Application of a recyclable fluorous oxime in the convenient synthesis of 3-amino-1,2-benzisoxazoles and 4-amino-1H-2,3-benzoxazines

Article abstract of DOI:10.1039/c3gc36966h

A microwave-assisted, fluorous synthetic route to 3-amino-1,2- benzisoxazoles and 4-amino-1H-2,3-benzoxazines has been developed. The strategy comprises linking the respective 2-fluorobenzonitrile or 2-(bromomethyl) benzonitrile to a fluorous oxime tag to give an aryloxime intermediate which then undergoes cyclization with concomitant cleavage of the substrate-tag in acidic conditions to provide the desired product in good to moderate yields. In addition, the aryloxime intermediate could be subjected to further reactions to expand the compound library. The product could be easily separated using fluorous solid-phase extraction (F-SPE) and the fluorous ketone recovered could be converted back to the fluorous oxime and reused in the next run of the synthesis.

Full text of DOI:10.1039/c3gc36966h

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Application of a recyclable fluorous oxime in the
convenient synthesis of 3-amino-1,2-benzisoxazoles
Cite this: Green Chem., 2013, 15, 780
and 4-amino-1H-2,3-benzoxazines†
Wei Jie Ang,a Chi-Yuan Chu,b Tzyy-Chao Chou,b Lee-Chiang Lo*b and Yulin Lam*a
A microwave-assisted, fluorous synthetic route to 3-amino-1,2-benzisoxazoles and 4-amino-1H-2,3-
benzoxazines has been developed. The strategy comprises linking the respective 2-fluorobenzonitrile or
2-(bromomethyl)benzonitrile to a fluorous oxime tag to give an aryloxime intermediate which then
undergoes cyclization with concomitant cleavage of the substrate-tag in acidic conditions to provide the
desired product in good to moderate yields. In addition, the aryloxime intermediate could be subjected
to further reactions to expand the compound library. The product could be easily separated using
fluorous solid-phase extraction (F-SPE) and the fluorous ketone recovered could be converted back to the
fluorous oxime and reused in the next run of the synthesis.
Received 11th October 2012,
Accepted 16th January 2013
DOI: 10.1039/c3gc36966h
www.rsc.org/greenchem
lithium amides6 and borane carriers7 have been recently
reported. Our interest in the development of environmentally
Introduction
In recent years, molecular tagging strategies, which aim to friendly reagents8 has led us to explore the possibility of a
facilitate purification by changing the physical properties or recyclable fluorous oxime tag 18a for the synthesis of two series
partitioning behaviour of the compound through the attach- of compound libraries carrying 3-amino-1,2-benzisoxazoles 2
ment of a suitable tag, have been introduced. The compounds and 4-amino-1H-2,3-benzoxazines 3, respectively, as the core
which are commonly tagged are the substrate, reagent, catalyst structures (Fig. 1).
or scavenger. The tags that are typically used are either the
Fluorous oxime tag 1 is classified as a “light fluorous” com-
polymer- or fluorous-support. In polymer-supported reac- pound (fluorine content <60%).9 “Light fluorous” compounds
tions,1 the heterogeneous reaction mixture eases product sep- have lower toxicity and persistence compared to “heavy fluo-
aration but causes disadvantages such as slow reaction rates rous” ones. The high chemical and thermal stability of fluorous
and the need to use a large excess of the reagent. In fluorous tags also allows the reactions to be carried out under micro-
synthesis, perfluoroalkyl chains are used as tags. The high wave irradiation. Fluorous oxime tag 1 is soluble in common
solubility of the fluorous tags in organic solvents results in the organic solvents. Products derived from 1 can be separated
reaction occurring in a homogeneous reaction mixture with from the non-fluorous compounds using fluorous silica gel
solution-phase reaction kinetics and the distinct physical prop- eluted with THF–H2O, which eliminates the use of environ-
erties of the fluorous compounds enable easy separation, via mentally persistent perfluorinated solvents.10 In addition, the
fluorous liquid–liquid extraction or fluorous solid-phase fluorous silica gel can be reused multiple times after washing
extraction (F-SPE), from the non-fluorous species. The poten- with THF and acetone.11 Thus, compared to conventional
tial of the fluorous tagging strategy has thus inspired the deve- column chromatography, this technique reduces the volume of
lopment of many fluorous reagents, catalysts and scavengers solvent used and also the waste generated.
to facilitate chemical synthesis.2 As a result, a wide variety of
Compound 2 constitutes an important class of heterocycles
recyclable fluorous catalysts,3 oxidants,4 chlorinating agents,5 because they are found in many pharmacologically active sub-
stances. The scaffold has been integrated into the structure of
aDepartment of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore 117543, Singapore. E-mail: chmlamyl@nus.edu.sg; Fax: +65-67791691;
Tel: +65-65162688
bDepartment of Chemistry, National Taiwan University, No. 1, Sec. 4 Roosevelt Road,
Taipei 106, Taiwan. E-mail: lclo@ntu.edu.tw; Fax: +886-233668670;
Tel: +886-233661669
†Electronic supplementary information (ESI) available: Full spectroscopic data
for all compounds. See DOI: 10.1039/c3gc36966h
Fig. 1 Structures of 1–3.
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Table 1 Optimization of the nucleophilic aromatic substitution reaction
Entry Solvent Base
Reaction conditiona
Yield of 6ab (%)
1
2
3
4
5
6
7
8
9
DMF
CH2Cl2 t-BuOK
t-BuOK
A
A
A
A
A
45
79
90
10
43
Scheme 1 Synthetic route towards compound 2, in which compound 4 could
be recycled.
THF
THF
THF
THF
THF
THF
t-BuOK
LiHMDS
DBU
t-BuOK
t-BuOK
t-BuOK
a potent LTB4 receptor antagonist12 and various analogs of 2
are known to possess anti-thrombin activities.13 Furthermore,
they are key intermediates in the synthesis of compounds exhi-
biting dual inhibition of acetylcholinesterase (AChE) and
monoamine-oxidase (MAO), and are potential therapeutic
agents for the treatment of senile dementia of the Alzheimer
type (SDAT).14 Because of their interesting biological proper-
ties, various synthetic routes to compound 2 have been
reported. The three most common methods used are via the
formation of the 1–1a or 1–2 bond or through the simul-
taneous formation of the 1–1a and 3–3a bonds.15 The solution-
phase synthesis of 2 via the formation of the 2–3 bond is less
commonly reported.16 Nevertheless, there is one earlier report
on the synthesis of 2 via the formation of the 2–3 bond using a
Kaiser oxime resin.16c However, no attempts were made to
recycle the resin. In this paper, we present fluorous oxime tag
1 as a convenient and regenerable substrate-tag for the syn-
thesis of 2 and 3. We also demonstrate a greener procedure to
synthesize fluorous oxime tag 1 from its precursor fluorous
ketone 4 as well as a procedure that enables the spent fluo-
rous-tag to be readily recovered in stoichiometric amounts,
regenerated and reused (Scheme 1). In addition, this protocol
is highly atom economical as the N–O moiety in compounds 2
and 3 is derived from the hydroxylamine.
M.W., 75 °C, 115 min 90
M.W., 85 °C, 70 min
M.W., 95 °C, 40 min
M.W., 95 °C, 70 min
M.W., 95 °C, 50 min
88
96
84
58
CH2Cl2 t-BuOK
DMF t-BuOK
10
a Condition A: conventional heating at 55 °C for 12 h. b Isolated yields.
with 4 : 1 TFA–aqueous 5 M HCl at 55 °C for 2 h gave the
product 2a in 95% yield. To optimize the nucleophilic aro-
matic substitution reaction, we first varied the solvent and
found that the reaction proceeded most efficiently in THF
(Table 1, entries 1–3). Next, we explored different bases. Non-
nucleophilic bases such as LiHMDS and DBU were shown to
afford 6a in lower yields than t-BuOK (Table 1, entries 3–5).
To facilitate a rapid synthesis of 2a, we explored microwave
irradiation (Table 1, entries 6–10) and found that compound
6a was obtained in the highest yield (96%) when the reaction
was performed in THF at 95 °C for 40 min (Table 1, entry 8).
Subsequent reaction of 6a with 4 : 1 TFA–aqueous 5 M HCl
under microwave irradiation at 95 °C for 8 min resulted in the
cyclization of 6a with concomitant cleavage of the fluorous-tag
to give product 2a and fluorous ketone 4 (Scheme 1), which
could be easily separated using F-SPE to provide pure 2a in
quantitative yield. Not only is the overall yield of 2 obtained
under microwave synthesis higher than that achieved via con-
ventional heating, it also offered a much more efficient proto-
col in terms of time consumed.
Results and discussion
Since microwave irradiation conditions could be applied to
Previously, the synthesis of fluorous oxime tag 1 was achieved both steps of the synthesis, we further explored the possibility
by refluxing 4 with H2NOH·HCl and TEA in a benzene–EtOH of synthesizing compound 2a directly from 5a without isolat-
mixture for 16 h.8a Since benzene is carcinogenic, we decided ing and purifying intermediate 6a. Gratifyingly, this methodo-
to explore a greener approach to prepare fluorous oxime tag 1 logy proceeded efficiently and provided compound 2a in
from fluorous ketone 4. We found that by using NaOH as the comparable yield (Table 2, entry 1) to the two-step microwave
base and ethanol–water as the solvent under microwave irradiation procedure. Using the optimized reaction con-
irradiation at 100 °C for 15 min, fluorous oxime tag 1 could be ditions, we extended the reaction to various substituted
smoothly prepared in 91% yield.
o-fluorobenzonitriles 5 which gave the respective compound 2
In our initial studies on the synthesis of 2, the key inter- in moderate to good yields (Table 2). It is worth noting that a
mediate 6a was synthesized and isolated using a conventional comparison of fluorous oxime tag 1 with its Kaiser oxime resin
heating procedure16c that involved treating fluorous oxime tag counterpart showed that 1 provided 2 in higher yields (Table 2,
1 with 2-fluorobenzonitrile 5a and t-BuOK in DMF at 55 °C. entries 1–2 and 4–5). In addition, ketone 4 was also recoverable
The reaction was monitored by TLC, was found to be com- by F-SPE in very good yield (Table 2).
pleted after 12 h and provided intermediate 6a in 45% yield
Next, we proceeded to investigate the reuse of fluorous
(Table 1, entry 1). Subsequent treatment of intermediate 6a oxime tag 1. Compound 5a was used for the model recycling
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Table 2 Synthesis of the compound 2 librarya
Table 3 Regeneration of 1 under microwave irradiation
Entry
1
Substrate
Yield of 2b (%)
Recovered 4c (%)
Run
Yield of 2aa (%)
Recovered 4b (%)
97
1
2
3
4
94
92
92
91
97
95
96
96
2
95
a Isolated yields over two steps. b Recovered via F-SPE.
3
4
5
6
97
96
96
96
fluorous ketone 4 recovered after each run was also consist-
ently very high (95–97%).
Encouraged by these results, we extended our studies to the
synthesis of 4-amino-1H-2,3-benzoxazines 3, a class of com-
pounds which have previously been demonstrated to possess
hypotensive, myorelaxant, anti-inflammatory and sedative
activities.17 Compound 3 is typically prepared via a multi-step
synthesis which first involves the cyclization of α-aminoxy-o-
toluic acid to yield 1H-2,3-benzoxazine-4(3H)-one.18 The latter
compound was then chlorinated with phosphorus pentachlo-
ride to give 4-chloro-1H-2,3-benzoxazine19 which was then
treated with ammonium acetate to facilitate a nucleophilic
substitution reaction to provide compound 3.17 Herein we
applied the reaction condition used for the synthesis of 2 to
2-(bromomethyl)benzonitrile 7a and fluorous oxime tag 1. The
first O-benzylation step went well under microwave conditions.
However, the second cyclization and hydrolytic step proceeded
more slowly than in the synthesis of compound 2 and required
30 min to complete. Compound 3a was obtained in 61% yield
and fluorous ketone 4 was recovered in 95% yield (Table 4,
entry 1). To demonstrate the versatility of this procedure, we
have applied it to other analogs of 2-(bromomethyl)benzo-
nitrile which gave the corresponding compound 3 derivatives
in good to moderate yields (Table 4, entries 2–5). It should be
emphasized that the newly developed synthetic route to com-
pound 3 presented here not only is shorter and more con-
venient, but also offers much more sense of green chemistry
as compared to the previous methods.
7
8
9
97
97
98
10
97
Since the fluorous aryloxime intermediate 6, unlike its
Kaiser oxime resin counterpart, could be isolated and purified
using F-SPE, this prompted us to examine the possibility of
subjecting 6 to further reactions before cyclization and release
of fluorous ketone 4. First, we explored the application of the
Suzuki coupling reaction on the aryloxime intermediate 6. As a
model study, the aryloxime intermediate of 4-bromo-2-fluoro-
nitrile 5d was coupled with phenylboronic acid using Pd
(PPh3)4 as the catalyst and under microwave irradiation at
a Intermediate 6 was not isolated. b Isolated yields over two steps. c Via
F-SPE. d Conventional heating conditions: (1) 55 °C, 12 h; (2) 55 °C,
2 h. e Using Kaiser oxime resin reaction conditions.16c f Conventional
heating conditions: (1) 55 °C, 15 h; (2) 55 °C, 2 h.
study under the optimized reaction conditions. The recycling 95 °C for 60 min. Subsequent cyclization and release of fluo-
experiments were carried out over 4 runs which provided com- rous ketone 4 afforded the biaryl product 2c in 47% overall
pound 2a in 91–94% overall yields (Table 3). The amount of yield (over 3 steps) (Table 5, entry 1). The Suzuki coupling
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Table 4 Synthesis of the compound 3 library
Yield of 3a (%)
Recovered 4b (%)
Scheme 2 Extension of the diversity of compound 2 via the nucleophilic aro-
Entry
1
Substrate
matic substitution reaction.
96
nucleophilic substitution reaction with phenol using potass-
ium hydroxide as the base and DMF as the solvent under
microwave irradiation followed by cyclization and release of
fluorous ketone 4 to afford the phenoxy compound 2f in 31%
yield (over 3 steps) (Scheme 2). These results demonstrate that
the fluorous aryloxime intermediate 6 could play a versatile
role and have great potential in expanding the structural vari-
ation of the compound library.
2
3
4
5
98
97
97
96
Conclusions
We have developed a greener procedure for the preparation of
fluorous oxime 1 and utilized it in a convenient, atom econo-
mical and environmentally friendly synthesis of 3-amino-1,2-
benzisoxazoles 2 and 4-amino-1H-2,3-benzoxazines 3. The by-
product fluorous ketone 4 could be recovered in high yield,
converted back into 1, and reused 4 times without significant
loss in quantity. In addition, the aryloxime intermediate 6
could be subjected to further reactions resulting in greater
diversity of the compound library. More importantly, the sense
of green chemistry offered by fluorous oxime 1 would certainly
create new opportunities for those previously considered for-
midable reactions utilizing benzophenone oxime and make
them more accessible.
a Isolated yields over two steps. b Via F-SPE.
Table 5 Extension of the diversity of compounds 2 and 3 via the Suzuki
coupling reaction
Entry
1
X
F
n
Yield of 2 or 3a (%)
Recovered 4b (%)
Experimental
0
92
General
All chemicals purchased were used without further purifi-
cation. Starting materials 5c, 5i, 7b, 7c, 7d and 7e were syn-
thesized by adapting literature procedures.8,21 Reactions were
carried out under nitrogen with commercially obtained an-
hydrous solvents. Analytical thin-layer chromatography (TLC)
was carried out on precoated F254 silica plates and visualized
with UV light. F-SPF was performed with FluoroFlash® silica
gel (40 micron). 1H and 13C NMR spectra were recorded at
298 K. Chemical shifts are expressed in terms of ppm relative
to the internal standard tetramethylsilane (TMS). Mass spectra
were performed under EI and ESI mode. Microwave reactions
were performed on the Biotage Initiator™ microwave synthesi-
zer in quartz pressure tubes.
2
Br
1
93
a Isolated yield. b Via F-SPE. c Suzuki coupling reaction: PhB(OH)2,
Pd(PPh3)4, 2 M Na2CO3, THF, M.W., 95 °C, 60 min. d Using Kaiser
oxime resin reaction conditions.20 e Suzuki coupling reaction:
PhB(OH)2, Pd(PPh3)4, 2 M Na2CO3, THF, M.W., 95 °C, 110 min.
reaction was also demonstrated with the aryloxime intermedi-
ate of 4-bromo-2-(bromomethyl)benzonitrile 7b which
afforded the biaryl product 3c in 34% overall yield (over
3 steps). Next we assessed the compatibility of the fluorous
aryloxime intermediate 6 for a second nucleophilic aromatic
substitution reaction. Compound 5h was used to prepare the
aryloxime intermediate which was then subjected to the
Synthesis of fluorous oxime tag 1
A mixture of fluorous ketone 4 (312 mg, 0.275 mmol), NaOH
(55 mg, 1.375 mmol) and hydroxylamine hydrochloride
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(30 mg, 0.43 mmol) in 25 : 1 95% EtOH–H2O (520 μL) was (5 mL) was added to the crude product and the reaction
heated in a pressure tube at 100 °C under microwave mixture was heated in a pressure tube at 95 °C under micro-
irradiation for 15 min. The reaction was monitored by TLC. wave irradiation for 30 min. The reaction was monitored by
When the reaction was complete, the reaction mixture was TLC. When the reaction was complete, the mixture was cooled
cooled to room temperature and concentrated to dryness. 2 M to room temperature and basified with 10% NaOH. The
HCl was added and the resulting mixture was extracted with aqueous solution was extracted with ethyl acetate (50 mL),
EtOAc. The organic phase was washed consecutively with 2 M washed with water (20 mL), dried over anhydrous MgSO4,
HCl, 5% NaHCO3, water and brine. Thereafter, the organic filtered, concentrated and subjected to F-SPE.
layer was dried over anhydrous MgSO4, filtered and concen-
trated to provide fluorous oxime tag 1 in 91% yield (296 mg).
General procedure for the Suzuki coupling reaction with
fluorous-tagged aryloxime intermediate, cyclization and
concomitant cleavage of the substrate-tag
General procedure for the synthesis of 3-amino-1,2-
benzisoxazoles under microwave irradiation
A mixture of the fluorous aryloxime intermediate (0.20 mmol),
phenylboronic acid (24 mg, 0.20 mmol), 5 mol% Pd(PPh3)4,
and Na2CO3 (535 μL, 2 M in H2O, 0.20 mmol) in THF
(2.60 mL) was heated in a pressure tube at 95 °C under micro-
wave irradiation (reaction time for 5d: 60 min and reaction
time for 7b: 110 min). The reaction was monitored by TLC.
When the reaction was complete, the mixture was cooled to
room temperature and concentrated to dryness. The reaction
mixture was diluted with ethyl acetate (20 mL) and washed
with water (10 mL) and brine (10 mL). The organic layer was
separated, dried over anhydrous MgSO4, filtered, concentrated
and subjected to F-SPE. A solution of 4 : 1 TFA–5 M HCl (aq.)
(5 mL) was added to the Suzuki coupled fluorous aryloxime
intermediate and the reaction mixture was heated in a
pressure tube at 95 °C under microwave irradiation for 8 min.
The reaction was monitored by TLC. When the reaction was
complete, the mixture was cooled to room temperature and
basified with 10% NaOH. The aqueous solution was extracted
with ethyl acetate (50 mL), washed with water (20 mL), dried
over anhydrous MgSO4, filtered, concentrated and subjected to
F-SPE.
A mixture of fluorous tag 1 (230 mg, 0.20 mmol), the respective
o-fluorobenzonitriles 5 (0.20 mmol) and potassium tert-but-
oxide (27 mg, 0.24 mmol) in THF (2.60 mL) was heated in a
pressure tube at 95 °C under microwave irradiation for 40 min.
The reaction was monitored by TLC. When the reaction was
complete, the mixture was cooled to room temperature and
concentrated to dryness. The reaction mixture was diluted with
ethyl acetate (20 mL) and washed with sodium bicarbonate
(10 mL), water (10 mL) and brine (10 mL). The organic layer
was separated, dried over anhydrous MgSO4, filtered and con-
centrated. A solution of 4 : 1 TFA–5 M HCl (aq.) (5 mL) was
added to the crude product and the reaction mixture was
heated in a pressure tube at 95 °C under microwave irradiation
for 8 min. The reaction was monitored by TLC. When the reac-
tion was complete, the mixture was cooled to room tempera-
ture and basified with 10% NaOH. The aqueous solution was
extracted with ethyl acetate (50 mL), washed with water
(20 mL), dried over anhydrous MgSO4, filtered, concentrated
and subjected to F-SPE.
General procedure for F-SPE
The crude product was first diluted with THF–H2O = 1 : 1 and
loaded into F-SPE fluorous silica. The product was then eluted
using THF–H2O = 1 : 1 and the recovered ketone 6 was sub-
sequently eluted with THF and concentrated. The product was
General procedure for the nucleophilic aromatic substitution
reaction with the fluorous tagged aryloxime intermediate of
5h, cyclization and concomitant cleavage of the substrate-tag
then concentrated, diluted with ethyl acetate (50 mL) and A mixture of the fluorous aryloxime intermediate (0.40 mmol),
washed with water (20 mL). The organic layer was separated, phenol (38 mg, 0.40 mmol) and KOH (23 mg, 0.40 mmol) in
dried over anhydrous MgSO4, filtered and concentrated to give DMF (2.60 mL) was heated in a pressure tube at 95 °C under
the pure, desired product.
microwave irradiation for 150 min. The reaction was moni-
tored by TLC. When the reaction was complete, the mixture
was cooled to room temperature and concentrated to dryness.
The reaction mixture was diluted with ethyl acetate (20 mL)
General procedure for the synthesis of 4-amino-1H-2,3-
benzoxazines under microwave irradiation
A mixture of fluorous tag 1 (230 mg, 0.20 mmol), the respective and washed with water (10 mL) and brine (10 mL). The
2-(bromomethyl)benzonitrile 7 (0.20 mmol) and potassium organic layer was separated, dried over anhydrous MgSO4, fil-
tert-butoxide (27 mg, 0.24 mmol) in THF (2.60 mL) was heated tered, concentrated and subjected to F-SPE. A solution of 4 : 1
in a pressure tube at 95 °C under microwave irradiation for TFA–5 M HCl (aq.) (5 mL) was added to the crude product and
40 min. The reaction was monitored by TLC. When the reac- the reaction mixture was heated in a pressure tube at 95 °C
tion was complete, the mixture was cooled to room tempera- under microwave irradiation for 8 min. The reaction was moni-
ture and concentrated to dryness. The reaction mixture was tored by TLC. When the reaction was complete, the mixture
diluted with ethyl acetate (20 mL) and washed with sodium was cooled to room temperature and basified with 10% NaOH.
bicarbonate (10 mL), water (10 mL) and brine (10 mL). The The aqueous solution was extracted with ethyl acetate (50 mL),
organic layer was separated, dried over anhydrous MgSO4, washed with water (20 mL), dried over anhydrous MgSO4,
filtered and concentrated. A solution of 4 : 1 TFA–5 M HCl (aq.) filtered, concentrated and subjected to F-SPE.
784 | Green Chem., 2013, 15, 780–785
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