An efficient ecofriendly protocol for the synthesis of novel fluoro isoxazoline and isoxazolidines using N-benzyl fluoro nitrone via cycloaddition reactions

Article abstract of DOI:10.1016/j.tetlet.2012.11.111

1-Butyl-3-methylimidazolium based ionic liquids are found to accelerate significantly the intermolecular 1,3-dipolar cycloaddition of N-benzyl fluoro nitrones derived in situ from aldehydes and benzylhydroxylamine, with electron deficient alkynes to affor

Full text of DOI:10.1016/j.tetlet.2012.11.111

Tetrahedron Letters 54 (2013) 765–770
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Tetrahedron Letters
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An efficient ecofriendly protocol for the synthesis of novel fluoro
isoxazoline and isoxazolidines using N-benzyl fluoro nitrone via cycloaddition
reactions
⇑
Bhaskar Chakraborty , Govinda Prasad Luitel
Organic Chemistry Laboratory, Sikkim Government College, Gangtok, Sikkim 737102, India
a r t i c l e i n f o
a b s t r a c t
Article history:
1-Butyl-3-methylimidazolium based ionic liquids are found to accelerate significantly the intermolecular
1,3-dipolar cycloaddition of N-benzyl fluoro nitrones derived in situ from aldehydes and benzylhydroxyl-
amine, with electron deficient alkynes to afford enhanced rates and improved yields of isoxazolines while
with enals exclusively endo isoxazolidines are obtained with high selectivity. Synthetic potentiality of the
novel isoxazolines and nitrones has also been tested successfully.
Received 10 September 2012
Revised 9 November 2012
Accepted 11 November 2012
Available online 1 December 2012
Ó 2012 Elsevier Ltd. All rights reserved.
keywords:
N-Benzyl fluoro nitrone
Cycloaddition reaction
Novel isoxazoline and isoxazolidines
Ionic liquid
1,3 Amino alcohol
Aldehyde/ketone synthesis
The 1,3-dipolar cycloaddition reactions represent the favourite
method for the construction of five-membered heterocycles, the
important frameworks of various natural products.1 In particular
the 1,3-dipolar cycloadditions of nitrones with alkenes and alkynes
afforded isoxazolidines and isoxazolines which are interesting
intermediates for the synthesis of b-amino alcohols and alka-
loids.2,3 Isoxazoline and isoxazolidines possess medicinal activities
such as antibacterial, anticonvulsant, antibiotic, antitubercular and
antifungal.4,5 Despite their potential utility, many of these proce-
dures require high temperature and prolonged reaction times
(drastic experimental conditions) and also suffer from poor regi-
oselectivity and lack of simplicity. In a few cases, the yields and
selectivities reported are far from satisfactory due to the occur-
rence of several side reactions.6
In recent times, ionic liquids have emerged as green solvents
with desirable properties such as good solvating ability, wide liqui-
dious range, tunable polarity, high thermal stability, negligible va-
pour pressure and ease of recyclability.7 Therefore, classical
organic reactions can be performed in these media with great
advantages (yield and selectivity) as compared to conventional
conditions. They are referred to as ‘designer solvents’ as their prop-
erties such as hydrophilicity, hydrophobicity, Lewis acidity, viscos-
ity and density can be altered by the fine-tuning of parameters
such as the choice of organic cation, inorganic anion and the length
of alkyl chain attached to an organic cation (Fig. 1).
These structural variations offer flexibility to the chemist to de-
vise the most idealized solvent, catering to the needs of any partic-
ular process. Since ionic liquids are entirely composed of non-
coordinating ions, they can provide an ideal reaction medium for
reactions that involve reactive ionic intermediates. Due to the sta-
bilization of charged intermediates by ionic liquids, they can pro-
mote unprecedented selectivities and enhanced reaction rates.
Consequently, ionic liquids are being used as recyclable solvents
for the immobilization of transition metal based catalysts, Lewis
acids and enzymes.8 As a result of their green credentials and po-
tential to enhance reaction rates and selectivities, ionic liquids
are finding increasing applications in organic synthesis9 with an
ever-increasing quest for exploration of newer reactions in ionic
liquids.10
It is known that introduction of fluorine atom into a specific
position of organic molecule may cause significant changes in the
stability, lipophilicity and biological activities of the resulting
molecules.11 This has been attributed to the high electro negativity
of the halogen, the strong C–F bond and the similar size of the
-
N
N
BF4
⇑
Corresponding author. Tel: +91 9434318330; fax:+91 3592231917.
E-mail address: bhaskargtk@yahoo.com (B. Chakraborty).
Figure 1. Chemical structure of ionic liquid.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.111

766
B. Chakraborty, G. P. Luitel / Tetrahedron Letters 54 (2013) 765–770
halogen and hydrogen atoms. For these reasons great efforts have
been placed on the development and evaluation of biologically ac-
tive fluorinated materials.12 The biological properties of multifluo-
rine containing compounds have been recently investigated.
Owing to their unique properties, such as high thermal stability
and lipophilicity, fluoro-organic compounds have been frequently
used as biorelated material, medicine and agrochemicals.13 The
presence of a fluoro group (C–F group) due to low polarizability
and high lipophilicity induces a relative metabolic stability and im-
proves the bioavailability of the modified heterocycles compared
to its hydrocarbon analogues.14,15
In continuation of our effort to establish green methodologies in
nitrone cycloaddition reactions,16–20 herein we wish to report the
use of ionic liquids as recyclable solvents for 1,3-dipolar cycloaddi-
tion reactions of N-benzyl fluoro nitrone (having vast synthetic
potentials) with electron deficient alkynes and alkenes (enals) to
produce novel isoxazoline and isoxazolidine derivatives in a one-
pot operation (Scheme 1).
an example, the reaction between 1 and alkynes, afforded the
cycloaddition derivative 2 after 17 h in CH2Cl2 in 67% yield and
93% yield (entry 1) in [bmim]BF4 at room temperature after
26 min respectively. In a typical procedure 1 mmol of nitrone
was mixed with 1 equiv of alkynes in [bmim]BF4 (2 ml) under stir-
ring, at room temperature. After the development of nitrone (mon-
itored by TLC), 1 mmol of dipolarophile was added and the
progress of the reaction was monitored by TLC. After completion
of reaction, the reaction mixture was washed with diethyl ether
(3 Â 10 ml). The combined ether extracts were concentrated in va-
cuo and the resulting product was directly charged on silica gel
column and eluted with a mixture of ethyl acetate:n-hexane
(1:8) to afford pure isoxazoline. The rest of the viscous ionic liquid
was further washed with diethyl ether and dried at 80 °C under re-
duced pressure to retain its activity in subsequent runs and was re-
used up to five times without loss of activity or selectivity after five
cycles. We have intentionally stopped the recycle at the fifth cycle,
however we are convinced that this process may be carried on
many more times. Several butylmethylimidazolium based ILs,
[bmim]X, with varying anions (X = PF6À, BrÀ, BF4À) were screened
for this reaction. Evidently, [bmim]BF4 was found to be superior
in terms of yield (93%) and reaction time (26 min) as compared
with [bmim]PF6 (84%; 43 min; entry 1). For optimizing the condi-
tions, we used the substrates in different ratios. It was found that
best results were obtained using 1:1 reactant ratio. The reaction
in [bmim]BF4 was also conducted at elevated temperatures for
optimizing the conditions and no significant improvements were
observed in yields and reaction times. We examined the reaction
under neat condition also, without using IL, to demonstrate the
catalytic ability of [bmim]BF4 This result clearly indicates that
[bmim]BF4 has significant catalytic role in this reaction (Table 1).
The addition of nitrone 1 to alkynes and alkenes (enals) can be
rationalized by an exo and endo approach of the nitrone which
has the Z configuration (transition state I and II).21
Compared to conventional conditions the cycloaddition reac-
tions performed in ionic liquids are much faster and selective. As
Bn
H
R
[bmim]BF4
r.t
N+
C6H5CH2NHOH
RCHO
Bn
C
O-
1
CHO
R2
R
H3
N
2
O
3
1
R1
5
4
Bn
R1
R2
R
H3
N
2
2
O
3
4
1
5
H3C
(endo)
3
H
CHO
H
R = 2,6 difluoro, 2-fluoro, 3,4-difluoro,
C6H5H2C
R
O
C
R
C
4-fluoro, 2-hydroxy, 4-hydroxy benzene/ phenyl
R1 = C6H5 ; R2 = COOCH3
R1 = COOCH3 ; R2 = COOCH3
R1 = R2 = COOH
N
CH2C6H5
R2
N
O
R1
Bn = Benzyl
H3C
TS II
TS I
CHO
Scheme 1.
Table 1
1,3-Dipolar cycloaddition reactions of N-benzyl fluoro nitrone with alkynes in ionic liquid.
Entry
Nitronea
Alkyne
Productb (2)
CH2C6H5
Time (min)
26 (1020)
Yieldc (%)
93 (67)
F
Ph
COOCH3
CH2C6H5
H
C
F
N+
F
O-
N
2
O
3
1
F
H3
1
5
4
Ph
COOCH3
F
HOOC
COOH
CH2C6H5
CH2C6H5
H
C
F
N+
F
O-
N
2
O
3
1
2
F
H3
35 (1080)
91 (64)
5
4
HOOC
COOH

B. Chakraborty, G. P. Luitel / Tetrahedron Letters 54 (2013) 765–770
767
Table 1 (continued)
Entry
Nitronea
Alkyne
Productb (2)
Time (min)
28 (1080)
Yieldc (%)
F
H3COOC
COOCH3
CH2C6H5
CH2C6H5
H
C
F
N+
F
O-
N
2
O
1
3
F
3
89 (63)
90 (66)
93 (62)
90 (61)
88 (61)
92 (62)
90 (59)
88 (57)
H3
5
4
H3COOC
CH2C6H5
COOCH3
CH2C6H5
Ph
COOCH3
H
C
F
N+
O-
F
N
2
O
3
1
4
28 (1140)
33 (1140)
35 (1260)
32 (1200)
24 (1320)
35 (1260)
36 (1380)
H3
5
4
Ph
COOCH3
CH2C6H5
CH2C6H5
N+
O-
H3COOC
HOOC
Ph
COOCH3
H
C
F
F
N
2
O
3
1
5
H3
5
4
H3COOC
COOCH3
CH2C6H5
N+
O-
COOH
CH2C6H5
H
C
F
F
N
2
O
1
3
6
H3
5
4
HOOC
COOH
CH2C6H5
CH2C6H5
N
COOCH3
H
F
C
N+
N+
N+
O-
2
O
1
3
F
7
H3
5
4
Ph
COOCH3
CH2C6H5
CH2C6H5
O-
H3COOC
COOCH3
H
H
H
F
C
N
2
O
3
F
1
8
H3
5
4
H3COOC
COOCH3
F
CH2C6H5
O-
HOOC
COOH
CH2C6H5
C
N
2
3
O
1
F
9
H3
5
4
HOOC
COOH
F
Ph
COOCH3
CH2C6H5
CH2C6H5
N
F
C
N+
O-
2
F
O
1
3
10
H3
F
5
4
Ph
COOCH3
(continued on next page)

768
B. Chakraborty, G. P. Luitel / Tetrahedron Letters 54 (2013) 765–770
Table 1 (continued)
Entry
Nitronea
Alkyne
Productb (2)
Time (min)
36 (1320)
Yieldc (%)
89 (57)
H3COOC
COOCH3
F
CH2C6H5
CH2C6H5
N
H
N+
O-
F
C
2
F
O
1
3
11
H3
F
5
4
H3COOC
COOCH3
HOOC
COOH
F
CH2C6H5
CH2C6H5
H
N+
O-
F
C
N
2
3
O
12
13
14
15
1
27 (1380)
25 (960)
23 (900)
40 (1200)
85 (55)
92 (66)
90 (67)
87 (62)
F
H3
5
4
F
HOOC
COOH
CH2C6H5
HO
CH2C6H5
Ph
COOCH3
H
C
N+
O-
N
2
O
1
3
OH
H3
5
4
Ph
COOCH3
CH2C6H5
HOOC
COOH
CH2C6H5
H
N+
OH
C
O-
N
2
3
O
1
HO
H3
5
4
HOOC
COOH
CH2C6H5
CH2C6H5
H3COOC
COOCH3
H
C
N+
O-
N
2
3
O
1
H3
5
4
H3COOC
COOCH3
a
b
c
Reaction conditions: nitrone (1 mmol), alkynes (1 equiv), [bmim]BF4 (2 ml), N2 atmosphere, rt.
All products were characterized by IR,1H NMR, 13C NMR and MS spectral data.
Isolated yield after purification. Figures in parentheses indicate reactions performed in conventional methods.
Furthermore, the novel isoxazoline derivatives (2) are found to
Treatment of N-benzyl fluoro nitrone with methacrolein in
[bmim]BF4 for 28 min gave the corresponding 5-substituted endo
isoxazolidine (entry 1, product 3) in 90% yield with excellent regi-
oselectivity (Table 2, entry 1). However, in the absence of ionic liq-
uids, the reaction did not yield any product even after a long
reaction time (30 h). Under conventional conditions (CH2Cl2 as sol-
vent, rt, 42 h), the products were obtained as a mixture of endo and
exo-isomers (80:20) favouring the endo-diastereomer. Signals for
the endo and exo diastereomers were assigned by 1H NMR analy-
sis.23 Nitrones bearing benzyl groups on the nitrogen atom are ex-
have vast synthetic potential as they could be converted into 1,3
difunctional amino alcohols (Scheme 2). Studies are in progress.
Nitrone 1 may also be used as an oxidizing reagent in the con-
version of alkyl halides to aldehydes and ketones (Scheme 3) fol-
lowing a pattern of atom efficient reactions reported by our
group.16 We have already reported the synthesis of various alde-
hydes and ketones from alkyl halides using
a-chloro nitrones in
atom efficient reactions.16,22
To explore the potential of this procedure we have extended the
protocol, with activated alkenes (enals) and to N-benzyl–C-phenyl
nitrones (with fluoro and hydroxy derivatives in phenyl ring) for
the synthesis of exclusively endo isoxazolidine and isoxazolines
in rather good conversions and yields (Tables 1 and 2).
R3
Bn
-O
R
C
H
X
R
N
Bn
R4
N+
C
C
H
R3CHO / R3 R4CO +
SN2
R1
R2
R1
R2
H
R1
Zn/CH3COOH
OH
O
O
Hydrolysis
H
H
Bn
N
N
Bn
N
Bn
R2
R
H
R
RCHO + BnNH2
H
R
H
Scheme 2.
Scheme 3.

B. Chakraborty, G. P. Luitel / Tetrahedron Letters 54 (2013) 765–770
769
Table 2
1,3-Dipolar cycloaddition reactions of N-benzyl fluoro nitrone with alkenes (enals) in ionic liquid.
Entry
Nitronea
Alkyne
Productb (3)
CH2C6H5
Time (min)
28 (2520)
Yieldc (%)
F
CH3
CH2C6H5
H
C
F
N+
F
O-
N
2
CHO
O
3
1
F
1
90 (62)
88 (60)
88 (57)
84 (57)
H3
5
4
H3C
CHO
CH2C6H5
CH2C6H5
N+
O-
CH3
H
C
F
F
N
2
CHO
O
3
1
2
3
4
30 (2640)
33 (2640)
32 (2640)
H3
5
4
H3C
CHO
CH2C6H5
CH2C6H5
N+
O-
CH3
H
C
F
N
2
CHO
O
3
F
1
H3
5
4
H3C
CHO
CH2C6H5
CH3
CH2C6H5
F
H
N+
O-
F
C
N
2
3
CHO
O
F
1
H3
F
5
4
H3C
CHO
a
b
c
Reaction conditions: nitrone (1 mmol), methacrolein (1 equiv), [bmim]BF4 (2 ml), N2 atmosphere, rt.
All products were characterized by IR, 1H NMR, 13C NMR and MS spectral data.
Isolated yield after purification. Figures in parentheses indicate reactions performed in conventional methods.
tremely valuable for applications in synthesis as these moieties act
as versatile protecting groups. We are pleased to find that the N-
benzyl fluoro nitrones afforded the expected endo substituted isox-
azolidines (3) with high yields. The anticipated 1,3-dipoles exhibit
enhanced reactivity in ionic liquid thereby reducing the reaction
times and improving the yields significantly. Furthermore, the io-
nic liquids were found to give better regioselectivity than organic
solvents. In addition, these molten salts could be easily recovered
on work-up. Since the products are fairly soluble in ionic media;
they could be easily extracted with ether. The rest of the ionic liq-
uids was further washed with ether and recycled in three to four
subsequent runs without loss of activity. Enhanced reaction rates,
excellent yields and high cis-selectivity are the features observed
in these ionic solvents. All products were characterized by 1H
NMR, IR and mass spectroscopic data.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
11.111.
References and notes
1. Frederickson, M. Tetrahedron 1997, 53, 403–425.
2. For
a general review of this area see: (a) Tufariello, J. J. 1,3-Dipolar
Cycloaddition Chemistry In Padwa, A., Ed.; Wiley: New York, 1984; (b)
Torssell, K. B. G. Nitrile Oxides, Nitrones and Nitronates in Organic Synthesis;
VCH: New York, 1988.
3. (a) Seerden, J. P. G.; Boeren, M. M. M.; Scheeren, H. W. Tetrahedron 1997, 53,
11843–11852; (b) Gothelf, K. V.; Jørgensen, K. A. Chem. Rev. 1998, 98, 863–910.
4. Patterson, W.; Cheung, P. S.; Ernest, M. J. J. Med. Chem. 1992, 35, 507–511.
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6. Confalone, P. N.; Huie, E. M. Org. React. 1988, 36, 1–173. Chapter 1.
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Angew. Chem., Int. Ed. 2000, 39, 3772–3777.
In conclusion, we have shown that 1,3-dipolar cycloadditions of
fluoro nitrones with alkene and alkynes may be conveniently car-
ried out in RTILs with the obtainment of corresponding novel flu-
oro isoxazolidine and isoxazolines in good conversions and
yields24,25 with high synthetic potentials. The ionic liquid may be
recycled several times without loss of activity or selectivity.
8. Sheldon, R. Chem. Commun. 2001, 2399–2403.
9. Gordon, C. M. Appl. Catal., A: Gen. 2001, 222, 101–105.
10. (a) Yadav, J. S.; Reddy, B. V. S.; Baishya, G. J. Org. Chem. 2003, 68, 7098–7104; (b)
Yadav, J. S.; Reddy, B. V. S.; Reddy, J. S. S.; Srinivas Rao, R. Tetrahedron 2003, 59,
1599–1603.
11. See Bach, D. Angew. Chem., Int. Ed. Engl. 1990, 29, 1320–1325.
12. Welch, J. T. Tetrahedron 1987, 43, 3123–3128.
13. Cao, S.; Qian, X. H.; Song, G. H.; Huang, Q. C. J. Fluorine Chem. 2002, 63, 117–123.
14. Arnone, A.; Bernardi, R.; Blasco, F.; Cardillo, R.; Gerus, I. I.; Kukhar, V. P.
Tetrahedron 1998, 54, 2809–2814.
15. (a) Chambers, R. D. Fluorine in Organic Chemistry; Blackwell Publishing: Oxford,
UK, 2004; (b) Kirsch, P. Modern Fluoroorganic Chemistry; Wiley-VCH:
Weinheim, Germany, 2004.
Acknowledgements
We are pleased to acknowledge the financial support from the
Department of Science & Technology, Government of India, New
Delhi (Grant No. SR/S1/OC-34/2011). We are also grateful to CDRI,
Lucknow for providing spectral data.
16. Chakraborty, B.; Sharma, P. K. Synth. Commun. 2012, 42, 1804–1812.

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1260–1265.
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dx.doi.org/10.1080/17518253.2012.720719, publishing online soon.
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(aromatic carbons). Spectroscopic data for isoxazoline 2 (entry 1): IR (KBr):
max 3010 (m), 2246 (m), 1813 (m), 1774 (s), 1610 (s), 1480 (s), 1324 (s), 1215
(s), 1105 (s), 993 (m), 782 (s) cmÀ1 1H NMR (CDCl3): d 7.87–7.80 (m, 3H,
C6H3F2), 7.68–7.21 (m, 2 Â 5H, C6H5), 3.38 (s, 3H, –COOCH3), 2.68 (s, 2H,
C6H5CH2), 1.25 (s, 1H, C3H). 13C NMR (CDCl3): d 168.52 (–COOCH3), 137.20,
137.04, 136.87, 136.66, 135.65, 135.48, 135.20, 134.93, 132.77, 132.35, 132.08,
131.78, 130.80, 129.90 (aromatic carbons), 88.16 (C5), 73.60 (C3), 58.45 (C4),
45.17 (–COOCH3), 36.80 (benzylic carbon). FAB-MS (m/z): 407 (M+), 330, 294,
211 (bp), 203, 105, 91, 77. Anal. Calcd. for C24H19O3F2N: C, 70.76; H, 4.66; N,
3.43%. Found: C, 70.63; H, 4.61; N, 3.35%.
m
.
25. Representative experimental procedure for fluoro isoxazolidine synthesis (Table 2;
entry 1): 2,6-Difluoro benzaldehyde (1 mmol) and N-benzylhydroxylamine
(1 equiv) were added to [bmim]BF4 (2 mL) in a 10 mL conical flask, mixed
thoroughly and stirred at rt for 2 h. The formation of nitrone was monitored by
TLC (Rf = 0.40). Methacrolein (1 mmol) was added dropwise, by syringe, at rt at
the time of development of nitrone and the reaction mixture was further
stirred at room temperature for an appropriate time (Table 2). After
completion of reaction, as indicated by TLC (Rf = 0.50), the reaction mixture
was washed with diethyl ether (3 Â 10 ml). The combined ether extracts were
concentrated in vacuo and the resulting product was directly charged on silica
gel column and eluted with a mixture of ethyl acetate:n-hexane (1:8) to afford
pure isoxazolidine 3 (Table 2, entry 1, 90%). The rest of the viscous ionic liquid
was further washed with ether and dried at 80 °C under reduced pressure to
retain its activity in subsequent runs. Spectroscopic data for isoxazolidine 3
23. Andrei, B.; Kundig, P. E. Org. Biomol. Chem. 2012, 10, 114–121.
24. Representative experimental procedure for nitrone and fluoro isoxazoline synthesis
(Table 1; entry 1): 2,6-Difluoro benzaldehyde (1 mmol) and N-
benzylhydroxylamine (1 equiv) were added to [bmim]BF4 (2 mL) in a 10 mL
conical flask, mixed thoroughly and stirred at rt for 2 h. The formation of
nitrone was monitored by TLC (Rf = 0.40). The nitrone was isolated as a white
crystalline solid (mp 42 °C, uncorrected) following the methodology as
adopted in the following cycloaddition reactions. As the nitrone decomposes
on keeping at room temperature, in situ reactions were performed with
alkynes. Methyl phenyl propiolate (1 mmol) was added at the time of
development of nitrone and the reaction mixture was further stirred at room
temperature for an appropriate time (Table 1). After completion of reaction, as
indicated by TLC (Rf = 0.58), the reaction mixture was washed with diethyl
ether (3 Â 10 mL). The combined ether extracts were concentrated in vacuo
and the resulting product was directly charged on silica gel column and eluted
with a mixture of ethyl acetate:n-hexane (1:8) to afford pure isoxazoline 2
(Table 1, entry 1, 93%). The rest of the viscous ionic liquid was further washed
with ether and dried at 80 °C under reduced pressure to retain its activity in
subsequent runs. Spectroscopic data for nitrone 1: UV kmax 238 nm; IR (KBr):
(entry 1): IR (KBr):
m
max 2994 (m), 2970 (m), 1730 (s), 1615 (s), 1462 (s), 1324
(s), 1145 (s), 980 (m), 788 (s) cmÀ1
.
1H NMR (300 MHz, CDCl3): d 9.82 (s, 1H,
CHO), 8.13–7.86 (m, 3H, C6H3F2), 7.64–7.33 (m, 5H, C6H5), 3.89 (dd, 1H, J = 5.20,
5.60 Hz, C4H), 3.71 (dd, 1H, J = 5.26, 5.60 Hz, C4H, endo), 2.95 (s, 2H, C6H5CH2),
1.60 (s, 3H, CH3), 1.27 (t, 1H, J = 5.14 Hz, C3H). 13C NMR (75 MHz, CDCl3): d
203.24 (–CHO), 134.53, 134.38, 134.28, 134.17, 132.12, 132.04, 131.95, 131.90,
131.86, 131.72 132 (aromatic carbons), 80.57(C5), 74.22 (C3), 55.40 (C4), 34.90
(benzylic carbon), 23.17 (CH3). FAB-MS (m/z): 317 (M+), 226, 204, 113 (bp), 83,
91, 77. Anal. Calcd. for C18H17O2F2N: C, 68.11; H, 5.39; N, 4.41%. Found: C,
68.02; H, 5.21; N, 4.28%.
m
max 2965 (m), 1610 (s), 1440 (m), 1154 (m), 784 (s) cmÀ1 1H NMR (300 MHz,
.
CDCl3): d 7.96–7.79 (m, 3H, C6H3F2), 7.67–7.35 (m, 5H, C6H5CH2), 6.98 (s, 1H, –
CH@N+), 3.37 (s, 2H, C6H5CH2). 13C NMR (75 MHz, CDCl3): d 142.04 (CH@N+),
134.80, 134.34, 134.12, 133.93, 131.60, 130.00, 129.55, 129.46, 128.67, 128.22