NR
strong bases
Cl
O
Friedel-Crafts
acylation
O
compounds are rare10. Consequently,
a complementary
NR
NR
strong acids
method to the synthesis of 2,3-dihydro-4(1H)-isoquinolones
are still in high demand.
O
O
lithiation-acylation
reactions
Li
Sequential reaction11 has great advantages in the
OR
NR
NR
strong bases
construction of complicated cyclic compounds from readily
available materials due to their high efficiency in the formation
of chemical bonds. Recently, we have described two examples
Scheme 2 Typical strategies for 2,3-dihydro-4(1H)-isoquinolones.
of sequential cyclization reactions based on ring opening of
aziridines12 and Michael addition of electron-deficient
alkenes13. Under the catalysis of AgOTf, ring opening of
aziridines with amines followed by aza-Michael addition led to
tetrahydroisoquinolines and isoindolines in good yields with
excellent cis-diastereoselectivities14 (Scheme 3, eq 1 and eq 2).
Under the catalysis of KOH, sequential Michael addition with
malonic esters, ring opening of aziridines and lactamization
afforded indane-fused pyrrolidin-2-ones in good yields with
good diastereoselectivities15 (Scheme 3, eq 3). In the process
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R
R
the sequential reaction using 10 equiv of DMSO in DCE, but
only a poor yield was observed (Table 1, entry 11).
N
Ts
N
R2NH
CO2Me
CO Me
eq 1
eq 2
NTs
Table 1 Optimization of the reaction conditions for the sequential reactiona
LA
previous work
2
MeO2C
CO2Me
NHTs
Ts
O
N
Ts
N
RNH2
Catalyst
CO2Me
CO2Me
N
R
NTs
CO2Me
LA
Solvent
CO2Me
previous work
CO2Me
NTs
MeO2C
CO2Me
MeO2C
1a
2a
Ts
N
CH2(CO2R)2
Base
CO2Me
Entry Catalyst
Solvent
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO/DCE
DCE
T (h) T (oC) Yieldb (%)
O
eq 3
CO2Me
1
2
AgOTf
BF3ꢀOEt2
SnCl4
7
7
70
70
70
70
70
70
70
70
70
70
70
67
61
17
5
CO2Me previous work
CO2R
RO2C
3
7
Scheme 3 Sequential reaction involving aziridines and electron-deficient alkenes.
4
In(OTf)3
Yb(OTf)3
DBU
7
5
7
40
0
O
O
Ts
N
6
7
O
S
7
NaH
7
0
Me
Me
CO2Me
CO2Me
NHTs
NTs
8
t-BuOK
AgOTf
AgOTf
AgOTf
7
0
oxidative ring
Aza-Michael
addition
opening of aziridines
9
18
18
18
84
64
26
MeO2C
CO2Me
MeO2C
CO2Me
10c
11d
Scheme 4 Sequential reaction towards the 2,3-dihydro-4(1H)-isoquinolones.
a
Reaction conditions: 1a (1 equiv., 0.05 mmol), the catalyst (20 mol%), the
solvent (1 mL), in the open air. Determined by 1 H NMR using 1-chloro-2,4-
dinitrobenzene as the internal standard. c DMSO:DCE=1:1. d DMSO (10 equiv., 0.5
mmol) was used in DCE (1 mL).
b
of exploring these two sequential reactions, we found an
interesting cyclization reaction involving sequential oxidative
ring opening16 of aziridines by the solvent DMSO and aza-
Michael addition17 (Scheme 4). In the presence of 20% mol
AgOTf
a series of 2,3-dihydro-4(1H)-isoquinolones were
afforded in moderate to good yields in mild conditions by the
formation of one new C=O bond and one new C-N bond.
Undoubtedly this method provided a good choice for the rapid
and efficient synthesis of 2,3-dihydro-4(1H)-isoquinolones. It
was particularly well suited for the preparation of
corresponding compound libraries. Herein, we hope to report
the results of this sequential reaction.
Results and discussion
Figure 1 ORTEP drawing for the product 2a.
N-sulfonyl aziridine 1a was selected as the model substrate
to investigate the feasibility of this sequential reaction (Table
1). Initially, 1a was treated with AgOTf (20 mol%) in dimethyl
sulfoxide (DMSO) at 70 oC for 7 h, and to our delight cyclization
product 2a was successfully obtained in 67% yield (Table 1,
entry 1). The structure of 2a was unambiguously confirmed by
X-ray crystal structure analysis18 (Figure 1). Then a series of
Lewis acids were screened to further improve the yield of this
sequential reaction. Relatively lower yields were obtained
when BF3·OEt2, SnCl4, Yb(OTf)3 and In(OTf)3 were used to
catalyze the sequential reaction (Table 1, entries 2~5). Several
bases such as DBU, NaH and t-BuOK were also selected to
promote this sequential reaction, but no desired products
were isolated (Table 1, entries 6~8). When we prolonged the
reaction time to 18 h in the presence of AgOTf, the best result
was obtained and 2a was generated in 84% yield (Table 1,
entry 9). In addition, the sequential reaction successfully
afforded the product 2a in a slightly decreased yield in a
DMSO-DCE mixed solvent (Table 1, entry 10). We also tested
With the optimized conditions established, we subsequently
explored the generality of the sequential reaction by using a
series of substrates
1 bearing electron-donating and electron-
withdrawing on the aromatic ring. The results are given in
Table 2. We found that various substituents residing at
different positions (R1~R4) on the phenyl ring were compatible
(entries 2~16). In general, the substrates with different
electron-donating substituents like methyl (1c and 1f), phenyl
(
1h and 1n), p-methoxyphenyl
successfully reacted with DMSO to afford the corresponding
products in good yields. The exception to this is that the
(1o) and methoxyl (1i)
2
electron-rich substrates 1g and 1m only led to products 2g and
2m in moderate yields. The electron-withdrawing substituents
seemed to have a bad effect on the sequential reaction. The
substrates 1b, 1d, 1e, 1j, 1k and 1p with electron-withdrawing
substituents gave slightly lower yields than those substrates
with electron-donating substituents. It should be noted that
the substrate 1h with the nitro group reacted with DMSO to
afford product 2h in 20% yield. At last, the sequential reaction
of the naphthalene-containing substrate 1q with DMSO was
2 | J. Name., 2012, 00, 1-3
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Table 3 Examining the substituent effect on the aziridines and the Michael acceptorsa
carried out under the optimized reaction conditions. Product
2q was successfully obtained in a 50% yield (entry 17).
O
NR1
R3
NR1
R3
CO2R2
Table 2 Testing the electronic effects on the aromatic ring a
AgOTf
DMSO
CO2R2
70 o
C
R2O2C
CO2R2
1
2
Entry 1 R1/R2/R3
Product
Yieldb (%)
1
2
3
4
5
6
1r Bs/Me/H
1s Ns/Me/H
1t Ts/Et/H
2r
2s
2t
78
93
60
49
62
88
Entry 1 R1/R2/R3/R4
Product
2a
Yieldb (%)
84
1u Ts/i-Pr/H
1v Ts/Bn/H
2u
2v
1
2
1a H/H/H/H
1b F/H/H/H
2b
2c
58
3
1c Me/H/H/H
1d H/F/H/H
66
4
2d
2e
2f
60
5
1e H/Cl/H/H
1f H/ Me/H/H
1g H/t-Bu/H/H
1h H/Ph/H/H
1i H/OMe/H/H
1j H/H/F/H
55
6
82
7
8
0
7
2g
45
8
2h
2i
87
9
68
48c
1y Ts/Me/Me
2y
10
11
12
13
14
15
16
17
2j
63
a Reaction conditions: 1 (1 equiv., 0.2 mmol), AgOTf (20 mol%, 0.04 mmol), 4 mL
DMSO, 70 oC, 18 h, in the open air. b Isolated yields. c dr=5:1.
1k H/H/Cl/H
2k
59
1l H/H/NO2/H
1m H/H/Me/H
1n H/H/ Ph/H
1o H/H/4-OMeC6H4/H
1p H/H/H/F
2l
20
2m
2n
2o
2p
45
of AgOTf, the reaction of 1a with DMSO still led to product 2a
in 75% yield (Scheme 5, eq 1). However, when 1a was treated
with 20 mol % of AgOTf in the solvent DCE at 70 oC, no desired
product 2a was isolated (Scheme 5, eq 2).The two control
experiments indicated that DMSO played a vital role in the
formation of product 2a. AgOTf was not necessary for the
formation of product 2a. But it was helpful for improving the
yield of the sequential reaction. Moreover, there are two
possible oxygen sources in the sequential reaction: molecular
oxygen in air and DMSO. We first run the reaction of 1a using
degassed and dry DMSO in Ar (Scheme 5, eq 3). Similar with
the reaction in air, product 2a was afforded in 84% yield. Then
the sequential reaction of 1a was performed with 18O-labeled
DMSO (Scheme 5, eq 4). Product 18O-2a and 16O-2a was
obtained in 84% yield with a ratio of 1.1:1 based on HRMS
analysis (see Supporting Information). The two experiments
suggested that the additional oxygen atom in product 2a is
original form DMSO.
64
68
55
50
a Reaction conditions: 1 (1 equiv., 0.2 mmol), AgOTf (20 mol%, 0.04 mmol), 4 mL
DMSO, 70 oC, 18 h, in the open air. b Isolated yields.
The substituent effect on the aziridines and the electron-
deficient alkenes was also investigated to further extend the
scope of this sequential reaction (Table 3). First, the influence
of the protecting group of the N-atom of the aziridines for the
sequential reaction was examined (entries 1 and 2). For the
substrates 1r and 1s, the products 2r and 2s were successfully
obtained in good yields. Then substrates 1t~1x with different
Michael acceptors were employed to react with DMSO under
the optimized reaction conditions (entries 3~7). The diethyl
ester substrate 1t, the diisopropyl ester substrate 1u and the
dibenzyl ester substrate 1v generated product 2t, 2u and 2v in
moderate to good yields. The treatment of the monomethyl
ester substrate 1w by DMSO only led to oxidative ring opening
intermediate 3w in 88% yield, instead of the desired product
2w. When the monoketone substrate 1v was subjected to the
sequential reaction, a complex mixture was formed and the
desired product was not isolated. Finally, the substrate 1y with
two neighboring substituent groups on the aziridine ring was
used in the sequential reaction as well (entry 8). Under the
optimized reaction conditions product 2y was afforded in a
moderate yield with a good diastereoselectivity (dr=5:1).
To understand the mechanism of the sequential reaction,
several control experiments were investigated. In the absence
Scheme 5 The controlled experiment of mechanistic studies
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A possible mechanism was proposed for the present
sequential reaction (Scheme 6). Regioselective nucleophilic
ring-opening reaction of 1a with DMSO was an initial starting
point to drive this whole sequence of reactions. Then the ring-
Journal Name
temperature and poured into water (20 mL) and extracted
with ethyl acetate (5 × 10 mL). The combined organic extract
was washed with brine (20 mL), dried over Na2SO4 and
evaporated under reduced pressure. The residue was purified
by flash column chromatography on silica gel (petroleum
opening intermediate
six-membered transition state16f, g to afford the intermediate
Further removal of Me2S from the intermediate followed by
tautomerization led to the intermediate . At last, conjugate
addition between the newly generated nitrogen anion and
highly activated Michael-type acceptor in intermediate
A underwent hydrogen migration in a
B
.
ether/ethyl acetate, 5:1) to afford product 2.
B
D
Acknowledgements
D
would provide product 2a
.
This work was supported by the Foundation of NSFC
(21302140, 21402141 and 21502141), the Foundation of
Talent Introduction in Tianjin Normal University (5RL121 and
Me
S
Me
S
LA
O
Me
O
N
O
LA-
Tol
Me
O
Tol
S
N
Tol
S
O
N
O
O
S
H
H
O
O
S
LA
5RL122)
and
Tianjin Normal University Doctoral
CO2Me
CO2Me
Me
Me
Foundation (52XB1301), which are gratefully acknowledged.
MeO2C
CO2Me
O
MeO2C
CO2Me
B
A
1a
O
O
Tol
N
S
O
-Me2S
LA
tautomerization
NHTs
OMe
O
OH
NHTs
CO2Me
Notes and references
MeO
O
MeO2C
CO2Me
MeO2C
1
J. M. Bobbitt, Adv. Heterocycl. Chem., 1973, 15, 99.
C
D
LA
2
M. S. Christodoulou, F. Calogero, M. Baumann, A. N. García-
Argáez, S. Pieraccini, M. Sironi, F. Dapiaggi, R. Bucci, G.
Broggini, S. Gazzola, S. Liekens, A. Silvani, M. Lahtela-
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Navarro, I. R. Baxendale, L. D. Via, D. Passarella, Eur. J. Med.
Chem., 2015, 92, 766.
O
Aza-Michael
addition
NTs
CO2Me
MeO2C
2a
3
S. Zhao, M. J. Totleben, J. P. Freeman, C. L. Bacon, G. B. Fox,
E. O'Driscoll, A. G. Foley, J. Kelly, U. Farrell, C. Regan, S. A.
Mizsak, J. Szmuszkovicz, Bioorg. Med. Chem., 1999, 7, 1637.
Scheme 6 A possible mechanism for the sequential reaction
Two one-step postfunctionalizations on the keto carbonyl
4
5
6
B. J. D. Wright, C. Chan, S. J. Danishefsky, J. Nat. Prod., 2008
71, 409.
,
group of 2a were carried out (Scheme 7). 2a reacted with
hydroxylamine hydrochloride in EtOH to afford product
4 in
T. Onoda, Y. Takikawa, T. Fujimoto, Y. Yasui, K. Suzuki, T.
Matsumoto, Synlett, 2009, 1041.
(a) Y. -R. Wu, Y. -B. Ma, Y. -X. Zhao, S. -Y. Yao, J. Zhou, Y.
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A. H. Jackson, N. A. Khan, D. S. Millington,
78% yield. The keto carbonyl group of 2a was reduced by
NaBH4 to provide product
OH
5
in 30% yield.
O
OH
N
NaBH4
NTs
NH2OH.HCl
Lloydia, 1974, 37, 581.
NTs
NTs
MeOH, 0 o
C
NaOAc
EtOH, reflux
7
8
(a) E. D. Phillips, S. C. Hirst, M. W. D. Perry, J. Withnall, J. Org.
Chem., 2003, 68, 8700; (b) M. S. Allen, P. Skolnick, J. M.
Cook, J. Med. Chem., 1992, 35, 368.
30%
MeO2C
CO2Me
MeO2C
CO2Me
78%
MeO2C
CO2Me
5
2a
4
(a) Y. Yuan, S. A. Zaidi, D. L. Stevens, K. L. Scoggins, P. D.
Mosier, G. E. Kellogg, W. L. Dewey, D. E. Selley, Y. Zhang,
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3697; (e) G. Grethe, H. L. Lee, M. Uskoković, A. Brossi, J. Org.
Chem., 1968, 33, 491.
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Kruczynski, N. Guilbaud, C. Bailly, Bioorg. Med. Chem. Lett.,
2015, 25, 184; (b) H. Faltz, C. Bender, B. M. Woehrl, K. Vogel-
Bachmayr, U. Huebscher, K. Ramadan, J. Liebscher, Eur. J.
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Liebscher, Synlett, 1997, 1071.
Scheme 7 Derivatization of 2a
Conclusions
In conclusion, an efficient AgOTf-catalyzed reaction
involving sequential oxidative ring opening of aziridines and
aza-Michael addition has been developed. A series of 2,3-
dihydro-4(1H)-isoquinolones with structural diversity were
synthesized in moderate to good yields under mild conditions.
Further investigations on this kind of sequential reaction based
on ring opening of aziridines and Michael addition of electron-
deficient alkenes are ongoing in our laboratory.
9
10 (a) K. R. Prasad, C. Nagaraju, Org. Lett., 2013, 15, 2778; (b) L.
Wei, J. Zhang, Chem. Commun., 2012, 48, 2636.
Experimental
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General procedure for synthesis of compound 2.
In the open air, AgOTf (0.04 mmol, 20% cat.) was added to a
solution of aziridine
1 (0.2 mmol) in dimethyl sulfoxide (4 mL)
at room temperature. The reaction mixture was stirred at 70
oC for 18h. Then the reaction mixture was cooled to room
4 | J. Name., 2012, 00, 1-3
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18 CCDC
1555724
2a
contain
the
supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystal-
lographic
www.ccdc.cam.ac.uk/data_request/cif.
Data
Centre
via
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