Preparation of cyclopentene

Alternatively, a cascade Michael/aldol process has also been devised for the preparation of cyclopentenes also using this iminium/enamine manifold. In this reaction, a malonate reagent containing a functionalized side chain incorporating a formyl group at the appropriate position has been used as the nucleophile initiating the cascade process (Scheme 7.20). The reaction started with the Michael addition of the malonate to the a,(3-unsaturated aldehyde... 

The original application of the Danheiser cyclization was for the preparation of cyclopentenes employing a, -unsaturated ketones as the electophilic component of the annulation process. A variety of a, y9-unsaturated ketones with various alkyl substitution patterns readily participate in the Danheiser annulation. ... 

Isomerization of unactivated vinyl cyclopropanes to cyclopentanes using nickel carbene complexes has been accomplished. The nickel carbene catalyst was generated in situ from Ni(cod)2, PPBF4 salt and base. These reactions constitute a simple protocol for the preparation of cyclopentenes by the isomerization of vinyl cyclopropanes. This result, combined with recent developments in the preparation of vinyl cyclopropanes,may provide a powerful new approach to the preparation of five-membered ring structures. 

Zuo and Louie discovered that the combination of Ni° with a sterically hindered NHC ligand catalyzed the isomerization of a variety of activated or unactivated vinylcyclopropanes under mild conditions to afford the corresponding cyclopentenes in very good yields (Equation (10.26)). This process also allowed for the efficient preparation of cyclopentene possessing a bicyclic framework, useful for potential biological applications. 

The dicarboxylation of cyclic alkenes is a useful reaction. All-c.vo-methyl-7-oxabicyclo(2.2.1]heptane-2,3,5,6-tetracarboxylate (233) was prepared from the cyclic alkene 232 using Pd on carbon and CuCh in MeOH at room temperature with high diastereoselectivity[216]. The dicarbonylation of cyclopentene... 

In a 200-ml three-necked flask fitted with a dropping funnel (drying tube) is placed a solution of 13.4 g (0.12 mole) of 1-octene in 35 ml of THF. The flask is flushed with nitrogen and 3.7 ml of a 0.5 M solution of diborane (0.012 mole of hydride) in THF is added to carry out the hydroboration. (See Chapter 4, Section I regarding preparation of diborane in THF.) After 1 hour, 1.8 ml (0.1 mole) of water is added, followed by 4.4 g (0.06 mole) of methyl vinyl ketone, and the mixture is stirred for 1 hour at room temperature. The solvent is removed, and the residue is dissolved in ether, dried, and distilled. 2-Dodecanone has bp 119710 mm, 24571 atm. (The product contains 15 % of 5-methyl-2-undecane.) The reaction sequence can be applied successfully to a variety of olefins including cyclopentene, cyclohexene, and norbornene. 

A dry 5(X)-mI flask equipped with a thermometer, pressure-equalizing dropping funnel, and magnetic stirrer is flushed with nitrogen and then maintained under a static pressure of the gas. The flask is charged with 50 ml of tetrahydrofuran and 13.3 ml (0.15 mole) of cyclopentene, and then is cooled in an ice bath. Conversion to tricyclo-pentylborane is achieved by dropwise addition of 25 ml of a 1 M solution of diborane (0.15 mole of hydride see Chapter 4, Section 1 for preparation) in tetrahydrofuran. The solution is stirred for 1 hour at 25° and again cooled in an ice bath, and 25 ml of dry t-butyl alcohol is added, followed by 5.5 ml (0.05 mole) of ethyl bromoacetate. Potassium t-butoxide in /-butyl alcohol (50 ml of a 1 M solution) is added over a period of 10 minutes. There is an immediate precipitation of potassium bromide. The reaction mixture is filtered from the potassium bromide and distilled. Ethyl cyclopentylacetate, bp 101730 mm, 1.4398, is obtained in about 75% yield. Similarly, the reaction can be applied to a variety of olefins including 2-butene, cyclohexene, and norbornene. 

Many authors have observed that the cis-trans ratio of the products of the metathesis reaction is equal to the thermodynamic equilibrium value. This suggests that the reaction is not highly stereoselective. However, under certain conditions the product distribution is influenced by kinetic factors. For instance, it proves to be possible to prepare from cyclopentene... 

Ruthenium hydride complexes, e.g., the dimer 34, have been used by Hofmann et al. for the preparation of ruthenium carbene complexes [19]. Reaction of 34 with two equivalents of propargyl chloride 35 gives carbene complex 36 with a chelating diphosphane ligand (Eq. 3). Complex 36 is a remarkable example because its phosphine ligands are, in contrast to the other ruthenium carbene complexes described so far, arranged in a fixed cis stereochemistry. Although 36 was found to be less active than conventional metathesis catalysts, it catalyzes the ROMP of norbornene or cyclopentene. 

Johnson, C. R. el. al, Tetrahedron Lett., 1964, 45,3327 Preparation of the 4-bromo compound by partial debromination of crude 3,5-dibromo-cyclopentene by addition of its ethereal solution to the aluminate in ice-cold ether is hazardous. Explosions have occurred on 2 occasions about 1 h after addition of dibromide. 

The experimental procedure described is essentially one reported by Ziegler and Wilms 8 as subsequently modified,9 except that the glutaraldehyde is prepared from 2-ethoxy-3,4-dihydro-2H-pyran instead of cyclopentene ozonide 8 or pyridine via dihydropyridine and glutaraldehyde dioxime.9 Essentially these procedures have also been reported briefly by other investigators.10... 

Castagnino, E., Corsano, S., and Strappavecchia, G.P., The preparation of a novel oxo-cyclopenten-2-phosphonate derivative, useful intermediate for 2-alkyl-substituted cyclopentenones synthesis, Tetrahedron Lett., 93, 1985. 

Cyclopentanecarboxaldehyde has been prepared by the procedure described above 2 3 by the reaction of aqueous nitric acid and mercuric nitrate with cyclohexene 6 by the action of magnesium bromide etherate 6 or thoria 7 on cyclohexene oxide by the dehydration of frarei-l, 2-cyclohexanediol over alumina mixed with glass helices 8 by the dehydration of divinyl glycol over alumina followed by reduction 9 by the reaction of cyclopentene with a solution of [HFe(CO)4] under a carbon monoxide atmosphere 10 and by the reaction of cyclopentadiene with dicobalt octacarbonyl under a hydrogen and carbon monoxide atmosphere.11... 

Novel thermal and metal-catalyzed di-tert-butylsilylene 161 transfer reactions have been reported by Woerpel < / /.308-312 The transfer reactions required the inital preparation of cyclohexene-derived silacyclopropanes 169-171, which has been achieved by trapping of di-fert-butylsilylenoid, generated from /-Bu2SiCl2 and lithium, with cyclohexenes (Scheme 26).305 It is noteworthy that these reactions occur with remarkably high diastereoselectivities when 2-substituted cyclohexenes are used. The silacyclopropanation of 169 with functionalized cyclopentenes under thermal conditions (115°C) has provided /razy-silacyclopropanes, such as 172, with diastereoselectivities up to 96 4, whereas no silacyclopropanes were obtained from the direct reaction of the same cyclopentenes with /-Bu2SiCl2 in the presence of lithium (Scheme 26).308... 

The cis form has been prepared by the syn hydroxylation of cyclopentene by osmium tetroxide as follows ... 

Volume 75 concludes with six procedures for the preparation of valuable building blocks. The first, 6,7-DIHYDROCYCLOPENTA-l,3-DIOXIN-5(4H)-ONE, serves as an effective /3-keto vinyl cation equivalent when subjected to reductive and alkylative 1,3-carbonyl transpositions. 3-CYCLOPENTENE-l-CARBOXYLIC ACID, the second procedure in this series, is prepared via the reaction of dimethyl malonate and cis-l,4-dichloro-2-butene, followed by hydrolysis and decarboxylation. The use of tetrahaloarenes as diaryne equivalents for the potential construction of molecular belts, collars, and strips is demonstrated with the preparation of anti- and syn-l,4,5,8-TETRAHYDROANTHRACENE 1,4 5,8-DIEPOXIDES. Also of potential interest to the organic materials community is 8,8-DICYANOHEPTAFULVENE, prepared by the condensation of cycloheptatrienylium tetrafluoroborate with bromomalononitrile. The preparation of 2-PHENYL-l-PYRROLINE, an important heterocycle for the synthesis of a variety of alkaloids and pyrroloisoquinoline antidepressants, illustrates the utility of the inexpensive N-vinylpyrrolidin-2-one as an effective 3-aminopropyl carbanion equivalent. The final preparation in Volume 75, cis-4a(S), 8a(R)-PERHYDRO-6(2H)-ISOQUINOLINONES, il lustrates the conversion of quinine via oxidative degradation to meroquinene esters that are subsequently cyclized to N-acylated cis-perhydroisoquinolones and as such represent attractive building blocks now readily available in the pool of chiral substrates. 

One of the first applications envisioned of ROMP was the preparation of polyenes with rubber-like properties by ROMP of cyclopentene. Although this polymer did not have the optimum product profile, the polymer obtained by ROMP of cyclooctene can indeed be used in blends with other rubbers, conferring on the final product improved mechanical properties and simplifying its processing. This polymer, called Vestenamer 8012, is being produced by Chemische Werke Hiils, Marl, Germany, and the annual capacity in 1990 reached 12000t. 

The synthesis of diarylethenes from hetaryl bromides and octafluoro-cyclopentene using a microflow system allows the preparation of various symmetrical and unsymmetrical diarylperfluorocyclopentenes in 47-87% yields at 0°C (07CC2947). 

The synthesis of dithienylethenes bound to heteroanalogs of cyclopentene, in particular to the 2,5-dihydrothiophene ring, also deserves notice. Diketone 135 was prepared by the replacement of the bromine atoms in two molecules of 134 by the sulfide anion followed by the McMurry cyclization to form thiacyclopentene derivative 136 (03OL1435). Diiodide 137 is also successfully used as the synthon for the subsequent functionalization this compound is involved in the Suzuki reaction to... 

The double )5-scission pathway becomes dominant in bicyclic systems (Equations (7)-(9) and Scheme 13). Thus, cyclopentene ozonide (69) gives cyclopropane (Equation (7)) <68TL329l>. Photolysis of the ozonide derived from 1,4-benzodioxins (70) provides a method for the preparation of labile o-benzoquinones (71) (Scheme 13) <87JOC56l6>. Photolysis can also provide a route to unstable compounds and transient species such as the aziridine-2,3-dione (72) (Equation (8)), identified at 77 K using infrared spectroscopy <80JA6902>. Relatively unstable azacarbapenems (73) have been prepared by photolysis of tricyclic compounds containing a cyclobutene ozonide (Equation (9)). On silica, the 1,2,4-trithiolane (74) underwent photo-equilibration (Equation (10)) with the 1,3-dithetane (75) and sulfur. 

The last synthesis to evolve which is due to Ito and his coworkers is interesting in that it relies on a stereospecific skeletal rearrangement of a bicyclo[2.2.2]octane system which in turn was prepared by Diels-Alder methodology (Scheme XLVIII) Heating of a toluene solution of cyclopentene 1,2-dicarboxylic anhydride and 4-methylcyclohexa-l,4-dienyl methyl ether in the presence of a catalytic quantity of p-toluenesulfonic acid afforded 589. Demethylation was followed by reduction and cyclization to sulfide 590. Desulfurization set the stage for peracid oxidation and arrival at 591. Chromatography of this intermediate on alumina induced isomerization to keto alcohol 592. Jones oxidation afforded diketone 593 which had earlier been transformed into gymnomitrol. 

The only practical laboratory preparation of cyclopentadiene is by the depolymerization of dicyclopentadieneA M 3-Chloro-cyclopentene has been prepared by the addition of hydrogen chloride to cyclopentadiene.2 7 8 9 10... 

Gem-Dihalocydopropanes belong to the most readily available cyclopropane derivatives known today. They have been shown to be extremely valuable starting materials for the preparation of cyclopropanes and cyclopropenes, they may be converted to bicyclobutane derivatives and spiropentanes, can lead to allenes and the higher cumulenes, cyclopentenes and cyclopentadienes, and many other classes of compounds, both hydrocarbon systems and derivatives with valuable functional groups. The article summarizes the preparative developments in the area of gem-dihalocyclopropane chemistry during the last decade. 

The methyl ester has also been obtained by esterification of cyclopentanecarboxylic acid.8 The acid, in turn, has been prepared by the Favorskii rearrangement,6 7 9-11 by the reaction of cyclopentyl Grignard reagent with carbon dioxide,12 by the carbonylation of cyclopentyl alcohol with nickel carbonyl13 or with formic acid in the presence of sulfuric acid,14 and by the hydrogenation of cyclopentene-1-carboxylic acid prepared from ethyl cyclopentanone-2-carboxylate 15 or from cyclopentanone cyanohydrin.16... 

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