Highly regioselective Friedel-Crafts reactions of electron-rich aromatic compounds with pyruvate catalyzed by Lewis acid-base: Efficient synthesis of pesticide cycloprothrin

Article abstract of DOI:10.1002/adsc.200505450

An efficient synthesis of aromatic lactate esters is reported via highly regioselective Friedel-Crafts reactions of electron-rich aromatic compounds with pyruvate ester promoted by TiCl₄ in the presence of basic Al ₂O₃. The utility of the reaction is shown by the efficient synthesis of the pesticide cycloprothrin in high yield.

Full text of DOI:10.1002/adsc.200505450

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DOI: 10.1002/adsc.200505450
Highly Regioselective Friedel–Crafts Reactions of Electron-Rich
Aromatic Compounds with Pyruvate Catalyzed by Lewis
Acid-Base: Efficient Synthesis of Pesticide Cycloprothrin
Yu-Gui Si,a Jun Chen,a Fan Li,a Jin-Hua Li,b Ye-Jun Qin,b Biao Jianga,*
a
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032,
Peopleꢀs Republic of China
Fax: (þ86)-21-64166128, e-mail: Jiangb@mail.sioc.ac.cn
b
Byelen Chemical Corporation, 500 Caobao Road, Shanghai 200233, Peopleꢀs Republic of China
Received: November 23, 2005; Accepted: March 16, 2006
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: An efficient synthesis of aromatic lactate es- Keywords: alumina; aromatic lactate esters; Friedel–
ters is reported via highly regioselective Friedel– Crafts reactions; pyruvate ester; titanium(IV) chlor-
Crafts reactions of electron-rich aromatic compounds ide
with pyruvate ester promoted by TiCl4 in the presence
of basic Al2O3. The utility of the reaction is shown by
the efficient synthesis of the pesticide cycloprothrin in
high yield.
Introduction
pyruvate esters and dehydration of 2-aryllactate esters.
It has been reported that the addition of a phenol to pyr-
2-Arylpropanoic acids signify a medicinally important uvic acid under acidic conditions generated a polymeric
class of non-steroidal, anti-inflammatory agents.[1] Espe- material or bis-adducts in the para position of phenol.[7]
cially, the 2-arylpropenoic acid possessing an ethoxy When the reaction was carried out in the presence of
group in the para position of the aromatic ring is an im- Lewis acidic conditions, such as with chlorides of a metal
portant intermediate for the preparation of the pesticide in a high oxidation state, e.g., tetravalent titanium, zirco-
cycloprothrin, which is used in controlling insects and nium or trivalent boron and aluminum, the aromatic
acaroids of crops. Each year these pests destroy an esti- substitution resulted in an aldol-type coupling product
mated 15% of agricultural crops in the United States in the ortho position of the phenolic oxygen atom
and even more than that in developing countries.[2] (Scheme 1).[8]
The classical procedure that is frequently used in the
The mechanism of acid-catalyzed diphenylation of
preparation of the compound cycloprothrin, involves pyruvate ester has been proposed by Ohwada through
the reaction of an appropriate phenylacetic acid ester
with ethyl oxalate under basic conditions, followed by
treating the resultant oxalo ester with aqueous formal-
dehyde in the presence of K2CO3.[3] Contrarily, 2-aryl-
propenoic acids can be generated from 2-aryllactate es-
ters, reported to be produced by the reaction of aromatic
Grignard reagent with pyruvate esters,[4] benzoyl for-
mate ester with methyl Grignard reagent[5] or alkylation
of a-hydroxyphenylacetic acid ester with iodome-
thane;[6] however, all these methods are not straightfor-
ward.
The most atom-economic and efficient method for the
formation of 2-arylpropenoic acid esters is through the
Friedel–Crafts reaction of an aromatic compound with by a Brønsted acid or a Lewis acid.
Scheme 1. Reaction of phenol with pyruvate esters catalyzed
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the observation of the dication formed from 1,2-diones tion conditions using a Brønsted acid or a Lewis acid
in Friedel–Crafts reactions of benzene. The reactive pyr- as catalyst (Table 1). When the mixture of ethoxyben-
uvate esters in the Friedel–Crafts reactions of aromatic zene and ethyl pyruvate in CH2Cl2 was treated with con-
substrates under acid conditions are considered to be centrated H2SO4 at 08C for 4 h, a bisadduct 4 was isolat-
O,O-diprotonated 1,2-dicarbonyl species to give gem- ed in 12% yield (entry 1). The efficacy of various Lewis
diphenyl propionates.[9] Taking into account the subse- acids was tested for the reaction. All reactions gave the
quent steps of the mechanism, we conceived that remov- bisadducts as the major component. Among these cata-
al of the proton acid released from the Friedel–Crafts re- lysts, BF3 ·OEt2, FeCl3 and AlCl3 were found to give the
action and masking the hydroxy group of phenol, would bisadduct 4 in high yield along with monophenylation
allow discrimination of bisadduct on the one hand, product 3 (entries 2 to 4). When the reaction was per-
whereas it would disallow the aldol-type addition on formed using TiCl4 at 08C or at lower temperature
the other hand, affording a regioselective monophenyla- (À158C), the bisadduct was not only produced as a ma-
tion product in the para position. Thus, we undertook a jor product, but also a triadduct 5 was found (entries 5
reinvestigation of the Friedel–Crafts reaction of the and 6). Astepwise Friedel–Crafts reaction was the pre-
electron-rich alkoxybenzenes with pyruvate esters, ferred reaction based on these observations. The first
thereby observing that when this reaction was carried Friedel–Crafts reaction of ethoxybenzene with ethyl
out in a Lewis acid-base system it afforded the monoad- pyruvate was promoted by a Lewis acid to give 2-aryllac-
duct in the para position. The mechanism of this reaction tate 3, which underwent a second Friedel–Crafts reac-
is different from the previously reported Friedel–Crafts tion with pyruvate ester catalyzed by the proton acid re-
reactions of aromatic compound with pyruvate ester, leased from the first step. Thus, we supposed that with
which was considered as a Lewis acid-catalyzed aldol re- the proton acid being trapped with a base, the formation
action mechanism.[8]
of the bisadduct 4 would be suppressed. Additionally,
the base was applied as an additive in the reaction.
When an organic base was employed as an additive,
such as triethylamine and pyridine, the Friedel–Crafts
reaction failed (entries 7 and 8). However, to our de-
Results and Discussion
Initially, the reaction of ethoxybenzene and ethyl pyru- light, when the base was changed to a solid inorganic
vate was carried out under normal Friedel–Crafts reac- base, the reaction gave the expected monoadduct 2-(p-
Table 1. Friedel-Crafts reaction of ethoxybenzene with ethyl pyruvate under various reaction conditions.[a]
Entry
Lewis Acid
Base
Temp.
Reaction
Time [h]
Yield [%][b]
3
4
5
1
2
3
4
5
6
7
8
9
10
11
12
13
H2SO4
BF3 ·OEt2
FeCl3
AlCl3
TiCl4
TiCl4
TiCl4
TiCl4
TiCl4
TiCl4
TiCl4
AlCl3
FeCl3
–
–
–
–
–
–
08C
4
4
4
4
4
8
4
4
7
5
5
2
2
0
5
5
15
15
5
12
68
65
41
60
34
0
0
3
9
7
0
0
0
0
11
3
0
0
0
0
À58C
À158C
08C
08C
À158C
À158C
À158C
À158C
À108C
À58C
À158C
À158C
NEt3
Py
0
0
Al2O3
Na2CO3
BaO
Al2O3
Al2O3
78
55
47
40
20
0
0
0
39
41
[a]
All reactions were carried out with ethoxybenzene (2 mmol), ethyl pyruvate (2.2 mmol) and Lewis acid (2 mmol) in 6 mL of
CH2Cl2 under argon. The concentration of the additive base was 5 mmol when required.
Yield of isolated product.
[b]
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ethoxyphenyl)lactate ester 3 as the major product. The
best result was obtained when Al2O3 was used as base to-
gether with TiCl4 as Lewis acid; monoadduct 3 was iso-
lated in 78% yield along with 3% of bisadduct 4 (en-
try 9). Consequently, the ratio of monoadduct to bisad-
duct was greatly improved to give the major monoad-
duct by using TiCl4/Na2CO3, A 2lO3 and BaO, FeCl3-
Al2O3 or AlCl3-Al2O3 as promoter (entries 10 to13).
Next, we carried out the TiCl4-Al2O3 catalyzed elec-
trophilic addition reaction of methyl pyruvate with a va-
riety of electron-rich aromatic compounds to better un-
derstand both the scope and the generality of this meth-
od (Table 2). As shown in Table 2, the reaction of ethoxy-
benzene with methyl pyruvate gave the 2-(p-ethoxy-
phenyl)lactate ester 6a in 85% yield (entry 1).
Treatment of anisole afforded the monophenylation
product 6b in 91% yield (entry 2). Furthermore, 1,2-
and 1,4-dimethyoxybenzenes yielded the respective
monophenylation product in high yields without isola-
tion of any diphenylation product (entries 3 and 4). Thio-
anisole was subject to the reaction conditions to give
monoadduct 6e in 87% yield (entry 5). Aiming at devel-
Figure 2. (a) Methyl pyruvate in CDCl3; (b) 3 hours after
TiCl4 was added to the solution of methyl pyruvate in CDCl3.
Figure 3. (a) Mixture of anisole with methyl pyruvate in
CDCl3; (b) 3 hours after TiCl4 was added to the mixture of
anisole and methyl pyruvate in CDCl3.
oping an asymmetrical versions of this reaction, the l- 2.48 ppm (CH3COCO2CH3) and 3.87 ppm (CH3-
(À)-menthyl pyruvate was synthesized and used to COCO2CH3) were downshifted to 2.97 ppm and
probe the asymmetric induction. The asymmetric induc- 4.46 ppm, respectively. This observation indicated that
tion obtained was reflected in a ratio of 1:3 of the dia- there was a certain action between TiCl4 and the pyru-
stereoisomers (entry 6). It was reported that the reac- vate ester to generate a complex compound. Finally,
tion of indole with carbonyl compounds in the presence the mixture of anisole with methyl pyruvate in the pres-
of a Lewis acid or a proton acid afforded the bis-indo- ence of TiCl4/Al2O3 in CDCl3 was tracked by 1H NMR
lealkane derivatives.[10] Extending this Lewis acid-base (Figure 3). The complex of TiCl4 with methyl pyruvate
reaction system to the addition reaction of indole deriv- ester was first observed at 2.96 ppm and 4.44 ppm. After
atives with methyl pyruvate ester, the reaction gave only three hours, the signal of the monophenylation product
the monoadducts 6g–i in good yield (entries 7–9).
appeared at 1.88 ppm [CH3C(Ar)OHCO2CH3] and
In order to understand the mechanism of the TiCl4- 3.96 ppm [CH3C(Ar)OHCO2CH3]. The above observa-
base-catalyzed Friedel–Crafts reaction of electron-rich tion indicated that primarily a complex of TiCl4 with
aromatic compounds with pyruvate esters, the reaction methyl pyruvate was produced, followed by the attach-
was carried out with NMR monitoring. Initially, a 1:1 ment of the complex to the electron-rich site of the aro-
mixture of anisole and TiCl4 in CDCl3 was stirred for 3 matic compounds to give the monoadduct.
hours at 08C and monitored by 1H NMR. As illustrated
Based on the above observations, a tentative mecha-
in Figure 1, comparison of the 1H NMR spectrum of the nism for this Friedel–Crafts reaction was proposed as
1
mixture of anisole-TiCl4 with the standard H NMR shown in Scheme 2. At first, TiCl4 was bound to the car-
spectrum of anisole showed that both 1H NMR signals bonyl of methyl pyruvate to give an active Ti(IV)-pyru-
of anisole were the same. This indicated that TiCl4 did vate complex cation A. This coordination was followed
not directly bind to and react with anisole. Asimilar
1H NMR experiment was carried out for methyl pyru- compounds to generate an aromatic cation intermediate
vate-TiCl4 in CDCl3 (Figure 2). Agreat chemical shift B. The aromatic cation intermediate B underwent aro-
by the addition to the electron-rich site of the aromatic
change was observed in Figure 2. The signals at matization by elimination of hydrogen chloride to give
the titanium trichloride-monoadduct compound C. If
the reaction was carried out without a base, the released
HCl from the reaction would decompose C to give the
monoadduct, which would undergo dehydration imme-
diately catalyzed by the acid to afford the cation D. The
cation D further attacked a second aromatic molecule to
give a bisadduct. Thus, when a base was added to remove
HCl, the second step of the Friedel–Crafts was sup-
pressed, terminating the reaction in intermediate C to
give a monoadduct after reaction.
Figure 1. (a) Anisole in CDCl3; (b) 3 hours after TiCl4 was
added to the solution of anisole in CDCl3.
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Table 2. Friedel–Crafts reactions of electron-rich aromatic compounds with methyl pyruvate.[a]
Entry
1
ArH
Reaction Time [h]
8
Product
Yield [%][b]
6a
85
91
82
2
3
6
5
6b
6c
4
5
6
5
6d
6e
6f
74
12
3
87
91[c]
7
8
7
5
5
6g
6h
6i
95[d]
52[e]
74[d]
9
[a]
All reactions were carried out with ArH (2 mmol), pyruvate (2.2 mmol), TiCl4 (2 mmol) and Al2O3 (5 mmol) in 6 mL
CH2Cl2 under argon.
[b]
[c]
[d]
[e]
Yield of isolated product.
A1: 3 mixture of diastereoisomers as determined by 1H NMR analysis of the reaction crude product.
Ts¼p-toluenesulfonyl.
TBDMS¼tert-butyldimethylsilyl.
Since the synthesis of the 2-aryllactate ester consti- biologically important pesticide, cycloprothrin. Previ-
tutes an important target in organic synthesis and there- ous syntheses of this compound were carried out by sev-
fore better evaluates the usefulness of the present meth- eral methods, however, these routes were either labori-
odology, we focused our attention on the synthesis of the ous, or comprised of steps of low yields, or required a
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Yu-Gui Si et al.
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Scheme 2. Tentative mechanism for the Friedel–Crafts reaction of aromatic compounds with pyruvate esters.
harsh reagent and conditions.[4,5,6] Consequently, we in 65% yield.[11] The addition of dichlorocarbene was
were interested in the preparation of this compound, induced at 608C with extremely exothermic conditions
and we report hereafter an improved route to cyclopro- that caused a vigorous reflux of the solvent to give a
thrin by utilizing the relatively eco-friendly reaction black tar product. Thus, the research for safe conditions
conditions. The strategy proceeds through the 2-(p- for dichlorocyclopropanation is very important due to
ethoxylphenyl)lactate ester (6a) and provides the short- the exothermic reaction when the reaction is scaled up
est routes and gives the highest yield of cycloprothrin to the tons scale. It is known that KF/Al2O3 is a useful
achieved to date (Scheme 3). The phenyllactate 6a was solid-supported reagent for base-induced organic reac-
dehydrated simply by heating in toluene along with a tions.[12] When phenylpropenate 7 was treated with
catalytic amount of p-toluenesulfonic acid to yield the chloroform in the presence of KF/Al2O3 in acetonitrile
phenylpropenate 7 in nearly quantitative yield. The ad- at room temperature, gem-dichlorocyclopropanecar-
dition of 7 to dichlorocarbene generated from chloro- boxylate 8 was obtained in 90% yield. The KF/Al2O3-
form in the presence of 50% sodium hydroxide and promoted dichlorocyclopropanation was carried out
the phase-transfer catalyst triethylbenzylammonium with simple manipulations and proceeded very smooth-
chloride gave gem-dichlorocyclopropanecarboxylate 8 ly under mild conditions. These salient advantages led to
Scheme 3. Synthesis of the pesticide cycloprothrin.
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a process for the preparation of cycloprothrin that is Methyl 1-(4-Ethoxyphenyl)-2,2-dichlorocyclopropane-
more easily to scale up in a very efficient way. Finally, cy- 1-carboxylic Acid (8)
cloprothrin was synthesized in 62% total yield in 5 steps
To a solution of methyl 2-(4-ethoxyphenyl)lactate (6a) (4.48 g,
20 mmol) in toluene (20 mL) were added p-toluenesulfonic
acid (0.20 g, 1.45 mmol) and hydroquinone (20 mg). The mix-
(starting from ethoxybenzene) compared with the exist-
ing procedure in 9 steps.[3]
ture was heated at reflux for 3 hours under a Dean–Stark
trap. The cooled reaction mixture was washed with 10%
NaOH (10 mLꢀ2). The organic layer was evaporated under
reduced pressure. The residue in CH3CN (40 mL) was treated
Conclusion
with H2O (1.1 mL), CHCl3 (2.4 mL) and KF/Al2O3 (20.0 g).
The mixture was stirred at room temperature overnight. The
In conclusion, we have shown the monophenylation of
KF/Al2O3 was removed by filtration and washed with
electron-rich aromatic compounds with pyruvate ester
CH3CN. The combined filtrate was evaporated and purified
using a Lewis acid-base system. This efficient method
provides a simple and practical procedure for the syn-
thesis of 2-aryllactate esters in high regioselectivity. Pre-
liminary mechanistic studies indicate the involvement of
a Ti(IV)-pyruvate complex species as the reaction inter-
mediate. The mildness of the reaction conditions and
low cost of reagents should make the present methodol-
ogy synthetically useful. This search for the construction
of organic molecules allowed us to develop a new proto-
col for the efficient synthesis of the important pesticide
cycloprothrin in high yield.
by recrystallization from methanol and H2O to give the prod-
uct; yield: 4.95 g (85%).
1-(4-Ethoxyphenyl)-2, 2-dichlorocyclopropane-1-
carboxylic Acid (9)
To a solution of methyl 1-(4-ethoxyphenyl)-2, 2-dichlorocyclo-
propane-1-carboxylic acid (8) (11.0 g, 38 mmol) in MeOH
(4 mL) was added 10% NaOH (50 g). The mixture was re-
fluxed for 3 hours. The cooled reaction mixture was extracted
with toluene (10 mLꢀ2). The aqueous phase was acidified
with 6 N HCl slowly at 08C. The precipitated acid 9 was
filtered and crystallized in the pure state from the methanol
and water; yield: 9.94 g (95%).
Experimental Section
Cycloprothrin
General Experimental Procedures
The compound 9 (1.4 g, 5.1 mmol) was treated with thionyl
chloride (3 mL) and refluxed for 1 hour. The excess thionyl
chloride was then evaporated under vacuum to give the car-
bonyl chloride 10, which was used for the next step of the reac-
tion without further purification.
Sodium cyanide (0.25 g, 5.1 mmol) in water (5 mL) was add-
ed to a solution of m-phenoxybenzaldehyde and tetrabutylam-
monium bromide (50 mg) in toluene (5 mL) at 08C for 1 h.
Then, the above prepared carbonyl chloride 10 was added
and the mixture allowed to stand at room temperature for 2
hours. After extracting with toluene, the combined organic
phase was washed consecutively with aqueous NaHCO3 solu-
tion, water, brine and finally dried over anhydrous sodium sul-
fate. The solvent was evaporated under vacuum to give the cy-
cloprothrin as an oil; yield: 90%.
Infrared (IR) spectra were determined with a FT-IR spectrom-
eter. 1H NMR and 13C NMR spectra were recorded at 300 MHz
in CDCl3 using TMS as an internal standard. The chemical
shifts are expressed in ppm and coupling constants are given
in Hz. Mass spectra were obtained by the EI method. Micro-
analyses were carried out on a Heraeus Rapid-CHNO instru-
ment. All moisture-sensitive reactions were done under an ar-
gon atmosphere in oven-dried (1508C) glassware. Flash chro-
matography was performed using silica gel H (10–40 mm).
Standard reagents and solvents were purified according to
known procedures.
General Procedure for the Friedel–Crafts Reaction of
Electron-Rich Aromatic Compounds to Pyruvate
Esters
Characterization data for all products are given in the Sup-
porting Information.
To a solution of pyruvate ester (2.2 mmol) and aromatic com-
pound (2 mmol) in dried CH2Cl2 (5 mL) was added basic
Al2O3 (510 mg, 5 mmol). The mixture was cooled to À158C.
TiCl4 (416 mg, 2.2 mmol) was added dropwise to the reaction
mixture under an argon atmosphere. The mixture was stirred
for 3–12 hours at À158C and poured into the ice-water
(10 mL). The mixture was filtered and the filtrate was extract-
ed with CH2Cl2 (15 mLꢀ2). The combined organic phase was
washed with 1 N NaOH and brine solutions. The organic phase
was dried with Na2SO4. After removal of the solvent, the crude
product was purified by flash chromatography on silica gel
(hexane:EtOAc¼12:1) to afford the aromatic lactate ester.
Acknowledgements
Shanghai Byelen Chemicals Co., Ltd. is gratefully acknowl-
edged for financial support.
References
[1] a) H. R. Sonawane, N. S. Bellur, J. R. Ahuja, D. G. Kul-
karni, Tetrahedron: Asymmetry 1992, 3, 163; b) S. C. Stin-
Adv. Synth. Catal. 2006, 348, 898 – 904
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
903

Yu-Gui Si et al.
FULL PAPERS
son, Chem. Eng. News 1992, September 28, 46; c) J. P.
Rieu, A. Boucherle, H. Cousse, G. Mouzin, Tetrahedron
1986, 42, 4095.
[8] a) G. Erker, A. A. H. van der Zeijden, Angew. Chem.
Int. Ed. 1990, 29, 512; b) G. Casiraghi, F. Bigi, G. Casnati,
G. Sartori, P. Soncini, G. Gasparri Fava, M. Ferrari Belic-
chi, J. Org. Chem. 1988, 53, 1779; c) A. Citterio, M. Gan-
dolfi, O. Piccolo, L. Filippini, L. Tinucci, E. Valoti, Syn-
thesis 1984, 760; d) G. Casiraghi, G. Sartori, G. Casnati,
F. Bigi, J. Chem. Soc. Perkin Trans. 1 1983, 1649.
[9] T. Ohwada, T. Yamazaki, T. Suzuki, S. Saito, K. Shudo, J.
Am. Chem. Soc. 1996, 118, 6220.
[10] Z.-H. Zhang, L. Yin, Y.-M. Wang, Synthesis 2005, 1949.
[11] a) F. Sirovski, M. Gorokhova, S. Ruban. J. Mol. Catal. A:
Chemical 2003, 197, 213 and references cited therein;
b) S. Kagabu, C. Ando, J. Ando, J. Chem. Soc. Perkin
Trans. 1 1994, 739.
[2] R. L. Metcalf, in: Research in Pesticides, (Ed.: C. O. Chi-
chester), Academic Press, New York, 1965, pp. 18–19.
[3] G. Holan, R. A. Walser, US Patent 4,262,014, 1981.
[4] M.-Y. Chang, S.-T. Chen, N.-C. Chang, Tetrahedron 2002,
58, 3623.
[5] X.-C. He, E. L. Eliel, Tetrahedron 1987, 43, 4979.
[6] G. Blay, I. Fernandez, P. Formentin, J. R. Pedro, A. L.
Rosello, R. Ruiz, Y. Journaux, Tetrahedron Lett. 1998,
39, 3327–3330.
[7] C. L. Parris, R. Dowbenko, R. V. Smith, N. A. Jacobson,
J. W. Pearce, R. M. Christenson, J. Org. Chem. 1962, 27,
455.
[12] M. Mihara, Y. Ishino, S. Minakata, M. Komatsu, J. Org.
Chem. 2005, 70, 5320 and references cited therein.
904
asc.wiley-vch.de
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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