Cyclopentenones, from

Cyclopentenones. from 1.4-diketones. 886-887 Cyclopropane, angle strain in, 115 bent bonds in. 115 from alkenes. 227-229 molecular model of, 111. 115 strain energy of, 114 torsional strain in, 115 Cystathionine, cysteine from. 1177 Cysteine, biosynthesis of, 1177 disulfide bridges from, 1029 structure and properties of, 1018 Cytosine, electrostatic potential map of, 1104... 

Separation of 3-methyl-2-cyclopentenone from the reaction mixture... 

The resulting solutions ixom above experiments were saturated with sodium chloride and then diethyl ether was added to extract 3-methyl-2-cyclopentenone from aqueous NaOH solution. The diethyl ether in the oiganic phase was then removed by a rotary vacuum evaporator. The heavy cut residue was separated by a simple distillation. The collected 3-methyl-2-cydopentenone was dried over solid anhydrous MgS04 and filtered. 

The Pauson-Khand reaction is the Co-induced formation of cyclopentenones from ene-ynes and CO. One impressive example of a domino Pauson-Khand process is the synthesis of fenestrane 6/4-15, as reported by Keese and colleagues [278]. The transformation is initiated by a double Grignard reaction of 4-pentynoic acid 6/4-12, followed by protection of the formed tertiary hydroxyl group to give 6/4-13. The Co-induced polycyclization of 6/4-13 led directly to the fenestrane 6/4-15... 

Rautenstrauch in 1984, this isomerization initially served for the synthesis of cyclopentenones from 1,4-... 

The availability of cyclopentenones from butanolides allows the lactone annulation to facilitate the synthesis of cyclopentyl natural and unnatural products. An example that highlights the latter is dodecahedrane (178) for which 179 constitutes a critical synthetic intermediate 136,137). Lateral fusion of cyclopentenones as present in 179 can arise by acid induced reorganization and dehydration of 180. While a variety of routes can be envisioned to convert a ketone such as 182 into 180, none worked satisfactorily137 On the other hand, the cyclobutanone spiro-annulation approach via 181 proceeds in 64 % overall yield. Thus, the total carbon cource of dodecahedrane derives from two building blocks — cyclopentadiene and the cyclopropyl sulfonium ylide. 

Acid-catalyzed electrocyclic formation of cyclopentenone from di-vinyl ketone. 

Fig. 4.2 Possible mechanisms for the formation of (a) cyclopentanones from 4-all

CYCLOPENTENONES FROM , ot -DIBROMOKETONES AND ENAMINES 2,5-DIMETHYL-3-PHENYL-2-CYCLOPENTEN-l-ONE... 

Cyclopentenones from Nazarov and Nazarov-type cyclizations Ferric chloride, 133 Organotin reagents, 211 Trifluoroacetic anhydride-2,6-Luti-dine, 177... 

Mierau V, Sterner O, Anke T (2004) Two New Biologically Active Cyclopentenones from Dasyscyphus sp. A47-98. J Antibiot 57 311... 

Rether J, Erkel G, Sterner O, Anke T (2005) Inhibition of TNF-a Promoter Activity and Synthesis by All-99-1, a New Cyclopentenone from the Ascomycctc Mollisia melaleuca. Z Naturforsch 60c 478... 

Cyclopentenones from carbohydrates.9 This transformation involves glycosides (2) obtained by reaction of 6-hydroxy-2,3-dihydro-6//-pyranones (1) with an... 

Kao, T.-C., Chuang, G.J. and Liao, C.-C. (2008) Photooxygenation of masked o-benzoquinones an efficient entry into highly functionalized cyclopentenones from 2-methoxyphenols. Angewandte Chemie, International Edition, 47 (38), 7325-7327. 

The synthesis of 2-HYDROXYMETHYL-2-CYCLOPENTENONE from cyclopentenone illustrates a general strategy and method for the synthesis of an effective latent synthon of an a-ketovinyl anion. The synthesis of CYCLODODECYL MERCAPTAN from cyclodecanone provides a method for the preparation of secondary or hindered mer-captans which cannot be prepared by traditional displacement reactions. 

Pauson-Khand Cycloaddition. Pauson Khand cycloaddition (see Pauson-Khand Reaction) is a cobalt-mediated method to prepare cyclopentenone from the cyclization of an alkyne with an alkene and CO (equation 14). This method is widely used to produce cychc ketones. Originally, stoichiometric amounts of Co2(CO)g were used in these reactions with the cobalt carbonyl being the CO source. However, it was shown that a strict temperature profile and high-purity reagents allowed the use of catalytic amounts of Co2(CO)g for reactions with 1 atm of CO. Currently, there is intense interest in developing catalytic cobalt starting materials for use in Pauson-Khand reactions. 

The mechanism is believed to begin with dissociation of CO, leaving Co2(CO)6 as the active catalyst. This carbonyl species adds an alkyne, forming a tetrahedrane complex (Section 2.4.1). Insertion of the alkene, upon CO loss, followed by CO insertion and reductive ehmination from one of the Co units results in a cyclopentenone. Dissociation of the cyclopentenone from the other Co unit regenerates Co2(CO)e (Scheme 1). 

Intramolecular Pauson-Khand reactions usually afford 2-substituted 2-cyclopentenones from terminal alkynes. Mixtures of products are often seen from reactions of internal alkynes with little or no stereo- or polarity differentiation between the two sides of the alkyne. Electron-rich and hydrocarbon-substituted alkynes are usually employed but electron-deficient alkenes such as esters, sulfones, and nitriles also participate in the Pauson-Khand reaction. 

In some of these reactions, substantial loss of the enantiomeric excess was observed,but for 1,4-enynes substituted at the propargylic position with a carboxylate (equations 76-78), the reaction proceeds with remarkable transfer of chirality allowing the efficient enantioselective synthesis of cyclopentenones from the corresponding enantiomerically emiched propargylic pivalates. The enantioselectivity can be explained by a remarkable center to helix chirality transfer, which was supported by DFT calculations. " These results indicate that the cyclization is faster than rates of helix interconversion and carboxylate rotation. " ... 

The Woodward-Hoffman rules also predict that in a given cyclization mode a permutation of alkene geometry must be reflected in the configuration of the products. This test is precluded under the normal reaction conditions (acid, light) which would isomerize the dienone double bonds. However, Corey recently reported the formation of a c -disubstituted cyclopentenone from a (Z, )-precursor, derived fiom an allene oxide, which cyclizes via the 2-oxido pentadienylic cation (Section 6.3.8). ... 

Although it has been known since 1938 that alkoxy-substituted alkyl cyclopropanecar-boxylates can be opened to 1,4-dicarbonyl compounds it was not until 1970 that the synthetic merit of this route to a valuable class of intermediates was recognized. In this year Wenkert and coworkers described the preparation of cyclopentenones from this type of cyclopropane via 1,4-diketones as outlined in equation 83 " . Shortly later McMurry and Glass have published a ds-jasmone synthesis following the same principle ... 

Pauson-Khand reaction Formation of cyclopentenones from alkenes, aikynes and CO. 334... 

---