HALOACETYLATED ENOL ETHERS: 16[5] REGIOSPECIFIC SYNTHESIS OF 5-TRICHLOROMETHYL-PYRAZOLES

HALOACETYLATED ENOL ETHERS:

16[5] REGIOSPECIFIC SYNTHESIS OF

5-TRICHLOROMETHYL-PYRAZOLES

Alex F. C. Flores,* Marcos A. P. Martins,

Adriano Rosa, Darlene Correia Flores, Nilo Zanatta,

and Helio G. Bonacorsso

Departamento de Quımica, Universidade Federal deSanta Maria, 97.105-900-Santa Maria, RS, Brazil

ABSTRACT

The regiospecific synthesis and isolation of three series of 5-trichloromethyl-pyrazoles 2f–j and 3, 4a–j from the cyclo-condensation of 1,1,1-trichloro-4-alkoxy-3-alken-2-ones(1a–f) or trichloroacetyl containing b-diketones (1g–j) withdry hydrazine and phenyl-hydrazine is reported. It was estab-lished by 1H- and 13C-NMR spectroscopy that the 5-hydroxy-5-trichloromethyl-4,5-dihydro-1H-pyrazole intermediates 2a–j

were formed quantitatively.

In a recent publication a convenient a-trichloroacetylation ofacetals derived from methyl-ketones leading to the isolation of 1,1,1-trichloro-4-alkoxy-3-alken-2-ones, was reported.1 The 1,1,1-trichloro-4-alkoxy-3-alken-2-ones has proven to be important building blocks for

+ [1.3.2002–11:34am] [1585–1594] [Page No. 1585] i:/Mdi/Scc/32(10)/120004150_SCC_32_010_R1_X0.kwd.3d Synthetic Communications (SCC)

1585

Copyright & 2002 by Marcel Dekker, Inc. www.dekker.com

*Corresponding author. E-mail: [email protected]

SYNTHETIC COMMUNICATIONS, 32(10), 1585–1594 (2002)

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120004150_SCC_032_010_R1.pdf

1586 FLORES ET AL.

the regiospecific synthesis of isoxazoles,2,4 as well as for the synthesis ofpyrimidines5–7 and benzodiazepines8 bearing a trichloromethyl group.

A few years ago, the regiospecific reactions of similar fluorinatedcompounds, that is 1,1,1-trifluoro-4-alkoxy-3-alken-2-ones, with 1,2-dinucleophiles such as hydroxylamines and hydrazines, was reported as arecent advance in heterocyclic synthesis.9,10

Although there is ample information on the reactions of 1,1,1-trifluoro-4-alkoxy-3-alken-2-ones with hydrazines for the synthesis of pyra-zoles,9–11 the use of 1,1,1-trichloro-4-alkoxy-3-alken-2-ones in similarreactions is reported only in three papers.3,12,13 There are enough observa-tions, however, to conclude that these ketones react with hydrazines similarto the trifluorinated intermediates. The cyclocondensation conditions, how-ever, may promote the hydrolysis of the trichloromethyl group.3,12,15 Forexample, the cyclocondensations of the 1,1,1-trifluoro-4-methoxy-3-alken-2-ones with hydrazine hydrochloride or phenyl hydrazine hydrochloride givestrifluoromethyl pyrazoles.11 The chlorinated compounds also furnished pyr-azoles but the trichloromethyl group is converted to a carboxyl.3,12 Specialconditions are needed to obtain trichloromethyl pyrazoles from direct cycli-zation of 1,1,1-trichloromethyl-4-alkoxy-3-alken-2-ones and othertrichloromethyl 1,3-dielectrophiles with hydrazine.

Considering the above aspects we decided to apply the methodology[trichloromethyl-containing CCCþNH2NHR] to obtain a new series of5-trichloromethyl-1H-pyrazoles. In this work, we are using a large set oftrichloromethyl-containing 1,3-dielectrophiles 1 in order to study the effectof substituents on the stability of the intermediate compounds 2a–j.Furthermore, the reactions were monitored by NMR to observe the forma-tion of the reaction intermediates 2a–j.

The 1H- and 13C-NMR chemical shift assignment of pyrazoles 2f–j

and 3, 4a–j were obtained with the help of HMQC and HMBC 2D-NMRexperiments and by comparison with NMR data of other pyrazoles11–13 andisoxazoles2–4,16,17 previously synthesized in our laboratory.

The cyclocondensation reactions of dielectrophiles compounds 1a–j

with dry hydrazine were carried out in a molar ratio 1 : 1 using chloroformas solvent and temperatures ranging from 0 to �10�C. Immediately after theaddition of the reactants, under vigorous stirring, the products 2a–j preci-pitated. In this series, the intermediates 2a–e are unstable in chloroformsolutions at 25–30�C and it was not possible to isolate them. When allowingthe reaction mixture to reach room temperature (25–30�C) products 2a–e

melted leading to residual oils which were identified as 5-trichloromethyl-1H-pyrazoles 3a–e (Table 2). For compounds 2a–e (R1

¼H, alkyl andR2

¼H) the elimination of water was a rapid process in chloroform solution.However, compounds 2f (R1

¼Ph; R2¼H) and 2g–j (R1; R2

6¼H) were


T2

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120004150_SCC_032_010_R1.pdf

SYNTHESIS OF 5-TRICHLOROMETHYL-PYRAZOLES 1587

stable in chloroform solutions and they could be filtered at 0�C. Compounds2f–j resisted at room conditions, but they decomposed on heating formelting point determination. In order to obtain the aromatic derivatives3f–j the chloroform was evaporated and the intermediates 2f–j weredehydrated in acetone under reflux.

The synthesis of 5-trichloromethyl-1H-pyrazoles 2f–j and 3a–j arepresented in the Scheme 1 and the most satisfactory yields of these reactions,melting points and 1H/13C NMR data for 3a–j are shown in Table 2. TheNMR spectral data for the intermediate compounds 2a–j are presented inTable 1.

The 1H- and 13C-NMR spectra of 4,5-dihydro-5-hydroxy-5-trichloro-methyl-1H-pyrazoles 2a–j exhibited only one set of signals. For compounds2a–f the two doublets at 3.4–4.5 ppm with approximate J2

HH ¼ 18:5Hz forthe diastereotopic hydrogens on 4-position of the pyrazolinic ring is char-acteristic. The 1H-NMR spectra of compounds 2g–i exhibited only a doub-let of doublets at 3.5 ppm (J3

HH in Table 1) for the hydrogen of the ringjunction. For 2j only a quartet was observed by the coupling of the H4with the methyl group. In the 13C-NMR spectra just one line for each ofthese carbons was observed. These observations suggest that only onepair of diastereoisomers was obtained (4S, 5S/4R, 5R or 4S, 5R/4R, 5S)for 2g–j.

The reactions carried out with the dielectrophiles 1a–j and phenylhydrazine in a 1 : 1 molar ratio in chloroform at 0 to �10�C took directlyto the 1-phenyl-5-trichloromethyl-1H-pyrazoles 4a–j. In these cyclo-condensations the reaction medium remained homogenous after the mixtureof the reagents and it was not possible to observe the formation of the


S1

T1

AQ1

Scheme 1.

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103

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120004150_SCC_032_010_R1.pdf

1588 FLORES ET AL.


Table

1.

1H

and

13C

NM

Ra

of5-T

rich

loro

met

hyl-4,5

-dih

ydro

-pyra

zole

s2a–j

1H

NM

R(d

ppm

/Jn

Hz)

13C

NM

R(d

ppm

)

N�

H4

R3

R4

C3

C4

C5

CC

l 3

2a

3.0

2;J2¼

19.0

;3.5

2;J2¼

19.0

;J3¼

1.4

6.8

5(H

)J3¼

1.4

–142.6

945.2

99.5

103.9

2b

2.9

2;J

2¼

18.7

;3.5

2;J2¼

18.7

2.0

(CH

3)

–152.6

747.5

100.7

103.4

2c

3.0

0;J

2¼

18.5

;3.5

1;J2¼

18.5

2.2

9(C

H2)

–157.2

545.6

100.8

104.0

1.1

5(C

H3)

2d

2.9

6;J

2¼

18.5

;3.4

4;J2¼

18.5

2.6

5(C

H)

–161.1

444.0

100.3

103.6

1.1

5(C

H3)

2e

3.0

3;J

2¼

18.4

;3.4

8;J2¼

18.4

1.1

7(C

H3)

–163.9

644.2

100.0

106.5

2f

3.3

6;J

2¼

18.1

;3.8

2;J2¼

18.1

7.4

(3H

);7.6

(2H

)–

150.7

843.9

100.7

103.6

2g

3.2

2;J

3 aa¼

12:2

;J

3 ae¼

6:4

1.4

(2H

);1.6

(1H

);2.0

(2H

)157.5

451.5

99.7

104.6

2h

3.4

7;J

3 aa¼

9:4

4;J

3 ae¼

4:0

;J4¼

1.2

1.5

–2.1

(8H

);2.6

(2H

)160.9

752.7

100.7

105.0

2i

3.4

2;J

3 aa¼

7:5

0;J

3 ae¼

4:2

41.5

–1.8

(8H

);1.9

(1H

)160.7

653.1

100.7

105.0

2.1

(1H

);2.4

–2.6

(2H

)

2j

3.8

2;J

3¼

7.5

7.3

8(3

H);

1.2

2(C

H3)

152.7

246.0

106.5

101.2

7.6

2(2

H)J

3¼

7.5

aThe

NM

Rsp

ectr

aw

ere

reco

rded

on

aBru

ker

DPX

400

inC

DC

l 3/T

MS.

120004150_SCC_032_010_R1.pdf



Table

2.

Sel

ecte

dPhysica

land

Spec

trala

Data

of3a–j

Yie

ldb

M.p

.cM

ole

cula

r

13C

NM

Rd

(%)

(�C

)Form

ula

d1H

-NM

Rd,

J(H

z)C

3C

4C

5C

Cl 3

3a

85

75–77

C4H

3C

l 3N

26.6

3(H

4,d)J

3 HH¼

2:4

130.6

103.8

154.3

89.9

185.4

7.6

4(H

3,d)J

3 HH¼

2:4

3b

95

120–122

C5H

5C

l 3N

22.3

8(C

H3),

6.4

3(H

4)

141.3

102.7

154.7

90.8

199.5

3c

95

122–123

C6H

7C

l 3N

21.3

(CH

3),

2.8

(CH

2),

145.1

103.0

155.0

89.4

213.5

6.4

0(H

4)

3d

93

136–138

C7H

9C

l 3N

21.4

5(C

H3),

3.1

(CH

),148.2

106.4

160.3

90.4

227.5

6.3

8(H

4)

3e

95

Oil

C8H

11C

l 3N

21.3

5(C

H3),

6.4

(H4)

148.7

106.3

157.2

90.0

241.5

3fe

90

130–132

C10H

7C

l 3N

25.8

9(H

4),

7.3

1(p

H),

145.4

85.5

2161.8

79.3

7

261.5

7.4

0(o

H),

7.6

8(m

H)

3g

97

105–108

C8H

10C

l 3N

21.8

0–1.9

0-(C

H2) 2

-144.1

115.7

5146.5

686.2

239.5

2.8

0–2.9

5-(C

H2) 2

-

3h

95

117–119

C9H

11C

l 3N

21.5

–1.7

-(C

H2) 2

-,1.7

-(C

H2)-,

144.9

116.1

5146.5

86.6

253.6

2.3

-(C

H2)-,2.5

-(C

H2)-

3i

95

115–117

C10H

13C

l 3N

21.4

-(C

H2) 2

-,1.5

–1.6

-(C

H2) 2

-,145.0

115.9

146.6

86.9

267.6

2.3

-(C

H2)-,2.5

-(C

H2)-

3j

91

156–158

C11H

9C

l 3N

22.3

7(C

H3),

7.4

2–7.6

5(A

r)143.0

109.5

150.5

92.4

275.6

aN

MR

-spec

tra

wer

ere

cord

edon

aBru

ker

DPX

400

(1H

at

400.1

3M

Hz

and

13C

at

100.6

1M

Hz)

,in

CD

Cl 3/T

MS.

bY

ield

of

isola

ted

com

pounds

(hig

hpurity

—95–100%

).cM

elting

poin

tsare

unco

rrec

ted.

dSatisf

act

ory

elem

enta

lanaly

sis

per

form

edon

aV

ario

EL

Foss

Her

aeu

sappara

tus

(C�

0.4

%;H�

0.6

%;N�

0.6

%).

eD

MSO

-d6

solu

tion.

120004150_SCC_032_010_R1.pdf

1590 FLORES ET AL.


Table

3.

Sel

ecte

dPhysica

land

Spec

trala

Data

of4a–j

Yie

ldb

M.p

.cM

ole

cula

r

13C

-NM

R

(%)

(�C

)Form

ula

d1H

-NM

Rd,

J(H

z)C

3C

4C

5C

Cl 3

4a

80

108–110

C10H

7C

l 3N

26.8

7(d

,J

3 HH¼

1:6

),7.5

8(d

,J

3 HH¼

1:6

);138.0

108.9

144.5

86.7

261.5

3N

Ph

7.4

–7.5

(3H

),7.5

–7.6

(2H

)

4b

85

196–197

C11H

9C

l 3N

22.3

2(s

,C

H3),

6.6

7(s

,H

4);

NPh

7.5

2(2

H),

144.8

108.5

147.1

86.7

275.5

77.4

5(3

H)

4c

90

151–153

C12H

11C

l 3N

21.2

7(t,C

H3),

2.7

(q,C

H2),

7.1

(s,H

4);

145.3

107.2

149.4

84.3

289.5

9N

Ph

7.9

(2H

),7.8

(H),

7.3

(2H

)

4d

90

Oil

C13H

13C

l 3N

21.3

(d,2C

H3),

3.1

(m,C

H2),

7.2

5(s

,H

4);

145.8

107.5

162.7

89.0

289.5

9N

Ph

7.8

5(2

H),

7.4

(H),

7.3

(2H

)

4e

91

Oil

C14H

15C

l 3N

21.3

8(s

,3C

H3),

7.3

3(s

,H

4);

NPh

7.9

(2H

),145.5

108.4

153.0

89.4

317.6

47.6

(H),

7.3

(2H

)

4f

90

78–81

C16H

11C

l 3N

27.1

6(s

,H

4),

7.3

4(H

),7.4

(2H

),7.5

(2H

);145.8

106.1

149.8

86.8

337.6

3N

Ph

7.6

(2H

),7.4

5(H

),7.3

0(2

H)

4g

90

87–89

C14H

13C

l 3N

21.7

-(C

H2) 2

-,2.6

-(C

H2)-,2.9

-(C

H2) 2

-;140.4

117.1

148.6

88.1

315.6

2N

Ph

7.5

(2H

),7.4

(3H

)

4h

90

104–108

C14H

15C

l 3N

21.6

-(C

H2)-,1.8

–1.9

-(C

H2) 2

-,3.0

-(C

H2)-,

141.2

118.5

149.7

88.4

329.6

53.5

-(C

H2)-;N

Ph

7.3

(2H

),7.5

–7.6

(3H

)

4i

92

Oil

C16H

17C

l 3N

21.6

-(C

H2)-,1.8

–1.9

-(C

H2) 2

-,3.0

-(C

H2)-,

141.7

117.5

149.3

89.3

343.6

83.5

-(C

H2)-;N

Ph

7.3

1(2

H),

7.4

–7.6

(3H

)

4j

95

132–134

C17H

13N

2C

l 32.5

(s,C

H3),

7.4

0(H

),7.4

5(2

H),

7.5

(2H

);140.3

115.5

151.7

88.3

351.6

6N

Ph

7.6

0(2

H),

7.4

5(3

H)

aN

MR

-spec

tra

wer

ere

cord

edon

aBru

ker

DPX

400

(1H

at

400.1

3M

Hz

and

13C

at

100.6

1M

Hz)

,in

CD

Cl 3/T

MS.

bY

ield

sof

isola

ted

com

pounds

(hig

hpurity

—95–100%

).cM

elting

poin

tsare

unco

rrec

ted.

dSatisf

act

ory

elem

enta

lanaly

sis

(C�

0.4

%;H�

0.6

%;N�

0.6

%)

per

form

edon

aV

ario

EL

Foss

Her

aeu

sappara

tus.

120004150_SCC_032_010_R1.pdf


1-phenyl-5-hydroxy-5-trichloromethyl-4,5-dihydropyrazole intermediatesbecause the reactions were too fast to be followed by 1H NMR.

The synthesis of 5-trichloromethyl-1-phenyl-1H-pyrazoles 4a–j ispresented in the Scheme 2 and the most satisfactory yields of these reactions,melting points and 1H/13CNMR data are shown in Table 3.

This work showed that is possible to retain the trichloromethyl groupupon the cyclocondensation of trichloromethyl containing dielectrophileswith hydrazine. Using anhydrous chloroform as solvent allowed the isola-tion of aromatic 5-trichloromethyl pyrazoles 3, 4a–j. In addition, was pos-sible to observe and acquire 1H- and 13C-NMR data of the reactionintermediates 4,5-dihydro-5-hydroxy-5-trichloromethyl-1H-pyrazoles 2a–j,by carrying out the reaction in a NMR sample tube and following thecourse of the reaction.

EXPERIMENTAL

The synthesis of 1,1,1-trichloro-4-alkoxy-3-alken-2-ones 1a–f and tri-chloromethyl-b-diketones 1g–j has been reported elsewhere.1,18 Anhydroushydrazine was obtained from successive distillation of hydrazine mono-hydrate under KOH. CHCl3 99.99% was used as obtained from commercialsuppliers without further purification. All melting points were determinatedon a Reichert Thermovar apparatus and are uncorrected. 1H and 13C-NMRspectra were acquired on a Bruker DPX400 spectrometer in a 5 mm probe inCDCl3 solutions and TMS was used as the internal reference.


S2T3

Scheme 2.

127

128

129

130

131

132

133

134

135

136

137

138

139

140

141

142

143

144

145

146

147

148

149

150

151

152

153

154

155

156

157

158

159

160

161

162

163

164

165

166

167

168

120004150_SCC_032_010_R1.pdf

1592 FLORES ET AL.

Synthesis of 5-Trichloromethyl-1H-pyrazoles (3a–j)

General Procedure

The chloroform solutions of 1,1,1-trichloro-4-alkoxy-3-alken-2-ones1a–f or trichloromethyl-b-diketones 1g–j (10 mmol, 5 ml) were added drop-wise to a cooled stirred solution (0 to �10�C) of dry hydrazine (12 mmol;0.4 g) in chloroform (5 ml). The reaction mixture was stirred for 30 min., thechloroform was evaporated in rotatory evaporator (25–30�C; 10 mBar) andthe residue was dried under vacuum (25–30�C; 10�1 mBar). The aromaticpyrazole derivatives 3a–e were directly obtained from 1a–e as crystallinesolids and were purified by recrystallization from hexane/chloroform(5 : 1). The 3-t-butyl-5-trichloromethyl-1H-pyrazole (3e) was obtained asoil in high purity and needed no further purification. The 4,5-dihydro-pyrazole intermediates 2f–j were crystalline solids obtained in highpurity.19 They were quantitatively dehydrated under reflux in acetone over-night leading to the respective aromatic pyrazole derivatives 3f–j.

Synthesis of 5-Trichloromethyl-1-phenyl-1H-pyrazoles (4a–j)

General Procedure

The chloroform solutions of 1,1,1-trichloro-4-alkoxy-3-alken-2-ones1a–f or trichloromethyl-b-diketones 1g–j (10 mmol, 5 ml) were added drop-wise to a cooled stirred solution (0 to �10�C) of dry phenyl-hydrazine(12 mmol; 1.45 g) in chloroform (10 ml). The reaction mixture was stirredfor 30 min, the chloroform was evaporated in rotatory evaporator (25–30�C;10 mBar) and the residue was dried under vacuum (25–30�C; 10�1 mBar).The aromatic pyrazole derivatives 4a–j were directly obtained from 1a–e ascrystalline solids and were purified by recrystallization from hexane. The 3-t-butyl-5-trichloromethyl-1H-pyrazole (4e) was obtained as oil in highpurity and needed no further purification.

Observation of Intermediates 4,5-dihydro-5-trichloromethyl-

1H-pyrazoles 2a–j by 1HNMR

A 5mm NMR tube was charged with the ketones 1a–f (2� 10�4 Msolution in 0.5 ml of CDCl3) or the b-dicarbonyl 1g–j (10�4 M solution in0.5 ml of CDCl3). The solutions were cooled to �10�C and one equivalent ofdry hydrazine was added. The 1H NMR spectra were recorded on a Bruker


169

170

171

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173

174

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186

187

188

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190

191

192

193

194

195

196

197

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199

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207

208

209

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DPX-400 spectrometer (1H at 400 MHz) in chloroform-d1 and TMS wasused as the internal reference. The following acquisition parameters wereused for 1H: pulse width¼ 7.0 ms (90�), delay time¼ 1.0 s, power pulseattenuation¼�3.0 dB, acquisition time 6.60 s, sweep width of 220 ppm,digital resolution �0.01 ppm, and 8 scans were acquired for each experi-ment. Spectra were recorded in intervals of 5 min until the complete dis-appearance of the signals of the starting ketones.

ACKNOWLEDGMENTS

Financial support from Conselho Nacional de DesenvolvimentoCientıfico e Tecnologico (CNPq) and Fundacao de Amparo a Pesquisa doEstado do Rio Grande do Sul (FAPERGS) is gratefully acknowledged. Twoof us (A.R. and D.C.F.) thank the CNPq for a fellowships.

REFERENCES

1. Martins, M.A.P.; Bastos, G.P.; Flores, A.C.F.; Zanatta, N.;Bonacorso, H.G.; Siqueira, G.M. Tetrahedron Lett. 1999, 40,4309–4312.

2. Martins, M.P.M.; Flores, A.F.C.; Freitag, R.; Zanatta, N.J. Heterocyclic Chem. 1995, 32, 731.

3. Martins, M.A.P.; Flores, A.F.C.; Freitag, R.; Zanatta, N. Synthesis1995, 1491.

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5. Zanatta, N.; Madruga, C.C.; Marisco, P.C.; Flores, D.C.; Bonacorso,H.G.; Martins, M.A.P. J. Heterocyclic Chem. 2000, 37, 001–006.

6. Zanatta, N.; Madruga, C.C.; Clerici, E.; Martins, M.A.P. J.Heterocyclic Chem. 1996, 33, 735.

7. Zanatta, N.; Cortelini, M.F.M.; Carpes, M.J.S.; Bonacorso, H.G.;Martins, M.A.P. J. Heterocyclic Chem. 1997, 34, 509.

8. Bonacorso, H.G.; Bittencourt, S.T.; Wastowski, A.D.; Wentz, A.P.;Zanatta, N.; Martins, M.A.P. Tetrahedron Lett. 1996, 37(51), 9155.

9. Katritzky; Rees. Comprehensive Heterocyclic Chemistry, Vol. 1–8, 1stEd. 1984 and 2nd Ed. 1995; Pergamon Press: Oxford.

10. Nenajdenko, V.G.; Sanin, A.V.; Balenkova, E.S. Molecules 1997, 2,186–232.

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Received in the USA June 6, 2001


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