A facile route to diastereomeric phosphorus ylides. Chemoselective synthesis of dialkyl (E)-2-[1-(2-oxocyclopentylidene)ethyl]-2-butenedioates

Article abstract of DOI:10.3390/molecules13020331

2-Acetylcyclopentanone undergoes a smooth reaction with triphenylphosphine and dialkyl acetylenedicarboxylates to produce dialkyl 2-(1-acetyl-2- oxocyclopentyl)-3-(1,1,1-triphenyl-λ^5-phosphanylidene) succinates. These compounds undergo intramol

Full text of DOI:10.3390/molecules13020331

Molecules 2008, 13, 331-339
molecules
ISSN 1420-3049
© 2008 by MDPI
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Full Paper
A Facile Route to Diastereomeric Phosphorus Ylides.
Chemoselective Synthesis of Dialkyl
(E)-2-[1-(2-Oxocyclopentylidene)ethyl]-2-butenedioates
Sakineh Asghari*, Mohammad Bagher Poushani and Samaneh Ramezani
Department of Chemistry, University of Mazandaran, P. O. Box 453, Babolsar, Iran
E-mails: mbpoushani@gmail.com, sara.ramezani@gmail.com
* Author to whom correspondence should be addressed; E-mail: s.asghari@umz.ac.ir
Received: 24 December 2007; in revised form: 21 January 2008 / Accepted: 21 January 2008 /
Published: 7 February 2008
Abstract: 2-Acetylcyclopentanone undergoes a smooth reaction with triphenylphosphine
and dialkyl acetylenedicarboxylates to produce dialkyl 2-(1-acetyl-2-oxocyclopentyl)-3-
(1,1,1-triphenyl-λ5-phosphanylidene)succinates. These compounds undergo intra-
molecular Wittig reactions in boiling benzene to produce highly strained spirocyclobutene
derivatives, which spontaneously undergo ring-opening reactions to produce dialkyl (E)-2-
[1-(2-oxocyclopentyliden)ethyl]-2-butenedioates.
Keywords: 2-Acetylcyclopentanone, phosphorus ylides, spirocompounds, intramolecular
Wittig reaction.
Introduction
Phosphorus ylides are reactive compounds which participate in many valuable syntheses of
organic products [1-4]. Phosphorus ylides are interesting synthetic targets because of their importance
in a variety of industrial, biological and chemical synthetic usages [5-9]. These ylides are usually
prepared by treatment of a phosphonium salt with a base, and the phosphonium salts are usually
obtained from the phosphine and an alkyl halide and also Michael addition of phosphorus nucleophiles
to activated olefins.

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332
In recent years the three-component reactions of triphenylphosphine, electron-deficient derivatives
and ZH-acids (Z=C, O, N, S) that leads to phosphorus ylides have been reported by Yavari et al. [10-
15]. We here report the reaction of 2-acetylcyclopentanone (2, acting as a CH-acid) and dialkyl
acetylenedicarboxylates 1 in the presence of triphenylphosphine. These reactions lead to
diastereomeric phosphorus ylides 3. These compounds undergo intramolecular Wittig reactions in
boiling benzene to produce spirocompounds 4, which spontaneously undergo ring-opening reactions to
produce dialkyl (E)-2-[1-(2-oxopentyliden)ethyl]-2-butenedioates 5 (Scheme1).
Scheme1.
H
O
O
O
CO2R
O
C
PPh3
+
PPh3
+
C
H
CO2R
CO2R
CO2R
1
2
3
benzene
reflux
3-5
a
R
CH3
O
O
CH3 CO2R
-CH3
ring opening
CO2R
H
b
-CH2CH3
-C(CH3)3
CO2R
c
H CO2R
5
4
Results and Discussion
We have not established a mechanism for the formation of 5a-c experimentally, but a reasonable
possibility is indicated in Scheme 2. On the basis of the well established chemistry of trivalent
phosphorus nucleophiles [1, 5, 6], it is reasonable to assume that the initial addition of
triphenylphosphine to the acetylenic ester and subsequently the protonation of the 1:1 adduct was
followed by attack of the carbon moiety of the enolate of CH-acid to the vinylphosphonium cation 6 to
generate ylide 3.
Phosphorus ylides 3a-c then undergo smooth reactions in boiling benzene to produce
triphenylphosphine oxide and spirocompounds 4a-c. These compounds were unstable and were not
isolated, but rather they are spontaneously converted to functionalized 1,3-dienes 5a-c. (Scheme 2).
Compounds 3a-c possess two stereogenic centers, and two diastereomers are expected (I and II) to
form (Scheme 3), and indeed two diastereomers were isolated from the reaction mixtures. It should be
pointed out that both diastereomeric ylides (I and II) were converted to 1,3-dienes with the same
geometry, indicating that the ring opening reactions did not take place as a concerted reaction.

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333
Scheme 2.
H
O
O
(Ph)3P
(Ph)3P
+
RO2C C C CO2R
RO2C
CO2R
1
O
O
O
O
(Ph)3P
RO2C
H
PPh3
CO2R
H
CO2R
CO2R
6
3
benzene
reflux
CH3
O
O
CH3 CO2R
ring opening
H
CO2R
H CO2R
CO2R
4
5
Scheme 3.
O
O
O
O
∗
∗
PPh3
PPh3
∗
∗
+
H
CO2R
RO2C
CO2R
CO2R
H
Diastereomeric ylides (I and II)
benzene
reflux
Intramolecular
Wittig reaction
CH3
O
CH3
O
∗
∗
CO2R
CO2R
∗
+
∗
RO2C H
H CO2R
ring opening
O
CH3 CO2R
H
CO2R
5

Molecules 2008, 13
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1
The structures of the compounds 3a-c were deduced from their elemental analyses, high field H-
13
and 13C-NMR as well as IR spectra data. The 1H- and C-NMR spectra of diastereomeric ylides 3a-c
were consistent with the presence of two geometric isomers. The ylide moiety of these compounds is
strongly conjugated to the adjacent carbonyl group and a rotation around the partial double bond in the
3-(E) and 3-(Z) geometric isomers (Figure 1) is relatively slow on the NMR time scale at ambient
temperature.
Figure 1.
O
O
Ph3P
Ph3P
O
OR
O
OR
CO2R
O
O
CO2R
H
H
3-(Z)
3-(E)
1
The geometries of the double bonds in compound 5 were established using H-NMR data. For the
double bond bearing two ester groups, we considered the chemical shift of the vinylic proton. If the
vinylic proton is in a cis position relative to the ester group on the adjacent carbon atom, an anisotropy
effect is imposed by the carbonyl of the cis ester group which causes a deshielding effect on this
proton, so this proton usually appears at a frequency higher than 6.5 ppm [16], as it was clearly
1
observed in our H-NMR spectra of compounds 5a-c. Therefore, it can be concluded that the vinylic
proton is located in the cis position with respect to the vicinal esteric group. We also investigated the
stereochemistry of the compound 5 by using a NOE experiment. In the decoupling process of the
methyl protons, the NOE difference spectrum did not show a nuclear Overhauser enhancement of the
CH2 protons of the cyclopentanone moiety nor the vinylic proton. The irradiation of the CH2 protons
also the vinylic proton lead to no enhancement of the intensity of the CH3 protons. On the basis of
these results, the geometry for the compound 5 is as indicated in Scheme 3.
Conclusions
In conclusion, the present method may be considered as a practical route for the synthesis of the
stable phosphorus ylides and electron deficient 1,3-butadienes. This procedure has advantages of high
yeild, mild reaction conditions, and simple experimental and work-up conditions.
Experimental
General
Triphenylphosphine, dialkyl acetylenedicarboxylate and 2-acetyl cyclopentanone were obtained
from Fluka (Buchs, Switzerland) and were used without further purification. Melting points were
1
measured on an Electrotermal 9100 apparatus and are uncorrected. H- and 13C-NMR spectra were
measured with a Bruker DRX-500 AVANCE spectrometer at 500 and 125.8 MHz, respectively.

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Elemental analyses were performed using a Heraeus CHN-O-Rapid analyzer. IR spectra were
recorded on a Shimadzu IR-470 spectrometer.
General procedure for preparation of
dialkyl 2-(1-acetyl-2-oxocyclopentyl)-3-[1,1,1-triphenyl-λ5-phosphanylidene]succinates (exemplified
by 3a)
A mixture of dimethyl acetylenedicarboxylate (0.245 mL, 2 mmol) in CH2Cl2 (4 mL) was added
dropwise at -10 ºC over 10 min. to a magnetically stirred solution of 2-acetylcyclopentanone (2, 0.252
g, 2 mmol) and triphenyl phosphine (0.524 g, 2 mmol) in CH2Cl2 (10 mL). The mixture was allowed to
stand at room temperature along with stirring for 24 hours. The solvent was removed under reduced
pressure and the residue was purified by silica gel column chromatography (Merck silica gel 60, 230-
400 mesh) using ethyl acetate-hexane (30:70) as eluent. Two diastereomers were isolated. The solvents
were removed under reduced pressure to give ylides 3a-I and 3a-II as white powders.
Dimethyl 2-(1-acetyl-2-oxocyclopentyl)-3-[1,1,1-triphenyl- λ5-phosphanylidene] succinate (3a)
First diastereomer (3a-I): M.p. 126-128.5 ºC; yield 50 %; IR (KBr, νmax, cm-1): 1755, 1730 and 1705
(C=O), 1629 (C=C); MS m/z (%): 530 (M+, 10), 499 (M+-OCH3, 14), 471 (M+-CO2Me, 25), 487 (M+-
CH3CO, 18), 268 (M+- PPh3, 25), 209 [M+- (PPh3+CO2Me), 37], 43 (CH3CO+, 100); Anal. calcd. for
1
C31H31O6P (530.56); C, 70.18; H, 5.89%. Found: C 70.06; H, 5.83%; H-NMR (CDCl3) 3a-I (Z): δH
1.42-1.50 (2H, m, CH2), 1.60 (3H, s, CH3), 1.97-2.13 (2H, m, CH2), 2.58-2.82 (2H, m, CH2), 2.92 and
3
3.70 (6H, 2s, 2OCH3), 3.56 (1H, d, JPH=18.4 Hz, CH), 7.4-7.7 (15H, m, -Ph); 13C-NMR (CDCl3): δC
20.12 (CH2), 27.64 (CH3), 29.81 (CH2), 37.31 (CH2), 39.38 (d, 1JPC = 122.5 Hz, P=C), 40.09 (d, 2JPC =
13.4 Hz, CH), 48.71 and 51.83 (2OCH3), 73.94 (cyclopentanone quaternary carbon), 127.61 (d, 1JPC =
91.4 Hz, Cipso), 128.58 (d, 3JPC = 11.7 Hz, Cmeta), 131.9 (Cpara), 133.97 (d, 2JPC = 9.4 Hz, Cortho), 169.94
2
3
(d, JPC = 13.1 Hz, C=O ester), 174.24 (d, JPC = 5.3 Hz, C=O ester), 203.70 and 216.22 (2C=O,
1
ketones). 3a-I (E) (31 %) H-NMR (CDCl3): δH 1.55 (3H, s, CH3), 1.62-1.65 (2H, m, CH2), 2.2-2.40
(2H, m, CH2), 2.48-2.53 (2H, m, CH2), 3.43 and 3.68 (2OCH3), 3.59 (1H, d, 3JPH = 18.5 Hz, CH), 7.4-
7.67 (15H, m, -Ph); 13C-NMR (CDCl3): δC 21.5 (CH2), 25.44 (CH3), 29.14 (CH2), 37.5 (d, 1JPC = 127.5
2
Hz, P=C), 38.70 (CH2), 48.12 (d, JPC = 13.2 Hz, CH), 49.65 and 52.34 (2OCH3), 73.26 (cyclo-
1
3
pentanone quaternary carbon), 126.7 (d, JPC = 91.7 Hz, Cipso), 128.2 (d, JPC = 11.5 Hz, Cmeta), 131.6
2
2
3
(Cpara), 132.13 (d, JPC = 9.9 Hz, Cortho), 168.5 (d, JPC = 12.8 Hz, C=O ester), 174.8 (d, JPC = 5.2 Hz,
C=O ester), 203.65 and 215.02 (2C=O, ketones).
Second diastereomer (3a-II): M.p. 151-152 ºC; yield 45 %; IR (KBr, νmax, cm-1): 3059 and 2987 (CH),
1751, 1726 and 1689 (C=O), 1629 (C=C); MS m/z (%): 530 (M+, 8), 471 (M+-CO2Me, 18), 487 (M+-
CH3CO, 30), 268 (M+- PPh3, 21), 209 (M+- (PPh3+CO2Me), 28), 43 (CH3CO+, 100); Anal. calcd. for
1
C31H31O6P (530.56); C, 70.18; H, 5.89%; Found C 70.05, H 5.78 %; 3a-II (Z) (68 %) H-NMR
(CDCl3): δH 1.64 (3H, s, CH3), 1.82-1.95 (2H, m, CH2), 2.09-2.18 (2H, m, CH2), 2.88-2.96 (2H, m,
CH2), 2.80 and 3.69 (6H, 2s, 2OCH3), 3.49 (1H, d, 3JPH = 18.8 Hz, CH), 7.45-7.64 (15H, m, -Ph); 13C-
1
NMR (CDCl3): δC 20.33 (CH2), 25.45 (CH3), 29.12 (CH2), 39.84 (d, JPC = 121.9 Hz, CH), 40.07

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(CH2), 48.37 (d, 2JPC = 13.4 Hz, CH), 51.82 and 52.22 (2OCH3), 73.97 (cyclopentanone quaternary
1
3
4
carbon), 127.5 (d, JPC = 91.5 Hz, Cipso), 128.43 (d, JPC = 12.20 Hz, Cmeta), 131.92 (d, JPC = 2.9 Hz,
2
2
3
Cpara), 134.02 (d, JPC = 9.5 Hz, Cortho), 170.39 (d, JPC = 13.3 Hz, C=O ester), 175 (d, JPC = 6.5 Hz,
C=O ester), 202.8 and 215 (2C=O, ketones). 3a-II (E) (32 %) 1H-NMR (CDCl3): δH 1.65 (3H, s, CH3),
1.82-1.95 (2H, m, CH2), 2.09-2.18 (2H, m, CH2), 2.88-2.96 (2H, m, CH2), 3.47 and 3.69 (6H, 2s,
2OCH3), 3.64 (1H, d, JPH = 19.5 Hz, CH), 7.45-7.64 (15H, m, -Ph); 13C-NMR (CDCl3): δC 21.07
(CH2), 25.84 (CH3), 28.55 (CH2), 39.71 (CH2), 40.58 (d, JPC = 129.4 Hz, P=C), 48.37 (d, 2JPC = 13.4
3
1
1
Hz, CH), 51.82 and 52.22 (2OCH3), 73.97 (cyclopentanone quaternary carbon), 127.3 (d, JPC = 90.8
Hz, Cipso), 128.33 (d, 3JPC = 12.07 Hz, Cmeta), 131.97 (d, 4JPC = 2.83 Hz, Cpara), 132.12 (d, 2JPC = 9.3 Hz,
2
3
Cortho), 171.07 (d, JPC = 13.5 Hz, C=O ester), 172.5 (d, JPC = 6.9 Hz, C=O ester), 202.3 and 214.8
(2C=O, ketones).
Diethyl 2-(1-acetyl-2-oxocyclopentyl)-3-[1,1,1-triphenyl- λ5-phosphanylidene] succinate (3b)
First diastereomer (3b-I): M.p. 157.5-159 ºC; yield 53 %; IR (KBr, νmax, cm-1): 1745, 1730 and 1705
(C=O), 1632 (C=C); MS m/z (%): 558 (M+, 7), 530 (M+-C2H5, 14), 515 (M+-CH3CO, 31), 485 (M+-
CO2Et, 18), 296 (M+- PPh3, 20), 223 [M+- (PPh3+CO2Et), 37], 43 (CH3CO+, 100); Anal. calcd. for
1
C33H35O6P (530.56); C, 70.96; H, 6.32%; Found: C 70.88; H, 6.25%; 3b-I (Z) (64 %) H-NMR
3
(CDCl3): δH 1.18 (3H, t, 3JHH = 7 Hz, CH3), 1.25 (3H, t, JHH = 7.2 Hz, CH3), 1.51-1.55 (2H, m, CH2),
1.57 (3H, s, CH3), 1.85-1.97 (2H, m, CH2), 2.75-2.78 (2H, m, CH2), 3.56 (1H, d, 3JPH = 18.5 Hz, CH),
3.66-3.68 (2H, m, OCH2), 4.12-4.15 (2H, m, OCH2), 7.46-7.81 (15H, m, -Ph); 13C-NMR (CDCl3): δC
14.13 and 14.34 (2CH3), 20.17 and 27.70 (2CH2), 29.69 (CH3), 37.38 (d, 1JPC = 122.78 Hz, P=C),
2
37.78 (CH2), 49.12 (d, JPC = 12.96 Hz, CH), 57.54 and 61.35 (2OCH2), 74.17 (cyclopentanone
1
3
quaternary carbon), 128.16 (d, JPC = 92.98 Hz, Cipso), 128.53 (d, JPC = 11.82 Hz, Cmeta), 131.86
2
2
3
(Cpara), 134.07 (d, JPC = 9.4 Hz, Cortho), 166.03 (d, JPC = 12.8 Hz, C=O ester), 173.87 (d, JPC = 6.3
Hz, C=O ester), 204.51 and 216.40 (2C=O, ketones). 3b-I (E) (36 %); 1H-NMR (CDCl3): δH 1.21 (3H,
t, 3JHH = 7.1 Hz, CH3), 1.25 (3H, t, 3JHH = 7.2 Hz, CH3), 1.41-1.46 (2H, m, CH2), 1.62 (3H, s, CH3CO),
3
2.07-2.12 (2H, m, CH2), 2.44-2.52 (2H, m, CH2), 3.37-3.39 (2H, m, OCH2), 3.47 (1H, d, JPH = 19.8
Hz, CHCO2Et), 4.12-4.15 (2H, m, OCH2), 7.46-7.81 (15H, m, -Ph); 13C-NMR (CDCl3): δC 13.10 and
1
13.60 (2CH3), 19.28 and 25.86 (2CH2), 29.05 (CH3CO), 37.1 (d, JPC = 125.8 Hz, P=C), 37.26
2
(CH2CO), 48.5 (d, JPC = 12.6 Hz, CHCO2Et), 60.97 and 62.15 (2OCH2), 71.5 (cyclopentanone
quaternary carbon), 127.8 (d, JPC = 90.8 Hz, Cipso), 129.89 (3JPC=12.4 Hz, Cmeta), 132.13 (2JPC=9.81
1
2
3
Hz, Cortho), 134.83 (Cpara), 165.04 (d, JPC=13.1 Hz, C=O ester), 170.72 (d, JPC = 6.7 Hz, C=O ester),
203.84 and 215.01 (2C=O, ketones).
Di-tert-butyl 2-(1-acetyl-2-oxocyclopentyl)-3-[1,1,1-triphenyl- λ5-phosphanylidene] succinate (3c)
First diastereomer (3c-I): M.p. 149-149.5 ºC; yield 53 %; IR (KBr, νmax, cm-1): 1752, 1733 and 1705
(C=O), 1620 (C=C); MS m/z (%): 614 (M+, 5), 571 (M+-CH3CO, 25), 558 (M+-C4H8, 16), 513 (M+-
t
t
t
CO2 Bu, 28), 456 [M+-(CO2 Bu+C4H9), 38], 352 (M+- PPh3, 14), 251 [M+- (PPh3+CO2 Bu), 37], 43
(CH3CO+, 100); Anal. calcd. for C37H43O6P (614.73): C 72.29, H 7.05 %; Found C 72.15, H 6.95 %;
3c-I (Z) (59 %) 1H-NMR (CDCl3): δH 0.84 (9H, s, CMe3), 1.47 (9H, s, CMe3), 1.49 (3H, s, CH3), 1.87-

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1.88 (2H, m, CH2), 2.01-2.06 (2H, m, CH2), 2.78-2.83 (2H, m, CH2), 3.37 (1H, d, 3JPH = 16.8 Hz, CH),
7.4-7.55 (15H, m, -Ph); 13C-NMR (CDCl3): δC 20.28 and 25.83 (2CH2), 28 and 28.44 (2CMe3), 28.21
1
2
(CH3), 37.58 (d, JPC = 120.79 Hz, P=C), 38.76 (CH2), 44.92 (d, JPC = 14.28 Hz, CH), 74.49
(cyclopentanone quaternary carbon), 80.65 and 81.67 (2CMe3), 125.8 (d, 1JPC = 85.6 Hz, Cipso), 128.52
3
2
3
(d, JPC = 12.1 Hz, Cmeta), 131.96 (Cpara), 132.1 (d, JPC = 9.94 Hz, Cortho), 169.47 (d, JPC = 12.8 Hz,
C=O ester), 175.73 (d, 3JPC = 6.5 Hz, C=O ester), 204.19 and 216.98 (2C=O, ketones). 3c-I (E) (41 %)
1H-NMR (CDCl3): δH 1.36 (9H, s, CMe3), 1.40 (9H, s, CMe3), 1.54 (CH3CO), 2.01-2.06 (2H, m, CH2),
2.51-2.56 (2H, m, CH2), 2.90-2.94 (2H, m, CH2), 3.35 (1H, d, 3JPH = 16.8 Hz, CH), 7.65-7.78 (15H, m,
-Ph); 13C-NMR (CDCl3): δC 20.28 and 25.16 (2CH2), 28 and 28.04 (2CMe3), 28.13 (CH3), 38.3 (d, 1JPC
= 122 Hz, P=C), 38.29 (CH2), 49.1 (d, 2JPC = 14.1 Hz, CH), 74.11 (cyclopentanone quaternary carbon),
1
3
81.18 and 81.79 (2CMe3), 126.2 (d, JPC = 84.5 Hz, Cipso), 128.30 (d, JPC = 11.95 Hz, Cmeta), 131.66
2
2
3
(Cpara), 135.38 (d, JPC = 10.1 Hz, Cortho), 168.80 (d, JPC = 13 Hz, C=O ester), 172.56 (d, JPC = 6.5
Hz, C=O ester), 202.41 and 212.82 (2C=O, ketones).
Second diastereomer; (3c-II): M.p. 157.5-159ºC; yield 42 %; IR (KBr, νmax, cm-1): 1748, 1725 and
1695 (C=O), 1612 (C=C); Anal. calcd. for C37H43O6P (614.72): C 72.29, H 7.05 %; Found C 72.18, H
6.97 %. 3c-II (Z) (66 %) 1H-NMR (CDCl3): δH 0.85 and 1.50 (18H, 2s, 2CMe3), 1.56 (3H, s, CH3CO),
3
1.87-1.92 (2H, m, CH2), 1.97-2.03 (2H, m, CH2), 3.10-3.16 (2H, m, CH2), 3.31 (1H, d, JPH = 20 Hz,
13
CH), 7.40-7.52 (15H, m, -Ph); C-NMR (CDCl3): δC 20.25 and 25.03 (2CH2), 28.26 and 28.31
2
(2CMe3), 28.80 (CH3CO), 38.47 (d, 1JPC = 122.3 Hz, P=C), 40.23 (CH2CO), 50.03 (d, JPC = 14.1 Hz,
CH), 74.38 (d, 3JPC = 3.4 Hz, cyclopentanone quaternary carbon), 77.01 and 80.71 (2CMe3), 125.90 (d,
1JPC = 84.8 Hz, Cipso), 128.1 (d, 3JPC = 11.8 Hz, Cmeta), 131.95 (Cpara), 132.10 (d, 2JPC = 9.4 Hz, Cortho),
160.9 (d, 2JPC = 13.1 Hz, C=O ester), 173.66 (d, 3JPC = 6.8 Hz, C=O ester), 203.12 and 215.98 (2C=O,
1
ketones). 3c-II (E) (34 %) H-NMR (CDCl3): δH 1.37 and 1.50 (18H, 2s, 2CMe3), 1.52 (3H, s,
CH3CO), 2.05-2.19 (2H, m, CH2), 2.43-2.48 (2H, m, CH2), 2.89-2.98 (2H, m, CH2), 3.30 (1H, d, 3JPH =
21.9 Hz, CH), 7.82-8.1 (15H, m, -Ph); 13C-NMR (CDCl3): δC 20.40 and 24.88 (2CH2), 27.91 and 28.48
(2CMe3), 29.28 (CH3CO), 40.62 (d, 1JPC = 131.33 Hz, P=C), 39.42 (CH2CO), 49.39 (d, 2JPC = 13.8 Hz,
CH), 74.02 (d, 3JPC = 3.5 Hz cyclopentanone quaternary carbon), 77.78 and 80.60 (2CMe3), 125.90 (d,
1JPC = 84.8 Hz, Cipso), 128.52 (d, 3JPC = 12.1 Hz, Cmeta), 131.79 (Cpara), 135.36 (d, 2JPC = 9.9 Hz, Cortho),
2
3
170.83 (d, JPC = 13.3 Hz, C=O ester), 173.42 (d, JPC = 7 Hz, C=O ester), 202.61 and 215.43 (2C=O,
ketones).
Preparation of Dimethyl (E)-2-[1-(2-oxocyclopentylidene)ethyl]-2-butenedioate (5a)
Compound 3a (I or II) was refluxed in benzene for 24 hours. The solvent was removed under
reduced pressure and the viscous residue was purified by silica gel column chromatography (Merck
silica gel 60, 230-400 mesh) using ethyl acetate-hexane (30:70) as eluent. The solvents were removed
under reduced pressure to give the product. White powder; m.p. 67-69 ºC, yield 85 %; IR (KBr, νmax
,
1
3
cm-1): 1745, 1735 and 1710 (C=O), 1627 (C=C); H-NMR (CDCl3): δH 1.94 (2H, t, JHH = 7.4 Hz,
CH2), 1.98 (3H, s, CH3), 2.50 (2H, t, 3JHH = 7.3 Hz, CH2), 2.70 (2H, m, CH2), 3.66 (3H, s, OCH3), 3.73
13
(3H, s, OCH3), 6.71 (1H, s, CH); C-NMR (CDCl3): δC 19.58 and 22.61 (2CH2), 28.42 (CH3), 39.06
(CH2), 51.78 and 52.67 (2OCH3), 124.89, 134.74, 139.82 and 149.97 (olefinic carbons), 165.06 and

Molecules 2008, 13
338
165.22 (2C=O, esters), 206.29 (C=O, ketone); MS m/z (%): 252 (M+, 21), 237 (M+- Me, 25), 221 (M+-
OMe, 34), 193 (M+-CO2Me, 42), 162 [M+-(CO2Me+OMe), 48], 110 (M+-MeO2CCCCO2Me, 100);
Anal. calcd. for C13H16O5 (252.27): C 61.90, H 6.39 %; Found C 61.77, H 6.30 %. The following
compounds were prepared similarly:
Diethyl (E)-2-[1-(2-oxocyclopentylidene)ethyl]-2-butenedioate (5b). White powder; m.p. 66-68 ºC;
1
yield 80 %; IR (KBr, νmax, cm-1): 1740, 1730 and 1710 (C=O), 1620 (C=C); H-NMR (CDCl3): δH
1.25-1.44 (6H, m, 2CH3), 2 (2H, t, 3JHH = 7.8 Hz, CH2), 2.04 (3H, s, CH3), 2.30-2.40 (2H, t, 3JHH = 7.7
3
3
Hz, CH2), 2.75-2.77 (2H, m, CH2), 4.17 (2H, q, JHH = 7.1 Hz, OCH2), 4.25 (2H, q, JHH = 7.1 Hz,
13
OCH2), 6.77 (1H, s, CH); C-NMR (CDCl3): δC 14.12 and 14.14 (2CH3), 19.62 and 22.16 (2CH2),
28.45 (CH3CO), 39.08 (CH2), 60.07 and 61.60 (2OCH2), 125.18, 134.55, 140.1 and 149.92 (olefinic
carbons), 164.57 and 164.9 (2C=O, esters), 203.5 (C=O, ketone); ); MS m/z (%): 280 (M+, 16), 265
(M+-Me, 22), 207 (M+-CO2Et, 28), 162 [M+-(CO2Et+OEt), 48], 110 (M+-EtO2CCCCO2Et, 100); Anal.
calcd. for C15H20O5 (280.32): C 64.27, H 7.19 %; Found C 64.15, H 7.08 %.
Di-tert-butyl (E)-2-[1-(2-oxocyclopentylidene)ethyl]-2-butenedioate (5c). White powder; m.p. 64-67
ºC; yield 87 %; IR (KBr, νmax, cm-1): 1738, 1730 and 1715 (C=O), 1618 (C=C); 1H-NMR (CDCl3): δH
1.40 and 1.47 (2CMe3), 1.92 (2H, t, 3JHH = 7.4 Hz, CH2), 1.95 (3H, s, CH3CO), 2.22-2.27 (2H, t, 3JHH
=
7.3 Hz, CH2), 2.65-2.70 (2H, m, CH2), 6.54 (1H, s, CH); 13C-NMR (CDCl3): δC 19.62 and 22.44
(2CH2), 27.92 and 27.98 (2CMe3), 28.48 (CH3), 39.10 (CH2), 81.02 and 81.82 (2CMe3), 126.60,
133.94, 140.63 and 149.52 (olefinic carbons), 163.86 and 164.43 (2C=O, esters), 205.71 (C=O,
t
ketone); MS m/z (%): 336 (M+, 8), 280 (M+- C4H8, 22), 263 (M+-OtBu, 28), 235 (M+-CO2 Bu, 20), 179
t
t
t
[M+-(CO2 Bu+C4H8), 35], 162 [M+-(CO2 Bu+OtBu), 42], 110 (M+-tBuO2CCCCO2 Bu, 54); Anal. calcd.
for C19H28O5 (336.43): C 67.83, H 8.38 %; Found C 67.70, H 8.26 %.
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Sample Availability: Available from the authors.
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