A reductive Mizoroki-Heck approach to dibenzo[b,e]oxepine

Article abstract of DOI:10.1016/j.tetlet.2013.06.070

The synthesis of dibenzo[b,e]oxepine framework via palladium-mediated reductive Mizoroki-Heck reaction has been described. The procedure is simple, straightforward, and regioselective.

Full text of DOI:10.1016/j.tetlet.2013.06.070

Accepted Manuscript
A reductive Mizoroki-Heck approach to dibenzo[b,e]oxepine
K.C. Majumdar, Tapas Ghosh, Sudipta Ponra
PII:
S0040-4039(13)01037-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.06.070
TETL 43123
Reference:
To appear in:
Tetrahedron Letters
Please cite this article as: Majumdar, K.C., Ghosh, T., Ponra, S., A reductive Mizoroki-Heck approach to
dibenzo[b,e]oxepine, Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/j.tetlet.2013.06.070
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A reductive Mizoroki-Heck approach to dibenzo[b,e]oxepine
K. C. Majumdar,* Tapas Ghosh, Sudipta Ponra
Department of Chemistry, University of Kalyani, Kalyani-741235, W.B., India.
Abstract: The synthesis of dibenzo[b,e]oxepine framework via palladium-mediated reductive
Mizoroki-Heck reaction has been described. The procedure is simple, straightforward and
regioselective.
Keywords:
Dibenzo[b,e]oxepine,
Reductive
Mizoroki-Heck
cyclization,
tetrakis(triphenylphosphine)palladium(0), 2-bromobenzylbromide, Sonogashira coupling
∗Corresponding author: Tel (O): +91-3712-267008, Fax: (O): +91-3712-267005 E-mail:
kcm@klyuniv.ac.in

A wide range of natural products possess a seven-membered oxepin ring in their molecular
architecture.1 Moreover, this unit serves as target molecules in numerous synthetic studies.2
The benz[b]oxepine ring system occurs in a small number of biologically active natural
products isolated mainly from plant sources.3 Some of these compounds exhibit oral
hypotensive and antiulcer properties.4 Few medicinally as well as pharmacogenically
important compounds bearing benzoxepin skeleton are shown in Figure 1. Recently, pacharin
(1) and bauhiniastatins 4 (2) were isolated from the plant Bauhinia purpurea, and these
compounds were shown to significantly inhibit cancer cell growth.5 These compounds are
similar to the natural products bauhinoxepin B (6)6 which have been shown to exhibit anti-
mycobacterial activity. CGP 3466 (5) exhibits strong neuroprotective activity as the result of
its ability to prevent neuronal apoptosis in the adult brain.7 Asakawa et al. first isolated
radulanins H (3)8 and E (4),9 from the liverwort Radula perrottetii and Radula variabilis.
They are 3-methyl-2,5-dihydro-1-benzoxepin derivatives.10 It has been reported that
radulanin H (3) exhibits important calmodulin and cyclooxygenase inhibitory actions.11 The
fungal metabolites pterulone (7)12 prevents the eukaryotic respiratory chain at the NADH site
of the ubiquinone oxidoreductase, possesses potent antifungal activity and is only weakly
cytotoxic. Therefore, the syntheses of these units carrying different functionalities are
important.
OH
OR1
R1
O
O
Ph
R2O
R2
Pacharin (1): R1 = H, R2 = CH3
Bauhiniastatin 4 (2): R1 = CH3, R2 = H
Radulanin H (3): R1 = CO2H, R2 = H
Radulanin E (4): R1 = H, R2 = CO2H
MeO
O
O
OH
N
CGP 3466 (5)
OH
O
Bauhinoxepin B (6)
Cl
O
Pterulone (7)
Figure 1: Benzoxepine moiety in natural and medicinal
compounds.

The scope of conventional cyclization strategies to medium-sized rings is limited by
unfavorable entropy factors, enthalpy factors, transannular interaction and lack of functional
diversity of the reaction products.13 Many synthetic methods, including RCM (Ring-Closing
Metathesis)14 and transition-metal-mediated cyclizations,15 have been utilized for the
construction of medium-ring heterocycles, especially oxepine derivatives. Recently we have
successfully synthesized medium-ring heterocycles based on a sulfanyl radical addition and
cyclization procedure.16 However, sometimes these methods suffer from complex reaction
conditions, difficulty in separation of products and also from the toxicity of thiophenol. We
have also previously developed routes to benzoxepine derivatives based on intramolecular
iodocyclization reaction and intramolecular Heck reaction.17, 18 The reductive Mizoroki-Heck
reaction has rapidly gained acceptance as a key tactic in the construction of a wide range of
carbo- and heterocyclic ring systems.19 Literature search revealed that reductive Mizoroki-
Heck cyclization has not yet been explored for the construction of benzoxepine moiety. The
reductive Mizoroki-Heck reaction may provide an alternative and easy procedure for the
construction of some benzoxepine skeletons, which are present as a basic structural moiety in
a number of naturally occurring compounds. This has prompted us to investigate the
reductive Mizoroki-Heck reaction for the synthesis of dibenzoxepine skeleton. Herein we
report the results.
The precursors 3a-n for our present study were synthesized in good to excellent yields by the
Sonogashira cross coupling reaction of iodide 2 with appropriate alkynes as depicted in
Scheme 1. The iodo compound 2 was in turn prepared from easily available 2-iodophenol and
2-bromobenzylbromide (or 2-bromo-4-methoxy-benzylbromide) by refluxing in anhydrous
acetone in the presence of K2CO3 for 4h in 90% yield.
Br
Br
R'
R'
OH
O
O
R
I
I
(i)
(ii)
(85-92%)
90%
1
2
3a-n
R = C3H7, C4H9, C5H11, C6H13, Ph,
CMe2(OH), CHMe(OH), CH2OH,
SiMe3, p-MeC6H4, m-OMeC6H4,p-CMe3C6H4
R' = H, OMe
Scheme 1: Preparation of the starting materials: Reagents and conditions:
(i) 2-bromobenzylbromide/2-bromo-4-methoxy-benzylbromide, acetone,
K2CO3, reflux, 4h; (ii) Alkyne, Pd(PPh3)2Cl2, CuI, DMF, Et3N, r.t., 4h

Next we have optimized the reaction conditions using compound 3a as model substrate for
the reductive Mizoroki-Heck reaction. The reaction was performed under conventional
conditions by heating in DMF at 120 °C using Pd(OAc)2 as catalyst. The reaction failed in
the absence of water probably due to the ineffectiveness of the reducing agent in organic
solvent (entry 1). When the reaction was conducted in 1:1 aqueous solution of DMF using the
catalysts Pd(OAc)2 and Pd(PPh3)2Cl2, the starting material was recovered intact (entries 2, 3).
In 1:1 aqueous solution of DMF, tetrakis(triphenylphosphine)palladium(0) (10 mol%) as a
catalyst and sodium formate as a reducing agent was effective enough to furnish the desired
product in low yield (30%, entry 4). After careful screening we were able to establish the
o
optimal condition as follows: DMF/water (5:2) mixture, Pd(PPh3)4, HCOONa, at 100 C for
2h (entry 5). We also observed that temperature plays an important role in the cyclization
o
process. As the temperature was increased from 100 C, extensive decomposition of the
starting materials occurred leading to lower yields of the cyclized products (entry 6). On the
other hand, decreasing the temperature from the optimal temperature results in the recovery
of the starting materials. The cyclization process is extremely time dependent. The reaction
was run for about 2h. The decrease in the reaction time resulted in the lowering of the product
(60%, entry 7). Under the optimized reaction condition the alkyne 3a was regioselectively
cyclized to afford 7-exo product 4a in 76% yield along with the unreacted starting material.
Aqueous solutions containing acetonitrile, DME, CH3CN, THF, gave low yields of the
cyclized product (entries 8-10) (Table 1).
a
Table 1:
Optimization of the reductive Mizoroki-Heck reaction
Entry
Catalyst
Solvent
(v/v)
Time Temp. Yieldb
(h)
(0C)
(%)
1
Pd(OAc)2
Pd(OAc)2
DMF
2.0
2.0
2.0
2.0
2.0
120
100
100
120
100
---
---
---
30
76
2
DMF/H2O (1:1)
3
Pd(PPh3)2Cl2 DMF/H2O (1:1)
4
5c
Pd(PPh3)4
Pd(PPh3)4
DMF/H2O (1:1)
DMF/H2O (5:2)
6
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
DMF/H2O (5:2)
DMF/H2O (5:2)
DME/H2O (5:2)
2.0
1.5
2.0
140
100
100
100
100
DPd
7
60
8
30
9
CH3CN/H2O (5:2) 2.0
THF/H2O (5:2) 2.0
15
10
15
aIn all cases HCOONa was used as reducing agent. bIsolated yield.
cOptimized reaction condition. dDP = Decomposed product.

A number of other substrates (3b-n) were allowed to react under the aforementioned
condition to examine the scope of the reductive Mizoroki-Heck cyclization process (Scheme
2).
Br
R'
R'
O
O
R
Pd(PPh3)4, HCOONa
H
DMF/H2O (5:2)
100 oC, 2-4h
(67-76%)
R
3a-n
4a-n
4a, R = C3H7, R' = H
4b, R = C4H9, R' = H
3a, R = C3H7, R' = H
3b, R = C4H9, R' = H
4c
3c
, R = C5H11, R' = H
, R = C5H11, R' = H
3d, R = C6H13, R' = H
4d, R = C6H13, R' = H
4e, R = Ph, R' = H
3e, R = Ph, R' = H
4f, R = CMe2(OH), R' = H
3f, R = CMe2(OH), R' = H
, R = CHMe(OH), R' = H
4g
3g
, R = CHMe(OH), R' = H
4h, R = CH2OH, R' = H
4i, R = SiMe3, R' = H
4j, R = p-MeC6H4, R' = H
3h, R = CH2OH, R' = H
3i, R = SiMe3, R' = H
3j, R = p-MeC6H4, R' = H
4k
3k
, R = m-OMeC6H4, R' = H
, R = m-OMeC6H4, R' = H
4l, R =p-CMe3C6H4, R' = H
4m, R = C4H9, R' = OMe
3l, R =p-CMe3C6H4, R' = H
3m, R = C4H9, R' = OMe
4n =
, R C5H11, R' = OMe
3n
=
, R C5H11, R' = OMe
Scheme 2: Reductive Mizoroki-Heck reaction of compounds 3a-n
Both aliphatic and aromatic alkynes were used in the immediate starting materials 3a-n. The
substrates having aliphatic alkyne part provide higher yields of the cyclized products (68-
76%) and for aromatic alkynes the yield is lower (67-69%). The substrates with p-methoxy
substitution at the phenyl ring of benzyl bromide part (3m, 3n) also furnished higher yields of
cyclized products (74%). The results are summarized in Table 2.

Table 2
: Summarized results of the reductive Mizoroki-Heck reaction
Entry
Substrate
Br
Time (h)
Product
Yield(%)
O
O
H
1
2.0
76
(CH2)2CH3
(CH2)2CH3
3a
4a
Br
Br
O
O
O
2
3
2.0
2.0
76
76
H
(CH2)3CH3
(CH2)4CH3
(CH2)3CH3
3b
3c
4b
4c
O
H
(CH2)4CH3
Br
Br
O
O
O
O
O
O
H
4
2.5
3.0
3.5
74
67
74
(CH2)5CH3
(CH2)5CH3
4d
4e
3d
3e
3f
H
5
6
Ph
Ph
Br
H
OH
4f
OH
Br
Br
O
O
O
O
H
7
8
3.5
4.0
72
68
H
OH
H
4g
3g
3h
OH
H
OH
OH
4h

Br
O
O
O
O
3.5
68
9
H
SiMe3
SiMe3
4i
4j
3i
3j
Br
H
10
11
12
3.0
2.5
3.0
69
67
67
Br
O
OMe
O
H
3k
4k
Br
O
OMe
O
H
3l
4l
Br
Br
OMe
O
O
O
O
OMe
13
14
H
2.5
2.5
(CH2)3CH3
74
74
3m
3n
(CH2)3CH3
4m
OMe
OMe
H
(CH2)4CH3
(CH2)4CH3
4n
The regioselective formation of 7-exo-cyclized product during the course of the reaction can
be rationalized by a mechanism similar to the one proposed in previous reports.20 An aryl
palladium π-complex A generated initially from 321 and transformed readily into a σ-vinyl
palladium complex B via simultaneous syn-addition to the triple bond. Endo-cyclization via a
hypothetical intermediate C is unfavorable due to high strain exerted by the trans-geometry
around the double bond in the eight-membered intermediate. The σ-vinyl complex readily
gets reduced to regenerate the Pd(0) catalyst by means of the reducing agent HCOONa
present in the reaction. Syn-addition of palladium to the triple bond during the Mizoroki-Heck

reaction implies exclusive formation of dibenzoxepine compounds 422 possessing Z
configuration of the exo-cyclic double bond (Scheme 3).
Br
R'
R'
[Pd(0)]
R
O
O
H
R
3
4
R'
HCOONa
[Pd]Br
R'
O
R
O
[Pd]Br
R
A
B
O
R
Br[Pd]
C
R'
Scheme 3: Plausible mechanism of the reaction
Liu et al. have developed23 a general ligand-free palladium catalyzed reductive Heck reaction
that was applied to the synthesis of a variety of five- and six-membered oxygen heterocycles
in 70–74% yields. In our previous attempt during the synthesis of oxepine derivatives we
were only able to manage a mixture of both 7-exo and 8-endo products under normal Heck
reaction condition. Our results on the reductive Heck cyclization in the synthesis of
dibenzooxepine derivatives seem to be significant in view of the aforesaid limited literature
reports on oxygen containing heterocycles.
In summary, we have developed an efficient and straightforward method for the construction
of potentially bioactive dibenzoxepine framework via palladium-mediated reductive
Mizoroki-Heck cyclization. In this method the 7-exo-cyclization products have been obtained
regioselectively. The protocol is equally effective for both aliphatic and aromatic alkyne
containing substrates.

Acknowledgement:
We thank DST (New Delhi) for financial assistance. We are also thankful to UGC and CSIR
(K. C. M. to UGC, New Delhi for a UGC-Emeritus fellowship and T. G. and S. P. to CSIR,
New Delhi for their senior research fellowships). We thank DST (New Delhi) for providing
Bruker NMR (400 MHz), Perkin-Elmer CHN Analyser, FTIR and UV-Vis spectrometer.
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21. General procedure for the preparation of staring materials 3a-n:
A mixture of compound 2 (300 mg, 0.77 mmol), 1-pentyne (68 mg, 1.01 mmol), dry Et3N
(2 ml), Pd(PPh3)2Cl2 (27 mg, 5 mol%), and CuI (7 mg, 5 mol%) in dry DMF (5 ml) was

stirred at room temperature for 4 h. After completion of the reaction as monitored by TLC,
the reaction mixture was cooled and water (5 mL) was added. This was then extracted
with CHCl3 (3 x 15 mL). The CHCl3 extract was washed with water (2 x 10 mL) followed
by brine (10 mL). The organic layer was dried (Na2SO4). Evaporation of CHCl3 furnished
a crude mass which was subsequently purified by column chromatography using silica gel.
Elution of the column with petroleum ether afforded the product 3a. Similarly other
alkynes were treated with compound 2 to give the corresponding substrates 3b-n.
Compound 3a: Yellow gummy, yield = 92%, IR (KBr): 2988, 2933, 2230, 1599 cm-1, 1H
NMR (400 MHz, CDCl3): δ = 7.74 (d, 1H, J = 7.6 Hz), 7.50 (d, 1H, J = 8.0 Hz), 7.40 (d,
H
1H, J = 7.2 Hz), 7.32 (t, 1H, J = 7.6 Hz), 7.14-7.24 (m, 2H), 6.90 (t, 2H, J = 7.6 Hz), 5.18
13
(s, 2H), 2.46 (t, 2H, J = 6.8 Hz), 1.63-1.70 (m, 2H), 1.05 (t, 3H, J = 6.8 Hz). C NMR
(100 MHz, CDCl3): δ = 158.8, 136.5, 133.5, 132.3, 128.9, 128.8, 128.5, 127.5, 121.6,
C
121.1, 114.1, 112.7, 94.8, 69.7, 22.3, 21.8, 13.6. HRMS (ESI+): Calcd. for C18H17BrO:
[M+H+] 329.0536; found 329.0560.
22. General procedure for the reductive Mizoroki-Heck cyclization of compounds 4a-n:
A mixture of the compound 3a (100 mg, 0.32 mmol), HCOONa (32.9 mg, 0.48 mmol),
Pd(PPh3)4 (11 mg, 9.6 x10-3 mmol) in DMF/H2O (7 mL, 5:2) was heated under continuous
o
stirring at 100 C for 2h. After completion of the reaction as monitored by TLC, the
reaction mixture was cooled and water (5 mL) was added. This was then extracted with
CHCl3 (3 x 15 mL). The CHCl3 extract was washed with water (2 x 10 mL) followed by
brine (10 mL). The organic layer was dried (Na2SO4). Evaporation of CHCl3 furnished a
crude mass. This was purified by column chromatography over silica-gel. Elution of the
column with petroleum ether afforded the product 4a. Other substrates 3b-n were similarly
treated to give the corresponding products 4b-n.
1
Compound 4a: Gummy, yield = 76%, IR (KBr): 2929, 1602 cm-1, H NMR (400 MHz,
CDCl3): δ = 7.44 (t, 2H, J = 8.0 Hz), 7.34-7.39 (m, 2H), 7.12-7.21 (m, 2H), 6.86-6.96 (m,
H
2H), 5.68-5.73 (m, 1H), 5.10 (s, 2H), 2.29-2.29 (m, 2H), 1.43-1.53 (m, 2H), 0.95 (t, 3H, J
= 8.0 Hz). 13C NMR (100 MHz, CDCl3): δ = 155.5, 146.2, 139.0, 133.8, 133.4, 131.6,
C
129.1, 128.5, 127.5, 127.4, 127.2, 126.3, 120.3, 119.5, 70.4, 31.7, 23.3, 14.0. MS (EI): m/z
= 251 [M+H]+. Anal. Calcd. for C18H18O: C, 86.36; H, 7.25%. Found: C, 86.39; H, 7.22%.
23. Liu, P.; Huang, L.; Lu, Y.; Dilmeghani, M.; Baum, J.; Xiang, T.; Adams, J.; Tasker, A.;
Larsen, R.; Faul, M. M. Tetrahedron Lett. 2007, 48, 2307-2310.

Graphical abstract
A reductive Mizoroki-Heck approach to the dibenzo[b,e]oxepine
K. C. Majumdar,* Tapas Ghosh, Sudipta Ponra
Department of Chemistry, University of Kalyani, Kalyani-741235, W.B., India
R'
R'
O
O
Br
R
Pd(PPh3)4, HCOONa
DMF/H2O, 100 oC
H
R
R = C3H7, C4H9, C5H11, C6H13, Ph,
CMe2(OH), CHMe(OH), CH2OH,
SiMe3,p-MeC6H4, m-OMeC6H4, p-CMe3C6H4
R' = H, OMe
E-mail: kcm@klyuniv.ac.in