Synthesis of 1-(1H-Indol-3-yl)-l,2-dihydroisoquinolines via AgOTf-catalyzed three-component reactions of 2-alkynylbenzaldehydes, amines, and indoles

Article abstract of DOI:10.1021/cc9001263

Three-component reactions of 2-alkynylbenzaldehydes, amines, and indoles catalyzed by AgOTf under mild conditions afford the 1-(1H-indol-3-yl)-l,2- dihydroisoquinolines in good yields. This silver-catalyzed tandem reaction is found to be workable with var

Full text of DOI:10.1021/cc9001263

238
J. Comb. Chem. 2010, 12, 238–244
Synthesis of 1-(1H-Indol-3-yl)-1,2-dihydroisoquinolines via
AgOTf-Catalyzed Three-Component Reactions of
2-Alkynylbenzaldehydes, Amines, and Indoles
Xingxin Yu† and Jie Wu*,†,‡
Department of Chemistry, Fudan UniVersity, 220 Handan Road, Shanghai 200433, China, and State Key
Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 354 Fenglin Road, Shanghai 200032, China
ReceiVed August 17, 2009
Three-component reactions of 2-alkynylbenzaldehydes, amines, and indoles catalyzed by AgOTf under mild
conditions afford the 1-(1H-indol-3-yl)-1,2-dihydroisoquinolines in good yields. This silver-catalyzed tandem
reaction is found to be workable with various indoles, anilines or alkyl amines, and 2-alkynylbenzaldehydes
with electron-withdrawing groups attached on the aromatic backbone.
Introduction
hydes. The key intermediate in the reaction process was
believed to be ortho-alkynylaryl aldimine D.8-10 After
nucleophilic attack of nitrogen on the activated triple bond
via 6-endo cyclization, iminium E would be generated.
Subsequent nucleophilic addition would afford the expected
N-heterocycles (Scheme 2). Prompted by this result and the
advancement of multicomponent reactions, we envisioned
that in the presence of suitable catalyst, three-component
reactions of 2-alkynylbenzaldehyde, amine, and indole would
occur via a similar transformation to furnish the desired
For chemistry to have its maximal effect on biology, it
must provide efficient methods for discovering small mol-
ecules that are in great demand in the field of chemical
genetics.1 Among the strategies utilized, multicomponent
reactions (MCR) have emerged as a powerful tool for
delivering the molecular diversity needed in combinatorial
approaches for the preparation of bioactive compounds.2 As
a privileged scaffold, the 1,2-dihydroisoquinoline core
structure is found in a wide variety of biologically active
natural products and pharmaceuticals.3 The indole skeleton
is an important substructure as well in both natural products
and therapeutic agents.4 Recently, we reported a highly
efficient and practical approach to functionalized 1,2-
dihydroisoquinolines with broad structural diversity, starting
from 2-alkynylbenzaldehydes.5 Its reliability, the ready
availability of the starting materials, and the versatility of
the resulting products make it a very important process for
library generation. Biological screening of a small library
of these compounds (Scheme 1) in a PTP1B (protein tyrosine
phosphatase) inhibition assay led to the identification of a
hit (Compound B, IC50 4.68 µM). With an expectation to
find more active compounds by evaluation of analogous
structures, we conceived that the 1,2-dihydroisoquinoline
with an indole substituent might be a good candidate because
of the importance of the indole skeleton. Thus, in our ongoing
program aimed at developing new approaches to construct
natural product-like compounds, we started to explore the
methodology for the synthesis of 1,2-isoquinolines with
indole substituents.
Scheme 1. Synthetic Route for the Preparation of Diverse
1,2-Dihydroisoquinolines Screened for PTP1B Activity
Scheme 2. Proposed Mechanism for the Formation of
Substituted 1,2-Dihydroisoquinolines
As described, in our previous reports5 we disclosed the
tandem reactions6,7 for generation of 1,2-dihydroisoquinoline
and related compounds starting from 2-alkynylbenzalde-
* To whom correspondence should be addressed. E-mail: jie_wu@
fudan.edu.cn.
† Fudan University.
‡ Chinese Academy of Sciences.
10.1021/cc9001263  2010 American Chemical Society
Published on Web 01/08/2010

Synthesis of 1-(1H-Indol-3-yl)-1,2-dihydroisoquinolines
Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 2 239
Table 1. Initial Studies for Three-Component Reaction of 2-Alkynylbenzaldehyde 1a, p-Toluidine 2a, and Indole 3aa
entry
Lewis acid
solvent
MeCN
additive
product
yield (%)b
1
2
3
4
5
6
7
8
AgOTf
CuI
Cu(OTf)2
Pd(OAc)2
FeCl3
I-1
I-1
I-1
I-1
II
II
II
58
48
48
46
86
82
83
88
85
54
70
68
49
54
51
62
57
46
65
72
65
72
90
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DCE
DME
toluene
THF
1,4-dioxane
DMA
Zn(OTf)2
Yb(OTf)3
Bi(OTf)3
Dy(OTf)3
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
II
II
9
10
11
12
13
14
15
16
17
18
19
20c
21d
22c,e
23c,f
4 Å MS
Na2SO4
Na2SO4
Na2SO4
Na2SO4
Na2SO4
Na2SO4
Na2SO4
Na2SO4
Na2SO4
Na2SO4
Na2SO4
Na2SO4
Na2SO4
I-1
I-1
I-1
I-1
I-1
I-1
I-1
I-1
I-1
I-1
I-1
I-1
I-1
I-1
CHCl3
CH2Cl2
MeCN
MeCN
MeCN
MeCN
a Reaction conditions: 2-alkynylbenzaldehyde 1a (0.3 mmol), p-toluidine 2a (0.3 mmol, 1.0 equiv), indole 3a (0.3 mmol, 1.0 equiv), Lewis acid (10
mol %), solvent (2.0 mL), room temperature. b Isolated yield based on 2-alkynylbenzaldehyde 1a. c In the presence of 5 mol % of AgOTf. d In the
presence of 2 mol % of AgOTf. e In the presence of 1.5 equiv of indole 3a. f In the presence of 2.0 equiv of indole 3a.
indole-derivatized 1,2-dihydroisoquinolines. Thus, we started
to investigate the possibility of this designed transformation.
tion was found to be workable with different indoles 3a-3g
bearing electron-rich and electron-poor groups attached on
the aromatic ring (Table 2, entries 1-7). Thus, 2-alkynyl-
benzaldehyde 1a reacted with p-toluidine 2a and 2-methyl
indole 3a leading to the 1-(2-methyl-1H-indol-3-yl)-3-phenyl-
2-p-tolyl-1,2-dihydroisoquinoline I-2 in 86% yield (Table 2,
entry 2). The desired product I-3 was generated in 80% yield
when 4-methoxyindole 3c was employed as a substrate
(Table 2, entry 3). Excellent yields were observed as well
when bromo- or nitro-substituted indoles were used in the
reaction of 2-alkynylbenzaldehyde 1a with p-toluidine 2a.
Alkyl amine was a suitable partner as well in this transfor-
mation. For example, benzyl amine 2b was utilized in the
reaction of 2-alkynylbenzaldehyde 1a with 6-methoxyindole
3d instead of p-toluidine 2a, leading to the desired product
I-9 in 43% yield (Table 2, entry 9). Other anilines were tested
as well, and all reactions proceeded smoothly to generate
the expected products in good yields (Table 2, entries 9-13).
Reaction of 2-alkynylbenzaldehyde 1b, p-toluidine 2a, and
6-methoxyindole 3d occurred to give rise to the correspond-
ing product I-15 in 50% yield (Table 2, entry 15). Lower
yield was observed when benzyl amine 2b was employed
in the reaction of 2-alkynylbenzaldehyde 1b with 2-meth-
ylindole 3b (Table 2, entry 16). Meanwhile, fluoro- or chloro-
substituted 2-alkynylbenzaldehyde 1c-1g were tested as
substrates in the reaction of amines and indoles (Table 2,
entries 17-27). As expected, all reactions (including the
reaction of benzyl or n-octyl amine) worked well to generate
the corresponding products in good yields. However, 2-alky-
Result and Discussion
The reaction was initially studied with 2-alkynylbenzal-
dehyde 1a, p-toluidine 2a, and indole 3a, which was selected
as a suitable substrate for reaction development. At the outset,
different Lewis acids were examined as catalyst in this
reaction. To our delight, 58% yield of desired product I-1
was obtained when the reaction was catalyzed by AgOTf in
MeCN at room temperature (Table 1, entry 1). Similar results
were observed when CuI, Cu(OTf)2, or Pd(OAc)2 was
utilized as a replacement (Table 1, entries 2-4). Interestingly,
only double addition adduct II was isolated when other Lewis
acids were employed (Table 1, entries 5-9).11 Further
extensive studies showed that the yield could be improved
when Na2SO4 was included as an additive (Table 1, entry
11). Screening of solvents revealed that MeCN and DCE
were the best choice in this reaction (Table 1, entries 12-19).
The efficiency was not affected when the catalytic amount
of AgOTf was reduced to 5 mol % (Table 1, entry 20).
Increasing the amount of indole to 2.0 equiv in the
transformation afforded the expected product I-1 in 90%
yield (Table 1, entry 23).
To investigate the scope of this three-component reaction,
various 2-alkynylbenzaldehydes, amines, and indoles were
examined under the optimized conditions [AgOTf (5 mol
%), MeCN, room temperature] (Table 2). This silver-
catalyzed 1-(1H-indol-3-yl)-1,2-dihydroisoquinoline forma-

240 Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 2
Yu and Wu
Table 2. AgOTf-Catalyzed Three-Component Reactions of 2-Alkynylbenzaldehydes, Amines, and Indoles

Synthesis of 1-(1H-Indol-3-yl)-1,2-dihydroisoquinolines
Table 2. Continued
Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 2 241

242 Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 2
Table 2. Continued
Yu and Wu

Synthesis of 1-(1H-Indol-3-yl)-1,2-dihydroisoquinolines
Table 2. Continued
Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 2 243
a Isolated yield based on 2-alkynylbenzaldehyde 1.
nylbenzaldehyde with an electron-donating group attached
on the aromatic ring (such as compound 1h) has not been
workable as a substrate in the transformation, which might
be due to the lower electrophilicity toward nucleophilic attack
(Table 2, entry 28).
phenyl-2-p-tolyl-1,2-dihydroisoquinoline (I-1), 1H NMR (400
MHz, CDCl3): 2.16 (s, 3H), 6.36 (s, 1H), 6.59 (s, 1H), 6.68
(s, 1H), 6.90 (d, J ) 8.2 Hz, 2H), 6.98 (d, J ) 8.2 Hz, 2H),
7.09-7.26 (m, 10H), 7.49 (d, J ) 7.8 Hz, 2H), 7.84 (br,
1H), 8.11 (d, J ) 8.2 Hz, 1H); 13C NMR (100 MHz, CDCl3)
δ 20.7, 61.7, 111.5, 112.1, 118.7, 119.4, 119.7, 122.0, 123.3,
124.5, 125.7, 126.0, 126.4, 127.3, 127.7, 127.8, 128.3, 129.3,
131.1, 132.2, 136.5, 138.1, 141.7, 145.1; HRMS calcd for
C30H24N2 (M+ - H): 411.1861, found: 411.1883.
Conclusion
In summary, we have described an efficient method for
the synthesis of 1-(1H-indol-3-yl)-1,2-dihydroisoquinolines
via AgOTf-catalyzed three-component reactions of 2-alky-
nylbenzaldehydes, amines, and indoles. This silver-catalyzed
1-(1H-indol-3-yl)-1,2-dihydroisoquinoline formation is found
to be workable with different indoles bearing electron-rich
and electron-poor groups attached on the aromatic ring.
Anilines and alkyl amines are also suitable partners in the
transformation. However, 2-alkynylbenzaldehyde with an
electron-donating group attached on the aromatic backbone
is not workable as a substrate, which might be due to the
lower electrophilicity toward nucleophilic attack. We believe
the mild reaction conditions combined with the efficiency
of this synthetic route would be attractive and beneficial for
indole-derivatized 1,2-dihydroisoquinoline small library
construction.
Acknowledgment. Financial support from National Natu-
ral Science Foundation of China (No. 20972030) and the
Science & Technology Commission of Shanghai Municipal-
ity (No. 09JC1404902) is gratefully acknowledged. We thank
Dr. Jason Wong (Roche R&D Center, China) for the English
review.
Supporting Information Available. Experimental pro-
1
cedures, characterization data, H and 13C NMR spectra of
compound I. This material is available free of charge via
the Internet at http://pubs.acs.org.
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