Sc(OTf)3-catalyzed bicyclization of o-alkynylanilines with aldehydes: Ring-fused 1, 2-dihydroquinolines

Article abstract of DOI:10.1002/anie.201406959

A Sc(OTf)₃-catalyzed cascade Prins-type cyclization reaction of o-alkynylanilines, bearing a hydroxy or amine functionality, with aldehydes affords 1, 2-dihydroquinoline derivatives having an extra fused ring efficiently under mild reaction con

Full text of DOI:10.1002/anie.201406959

Angewandte
Chemie
DOI: 10.1002/anie.201406959
Heterocycles
Sc(OTf)3-Catalyzed Bicyclization of o-Alkynylanilines with Aldehydes:
Ring-Fused 1,2-Dihydroquinolines**
Can Zhu and Shengming Ma*
Dedicated to Professor Lixin Dai on the occasion of his 90th birthday
Abstract: A Sc(OTf)3-catalyzed cascade Prins-type cyclization
reaction of o-alkynylanilines, bearing a hydroxy or amine
functionality, with aldehydes affords 1,2-dihydroquinoline
derivatives having an extra fused ring efficiently under mild
reaction conditions. It is interesting to observe the reversed
reactivity in the highly selective formation of 1,2-dihydroqui-
noline derivatives instead of the formation of the usually
favored indole derivatives.
P
olycyclic structures are often important structural motifs
which exist in nature and display significant pharmacological
and biological activities.[1] Thus, much attention has been paid
to the development of new approaches to such skeletons.
Tandem reactions,[2,3] in which multistep reactions are com-
bined into one synthetic operation, have shown high effi-
ciency for the construction of complex skeletons. In contrast,
ring-fused indoles are a class of important heterocyclic
structural motifs found in many natural products, as well as
in bioactive substances.[4] An efficient coupling/cyclization
approach to the indole skeletons 2 by the cyclization of the o-
alkynylanilines 1 starting from 2-haloanilines and alkynes has
been well developed (Scheme 1a).[5] On this basis, we
envisioned that the o-alkynylaryl amines 3, having an extra
nucleophilic functionality, may undergo poly-heteroannula-
tion under the metal catalysis to provide the indolyl inter-
mediate Int-1,[6] which would sequentially react with the
aldehydes 4[6a,7] and the pre-installed nucleophilic unit (YH)
to afford ring-fused indole skeletons 5 (Scheme 1b).[8]
Scheme 1. Possibilities for forming different fused tricyclic skeletons.
room temperature for 10 hours unexpectedly afforded the
furanoquinoline 6a, albeit in only 23% yield (Table 1,
entry 1). It is surprising to observe that instead of undergoing
the most favored 5-endo cyclization mode between the NH
group of 3a with the triple bond, 3a first reacted with 4a to
produce Int-2 (Scheme 1c). Subsequent bicyclization trig-
gered the double endo-cyclization mode to afford 6a, thus
exhibiting reversed selectivity. Because of the importance of
1,2-dihydroquinoline derivatives,[9–11] we then turned to
optimizing the reaction conditions for the efficient formation
of 6a. FeCl3 failed to execute such a transformation (Table 1,
entry 2). Further catalyst screening showed that Sc(OTf)3 was
the best: the yield of 6a was improved to 72% at 458C with
5% recovery of 4a (entries 3–10). The reaction at a higher
concentration improved the yield further to 87% with
a reaction time of 6.5 hours (entry 11). The yield of 6a
decreased dramatically when using 5 mol% of Sc(OTf)3
(entry 12). Other solvents, such as THF, 1,4-dioxane, toluene,
and 1,2-dichloroethane (DCE) did not deliver better results
than CH2Cl2 (entries 13–16). An additive, such as 4 ꢀ M.S.
(M.S. = molecular sieves), did not improve the yield
(entry 17). Thus, Sc(OTf)3 (10 mol%) in CH2Cl2 at 458C
was defined as the optimal reaction conditions for further
study.
Our initial attempt began with the cyclization reaction of
the o-alkynylaniline 3a in the presence of 4-bromobenzalde-
hyde (4a) under the catalysis of Fe(OTf)3 (10 mol%).
Inspiringly, the reaction in dichloromethane (CH2Cl2) at
[*] C. Zhu, Prof. Dr. S. Ma
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Lu, Shanghai 200032 (P.R. China)
E-mail: masm@sioc.ac.cn
Prof. Dr. S. Ma
Department of Chemistry, Fudan University
220 Handan Lu, Shanghai 200433 (P.R. China)
[**] We thank the National Basic Research Program (2011CB808700)
and National Natural Science Foundation of China (21172192) for
financial support. We thank Youai Qiu of our research group for
reproducing the preparations of 6b of Table 2, 6h of Table 3, and 6p
of Scheme 3.
The scope of the reaction was then investigated by using
the optimal reaction conditions (Table 2). We first studied the
bicyclization reaction of 3a with different aldehydes (4). The
groups p-Br, p-MeO, m-MeO, and o-I may be introduced to
benzene ring of aldehydes, thus giving the corresponding
products in decent yields (entries 1–4). The reaction worked
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201406959.
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Table 1: Optimization of the reaction conditions for the bicyclization of
3a in the presence of 4a.[a]
observed that aliphatic aldehydes could also react with 3a to
give the corresponding products 6r and 6s in 64 and 30%
yield, respectively (entries 7 and 8).
Furthermore, for a class of aromatic aldehydes with
electron-withdrawing groups such as p-NC, p-MeO2C, p-Cl,
and m-Br on the phenyl ring, PhCO2H (10 mol%) together
with Sc(OTf)3 (10 mol%) was required to accelerate the
reaction rate, thus affording 6g (61%), 6h (60%), 6i (70%),
and 6j (69%), respectively (Table 3, entries 1–4). PhCO2H
probably helps to enhance the eletrophilic capability of
aldehydes, thus favoring the formation of the imine inter-
mediates Int-2 (Scheme 1c).[12] It is noteworthy that different
substituents on the benzene ring of the o-alkynylanilines (3),
such as fluoro and methoxy at the 4-position, turned out to be
compatible under the reaction conditions, thus leading to the
corresponding dihydroquinoline derivatives 6k and 6l in good
yields (Table 3, entries 5 and 6).
Entry Catalyst Solvent
T [oC] t [h]
6a
4a
Yield [%][b] Recovery [%][b]
1
2
3
4
5
6
7
8
9
10
Fe(OTf)3 CH2Cl2
FeCl3 CH2Cl2
RT
RT
RT
RT
RT
RT
RT
RT
RT
45
45
45
45
10
10
10
10
10
10
10
10
10
23
–
19
–
–
–
–
–
–
72
87
30
3
12
20
39
46
45
71
59
91
92
93
85
92
86
5
In(OTf)3 CH2Cl2
Zn(OTf)3 CH2Cl2
Cu(OTf)2 CH2Cl2
Yb(OTf)3 CH2Cl2
Er(OTf)3 CH2Cl2
Pd(OAc)2 CH2Cl2
CuI
CH2Cl2
Table 3: Sc(OTf)3/PhCO2H cocatalyzed one-pot synthesis of furanoqui-
nolines (6) from o-alkynylanilines (3) and aldehydes (4).[a]
Sc(OTf)3 CH2Cl2
10
11[c] Sc(OTf)3 CH2Cl2
12[c,d] Sc(OTf)3 CH2Cl2
13[c] Sc(OTf)3 THF
6.5
6.5
6.5
6.5
6.5
6.5
6.5
–
55
82
57
41
25
13
14[c] Sc(OTf)3 1,4-dioxane 45
15[c] Sc(OTf)3 toluene
16[c] Sc(OTf)3 DCE
17[c,e] Sc(OTf)3 CH2Cl2
45
45
45
Entry
R1 (3)
R2 (4)
t [h]
6
[a] The reaction was conducted at 458C CH2Cl2 with 4a (c=0.1m) and
3a (1.5 equiv) in the presence of Sc(OTf)3 (10 mol%). [b] Determined by
1H NMR analysis with CH3NO2 as the internal standard. [c] c=0.2m.
Yield [%][b]
1[c,d]
2
3
H (3a)
H (3a)
H (3a)
H (3a)
4-MeO (3b)
4-F (3c)
4-NC (4g)
4-MeO2C (4h)
4-Cl (4i)
3-Br (4j)
4-Br (4a)
4-Br (4a)
5.5
4
3
2.5
34
7.5
61 (6g)
60 (6h)
70 (6i)
69 (6j)
80 (6k)
73 (6l)
[d] Sc(OTf)3 (5 mol%) was used. [e] 4 ꢀ molecular sieves (25.0 mgmLÀ1
were added to the reaction. DCE=1,2-dichloroethane, Ms=methane-
sulfonyl, Tf=trifluoromethanesulfonyl, THF=tetrahydrofuran.
)
4
5[e]
6
equally well using benzaldehyde itself, thus producing 6e in
70% yield (entry 5). Heteroaryl-substituted aldehydes seem
to be compatible under the reaction conditions: 2-thiophe-
necarboxaldehyde produced 6 f in 50% yield (entry 6). It is
[a] The reaction was conducted at 458C in CH2Cl2 with the aldehyde 4
(c=0.2m) and 3 (1.5 equiv) in the presence of Sc(OTf)3 (10 mol%) and
PhCO2H (10 mol%). [b] Yield of isolated product. [c] Yield of product
isolated after chromatography on silica gel and then recrystallized from
dichloromethane and petroleum ether. [d] Only 32% yield of 6g was
obtained without the addition of PhCO2H, while 4g was recovered in
28% yield. [e] The reaction was carried out without PhCO2H.
Table 2: Sc(OTf)3-catalyzed one-pot synthesis of furanoquinolines 6
from o-alkynylaniline 3a and aldehydes 4.[a]
Further studies showed that the reaction worked equally
well using NHTs as the nucleophilic unit (YH = NHTs), and
the corresponding tricyclic dihydroquinoline derivative 6m
was obtained in 66% yield [Eq. (1)]. Finally, it is easy to
Entry
R2 (4)
t [h]
6
Yield [%][b]
1
2
3
4-BrC6H4 (4a)
4-MeOC6H4 (4b)
3-MeOC6H4 (4c)
2-IC6H4 (4d)
Ph (4e)
2-thienyl (4 f)
tBu (4k)
cPr (4l)
6.5
10
4
3.5
4.5
16
80 (6a)
72 (6b)
53 (6c
52 (6d)
70 (6e)
50 (6 f)
64 (6r)
30 (6s)
4[c]
5
6
7
8
18
14.5
[a] The reaction was conducted at 458C in CH2Cl2 with the aldehyde 4
(c=0.2m) and 3a (1.5 equiv) in the presence of Sc(OTf)3 (10 mol%).
[b] Yield of isolated product. [c] Yield of product isolated after chroma-
tography on silica gel and then recrystallized from dichloromethane and
petroleum ether.
2
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conduct the reaction on a one gram scale to afford 6i in 67%
yield [Eq. (2)].
The ring-fused 1,2-dihydroquinoline structure of the
products 6a–m has been determined by the an X-ray single-
crystal diffraction study of 6i (see the Supporting Informa-
tion).[13]
Further studies showed that this bicyclization reaction of
3a may also proceed with the N-Ts-substituted indole-3-
carbaldehyde 7c, while trace amounts of the corresponding
products were formed from the non-protected 7a and the N-
Bn-substituted indole-3-carbaldehyde 7b (Scheme 2).
In conclusion, we have developed a Sc(OTf)3-catalyzed
tandem aza-Prins cyclization[16] reaction of o-alkynylanilines
(3) with aldehydes (4) to construct fused tricyclic derivatives
with a potentially useful 1,2-dihydroquinoline unit under mild
reaction conditions. Considering the common knowledge that
nucleophilicity of the NH group is stronger than that of the
OH group in the substrates of the type 3, the formation of
indole-containing derivatives (5) should be, in principle, much
more favorable. However, the reversed selectivity on the
highly chemoselective formation of the 1,2-dihydroquinoline
derivatives 6 was observed. In view of the mild reaction
conditions and decent functional-group tolerance, this type of
transformation may be useful in organic synthesis and
medicinal chemistry. Further studies, including mechanistic
studies and asymmetric variants of such cyclization reactions,
are currently under way in our laboratory.
Scheme 2. Bicyclization reaction of the o-alkynylaniline 3a with indole-
3-carbaldehyde. [a] 7a was recovered in 35% as determined by
1H NMR analysis. [b] 7b was recovered in 84% as determined by
1H NMR analysis.
Received: July 7, 2014
Revised: August 15, 2014
Published online: && &&, &&&&
Six- and seven-membered oxacyclo-fused products may
also be easily obtained under the standard reaction condi-
tions, thus producing 6o and 6p in 93 and 81% yield,
respectively (Scheme 3).
Keywords: aldehydes · amine · cyclization · heterocycles ·
.
scandium
´
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113.4530(10),
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c = 14.3792(11) ꢀ,
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a =
V=
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Heterocycles
C. Zhu, S. Ma*
&&&& — &&&&
Sc(OTf)3-Catalyzed Bicyclization of o-
Alkynylanilines with Aldehydes: Ring-
Fused 1,2-Dihydroquinolines
Reversed selectivity in the Sc(OTf)3-cata-
lyzed cascade Prins-type cyclization reac-
tion of o-alkynylanilines, bearing a hy-
droxy or amine functionality, with alde-
hydes has been observed to afford 1,2-
dihydroquinoline derivatives. The reac-
tion proceeds through a bicyclization
pathway to construct tricycles in a one-
pot process under mild reaction condi-
tions.
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
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These are not the final page numbers!