Microwave-assisted, palladium-catalyzed intramolecular direct arylation for the synthesis of novel fused heterocycles

Article abstract of DOI:10.1055/s-2007-991053

An efficient synthesis of novel fused heterocycles has been established via microwave-assisted palladium-catalyzed intramolecular direct arylation. The acyclic aryl bromides are readily available from microwave-assisted one-pot annulation of N-(2-bromobenzyl)-2-aminophenols and ethyl 2-bromoalkanoates. The intramolecular direct arylation is then performed in the presence of palladium(II) acetate and dppf (10 mol% each) in toluene using potassium carbonate (2 equiv) as the base under microwave heating (150°C, 1 h) to afford the products in 43-99% yields. Steric effect is observed for Ar-Ar bond formation, giving a substantial amount of the debromination byproduct. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-991053

2728
LETTER
Microwave-Assisted, Palladium-Catalyzed Intramolecular Direct Arylation
for the Synthesis of Novel Fused Heterocycles
I
ntramolec
i
ular
D
n
irect Arylatio
l
n
ong Wu,a Liang Nie,a Jialu Luo,a Wei-Min Dai*a,b
a
Laboratory of Asymmetric Catalysis and Synthesis, Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. of China
Fax +86(571)87953128; E-mail: chdai@zju.edu.cn
b
Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR,
P. R. of China
Fax +85223581594; E-mail: chdai@ust.hk
Received 20 July 2007
24 h) using Cs2CO3 or NaOAc as the base.6 The yields
were moderate (56–64%). Fagnou and co-workers7 im-
proved efficiency of the synthesis of 5 (G = Ac, Ts, Boc;
90–99% yields) by using 2-(diphenylphosphino)-2¢-(N,N-
Abstract: An efficient synthesis of novel fused heterocycles has
been established via microwave-assisted palladium-catalyzed in-
tramolecular direct arylation. The acyclic aryl bromides are readily
available from microwave-assisted one-pot annulation of N-(2-bro-
mobenzyl)-2-aminophenols and ethyl 2-bromoalkanoates. The in- dimethylamino)biphenyl (10 mol%) as the ligand to
tramolecular direct arylation is then performed in the presence of
Pd(OAc)2 (5 mol%) and higher reaction temperature in
DMA (135 °C, 3 h). Zhu and co-workers8 reported a
palladium(II) acetate and dppf (10 mol% each) in toluene using po-
tassium carbonate (2 equiv) as the base under microwave heating
domino process for synthesis of the fused heterocycles 6
(150 °C, 1 h) to afford the products in 43–99% yields. Steric effect
(Scheme 1). It consists of the Pd-catalyzed sequential in-
is observed for Ar–Ar bond formation, giving a substantial amount
tramolecular amidation (path a) and intramolecular direct
arylation (path b). The bisiodobenzenes linked by a spacer
were treated with 5 mol% PdCl2(dppf) and 3 equivalents
of KOAc (DMSO, 120 °C) to furnish 6 in 51–94% yields.
However, it was reported that no reaction took place in re-
fluxing toluene.8 We report here an efficient synthesis of
the novel fused heterocycle 7 via Pd-catalyzed direct ary-
lation of 4-(2-bromobenzyl)-3,4-dihydro-3-oxo-2H-1,4-
benzoxazines 8 in toluene under microwave heating.9,10
Influence of the substituent X on the arylation is also
discussed.
of the debromination byproduct.
Key words: 3,4-dihydro-3-oxo-2H-1,4-benzoxazine, direct aryla-
tion, heterocycles, microwave, palladium
Dihydropyrrolo[3,2,1-de]phenanthridine is the skeleton
of the lycorine subgroup of amaryllidaceae alkaloids1
such as assoanine (1) and anhydrolycorine (2, Scheme 1).
A related tetracyclic structure without a nitrogen ring
juncture is found in the aporphine alkaloids2 such as apor-
phine (5: R1 = R2 = H, G = Me) and unciferine (5:
R1 = R2 = OMe, G = Me). These naturally occurring het-
erocycles have been prepared by intramolecular direct
arylation3 using Pd as the metal catalyst. Harayama and
co-workers4a used the aryl iodides 3 and the correspond-
ing bromides in their synthesis of 1 and 2 catalyzed by 5–
10 mol% Pd(OAc)2 and 10–20 mol% PCy3 (2 equiv
K2CO3, DMF, 125 °C). The desired products were ob-
tained in ≤50% yields along with the dehalogenation
byproducts. Alternatively, Garden and co-workers4b em-
ployed the spiro-dioxolanes 4 in the direct arylation under
the ligand-free conditions [10 mol% Pd(OAc)2, 1 equiv
n-Bu4NBr, 5 equiv KOAc, DMF, 100 °C] to give the
cyclization products in 87–95% yields. However, a bromo
analogue of 4 demonstrated diminished reactivity, giving
lower product yield even after prolonged reaction time.
Gómez-Lor and Echavarren used the direct arylation to
synthesize the [4,5]benzo derivatives of 1 (R1 = R2 = H).5
Cuny applied the combination of Pd(OAc)2 (20–50
mol%) and PCy3 (40–100 mol%) in the synthesis of 5
(G = CO2Me, Ts) via direct arylation of the 2-bromobenz-
yl-substituted tetrahydroisoquinolines in DMA (110 °C,
As a continuation of our effort in construction of a diverse
array of potentially bioactive heterocycles starting from 2-
aminophenols,11–15 we were interested in assembling the
novel tetracyclic heterocycle 7. As shown in Table 1, a se-
lected collection of substituted 2-aminophenols 9 were
converted into the N-(2-bromobenzyl)-substituted deriva-
tives 11 in excellent yields through reductive N-alkylation
with 10 by using NaBH(OAc)3 as the reductant.13b,16
A
competitive reduction of the aldehyde was observed for
the reactions of some 2-aminophenols. Formation of 2-
bromobenzyl alcohol could be suppressed by using 1,2-
dichloroethane (DCE)16 as the solvent (Table 1, entries 1,
2, and 5).
We have previously reported a microwave-assisted one-
pot annulation of N-arylmethyl-2-aminophenols with
ethyl 2-bromoalkanoates such as 12a,b in aqueous DMF
in the presence of K2CO3.13b By applying the same reac-
tion conditions, 4-(2-bromobenzyl)-3,4-dihydro-3-oxo-
2H-1,4-benzoxazines 8a–f were synthesized from 11 in
66–87% yields (Table 2). The annulation was generally
carried out at 120 °C for 20–90 minutes (Table 2, entries
1, 3, 4, and 6). For substrates 11b and 11e, heating at
180 °C for 20–30 minutes was required for producing
the products 8b and 8e in high yields (Table 2, entries 2
and 5). For the reaction of 11a with 12a, higher reaction
SYNLETT 2007, No. 17, pp 2728–2732
Advanced online publication: 25.09.2007
DOI: 10.1055/s-2007-991053; Art ID: W13407ST
© Georg Thieme Verlag Stuttgart · New York
1
6
.1
0
.
2
0
0
7

LETTER
Intramolecular Direct Arylation
2729
Table 1 Synthesis of N-(2-Bromobenzyl)-2-aminophenols 11a
4
R4
5
OH
NH
direct
arylation
R3
N
X
OH
CHO
Br
N
7
NaBH(OAc)3
solvent, r.t.
X
R2
+
NH2
R1
9
10
R1
I
R2
Br
11
3: R3 = R4 = H2
1: R1 = R2 = OMe
4: R3 = O; R4 = -O(CH2)2O-
2: R1–R2 = -OCH2O-
Entry
Product 11
Solvent
Time (h)
Yield (%)b
O
OH
Me
R1
N
a
R
81c
(88–89)
NH
N
direct
arylation
R2
G
1
DCE
1
2
N
b
O
Br
11a
5: R1, R2 = H, OH, OMe;
6 [a: amidation;
b: direct arylation]
Me
OH
G = CO2Me, Boc, Ac, Ts, Me
NH
Br
90
(80–81)
O
N
R
O
O
R
O
2
DCE
X
X
direct
arylation
N
11b
Me
Br
OH
NH
7
8
98
(77–78)
Scheme 1 Direct arylation approaches used for synthesis of dihy-
dropyrrolo[3,2,1-de]phenanthridine alkaloids (1 and 2), the aporphine
alkaloid skeleton (5), and novel fused heterocycles 6 and 7
Me
3
THF
3.5
Br
temperature and prolonged reaction time (140 °C, 90 min)
slightly improved the yield of 8a from 58% to 66%
(Table 2, entry 1). An N,O-bisalkylation byproduct13b was
obtained along with the desired product 8a.
11c
OH
Me
NH
90
(84–85)
4
5
6
THF
DCE
THF
6
Next, we carried out the key intramolecular Ar–Ar bond
formation within 8 (Scheme 2). In searching for a suitable
set of conditions, we tried Pd(OAc)2–BINAP (10 mol%
each) and K2CO3 (2 equiv) in toluene at 150 °C under mi-
crowave irradiation, which were previously developed in
our laboratories for intramolecular amidation of aryl bro-
mides.15 We found the catalyst system working for the mi-
crowave-assisted direct arylation while the best results
were obtained by replacing BINAP with dppf as the
ligand (Table 3).17 It’s interesting to note that the nonpolar
solvent, toluene, works well for our microwave-assisted
direct arylation although polar, aprotic solvents are much
more commonly used for reactions using conventional
thermal heating.3a In general, the microwave-assisted di-
rect arylation of 8 took place in the presence of 10 mol%
Pd(OAc)2, 10 mol% dppf, and 2 equiv K2CO3 (toluene,
150 °C for 1 h) to form the products 7 in 68–99% yields
(Table 3, entries 1, 2, 4, and 5). The catalyst loading can
Br
11d
OH
88c
(oil)
Cl
NH
1.5
Br
11e
OH
Ph
NH
Br
99
17d
(118–119)
11f
be reduced to 2.5 mol% for formation of 7c in 94% yield a All reactions were carried out at r.t. in the specified solvent. The ratio
of 9/10/NaBH(OAc)3 is 1.0:1.1:3.0.
(Table 3, entry 3). On the other hand, the yield of 7d in-
creased to 90% by using 15 mol% of the catalyst (Table 3,
entry 4). It seems that the weakly electron-donating meth-
yl group being para to the reacting C–H is beneficial for
b Isolated yields. Mp (°C) are given in parentheses.
c Estimated according to the 1H NMR spectrum of an inseparable
mixture with the minor byproduct, 2-bromobenzyl alcohol.
d The reaction was not complete after 7 h and was left for overnight.
Synlett 2007, No. 17, 2728–2732 © Thieme Stuttgart · New York

2730
J. Wu et al.
LETTER
Table 2 Synthesis of 3,4-Dihydro-3-oxo-2H-1,4-benzoxazines 8a
the direct arylation by comparing the formation of 7c and
7d (Table 3, entry 3 vs. entry 4). A steric effect was also
observed in the formation of the phenyl-substituted prod-
uct 7f (Table 3, entry 6). The latter was produced as an
inseparable mixture with the debromination byproduct. A
pure sample of 7f was prepared from 7e via Suzuki–
Miyaura cross-coupling using our Pd(OAc)2–Aphos cata-
lyst system18 in 99% yield (Scheme 3). It also provides a
ready access to other aryl-substituted analogues by using
various arylboronic acids.
OH
NH
O
N
R
O
X
X
Br
R
O
K2CO3
+
DMF–H2O (2:1)
120–180 °C
EtO
20–90 min, MW
Br
Br
12a: R = H
12b: R = Me
11
8
Entry Product 8
12
Temp Time Yield
(°C) (min) (%)b
O
O
N
R
O
O
N
R
O
Pd(OAc)2/dppf (1:1)
K2CO3 (2 equiv)
X
X
58c
N
O
12a 120 20
12a 140 90
1
2
66c
PhMe, 150 °C, 1 h
(112–113)
MW
Br
Br
8
7
8a
Me
O
N
Me
O
Scheme 2 Pd-catalyzed direct arylation of 8 under microwave
heating
84
(99–100)
12b 180 20
O
N
Me
O
O
N
Me
O
PhB(OH)2
1 mol% Pd(OAc)2
2 mol% Aphos
Br
Cl
Ph
8b
K3PO4 (3 equiv)
THF, 80 °C, 3 h (99%)
Me
O
N
Me
O
O
7e
7f
76
N(i-Pr)2
PCy2
12b 120 60d
Me
3
Aphos =
(112–113)
Scheme 3 Pd–Aphos-catalyzed Suzuki–Miyaura cross-coupling
of 7e
Br
O
8c
Me
O
Table 3 Synthesis of 7 via Microwave-Assisted Direct Arylation
of 8a
Me
N
87
4
5
6
12b 120 90
12b 180 30d
12b 120 60d
Entry Product 7
Pd(OAc)2 (mol%) Yield (%)b
(114–115)
O
Br
8d
N
O
93
(89–90)
O
Me
O
1
2
10
Cl
N
82
(140–141)
7a
Me
O
Me
O
Br
O
N
8e
N
76
Me
O
10
(112–113)
Ph
73
(125–126)
7b
Me
7c
Me
Br
O
N
Me
O
8f
94
a All reactions were carried out in closed pressurized vials on a tech-
nical microwave reactor with temperature measured by an IR sensor.
The ratio of 11/12/K2CO3 was 1.0:2.0:1.5.
3
2.5
(164–165)
b Isolated yields. Mp (°C) are given in parentheses.
c A bisalkylation byproduct was formed (see ref. 13b for detail).
d 1.2 equiv of 12b were used.
Synlett 2007, No. 17, 2728–2732 © Thieme Stuttgart · New York

LETTER
Intramolecular Direct Arylation
2731
Table 3 Synthesis of 7 via Microwave-Assisted Direct Arylation
of 8a (continued)
bromopropionate 12b in the presence of 2 equivalents of
K2CO3 at 120 °C for 30 minutes afforded 15 in 80% yield.
The bromide 15 was then treated with 10 mol% each of
Pd(OAc)2 and dppf, and 2 equivalents of K2CO3 in
toluene with microwave heating at 150 °C for 1 hour to
furnish the pentacyclic heterocycle 16 in 91% yield.
Entry Product 7
Pd(OAc)2 (mol%) Yield (%)b
O
N
Me
O
68
90
Me
10
15
In summary, we have established an intramolecular direct
arylation of aryl bromides by using Pd(OAc)2–dppf as the
catalyst in a nonpolar solvent, toluene, under microwave
heating (150 °C, 1 h). The synthesized polycyclic hetero-
cycles feature a fused ring system of 1,4-oxazine and 5H-
phenanthridine. Both electronic and steric effects on the
direct arylation are observed. Of particular synthetic inter-
est is the efficient formation of the chloro-substituted
product 7e, which offers further derivatization via the
Suzuki–Miyaura cross-coupling. Application of our
microwave-assisted direct arylation conditions to the
synthesis of other fused ring systems is in progress in our
laboratories.
4
(126–127)
7d
O
Me
O
Cl
N
99
5
10
(165–166)
7e
O
N
Me
O
43c
(150–151)
Ph
6
10
Acknowledgment
This work is supported by research grants provided by The National
Natural Science Foundation of China (Grant No. 20672092) and
Zhejiang University. W.-M. Dai is a recipient of the Cheung Kong
Scholars Award of The Ministry of Education of China.
7f
a All reactions were carried out in closed pressurized vials in toluene
at 150 °C for 1 h on a technical microwave reactor with temperature
measured by an IR sensor. The ratio of Pd(II):dppf is 1:1. Two equiv-
alents of K2CO3 were used.
References and Notes
b Isolated yields. Mp (°C) are given in parentheses.
(1) (a) Hoshino, O. In The Alkaloids, Vol. 51; Cordell, G. A.,
Ed.; Academic Press: San Diego, 1998, 323. (b) Lewis, J.
R. Nat. Prod. Rep. 2000, 17, 57. (c) Lewis, J. R. Nat. Prod.
Rep. 1998, 15, 107.
c An inseparable debromomination byproduct was obtained. The yield
was estimated according to the 1H NMR spectrum of the mixture.
(2) (a) Ríos, J. L.; Máñez, S.; Giner, R. M.; Recio, M. C. In The
Alkaloids, Vol. 53; Cordell, G. A., Ed.; Academic Press:
New York, 2000, 57. (b) Guinaudeau, H.; Leboeaf, M.;
Cavé, A. J. Nat. Prod. 1994, 57, 1033; and references cited
therein.
(3) For selected reviews, see: (a) Alberico, D.; Scott, M. E.;
Lautens, M. Chem. Rev. 2007, 107, 174. (b) Campeau,
L.-C.; Fagnou, K. Chem. Commun. 2006, 1253.
(c) Echavarren, A. M.; Gómez-Lor, B.; González, J. J.; de
Frutos, Ó. Synlett 2003, 585.
OH
OH
1. NaBH(OAc)3
DCE, r.t., 2.5 h
NH2
+
CHO
Br
NH·HCl
2. HCl (86%)
13
10
Br
14
(4) (a) Harayama, T.; Hori, A.; Abe, H.; Takeuchi, Y.
Tetrahedron 2004, 60, 1611. (b) Torres, J. C.; Pinto, A. C.;
Garden, S. J. Tetrahedron 2004, 60, 9889.
Pd(OAc)2
dppf
K2CO3
O
N
Me
O
Me
12b
(5) Echavarren, A. M.; Gómez-Lor, B. Org. Lett. 2004, 6, 2993.
(6) (a) Cuny, G. D. Tetrahedron Lett. 2003, 44, 8149.
(b) Cuny, G. D. Tetrahedron Lett. 2004, 45, 5167.
(7) (a) Lafrance, M.; Blaquière, N.; Fagnou, K. Chem. Commun.
2004, 2874. (b) For synthesis of 9-arylphenanthrenes, see:
García-Cuadrado, D.; de Mendoza, P.; Braga, A. A. C.;
Maseras, F.; Echavarren, A. M. J. Am. Chem. Soc. 2007,
129, 6880.
K2CO3 (2 equiv)
O
N
O
DMF–H2O (2:1)
120 °C, 30 min
MW (80%)
PhMe
150 °C, 1 h
MW (91%)
Br
16
15
(8) (a) Cuny, G.; Bois-Choussy, M.; Zhu, J. Angew. Chem. Int.
Ed. 2003, 42, 4774. (b) Cuny, G.; Bois-Choussy, M.; Zhu, J.
J. Am. Chem. Soc. 2004, 126, 14475.
(9) For examples of microwave-assisted C–H activation using
rhodium catalyst, see: (a) Tan, K. L.; Vasudevan, A.;
Bergman, R. G.; Ellman, J. A.; Souers, A. J. Org. Lett. 2003,
5, 2131. (b) Lewis, J. C.; Wu, J. Y.; Bergman, R. G.; Ellman,
J. A. Angew. Chem. Int. Ed. 2006, 45, 1589. (c) Using
palladium catalyst, see: Sridharan, V.; Martín, M. A.;
Scheme 4 Synthesis of the extended heterocyclic skeleton 16
Finally, we synthesized an extended heterocyclic skeleton
16 from 2-amino-1-naphthol 13 via the three-step se-
quence (Scheme 4). Thus, reductive N-alkylation of 13
with 10 gave N-(2-bromobenzyl)-2-amino-1-naphthol,
which is not stable in air and was isolated as the HCl salt
14 in 86% yield. One-pot annulation of 14 with ethyl 2-
Synlett 2007, No. 17, 2728–2732 © Thieme Stuttgart · New York

2732
J. Wu et al.
LETTER
(16) Abdel-Magid, A. F.; Carson, K. G.; Harris, B. D.;
Maryanoff, C. A.; Shah, R. D. J. Org. Chem. 1996, 61, 3849.
(17) General Procedure for Pd-Catalyzed Direct Arylation: A
10 mL pressurized process vial was charged with the
Menendez, J. C. Synlett 2006, 2375. (d) Using nickel
catalyst, see: Cioffi, E. A.; Bell, R. H.; Le, B. Tetrahedron:
Asymmetry 2005, 16, 471. Also, see: (e) Larhed, M.;
Moberg, C.; Hallberg, A. Acc. Chem. Res. 2002, 35, 717.
(f) Roberts, B. A.; Strauss, C. R. Acc. Chem. Res. 2005, 38,
653.
bromide 8 (0.50 mmol), Pd(OAc)2 (0.05 mmol), dppf (0.05
mmol), and K2CO3 (1.00 mmol) and it was sealed with a cap
containing a silicon septum. The vial was then evacuated and
backfilled with N2 (repeated for several times) through the
cap using a needle. To the degassed vial was added degassed
anhyd toluene (3 mL) through the cap using a syringe. The
loaded vial was then placed into the microwave reactor
cavity and was heated at 150 °C for 1 h. After cooled to r.t.
H2O (5 mL) was added to the reaction vial. The resultant
mixture was then extracted with EtOAc (3 × 10 mL). The
combined organic layer was washed with brine, dried over
anhyd Na2SO4, and evaporated under reduced pressure. The
residue was purified by column chromatography on silica
gel to furnish the product (see Table 3 and Scheme 4 for
details). Spectroscopic data for 7e: IR (KBr): 1672, 1236
cm–1. 1H NMR (400 MHz, CDCl3): d = 8.49 (d, J = 7.6 Hz,
1 H), 7.43–7.32 (m, 3 H), 7.16 (d, J = 8.4 Hz, 1 H), 6.89 (d,
J = 8.8 Hz, 1 H), 5.13 and 4.61 (ABq, J = 14.8 Hz, 2 H), 4.54
(q, J = 6.8 Hz, 1 H), 1.57 (d, J = 6.8 Hz, 3 H). 13C NMR (100
MHz, CDCl3): d = 164.6, 143.9, 132.2, 129.0, 128.5, 127.6,
127.5, 127.2, 126.8, 126.6, 124.4, 122.8, 116.2, 73.4, 42.6,
15.5. MS (ESI+): m/z (%) = 308 (100) [M + Na+]. Anal.
Calcd for C16H12ClNO2: C, 67.26; H, 4.23; N, 4.90. Found:
C, 67.28; H, 4.22; N, 4.88. The 1H NMR and 13C NMR of
7a–f and 16 can be obtained from the authors upon request.
(18) (a) Dai, W.-M.; Li, Y.; Zhang, Y.; Lai, K. W.; Wu, J.
Tetrahedron Lett. 2004, 45, 1999. (b) Dai, W.-M.; Zhang,
Y. Tetrahedron Lett. 2005, 46, 1377. (c) Jin, J.; Chen, Y.;
Li, Y.; Wu, J.; Dai, W.-M. Org. Lett. 2007, 9, 2585.
(10) For selected reviews on controlled microwave heating, see:
(a) Nüchter, M.; Ondruschka, B.; Bonrath, W.; Gum, A.
Green Chem. 2004, 43, 128. (b) Kappe, C. O. Angew.
Chem. Int. Ed. 2004, 43, 6250. For recent monographs, see:
(c) Kappe, C. O.; Stadler, A. Microwaves in Organic and
Medicinal Chemistry; Wiley-VCH: Weinheim, 2005.
(d) Microwaves in Organic Synthesis, Vol. 1 and 2; Loupy,
A., Ed.; Wiley-VCH: Weinheim, 2006.
(11) For synthesis of indoles, see: (a) Dai, W.-M.; Guo, D.-S.;
Sun, L.-P. Tetrahedron Lett. 2001, 42, 5275. (b) Dai, W.-
M.; Sun, L.-P.; Guo, D.-S. Tetrahedron Lett. 2002, 43,
7699. (c) Dai, W.-M.; Guo, D.-S.; Sun, L.-P.; Huang, X.-H.
Org. Lett. 2003, 5, 2919. (d) Sun, L.-P.; Huang, X.-H.; Dai,
W.-M. Tetrahedron 2004, 60, 10983. (e) Sun, L.-P.; Dai,
W.-M. Angew. Chem. Int. Ed. 2006, 45, 7255.
(12) For synthesis of benzo[b]furanes, see: Dai, W.-M.; Lai, K.
W. Tetrahedron Lett. 2002, 43, 9677.
(13) For synthesis of 1,4-benzoxazines, see: (a) Dai, W.-M.;
Wang, X.; Ma, C. Tetrahedron 2005, 61, 6879. (b) Feng,
G.; Wu, J.; Dai, W.-M. Tetrahedron 2006, 62, 4635.
(c) Xing, X.; Wu, J.; Feng, G.; Dai, W.-M. Tetrahedron
2006, 62, 6774. (d) Feng, G.; Wu, J.; Dai, W.-M.
Tetrahedron Lett. 2007, 48, 401.
(14) For synthesis of tetrahydroquinolines, see: Xing, X.; Wu, J.;
Dai, W.-M. Tetrahedron 2006, 62, 11200.
(15) For synthesis of the conjugates of dibenz[b,f][1,4]oxazepine
with 2-oxindole, see: Xing, X.; Wu, J.; Luo, J.; Dai, W.-M.
Synlett 2006, 2099.
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