A Facile Route to Diastereomeric Phosphorus Ylides. Chemoselective Synthesis of Dialkyl (E)-2-[1-(2-Oxocyclopentylidene)ethyl]-2-butenedioates

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.


Introduction
Phosphorus ylides are reactive compounds which participate in many valuable syntheses of organic products [1][2][3][4]. Phosphorus ylides are interesting synthetic targets because of their importance in a variety of industrial, biological and chemical synthetic usages [5][6][7][8][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.

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. The structures of the compounds 3a-c were deduced from their elemental analyses, high field 1 Hand 13 C-NMR as well as IR spectra data. The 1 H-and 13 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.

3-(E) 3-(Z)
O Ph 3 P The geometries of the double bonds in compound 5 were established using 1 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 observed in our 1 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 CH 2 protons of the cyclopentanone moiety nor the vinylic proton. The irradiation of the CH 2 protons also the vinylic proton lead to no enhancement of the intensity of the CH 3 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.

General
Triphenylphosphine, dialkyl acetylenedicarboxylate and 2-acetyl cyclopentanone were obtained from Fluka (Buchs, Switzerland) and were used without further purification. Melting points were measured on an Electrotermal 9100 apparatus and are uncorrected. 1 H-and 13 C-NMR spectra were measured with a Bruker DRX-500 AVANCE spectrometer at 500 and 125.8 MHz, respectively. Elemental analyses were performed using a Heraeus CHN-O-Rapid analyzer. IR spectra were recorded on a Shimadzu IR-470 spectrometer.