Efficient synthesis of [1,2,3]triazolo[5,1-c][1,4]benzoxazines through palladium-copper catalysis

Article abstract of DOI:10.1016/j.tetlet.2009.03.120

A wide variety of [1,2,3]triazolo[5,1-c][1,4]benzoxazines were synthesized through palladium-copper catalyzed reactions of 1-azido-2-(prop-2-ynyloxy)benzene with aryl/vinyl iodides. A plausible reaction mechanism has also been proposed.

Full text of DOI:10.1016/j.tetlet.2009.03.120

Tetrahedron Letters 50 (2009) 2678–2681
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Tetrahedron Letters
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Efficient synthesis of [1,2,3]triazolo[5,1-c][1,4]benzoxazines through
palladium–copper catalysis
*
Chinmay Chowdhury , Anup Kumar Sasmal, Pradeep Kumar Dutta
Chemistry Division, Indian Institute of Chemical Biology (unit of CSIR), 4, Raja S. C. Mullick Road, Jadavpur, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A wide variety of [1,2,3]triazolo[5,1-c][1,4]benzoxazines were synthesized through palladium-copper
catalyzed reactions of 1-azido-2-(prop-2-ynyloxy)benzene with aryl/vinyl iodides. A plausible reaction
mechanism has also been proposed.
Received 22 January 2009
Revised 12 March 2009
Accepted 17 March 2009
Available online 21 March 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
[1,2,3]Triazolo[5,1-c][1,4]benzoxazines
1-Azido-2-(prop-2-ynyloxy)benzene
1,3-Cycloaddition
Palladium
Copper
Because of the widespread applications of structurally novel
heterocycles in drug development, the establishment of new syn-
thetic methods for such molecules has been one of the frontier
areas in organic synthesis. In particular, synthetic approaches to
various nitrogen- and oxygen-containing heterocycles have been
extensively pursued in search of pharmaceutical leads and thera-
peutic agents. Toward this end, various palladium-catalyzed
heteroannulation strategies, especially the addition of nitrogen or
oxygen across the triple/double bonds have become very popular.1
In recent past, triazole derivatives have received considerable
attention due to their applications ranging from medicinal chemis-
try2 to material science.3 Fused triazoles endowed with wide range
of bioactivities4 are also interesting and some of them (e.g.,
Estazolam, Alprazolam, etc.) have achieved both clinical and com-
mercial success.5 Parallel to these, 1,4-benzoxazines and their
dihydro derivatives are frequently found in naturally occurring
compounds6 and pharmacologically active substances7 including
drugs7c (e.g., Levofloxacin). More importantly, 2,3-dihydro-1,4-ben-
zoxazines fused with triazole have featured in novel therapeutic
agents as reported in patents8 and utilized for structure based
design of fungicides through molecular modeling studies.9
harsh and/or sluggish reaction conditions. Moreover, reported
methods have been targeted toward the synthesis of some specific
[1,2,3]triazolo[5,1-c][1,4]benzoxazine derivatives and thus are lim-
ited in general synthetic utility. To the best of our knowledge, mod-
ern palladium-catalyzed reactions have completely been ignored
to develop a general method for the synthesis of [1,2,3]triazol-
o[5,1-c][1,4]benzoxazines.
Recently, inter and intramolecular version of azide-alkyne cyc-
loadditions in the presence of transition-metal catalysts has
emerged as useful tools in chemical synthesis13 and chemical biol-
ogy14 as well. In continuation of our efforts in the area of palla-
dium-catalyzed reactions7a,15 to develop heterocyclic compounds
of biological interest, we have recently reported16 isoindoline-
fused triazoles through successful exploration of intramolecular
[3+2] cycloaddition reactions. As a promising extension of our pre-
vious study, we felt that O-propargylation of 2-azido phenol and its
subsequent reaction with aryl/vinyl halides under palladium-cop-
per catalysis could result in the ultimate formation of 1,4-benzox-
azines fused with 1,2,3-triazole, provided appropriate reactions
and catalysts could be developed. Herein, we wish to report the
preliminary results obtained so far toward this goal.
While there are many synthetic routes toward individual 1,
4-benzoxazines10 and 1,2,3-triazoles,11 only a few traditional
synthetic methods are available for the synthesis of [1,2,3]triazol-
o[5,1-c][1,4]benzoxazines.8a,12 Some of these involve multi-step
procedure with attendant poor overall yields, while others require
The reactions of 1-azido-2-(prop-2-ynyloxy)benzene 1a with
iodobenzene 2a were chosen as a model system (Table 1). Initially,
we attempted to adopt our previously reported reaction protocol16
[Pd(PPh3)2Cl2/CuI/Et3N/DMF/115 °C] to synthesize the targeted
product 3a but to our surprise, 3a was formed only with 35% yield.
This disappointing result prompted screening studies (a selection
of which given in Table 1) in order to improve the yield of 3a
and deserves some comments in this context. We ran the reactions
* Corresponding author. Tel.: +91 33 2499 5862; fax: +91 33 2473 0284.
E-mail address: chinmay@iicb.res.in (C. Chowdhury).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.03.120

C. Chowdhury et al. / Tetrahedron Letters 50 (2009) 2678–2681
2679
Table 1
Optimization of the reaction conditions for the reaction of iodobenzene (2a) with 1-azido-2-(prop-2-ynyloxy)benzene (1a)a
N
N
N
Pd cat./
CuI
N
N
N
N
Ph
3
H
PhI
+
+
Base,
100 ºC
O
O
3a
O
2a
4
1a
Entry
Catalyst system
Co-cata-lyst
Base (h)
Timeb (h)
Yieldc 3a (%)
Yieldc 4 (%)
1d
2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)4
CuI
CuI
CuI
—
CuI
CuI
—
Et3N
11
4
4
4
4
4
9
4
4
37
45
27
15
49
64
37
0
44
0
0
35
0
0
4
5
10
K2CO3
K2CO3
K2CO3
Et3N
K2CO3
K2CO3
K2CO3
K2CO3
3e
4e
5
Pd(PPh3)4
Pd(OAc)2 /PPh3
Pd(OAc)2 /PPh3
Pd(OAc)2 /PPh3
Pd(OAc)2
6
7e
8e
9e
CuI
CuI
Pd/C
0
a
Acetylene 1a (0.57 mmol), phenyl iodide 2a (0.48 mmol), palladium catalyst (0.024 mmol), CuI (0.048 mmol, except entries 4 and 7), PPh3 (0.10 mmol, entries 5–7 only),
K2CO3 (0.72 mmol) or Et3N (0.72 mmol) in DMF (4 mL, except entry 1).
b
Total experimental time that includes time for stirring at rt followed by heating at 100 °C for 2 h.
Chromatographically isolated yield of pure product.
Et3N was used as solvent as well as base.
c
d
e
Except the products 3a and/or 4, no other significant side products were observed indicating the propensity of starting materials to undergo substantial degradation
under the reaction conditions.
through modification of our previously reported protocol16 with
Table 2
Palladium-copper catalyzed synthesis of [1,2,3]Triazolo[5,1-c][1,4]benzoxazines 3a
increased catalyst loading and manipulation of other parameters
(Table 1, entry 1). For example, treatment of azide 1 (0.57 mmol)
N
N
and iodobenzene 2a (0.48 mmol) in the presence of 5 mol % of
Pd(PPh3)2Cl2 (0.024 mmol) and 10 mol % of CuI (0.048 mmol) in
Et3N (solvent and base) with stirring at room temperature for 9 h
(until consumption of starting materials) followed by heating at
100 °C for 2 h led to the isolation of 3a (37%) along with 44% of
product 4, a self cycloaddition product of 1a. Use of stronger base17
such as K2CO3 in DMF resulted in slight improvement of the yield
of 3a, along with complete suppression of formation of the unde-
sired product 4 (Table 1, entry 2). Replacing the catalyst by
Pd(PPh3)4 with and without CuI furnished the product 3a with
27% and 15% yields, respectively (Table 1, entries 3 and 4). Employ-
ing Pd(OAc)2 combined with PPh3 as catalytic system and CuI as
co-catalyst and also using Et3N base in DMF afforded the product
3a with 49% yield (Table 1, entry 5). Gratifyingly, changing the base
to K2CO3 afforded a good yield of desired product of 3a (Table 1,
entry 6). Removal of either CuI or PPh3 resulted in low yield or
no formation of product 3a (Table 1, entries 7 and 8), indicating
that their cooperative assistance was crucial for the reaction to
proceed. Switching to Pd/C and PPh3 as catalytic system did not
produce any desired product 3a (Table 1, entry 9). Several other
reaction parameters were also tuned. DMF was found to be the sol-
vent of choice among various solvents examined.
N3
O
N
R
Pd(OAc)2/ PPh3
+
RI
CuI, K2CO3
O
3a-l
DMF, 100 ºC
2a-l
1a
Entry
1
R
Productb
Yieldc (%)
3a
3b
64
69
2
3
3c
75
38
N
CO Me
4d
3d
2
Shorter reaction time and lower temperature resulted in incom-
plete conversion. Immediate heating without stirring at room tem-
perature for requisite time gave lower yields of desired products.
With optimized conditions (Table 1, entry 6) in hand, the scope
of the reaction was explored by treating various aryl iodides 2a–l
with 1-azido-2-(prop-2-ynyloxy)benzene 1a. The results are sum-
marized in Table 2. All the products were characterized by spectral
and analytical data. In addition, unequivocal conformation of the
structure of a representative product 3d was obtained through
X-ray diffraction analysis18 (Fig. 1). It is obvious that a wide variety
of aryl, heteroaryl, and vinyl iodides possessing different functional
groups reacted successfully. Unfortunately, aryl bromides did not
react. Entries 4–12 in Table 2 showed the compatibility of reaction
with different functional groups (e.g., ester, nitro, fluoro, trifluoro,
methoxy, methyl, etc.). Apparently, an electron-withdrawing
NO
2
5
6
3e
3f
42
70
CF
3
7
3g
43
(continued on next page)
CH
3

2680
C. Chowdhury et al. / Tetrahedron Letters 50 (2009) 2678–2681
Table 2 (continued)
CuL
n
Entry
R
Productb
Yieldc (%)
R
N
N
3
3
CuI.Ln
K CO
II
Pd
2
3
O
O
N
1
3
A
8
9
3h
3i
47
RI
Pd(OAc)
+
2
2
OMe
N
O
II
C
RPd I
Pd(0)
R
OMe
N
PPh
3
B
R
N
54
56
N
N
N
N
N
R
I
N
u
C
3
MeO
O
O
O
O
3
E
D
CuI.Ln
Me
N
10
3j
Scheme 1. Proposed reaction mechanism.
O
N
Me
smoothly converted22 to the desired cyclized product 3 through
copper-coordinated intermediate species E. DMF may also facili-
tate the cycloadditions as it is a dipolar aprotic solvent.23
11
12
3k
3l
52
67
In conclusion, we have developed an efficient and general meth-
od for the synthesis of [1,2,3]triazolo[5,1-c][1,4]benzoxazines in
one-pot, from readily available starting materials. This method
operates through carbon–carbon bond formation followed by
intramolecular cycloaddition of aromatic azide with internal al-
kyne, generated in situ under palladium-copper catalysis. In spite
of the widespread popularity of ‘click reaction’, it is limited to ter-
minal alkynes only. To broaden the scope, several transition-metal-
catalyzed cycloadditions between internal alkyne and azide are
now being reported.24 In this context, our method adds one more
successful example in this area. We believe that it should find
applications in medicinal chemistry.
F
OMe
a
The mixture of 1-azido-2-(prop-2-ynyloxy)benzene 1a (0.57 mmol), aryl iodide
(0.48 mmol), 5 mol % Pd(OAc)2 (0.024 mmol), 21 mol % PPh3 (0.10 mmol),
2
10 mol % CuI (0.048 mmol), K2CO3 (0.72 mmol) in DMF (4 mL) was stirred at rt for
2 h, then heated at 100 °C for another 2 h.
b
Satisfactory spectroscopic (NMR, IR, mass) and analytical data were obtained
for all compounds.
c
Chromatographically isolated yield of pure product.
A major part of the starting materials was converted to a tar within 1 h resulting
d
Acknowledgments
in low yield of the desired product 3d and no other side product was observed.
A.S. thanks CSIR, India for the award of fellowship and C.C.
thanks Director, IICB for his support.
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Figure 1. Perspective view of compound 3d.
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0
0
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3
ꢀ
P1, a = 6.8601(4) AÅ, b = 10.1112(6) AÅ, c = 12.0728(7) ÅA, V = 741.57(8) ÅA , Z = 2,
Dcalcd = 1.376 Mg mꢀ3
,
T = 296(2) K,
l
= 0.097 mmꢀ1
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