Copper(I) mediated intramolecular cyclization of 2-(2-amino-phenylethynyl)benzoic and [2-(2-aminophenylethynyl)phenyl]acetic acid esters: A new synthetic step towards isoindolo[2,1-a]indoles and 5H-indolo[2,1-a]isoquinolines

Article abstract of DOI:10.1055/s-2003-40984

We describe herein parallel synthetic routes to isoindolo[2,1-a]indoles and 5H-indolo[2,1-a]isoquinolines from 2-(2-aminophenylethynyl)benzoic acid esters and [2-(2-aminophenylethynyl)phenyl]acetic acid esters, respectively.

Full text of DOI:10.1055/s-2003-40984

LETTER
1603
Copper(I) Mediated Intramolecular Cyclization of 2-(2-Amino-phenylethy-
nyl)benzoic and [2-(2-Aminophenylethynyl)phenyl]acetic Acid Esters: A
New Synthetic Step towards Isoindolo[2,1-a]indoles and 5H-Indolo[2,1-a]iso-
quinolines
C
opper(I) Med
r
iatedInt
a
ramolecula
n
r
Cyclizationcisco J. Reboredo, Mónica Treus, Juan C. Estévez, Luis Castedo, Ramón J. Estévez*
Departamento de Química Orgánica, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain
E-mail: qorjec@usc.es
Received 12 June 2003
doaniline (4a) afforded phenylethynylbenzoic acid ester
Abstract: We describe herein parallel synthetic routes to isoindo-
lo[2,1-a]indoles and 5H-indolo[2,1-a]isoquinolines from 2-(2-ami-
nophenylethynyl)benzoic acid esters and [2-(2-aminophenyl-
ethynyl)phenyl]acetic acid esters, respectively.
5a. Esterification of commercial 3,4-dimethoxybenzoic
acid and iodination of the resulting methyl 3,4-
dimethoxybenzoate with bis(pyridine)iodonium(I) tetra-
fluoroborate11b (IPy2BF4) gave 1b; 1b was converted into
3b in the same way as 1a into 3a; and Sonogashira cou-
pling of 3b with 4a and its dimethoxylated derivative 4b
(easily prepared by iodination of the corresponding
commercial aniline with IPy2BF4), afforded phenyl-
ethynylbenzoic acid esters 5b and 5c, respectively.
Key words: alkaloids, alkynes, cyclizations, indoles
Although considerable attention has been directed toward
the synthesis of compounds containing the indole nucleus
due to the chemical and biological importance of this unit,
the search for new methods for the simple and efficient
construction of functionalized indoles still constitutes a
major challenge in organic synthesis.1 Of particular inter-
est is the synthesis of polycyclic species such as isoindo-
lo[2,1-a]indoles, for which classical,2 photochemical3 and
radical4 methods all afford relatively low yields (although
efficient synthesis of a limited number of these hetero-
cycles has been achieved by palladium-mediated meth-
ods).5,6
When methyl 2-ethynylbenzoate 5a was treated with CuI
to form indole 6a by intramolecular attack of the amino
group on the triple bond,11b we found to our suprise that
isoindoloindole 7a was produced directly, as deduced
from analytical and spectroscopic data. The formation of
this compound probably involves initial formation of the
expected indole 6a followed by a spontaneous intra-
molecular cyclization to create the amide ring of 7a.
Compounds 5b and 5c were similarly converted directly
into the corresponding isoindoloindoles 7b and 7c, re-
spectively.
Among the most efficient procedures for indole system
synthesis are methods starting from 2-ethynylaniline de-
rivatives, which are easily prepared and modified, and can
be heteroannulated to give the indole system by many
types of reagents, among the most frequently used being
palladium complexes7–10 and, more recently, copper
salts.11 As part of a broader synthetic programme on in-
doles, we herein describe a copper-mediated heteroannu-
lation of o-aminophenylethynylbenzoic acid esters that
affords a new, short, efficient route to isoindolo[2,1-a]in-
doles and which generalizes to the preparation of indo-
lo[2,1-a]isoquinolines from 2-aminophenylethynyl-
phenylacetic acid esters.
o-Aminophenylethynylbenzoic acid esters 5a,12 5b and 5c
were obtained via o-iodobenzoic acid esters 1a and 1b as
follows. Esterification of commercial o-iodobenzoic acid,
followed by Sonogashira coupling13 of the resulting meth-
yl ester 1a with TMS-acetylene and subsequent removal
of the TMS group by treatment with TBAF, gave 2-ethy-
nylbenzoic acid ester 3a in moderate yield, and a second
Sonogashira reaction between 3a and commercial o-io-
Encouraged by the above results, we investigated whether
inclusion of a methylene group into the carboxylated side
chain of 5a–c would allow the same methodology to yield
indolo[2,1-a]isoquinolines 8, which have the tetracyclic
structure of the antileukaemic and antitumoral14 di-
benzopyrrocoline alkaloids cryptaustoline and cryptowo-
line.15 Several methods for the construction of these
structures16 have been reported, but to our knowledge
neither palladium- nor copper-based methodologies have
been tried.
To obtain the required esters 5d–f, compounds 1c and 1d
were prepared by esterification of the corresponding un-
iodinated commercial acids followed by iodination with
IPy2BF4, and compound 1e by treatment of commercial
o-iodobenzyl chloride (2a) with potassium cyanide, and
subsequent basic hydrolysis of the resulting nitrile and
esterification of the hydrolysis product.17 Sonogashira
coupling of 1c–e with TMS-acetylene and subsequent
removal of the protecting TMS group by treatment
with TBAF afforded compounds 3c–e, which were con-
verted into 5d–f by Sonogashira coupling with o-iodo-
aniline 4a.
Synlett 2003, No. 11, Print: 02 09 2003. Web: 05 08 2003.
Art Id.1437-2096,E;2003,0,11,1603,1606,ftx,en;D13803ST.pdf.
DOI: 10.1055/s-2003-40984
© Georg Thieme Verlag Stuttgart · New York

1604
F. J. Reboredo et al.
LETTER
Subjecting methyl 2-phenylethynylphenylacetates 5d–f active indoles such as benzo[b]carbazoles19 and the close-
to the conditions used to promote heteroannulation of 5a ly related pyrido[b]carbazoles,20 which have well-docu-
afforded the expected indoles 6d–f, and 6e cyclized spon- mented antitumour properties.
taneously to the desired indoloisoquinoline 8b. Upon
treatment with p-TsOH at room temperature, 6d and 6f
cyclized to 8a and 8c, respectively, but 8c proved to be un-
Acknowledgment
We thank the Spanish Ministry of Science and Technology and the
Xunta de Galicia for financial support, and the latter organisation
for a grant to Francisco J. Reboredo.
stable, affording the highly oxidized indoloisoquino-
linedione 9 when left standing at room temperature
(Scheme 1).
Suming up, we have developed new, simple syntheses of
isoindolo[2,1-a]indol-6-ones and indolo[2,1-a]isoquino-
lines that are closely related to our earlier synthesis18 of
5H-indeno[1,2-b]indol-10-ones. These results constitute
further confirmation of the usefulness of intramolecular
addition of anionic or heteroatomic centres to carbon-car-
bon triple bonds for the construction of ring systems in
natural product synthesis. Work is now in progress to es-
tablish the scope and limitations of this novel route to the
synthetic targets 7 and 8 in order to apply this methodolo-
gy to the preparation of the dibenzopyrrocoline alkaloids
cryptaustoline and cryptowoline and other biologically
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a)
TMS
CO2Me
R2
CO2Me
n
CO2Me
Pd Cl2(PPh3)2
n
R2
R1
R2
R1
CuI, Et2NH
THF, r.t., 1.5 h
n
R1
b) TBAF, CH2Cl2
r.t., 20 min
I
R3
R3
1
2
3:
a) R =R =H, n=0 (42% yield)
b) R1=R2=OMe, n=0 (49% yield)
c) R1=R2=OMe, n=1 (70% yield)
d) R1=H, R2=OMe, n=1 (79% yield)
e) R1=R2=H, n=1 (58% yield)
1
2
1:
a) R =R =H, n=0
Pd Cl2(PPh3)3, CuI
b) R1=R2=OMe, n=0
c) R1=R2=OMe, n=1
d) R1=H, R2=OMe, n=1
e) R1=R2=H, n=1
Et2NH, THF, r.t., 12 h
H2N
1
2
3
5:
a) R =R =R =H, n=0 (64% yield)
b) R1=R2=OMe, R3=H, n=0 (90% yield)
c) R1=R2=R3=OMe, n=0 (19% yield)
d) R1=R2=OMe, R3=H, n=1 (31% yield)
e) R1=R3=H, R2=OMe, n=1 (85% yield)
f) R1=R2=R3=H, n=1 (64% yield)
a) KCN, DMSO, 70 °C, 14 h
b) NaOH, EtOH/H2O, reflux, 6.5 h
c) H2SO4, MeOH, reflux, 2.5 h
I
R3
R3
H2N
2:
a) X=Cl
b) X=CN
c) X=CO2H
a) R3=H
X
4:
b) R3=OMe
I
O
R2
R1
R3
R3
N
CO2Me
1
2
3
R2
R1
7:
a) R =R =R =H (25% yield)
b) R1=R2=OMe, R3=H (42% yield)
c) R1=R2=R3=OMe (30% yield)
n
H
N
CuI, DMF
reflux, 2 h
R3
p-TsOH
CH2Cl2
O
R2
R1
O
r.t., 3.5 h
O
R3
1
2
3
6:
a) R =R =R =H, n=0
N
N
b) R1=R2=OMe, R3=H, n=0
c) R1=R2=R3=OMe, n=0
R3
d) R1=R2=OMe, R3=H, n=1 (40% yield)
e) R1=R3=H, R2=OMe, n=1
9
R3
f) R1=R2=R3=H, n=1 (45% yield)
1
2
3
8:
a) R =R =OMe, R =H (77% yield)
b) R1=R3=H, R2=OMe (33% yield, two steps)
c) R1=R2=R3=H (79% yield)
Scheme 1
Synlett 2003, No. 11, 1603–1606 © Thieme Stuttgart · New York

LETTER
Copper(I) Mediated Intramolecular Cyclization
1605
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H), 7.13 (dt, 1 H, J = 7.6 Hz, J = 1.2 Hz, Ar-H), 7.35 (dd, 1
H, J = 7.9 Hz, J = 1.4 Hz, Ar-H). MS: m/z (%) = 326 (100)
[M + 1]+. Compound 5e (oil). 1H NMR (CDCl3, ppm): d =
3.61 (s, 3 H, -OCH3), 3.68 (s, 3 H, -OCH3), 3.81 (s, 2 H,
-CH2-), 4.44 (br s, 2 H, -NH2), 6.61–6.67 (m, 2 H, 2 × Ar-H),
6.74 (dd, 1 H, J = 8.5 Hz, J = 2.4 Hz, Ar-H), 6.82 (d, 1 H,
J = 2.4 Hz, Ar-H), 7.05 (t, 1 H, J = 7.8 Hz, Ar-H), 7.33 (dd,
1 H, J = 8.2 Hz, J = 1.3 Hz, Ar-H), 7.42 (d, 1 H, J = 8.5 Hz,
Ar-H). MS (CI): m/z (%) = 296 (44) [M + 1]+, 19(100).
Compound 5f (oil). 1H NMR (CDCl3, ppm): d = 3.66 (s, 3 H,
-OCH3), 3.88 (s, 2 H, -CH2-), 4.43 (br s, 2 H, -NH2), 6.67–
6.72 (m, 2 H, 2 × Ar-H), 7.12 (dt, 1 H, J = 1.5 Hz, J = 8 Hz,
Ar-H), 7.25–7.37 (m, 4 H, 4 × Ar-H), 7.54 (dd, 1 H, J = 2.5
Hz, J = 5.3 Hz, Ar-H). MS (CI): m/z (%) = 266 (100) [M +
1]+, 234 (100). Compound 7a. Mp 149–151 ºC (CH2Cl2/
MeOH). 1H NMR (CDCl3/CD3SOCD3, ppm): d = 6.57 (s, 1
H, Ar-H), 7.10–7.49 (m, 6 H, 6 × Ar-H), 7.72 (d, 1 H, J = 7.5
Hz, Ar-H), 7.86 (d, 1 H, J = 7.9 Hz, Ar-H). MS: m/z (%) =
219(81) [M+], 163(100). Compound 7b. Mp 177–180 ºC
(CH2Cl2/MeOH). 1H NMR (CD3SOCD3, ppm): d = 3.96 (s,
3 H, -OCH3), 4.01 (s, 3 H, -OCH3), 6.49 (s, 1 H, Ar-H), 7.01
(s, 1 H, Ar-H), 7.11 (t, 1 H, J = 7.6 Hz, Ar-H), 7.22–7.28 (m,
2 H, 2 × Ar-H), 7.41 (d, 1 H, J = 7.6 Hz, Ar-H), 7.82 (d, 1 H,
J = 7.9 Hz, Ar-H). MS: m/z (%) = 279(100) [M+].
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Compound 7c. Mp 199–201 ºC (CH2Cl2/MeOH). 1H NMR
(CDCl3/CD3SOCD3, ppm): d = 3.91 (s, 3 H, -OCH3), 3.95 (s,
3 H, -OCH3), 3.98 (s, 3 H, -OCH3), 4.00 (s, 3 H, -OCH3),
6.37 (s, 1 H, Ar-H), 6.90 (s, 1 H, Ar-H), 6.94 (s, 1 H, Ar-H),
7.22 (s, 1 H, Ar-H), 7.38 (s, 1 H, Ar-H). MS: m/z (%) = 339
(100) [M+]. Compound 8a. Mp 180–181 ºC (MeOH). 1H
NMR (CDCl3, ppm): d = 3.85 (s, 3 H, -OCH3), 3.89 (s, 2 H,
-CH2-), 3.92 (s, 3 H, -OCH3), 6.53 (s, 1 H, Ar-H), 6.76 (s, 1
H, Ar-H), 7.11 (s, 1 H, Ar-H), 7.11–7.29 (m, 2 H, 2 × Ar-H),
7.45–7.49 (m, 1 H, Ar-H), 8.43–8.47 (m, 1 H, Ar-H). MS:
m/z (%) = 293 (100) [M+]. Compound 8b. Mp 171–172 ºC
(CH2Cl2/MeOH). 1H NMR (CDCl3, ppm): d = 3.83 (s, 3 H,
-OCH3), 4.02 (s, 2 H, -CH2-), 6.69 (d, 1 H, J = 2.5 Hz, Ar-
H), 6.83 (s, 1 H, Ar-H), 6.89 (dd, 1 H, J = 8.7 Hz, J = 2.5 Hz,
Ar-H), 7.25–7.32 (m, 2 H, 2 × Ar-H), 7.52–7.54 (m, 1 H, Ar-
H), 7.71 (d, 1 H, J = 8.7 Hz, Ar-H), 8.48–8.52 (m, 1 H, Ar-
H). MS: m/z (%) = 263(100) [M+]. Compound 9. Mp 254–
255 ºC (CH2Cl2/MeOH). 1H NMR (CDCl3/CD3SOCD3,
ppm): d = 7.17 (s, 1 H, Ar-H), 7.28–7.48 (m, 3 H, 3 × Ar-H),
7.58 (d, 1 H, J = 7.2 Hz, Ar-H), 7.71 (dt, 1 H, J = 1.1 Hz,
J = 7.5 Hz, Ar-H), 7.85 (d, 1 H, J = 8.0 Hz, Ar-H), 8.15 (d,
1 H, J = 8.0 Hz, Ar-H), 8.48 (d, 1 H, J = 7.9 Hz, Ar-H). MS:
m/z (%) = 247 (35) [M+], 219 (100).
(b) Ezquerra, J.; Pedregal, C.; Lamas, C.; Barluenga, J.;
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(12) All new compounds gave satisfactory analytical and
spectroscopic data. Selected physical and spectroscopic data
follow. Compound 5a. Mp 282–284 °C (CH2Cl2/MeOH). 1H
NMR (CDCl3, ppm): d = 3.87 (s, 3 H, -OCH3), 5.05 (br s, 2
H, -NH2), 6.65–6.70 (m, 2 H, 2 × Ar-H), 7.19 (dt, 1 H,
J = 1.5 Hz, J = 8.6 Hz, Ar-H), 7.34–7.45 (m, 2 H, 2 × Ar-H),
7.53 (dt, 1 H, J = 1.4 Hz, J = 7.6 Hz, Ar-H), 7.70 (dd, 1 H,
J = 1.1 Hz, J = 7.6 Hz, Ar-H), 8.06 (dd, 1 H, J = 1.2 Hz,
J = 7.9 Hz, Ar-H). MS: m/z (%) = 252 (6) [M + 1]+, 43 (100).
Compound 5b. Mp 177–180 °C (CH2Cl2/MeOH).1H NMR
(CDCl3, ppm): d = 3.91 (s, 3 H, -OCH3), 3.94 (s, 3 H,
-OCH3), 3.97 (s, 3 H, -OCH3), 5.10 (bs, 2 H, -NH2), 6.67–
6.73 (m, 2 H, 2 × Ar-H), 7.09–7.13 (m, 2 H, 2 × Ar-H), 7.37
(d, 1 H, J = 7.6 Hz, Ar-H), 7.51 (s, 1 H, Ar-H). MS (CI):
m/z (%) = 312(100) [M + 1]+. Compound 5c. Mp 254–255
ºC (CH2Cl2/MeOH). 1H NMR (CDCl3, ppm): d = 3.83 (s, 3
H, -OCH3), 3.87 (s, 3 H, -OCH3), 3.92 (s, 3 H, -OCH3), 3.95
(s, 3 H, -OCH3), 3.97 (s, 3 H, -OCH3), 6.29 (s, 1 H, Ar-H),
6.89 (s, 1 H, Ar-H), 7.08 (s, 1 H, Ar-H), 7.51 (s, 1 H, Ar-H).
MS (CI): m/z (%) = 372 (100) [M + 1]+. Compound 5d. Mp
125–127 ºC (CH2Cl2/MeOH). 1H NMR (CDCl3, ppm): d =
3.69 (s, 3 H, -OCH3), 3.84 (s, 2 H, -CH2-), 3.90 (s, 3 H,
-OCH3), 3.91 (s, 3 H, -OCH3), 4.80 (br s, 2 H, -NH2), 6.70–
6.73 (m, 2 H, 2 × Ar-H), 6.82 (s, 1 H, Ar-H), 7.03 (s, 1 H, Ar-
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Synlett 2003, No. 11, 1603–1606 © Thieme Stuttgart · New York

1606
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LETTER
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Synlett 2003, No. 11, 1603–1606 © Thieme Stuttgart · New York