Synthesis of 1,1,1-trihalo-4-methoxy-4-[2-heteroaryl]-3-buten-2-ones, the corresponding butan-1,3-dione and azole derivatives

Article abstract of DOI:10.1016/S0040-4039(02)02137-8

The synthesis of 1,1,1-trihalo-4-methoxy-4-[2-thienyl]-3-buten-2-ones, 1,1,1-trihalo-4-methoxy-4-[2-furyl]-3-buten-2-ones and the respective trihalomethyl 1,3-butanone derivatives by trihaloacylation of dimethoxy acetals derived from 2-acetylthiophene and 2-acetylfuran are reported. This is a convenient method to obtain fluorinated and chlorinated 1,3-dielectrophiles from 2-acetylthiophene and 2-acetylfuran. The fluorinated 1,3-dielectrophiles were used to synthesize five new 2-thienyl- and 2-furylazoles. The structure of all products were assigned by ¹HH- and ¹³C NMR and mass spectrometry.

Full text of DOI:10.1016/S0040-4039(02)02137-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 8701–8705
Synthesis of
1,1,1-trihalo-4-methoxy-4-[2-heteroaryl]-3-buten-2-ones, the
corresponding butan-1,3-dione and azole derivatives
Alex F. C. Flores,* Sergio Brondani, Nilo Zanatta, Adriano Rosa and Marcos A. P. Martins*
Departamento de Qu´ımica, Universidade Federal de Santa Maria, Campus Camobi, Santa Maria, 97105 900 RS, Brazil
Received 9 September 2002; revised 25 September 2002; accepted 26 September 2002
Abstract—The synthesis of 1,1,1-trihalo-4-methoxy-4-[2-thienyl]-3-buten-2-ones, 1,1,1-trihalo-4-methoxy-4-[2-furyl]-3-buten-2-ones
and the respective trihalomethyl 1,3-butanone derivatives by trihaloacylation of dimethoxy acetals derived from 2-acetylthiophene
and 2-acetylfuran are reported. This is a convenient method to obtain fluorinated and chlorinated 1,3-dielectrophiles from
2-acetylthiophene and 2-acetylfuran. The fluorinated 1,3-dielectrophiles were used to synthesize five new 2-thienyl- and 2-furyla-
1
zoles. The structure of all products were assigned by HH- and 13C NMR and mass spectrometry. © 2002 Elsevier Science Ltd.
All rights reserved.
The introduction of a trifluoromethyl or trichloro-
methyl groups into an organic compound can bring
about remarkable changes in the physical, chemical and
biological proprieties that result in new compounds or
materials suitable for diverse pharmacological,1 agro-
chemical,2 analytical3 and synthetic applications.4
The haloacetylation of acyclic enol ethers and acetals,
described elsewhere,7 affords 1,1,1-trihalo-4-alkoxy-3-
alken-2-ones which have been used as precursors for
heterocycles and acyclic compounds.9 In general, the
presence of a trihalomethyl group in the precursor is a
determining factor to establish the regiochemistry of the
heterocyclic ring. The transformation of the
trichloromethyl group, under mild conditions, into car-
boxylic groups led us to devote special attention to
these substrates.10 Despite extensive studies on the
applications of 1,1,1-trihalo-4-alkoxy-3-alken-2-ones in
heterocyclic chemistry, the strategy to synthesize het-
eroaryl substituted 1,1,1-trihalo-4-methoxy-3-alken-2-
ones and their usefulness to the synthesis of
heterocycles has not yet been developed. These com-
pounds are highly functionalized intermediates which
may be useful for further synthetic applications.
On the other hand, the most employed method to
obtain trifluoromethyl b-diketones is via acylation of
enolates with perfluoroalkanoates.5 The perfluorinated
esters are commercially available and they are usually
obtained by esterification of the respective perfluori-
nated acids, which is not an explicit reported method.
Strong bases such as sodium methoxide and sodium
amide are used to obtain enolates from ketones and the
overall yields for the enolate formation and subsequent
acylation are only moderate. The majority of methods
found in the literature referring to the synthesis of
4,4,4-trifluoro-1-(2-thienyl)-1,3-butanodione5 relies on
the Park’s work6 (Claisen condensation). We found
only one reference about trichloromethyl b-diketones in
the literature.7 The lack of literature data is probably
because the acylation of enolates by the ester method is
of limited use to obtain perchloroalkyl b-diketones, due
to the lability of the perchloroalkyl groups in basic
medium.8
In this paper, we report a new and efficient synthetic
approach for the synthesis of a series 1,1,1-trihalo-4-
methoxy-4-[2-thienyl]- (1a, 2a) and 4-[2-furyl]-3-buten-
2-ones (1b, 2b) and the respective 4,4,4-trihalo-1-
[2-heteroaryl]-1,3-butanediones derivatives (3a,b, 4a,b)
We show that trifluoromethyl substituted 1,3-dielec-
trophiles (1, 3) react with hydroxylamine and hydrazine
leading to a new series of biheterocycles 2-thienyl- and
2-furylazoles (5a,b, 6b, 7a,b) (Scheme 2).
The dimethoxy acetals derived from 2-acetylthiophene
and 2-acetylfuran were obtained from the reaction of
these ketones and trimethyl orthoformate in the pres-
* Corresponding authors. Tel.: +55 55 220 8741; fax: +55 55 220
8031; e-mail: alexflores@quimica.ufsm.br
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02137-8

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A. F. C. Flores et al. / Tetrahedron Letters 43 (2002) 8701–8705
ence of p-toluenesulfonic acid.11 The trihaloacylation
reactions were carried out in chloroform or
dichloromethane and pyridine at −10 to 30°C for 8–12
h. Two equivalents of acylating agent per acetal was
required to obtain the 1,1,1-trihalo-4-methoxy-4-[2-het-
eroaryl]-3-buten-2-ones (1, 2) since one molecule of the
acylant promotes the formation of the enol ether by
trapping the alkoxy group liberated by the acetal and
the second molecule of the acylant promotes the
acylation.12 The acylation products obtained without
hydrolysis furnished black oil mixtures of the (E/Z)-
1,1,1-trihalo-4-methoxy-4-[2-heteroaryl]-3-buten-2-ones
(1, 2) and the respective 4,4,4-trihalo-1-[2-het-
eroaryl]butan-1,3-dione (3, 4) with variable ratios
(Scheme 1). The E-configuration isomer was the major
compound in all mixtures. The configuration of isomers
thienyl)pyrazole 6a15 or 3(5)-trifluoromethyl-5(3)-(2-
furyl)pyrazole 6b.
The reaction of 3a,b with thiosemicarbazide hydrochlo-
ride led to a new series of 5-trifluoromethyl-5-hydroxy-
3 - [2 - heteroaryl] - 1H - 1 - pyrazolethiocarboxy amides
(7a,b) (Scheme 2).
Unless otherwise indicated all common reagents and
solvents were used as obtained from commercial suppli-
ers without further purification. All melting points were
measured using a Reichert–Thermovar melting-point
microscope and are uncorrected. 1H and 13C NMR
spectra were recorded on a Bruker DPX 400 (1H at
400.13 MHz and 13C at 100.62 MHz), 5 mm sample
tubes, 300 K, digital resolution 0.01 ppm, in CDCl3/
TMS. Mass spectra were recorded using an HP 5973
MSD connected to an HP 6890 GC and interfaced by a
Pentium PC. The GC was equipped with a split–split-
less injector, autosampler, cross-linked HP-5 capillary
column (30 m, 0.32 mm of internal diameter), and
helium was used as the carrier gas.
3
was assigned on the basis of JC2%–H3 coupling
constants.13
The acylated products obtained from the hydrolysis
path furnished only 4,4,4-trihalo-1-[2-hetaryl]butan-1,3-
diones 3a,b,14 4a,b in high purity (Scheme 1). For the
1,3-dicarbonyl compounds 3a,b,14 4a,b isolated from
1
the hydrolysis path, was observed only one set of H
and 13C NMR signals which indicates that only one
enolic form was obtained for each compound. 1H
NMR spectra showed the signal for the enolic hydro-
gen within 12–15 ppm for all compounds and the
chemical shifts for H2 and C2 in the range of 6.4–6.7
and 89–83.5 ppm, respectively (Table 1).
4,4,4-Trihalo-1-[2-heteroaryl]-1,3-butanediones 3a,b,
and 4a,b: general procedure
To a stirred solution of dimethoxy acetal derived from
2-acetylthiophene or 2-acetylfuran (30 mmol) and pyri-
dine (60 mmol, 4.8 g) in chloroform (30 ml) kept at
0°C, a solution of trichloroacetyl chloride or tri-
fluoroacetic anhydride (60 mmol) in chloroform (20 ml)
was added dropwise at −10°C. The mixture was stirred
for 8–12 h at room temperature (25–35°C). The mixture
was quenched with a 2 M sulfuric acid solution (30 ml)
and stirred for 2 h at 50°C. The organic layer was dried
with sodium sulfate, the solvent was evaporated and the
products were obtained in high purity (see Table 1).
The cyclization of compounds 1a,b (E/Z) or 3a,b with
hydroxylamine hydrochloride was carried out in the
presence of pyridine in a molar ratio of 1.0:1.2:1.2,
respectively, and the mixture was stirred at 50°C for 8
h to afford a new series of 3-[2-heteroaryl]-5-tri-
fluoromethyl-4,5-dihydroisoxazoles 5a or 5b in 95%
yield. Compounds 1a,b and 3a,b also reacted with dry
hydrazine to furnish 3(5)-trifluormethyl-5(3)-(2-
Scheme 1. Reagents and conditions: (i) (F3CCO)2O or Cl3CCOCl, pyridine, CHCl3, −10 to 0°C (4 h), 0–30°C (8 h); (ii) H2SO4 1
M, 50°C, 5 h.

A. F. C. Flores et al. / Tetrahedron Letters 43 (2002) 8701–8705
8703
Table 1. Yields and selected physical and NMRa data for 1,1,1-trihalo-4-methoxy-4-[2-thienyl]-3-buten-2-ones (1a,b) and
1,1,1-trihalo-4-methoxy-4-[2-furyl]-3-buten-2-ones (1b, 2b) and buten-1,3-dione derivatives 3a,b and 4a,b
Compd
M.F.,b M.W.
Yield (%)c
Ratiod E/Z/bd GC–MS m/z (%)
1H NMR l (ppm)e
13C NMR l (ppm)
[JCF Hz]e
1a
C9H7F3O2S,
236.21
84
5: 1: 1
236 M+ (10), 201 (100), 3.95 (3H, OMe), 6.2
117.7 [278] (C1), 178.2
[36.5](C2), 89.6 [2.5]
(C3), 168.6 (C4), 56.9
173 (30)
(H3); Tn: 7.12 (H4),
7.57 (H3), 8.28 (H5)
(OMe), Tn: 134.9 (C2),
131.5 (C3), 127.3 (C4),
133.8 (C5)
1b
2a
2b
3a
3b
4a
4b
C9H7F3O3,
220.15
83
89
87
82
79
86
85
10: 1: 1
221 M+ (30), 204 (30),
153 (70), 69 (100)
4.1 (3H, OMe), 6.7
117.5 [280] (C1), 183 [37]
(H3); Fu: 6.7 (H4), 7.1 (C2), 89.3 [2.4] (C3), 170
(H3), 7.65 (H5)
(C4), 56.4 (OMe), Fu:
149 (C2), 113.5 (C3),
114.2 (C4), 146.8 (C5)
98.2 (C1), 176.2 (C2),
C9H7Cl3O2S,
285.58
4: 1: 4
285 M+ (10), 251 (20),
207 (15), 153 (100)
3.95 (3H, OMe), 5.8
(H3); Tn: 7.13 (H4), 7.6 90.0 (C3), 169.2 (C4),
(H3), 8.5 (H5)
56.9 (OMe), Tn: 135.0
(C2), 132.3 (C3), 127.4
(C4), 134.3 (C5)
C9H7Cl3O3,
269.51
7: 1: 3
269 M+ (10), 235 (20),
191 (30), 137 (100)
4.0 (3H, OMe), 6.73
(H3); Fu: 6.6 (H4), 7.27 88.8 (C3), 170.5 (C4),
(H3), 7.65 (H5)
94.7 (C1), 185.5 (C2),
55.7 (OMe), Fu: 147.9
(C2), 113.1 (C3), 114.1
(C4), 147.2 (C5)
C8H5F3O2S,
222.19
–
–
–
–
222 M+ (30), 153 (60),
111 (40), 69 (100)
13.7 (OH), 6.46 (H2);
Tn: 7.2 (H4), 7.7 (H3), (C2), 171 [36.6] (C3),
7.85 (H5)
182.7 (C1), 93.4 [2.6]
117.6 [280.2] (C4), Tn:
139.3 (C2), 132.7 (C2),
128.8 (C4), 135.3 (C5)
176.7 (C1), 92.7 [2.4]
(C2), 173.5 [36.7] (C3),
118 [281] (C4); Fu: 148.9
(C2), 118.7 (C3), 113.3
(C4), 147.9 (C5)
C8H5F3O3,
206.12
206 M+ (80), 178 (10),
137 (100)
14.0 (OH), 6.5 (H2);
Fu: 6.64 (H4), 7.35
(H3), 7.7 (H5)
C8H5Cl3O2S,
271.54
272 M+ (25), 207 (30),
153 (100)
13.7 (OH), 6.7 (H2);
Tn: 7.2 (H4), 7.7 (H3), 176.5 (C3), 94.5 (C4),
7.85 (H5)
184.3 (C1), 89.3 (C2),
Tn: 137.2 (C2), 131.5
(C3), 128.7 (C4), 133.6
(C5)
185.4 (C1), 88.7 (C2),
170.4 (C3), 94.6 (C4),
Fu: 147.8 (C2), 117.1
(C3), 113.1 (C4), 147.2
(C5)
C8H5Cl3O3,
255.48
256 M+ (10), 191 (25),
137 (100)
13.8 (OH), 6.7 (H2);
Fu: 6.25 (H4), 7.25
(H3), 7.65 (H5)
a NMR spectra were recorded on a Bruker DPX 400 (1H at 400.13 MHz and 13C at 100.61 MHz) in CDCl3/TMS. Of the four b-diketones whose
NMR spectroscopy is described here for the first time.
b Satisfactory elemental analysis performed on a Vario EL Foss Heraeus apparatus (C 0.4%; H 0.6)
c Yields of isolated mixtures for 1 (E,Z) and 2 (E,Z) and pure diketones 3 and 4.
d Ratio determined by GC and 1H NMR integrals.
e Data for E isomers of 1 and 2.
Synthesis of 3-[2-heteroaryl]-5-trifluoromethyl-4,5-dihy-
droisoxazoles (5a,b)
C8H6F3NO2S; mp 119–123°C; 1H NMR: Isox l 3.6
(H4), 4.0 (H4), Thienyl l 7.7 (H5), 7.5 (H3), 7.2 (H4);
13C NMR: l Isox 153.0 (C3), 42.8 (C4), 103.7 (C5,
To a solution of 4,4,4-trifluoro-1-[2-heteroaryl]-1,3-
butanedione (3a,b) (5 mmol) in pyridine (0.52 g, 6.5
mmol) and methanol (5 ml) was added hydroxylamine
hydrochloride (0.45 g, 6.5 mmol). The mixture was
stirred for 8 h at 50°C. The solvent was evaporated and
the solid residue was washed with water and dried
under vacuum. Isoxazole compounds were fully charac-
terized by spectroscopic methods. Data for 5a:
JCF=33 Hz), 122.4 (CF3, JCF=284 Hz). Anal. calcd: C,
40.51; H, 2.55; N, 5.90. Found: C, 40.6; H, 2.7; N,
6.0%. Data for 5b: C8H6F3NO3; mp 117–120°C; 1H
NMR: Isox l 3.5 (H4), 3.8 (H4), Furyl l 7.7 (H5), 6.9
(H3), 6.6 (H4); 13C NMR: l Isox 148.7 (C3), 42.5 (C4),
103.6 (C5, JCF=33.6 Hz), 122.6 (CF3, JCF=282 Hz).
Anal. calcd: C, 43.45; H, 2.73; N, 6.33. Found: C, 43.6;
H, 2.7; N, 6.5%.

8704
A. F. C. Flores et al. / Tetrahedron Letters 43 (2002) 8701–8705
Synthesis of 3(5)-[2-heteroaryl]-5(3)-trifluoromethyl-1H-
pyrazoles (6a,b)
Acknowledgements
The authors thank the Conselho Nacional de Desen-
volvimento Cient´ıfico e Tecnolo´gico (CNPq/PADCT),
Fundac¸a˜o de Amparo a` Pesquisa do Estado do Rio
Grande do Sul (FAPERGS) for financial support. The
fellowships from CNPq and FAPERGS are also
acknowledged.
A solution of 4,4,4-trifluor-1-[2-heteroaryl]-1,3-butane-
dione (3a,b) (5 mmol) in chloroform (5 ml) was added
dropwise to a cooled stirred solution (−10°C) of dry
hydrazine (0.2 g, 5.5 mmol). The mixture was stirred
for 1 h and the solvent was evaporated. The solid
residue was washed with water and dried under vac-
uum. Pyrazole compounds were fully characterized by
spectroscopic methods. Data for 6a: C8H5F3N2S (lit.
15); 1H NMR: Pyz l 6.65 (H4), Thienyl l 7.35 (H5), 7.0
(H4), 7.3 (H3); 13C NMR: l Pyz 143.7 (C3), 101.5 (C4,
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