Nucleophilic reactions with α-trifluoromethyl imine and N,O-disubstituted aminal: Synthesis of β-trifluoromethyl β-anilino esters

Article abstract of DOI:10.1016/S0022-1139(01)00437-7

The Reformatsky reaction with 4-methoxy-N-(2,2,2-trifluoroethylidene)aniline (1) proceeds under mild conditions to yield ethyl 4,4,4-trifluoro-3-(N-4-methoxyphenyl)amino-2-alkylbutyrate (5) and 4-trifluoromethyl-3-alkyl-1-(4-methoxyphenyl)-2-azetidinone (

Full text of DOI:10.1016/S0022-1139(01)00437-7

Journal of Fluorine Chemistry 111 ,2001) 77±80
Preliminary note
Nucleophilic reactions with a-tri¯uoromethyl imine and
N,O-disubstituted aminal: synthesis of
b-tri¯uoromethyl b-anilino esters
Yuefa Gong*, Katsuya Kato
National Institue of Advanced Industrial Science and Technology ꢀAIST), Hirate-cho, Kita-ku, Nagoya 462-8510, Japan
Received 15 January 2001; received in revised form 10 May 2001; accepted 21 June 2001
Abstract
The Reformatsky reaction with 4-methoxy-N-,2,2,2-tri¯uoroethylidene)aniline ,1) proceeds under mild conditions to yield ethyl 4,4,4-
tri¯uoro-3-,N-4-methoxyphenyl)amino-2-alkylbutyrate ,5) and 4-tri¯uoromethyl-3-alkyl-1-,4-methoxyphenyl)-2-azetidinone ,6) as the
main products. Similarly, the reaction of diethyl malonate with 1 in the presence of sodium hydride affords a high yield of ethyl 4,4,4-
tri¯uoro-3-,N-4-methoxyphenyl)amino-2-carbethoxybutyrate ,5d), which is also provided in the reaction of diethyl malonate with 4-
methoxy-N-,1-ethoxy-2,2,2-tri¯uoroethyl)aniline ,2). The Reformatsky reactions of 4a±c with N-,1-ethoxy-2,2,2-tri¯uoroethyl)aniline ,3)
also proceed to give the corresponding ethyl 4,4,4-tri¯uoro-3-,N-phenyl)amino-2-alkylbutyrates ,11a±c). # 2001 Elsevier Science B.V. All
rights reserved.
Keywords: Reformatsky reaction; a-Tri¯uoromethyl imine; a-Tri¯uoromethyl N,O-disubstituted aminal; b-Anilino ester
1. Introduction
important intermediate in the syntheses of the highly bioac-
tive quinoline group of antimalarial and antibacterial drugs
[13]. For this reason, we have carried out the Reformatsky
reaction with 1. During the experiments, the corresponding
reaction with N,O-disubstituted aminals 2 and 3 was also
examined. The observed results are described in Section 2.
In recent years, the preparation of ¯uorinated amino acids
has received a great deal of interest because of their unique
chemical and biological properties [1]. Several synthetic
approaches of b-tri¯uoromethyl b-amino acids have been
reported [2±6]. N-,p-methoxyphenyl)imine of tri¯uoroace-
taldehyde 1 is an useful reagent for this purpose. The
reactions of 1 with lithium enolate of ethyl dibenzylami-
noacetate and with phenoxyacetyl chloride afforded good
yields of tri¯uoromethyl-substituted b-lactams [6,7]. In
addition, the nucleophilic addition with chiral Ni,II) com-
plex of the Schiff base of glycine [8] and with chiral
sulphoxide [9] gave b-tri¯uoromethyl-b-,4-methoxypheny-
l)amino aminoacids and a-tri¯uoromethyl b-sulphinyla-
mines, respectively, with excellent stereoselectivity.
However, little has been studied on its Reformatsky reaction,
although the reaction of non-¯uorinated imine or its syn-
thetic equivalents with ethyl bromodi¯uoroacetate was
reported [10,11]. On the other hand, b-anilino carboxylic
esters formed in the Reformatsky reaction are useful for the
preparation of 2,3-dihydroquinolones [12]. The latter is an
2. Experimental details
1H NMR spectra were recorded with tetramethylsilane
,TMS) as an internal standard at 90 MHz on a Hitachi R-90H
FT spectrometer. 19F NMR spectra were recorded with
hexa¯uorobenzene as an internal standard at 84.7 MHz on
the same spectrometer. Mass spectra ,70 eV) were measured
on a Hitachi M-80 instrument. High-resolution mass spectra
were measured on a JEOL JMS-SX102A MS spectrometer.
Zinc dust was activated by dilute aqueous hydrochloric acid,
and then washed free of the acid with distilled water, free of
water with acetone and free of acetone with anhydrous ether.
After dried under vacuum, the zinc was ready for use in these
experiments.
In a typical procedure of the Reformatsky reaction, a
mixture of 2.03 g ,10 mmol) of 4-methoxy-N-,2,2,2-tri¯uor-
oethylidene)aniline ,1), 1.84 g ,11 mmol) of ethyl 2-bro-
moacetate and 0.72 g ,11 mmol) of activated zinc dust in
* Corresponding author. Tel.: ‡81-52-911-3296; fax: ‡81-52-911-2428.
E-mail addresses: gong@nirin.go.jp ,Y. Gong), ktykato@nirin.go.jp
,K. Kato).
0022-1139/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 , 0 1 ) 0 0 4 3 7 - 7

78
Y. Gong, K. Kato / Journal of Fluorine Chemistry 111 ꢀ2001) 77±80
20 ml of anhydrous toluene was heated with vigorous stir-
ring at 1108C for 4 h.
,2H, d, J ˆ8.9 Hz), 6.87 ,2H, d, J ˆ 8:9 Hz), 4.17 ,1H,
m), 3.78 ,3H, s), 3.32 ,1H, d, J ˆ 7:0, 2.2 Hz), 1.84 ,2H, m,
J ˆ 7:0 Hz), 1.08 ,3H, t, J ˆ 7:0 Hz). 19F NMR ,CDCl3) d
‡
89.26 ,3F, d, J ˆ 6:8 Hz). MS m/e 273 ,M , 56), 203 ,51),
3. Results and discussion
149 ,100), 134 ,72). HRMS Found: 273.0975. Calc. for
C13H14F3NO2: 273.0977. 5d: Yellowish oil 1H NMR
,CDCl3) d 6.73 ,4H, s), 4.68 ,1H, br, s), 4.60 ,1H, m),
4.24 ,2H, q, J ˆ 7:2 Hz), 4.12 ,2H, q, J ˆ 7:2 Hz), 3.81
,1H, d, J ˆ 3:6 Hz), 3.74 ,3H, s), 1.27 ,3H, t, J ˆ 7:2 Hz),
1.15 ,3H, t, J ˆ 7:2 Hz). 19F NMR ,CDCl3) d 87.80 ,3F, d,
Imine 1, N-,2,2,2-tri¯uoroethylidene)-4-methoxyaniline,
was usually prepared from tri¯uoroacetaldehyde ethyl hemi-
acetal ,TFAE) and 4-methoxyaniline in the presence of
catalytic amounts of toluenesulphonic acid [6]. Recently,
‡
Ê
Ingrassia and Mulliez reported that the use of 5 A molecular
J ˆ 6:8 Hz). MS m/e 363 ,M , 100), 294 ,25), 204 ,51), 176
sieves markedly improves the yield of 1 instead of toluene-
sulphonic acid [14]. In our experiments, however, 4-
methoxy-N-,1-ethoxy-2,2,2-tri¯uoroethyl)aniline ,2) was
also afforded in 44% yield besides 1 in 40% yield. Under
the same conditions, N-,1-ethoxy-2,2,2-tri¯uoroethyl)ani-
line ,3) was generated in 81% yield in the reaction of aniline
with TFAE ,Scheme 1) 3: a colourless liquid, bp 72±758C/
,13), 134 ,11). HRMS Found: 363.1293. Calc. for
C16H20F3NO5: 363.1294. 7: colourless oil 1H NMR ,CDCl3)
d 4.25 ,4H, q, J ˆ 7:2 Hz), 4.21 ,4H, q, J ˆ 7:2 Hz), 4.05
,1H, m), 3.93 ,2H, d, J ˆ 6:6 Hz), 1.29 ,6H, t, J ˆ 7:2 Hz),
1.27 ,6H, t, J ˆ 7:2 Hz). 19F NMR ,CDCl3) d 94.95 ,3F, d,
‡
J ˆ 8:7 Hz). MS m/e 400 ,M , 2), 355 ,31), 326 ,18), 309
,83), 281 ,14), 263 ,69), 235 ,31), 195 ,40), 160 ,100).
HRMS Found: 400.1342 Calc. for C16H23F3O8: 400.1345.
9b: Yellowish oil 1H NMR ,CDCl3) d 6.73 ,4H, s), 4.68 ,1H,
br, s), 4.60 ,1H, q, J ˆ 7:0 Hz), 4.25 ,2H, q, J ˆ 7:2 Hz),
4.18 ,2H, q, J ˆ 7:2 Hz), 3.74 ,3H, s), 1.62 ,3H, s), 1.28
,3H, t, J ˆ 7:2 Hz), 1.20 ,3H, t, J ˆ 7:2 Hz). 19F NMR
1
8 mmHg. H NMR ,CDCl3) d 7.22 ,2H, m), 6.75 ,3H, m),
5.06 ,1H, dq, J ˆ 10:5, 5.8 Hz), 4.20 ,1H, br, d,
J ˆ 10:5 Hz), 3.75 ,2H, m), 1.20 ,3H, t, J ˆ 7:2 Hz). 19F
‡
NMR ,CDCl3) 81.50 ,3F, d, J ˆ 5:8 Hz). MS m/e 219 ,M ,
79), 173 ,58), 150 ,100.0). 5a: Yellowish oil 1H NMR
,CDCl3) d 6.73 ,4H, s), 4.35 ,1H, m), 4.13 ,2H, q,
J ˆ 7:0 Hz), 3.74 ,3H, s), 3.68 ,1H, br), 2.72 ,1H, d,
J ˆ 6:4 Hz), 2.64 ,1H, d, J ˆ 8:8 Hz), 1.20 ,3H, t,
‡
,CDCl3) d 87.83 ,3F, d, J ˆ 7:0 Hz). MS m/e 377 ,M , 56),
363 ,42), 308 ,12), 204 ,100), 174 ,19), 123 ,17). HRMS
Found: 377.1448. Calc. for C17H22F3NO5: 377.1450. 9c:
Yellowish oil 1H NMR ,CDCl3) d 6.77 ,2H, d, J ˆ 9:0 Hz),
6.63 ,2H, d, J ˆ 9:0 Hz), 4.60 ,1H, q, J ˆ 7:0 Hz), 4.50
,1H, br, s), 4.23 ,2H, q, J ˆ 7:2 Hz), 4.07 ,2H, q,
J ˆ 7:2 Hz), 3.73 ,3H, s), 2.20 ,3H, s), 1.25 ,3H, t,
J ˆ 7:2 Hz), 1.14 ,3H, t, J ˆ 7:2 Hz). 19F NMR ,CDCl3)
J ˆ 7:0 Hz). 19F NMR ,CDCl3)
d 85.82 ,3F, d,
‡
J ˆ 7:7 Hz). MS m/e 291 ,M , 100), 222 ,80), 204 ,43),
148 ,56), 134 ,60). HRMS Found: 291.1081. Calc. for
C13H16F3NO3: 291.1082. 6a: Yellowish oil 1H NMR
,CDCl3) d 7.34 ,2H, d, J ˆ 9:0 Hz), 6.87 ,2H, d,
J ˆ 9:0 Hz), 4.50 ,1H, m), 3.78 ,3H, s), 3.25 ,1H, d,
J ˆ 5:7 Hz), 3.20 ,1H, d, J ˆ 3:1 Hz). 19F NMR ,CDCl3)
‡
d 91.93 ,3F, d, J ˆ 7:0 Hz). MS m/e 421 ,M , 87), 379 ,20),
337 ,21), 204 ,100), 176 ,23), 123 ,14). HRMS Found:
421.1344. Calc. for C18H22F3NO7: 421.1348. 10: Colourless
oil 1H NMR ,CDCl3) d 7.19 ,2H, m), 6.77 ,3H, m), 5.04 ,1H,
br), 4.80 ,1H, m), 4.24 ,2H, q, J ˆ 7:2 Hz), 4.11 ,2H, q,
J ˆ 7:2 Hz), 3.84 ,1H, d, J ˆ 4:2 Hz), 1.27 ,3H, t,
J ˆ 7:2 Hz), 1.12 ,3H, t, J ˆ 7:2 Hz). 19F NMR ,CDCl3)
‡
d 88.15 ,3F, d, J ˆ 7:7 Hz). MS m/e 245 ,M , 100), 203
,78), 150 ,16), 134 ,83). HRMS Found: 245.0661. Calc. for
1
C11H10F3NO2: 245.0664. 6b: White needles, mp 868C. H
NMR ,CDCl3) d 7.36 ,2H, d, J ˆ 9:1 Hz), 6.88 ,2H, d,
J ˆ 9:1 Hz), 4.51 ,1H, m, J ˆ 6:3 Hz), 3.79 ,3H, s), 3.66
‡
,1H, m, J ˆ 7:4, 6.3 Hz), 1.45, 1.43 ,3H, d, J ˆ 7:4 Hz). 19
F
d 87.71 ,3F, d, J ˆ 6:8 Hz). MS m/e 333 ,M , 100), 264
NMR ,CDCl3) d 94.10 ,3F, d, J ˆ 6:3 Hz). MS m/e 259
,49), 174 ,38), 146 ,28), 104 ,19). HRMS Found: 333.1188.
1
‡
,M , 62), 203 ,67), 149 ,60), 134 ,100). HRMS Found:
Calc. for C15H18F3NO4: 333.1188. 11a: Colourless oil H
259.0820. Calc. for C12H12F3NO2: 259.0820. 5c: Yellowish
oil 1H NMR ,CDCl3) d 6.79 ,2H, d, J ˆ 9:1 Hz), 6.64 ,2H,
d, J ˆ 9:1 Hz), 4.24 ,1H, m), 4.16 ,2H, q, J ˆ 7:2 Hz), 3.74
,3H, s), 3.50 ,1H, br), 2.63 ,1H, m, J ˆ 7:2, 5.1 Hz), 1.78
,2H, m, J ˆ 7:2 Hz), 1.24 ,3H, t, J ˆ 7:2 Hz), 0.85 ,3H, t,
J ˆ 7:2 Hz). 19F NMR ,CDCl3) d 88.12 ,3F, d, J ˆ 7:8 Hz).
NMR ,CDCl3) d 7.20 ,2H, m), 6.72 ,3H, m), 4.43 ,1H, m),
4.12 ,2H, q, J ˆ 7:2 Hz), 3.80 ,1H, d, br, J ˆ 8:4 Hz), 2.76
,1H, d, J ˆ 4:8 Hz), 2.66 ,1H, d, J ˆ 8:4 Hz), 1.18 ,3H, t,
J ˆ 7:2 Hz). 19F NMR ,CDCl3) d 85.72 ,3F, d, J ˆ 7:1 Hz).
‡
MS m/e 261 ,M , 100), 192 ,19), 174 ,28), 93 ,43). HRMS
Found: 261.0975, Calc. for C12H14F3NO2: 261.0977. 11b:
Colourless oil 1H NMR ,CDCl3) d 7.20 ,2H, m), 6.72 ,3H,
m), 4.52 ,1H, m), 4.09 ,2H, q, J ˆ 7:2 Hz), 3.80 ,1H, d, br,
J ˆ 8:4 Hz), 2.93 ,1H, m, J ˆ 7:3 Hz), 1.29 ,3H, d,
‡
MS m/e 319 ,M , 69), 250 ,24), 204 ,100), 176 ,32), 134
,11). HRMS Found: 319.1390. Calc. for C15H20F3NO3:
319.1395. 6c: Yellowish oil 1H NMR ,CDCl3) d 7.36
Scheme 1.

Y. Gong, K. Kato / Journal of Fluorine Chemistry 111 ꢀ2001) 77±80
79
Scheme 2.
J ˆ 7:3 Hz), 1.17 ,3H, t, J ˆ 7:2 Hz). 19F NMR ,CDCl3) d
fraction 4-methoxyaniline. However, 19F NMR spectrum of
the second fraction clearly shows two absorption signals ,d)
appearing at 87.83 and 95.24 at the ratio of about 0.8:1 with
hexa¯uorobenzene as the internal standard. This means the
existence of two different components, respectively, with a
tri¯uoromethylgroup. MS, 1H and 19F NMRanalysesdemon-
strated that the absorption at 87.83 is assigned to 5d, and that
at 95.24 is attributed to 7 ,Table 2, run 1).
Further experiment demonstrated that the above reaction
also proceeded smoothly in the absence of zinc chloride.
When the reaction was performed at 1108C for 3 h, 5d
formed in high yield ,Scheme 2), whereas only a trace
amount of 7 was detected ,run 2). It was found that 5d
readily decomposed into 7 when it was heated together with
diethyl malonate and sodium hydride in the presence of zinc
chloride at about 908C. This result implies that 7 may be
generated along with the route described in Scheme 3, where
the elimination of 4-anisidinide anion was greatly promoted
in the presence of zinc chloride. Further experiment showed
that the reaction readily occurred even at room temperature,
5d being the only product ,runs 3±4). Under the same
conditions, the reaction between 1 and diethyl 2-substituted
malonates ,8b and c) also took place readily, respectively,
giving 9b and c in moderate yields ,runs 5±6).
‡
88.59 ,3F, d, J ˆ 7:1 Hz). MS m/e 275 ,M , 79), 206 ,29),
174 ,100), 132 ,26), 93 ,63). HRMS Found: 275.1133. Calc.
1
for C13H16F3NO2: 275.1133. 11c: Colourless oil H NMR
,CDCl3) d 7.20 ,2H, m), 6.71 ,3H, m), 4.42 ,1H, m), 4.16
,2H, q, J ˆ 7:2 Hz), 3.74 ,1H, d, br, J ˆ 8:4 Hz), 2.64 ,1H,
dt, J ˆ 8:1, 5.1 Hz), 1.85 ,2H, m), 1.24 ,3H, t, J ˆ 7:2 Hz),
0.92 ,3H, t, J ˆ 7:2 Hz). 19F NMR ,CDCl3) d 88.01 ,3F, d,
‡
J ˆ 7:0 Hz). MS m/e 289 ,M , 81), 220 ,15), 174 ,100),
146 ,26), 93 ,48). HRMS Found: 289.1286. Calc. for
C14H18F3NO2: 289.1290.
The Reformatsky reaction with 1 was carried out by
heating a mixture of 1 ,10 mmol), a-bromocarboxylic ester
,4) ,11 mmol) and activated zinc dust ,11 mmol) in anhy-
drous toluene ,15 ml) at 1108C. Product analysis showed
that b-amino ester ,5) and b-lactam ,6) are the main product
in the case of 4a±c ,Scheme 2), and the yields are given in
Table 1 ,runs 1±4). Under the same conditions, the reaction
of diethyl 2-bromomalonate ,4d) did not furnish such pro-
ducts as 5d and 6d.
An alternative way was therefore tried for preparing 5d. A
mixture of 1 ,10 mmol), sodium salt of diethyl malonate
,11 mmol) generated in situ from diethyl malonate and
sodium hydride, and zinc chloride ,11 mmol) was stirred in
toluene at 1108C for 4 h. The crude product was isolated by
column chromatograph, and three fractions were collected.
Spectrometric analysis clearly showed that the component of
®rst fraction is the starting material 1, and that of the third
As mentioned above, the N,O-disubstituted aminal ,2), a
stable synthetic equivalent of imine 1, was simultaneously
Table 2
Reformatsky reaction of diethyl malonates with 1±3a
Table 1
Reformatsky reaction with 1 or 3a
Run
Substrate
Malonate
Conditions
Product ,yield %)b
1c
2
3
4
5
6
7
8
1
1
1
1
1
1
2
3
8a
8a
8a
8a
8b
8c
8a
8a
908C, 4 h
Reflux, 3 h
RT, 4 h
5d ,30) 7 ,36)
5d ,92)
5d ,44)
Run
Substrate
Ester
Conditions
Product ,yield %)b
1
2
3
4
5
6
7
8
1
1
1
1
3
3
3c
3
4a
4b
4c
4d
4a
4b
4b
4c
Reflux, 4 h
Reflux, 4 h
Reflux, 4 h
Reflux, 4 h
Reflux, 6 h
Reflux, 6 h
Reflux, 6 h
Reflux, 6 h
5a ,26) 6a ,41)
5b ,37) 6b ,46)
5c ,17) 6c ,28)
None
RT, 36 h
RT, 48 h
RT, 48 h
RT, 36 h
RT, 36 h
5d ,90)
9b ,57)
9c ,61)
5d ,92)
10 ,88)
11a ,37)
11b ,45)
11b ,72)
11c ,27)
a Substrate ,10 mmol), 8 ,11 mmol) and NaH ,11 mmol) in toluene
,15 ml).
a Substrate ,10 mmol), 4 ,11 mmol) and zinc ,11 mmol).
b Isolated yields.
c 3 ,10 mmol), 4b ,22 mmol) and zinc ,22 mmol).
b Isolated yields.
c ZnCl2 ,11 mmol) was used, and the yields were determined by 19F
NMR.

80
Y. Gong, K. Kato / Journal of Fluorine Chemistry 111 ꢀ2001) 77±80
Scheme 3.
Chart 1.
generated. To our knowledge, the well-known N,O-disub-
stituted aminal compound 1,3-oxazolidine has been success-
fully used to react with several organometallic reagents
[15,16], involving the Reformatsky reagent prepared from
ethyl bromodi¯uoroacetate [10]. We therefore decided to
assess the reaction of diethyl malonate with 2. As a con-
sequence, 5d was also afforded in high yield when the
reaction of diethyl malonate with 2 in the presence of sodium
hydride was carried out in toluene at room temperature
,Table 2, run 7). With analogous N,O-disubstituted aminal
,3), compound 10 ,chart 1) was the only product ,run 8).
The Reformatsky reaction with 3 was next studied. Under
the conditions listed in Table 1, the reaction with 3 readily
proceeded. Substitution product 11a±c ,chart 1) was given as
the major product rather than the b-lactam, and the yields are
given in Table 1 ,runs 5±8). It was found that the yield of 11b
was markedly improved when excess amounts of 4b and
zinc dust were used ,run 7). It should be noted, however, that
complicated condensed products were also generated simul-
taneously in these reactions with 3, which led to the
remarked decrease of the yield of 11.
managed by the Research Development Corporation of
Japan ,JRDC) in cooperation with Japan International
Science and Technology Exchange Center ,JISTEC).
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In conclusion, we demonstrated that the reaction of
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like diethyl malonate with a-tri¯uoromethyl imine or N,O-
disubstituted aminal is a convenient way in the preparation
of the titled compounds.
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Acknowledgements
Y.G. thanks for the Science and Technology Agency of
Japan ,STA) for the award of Fellowship Program, which is
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