Cyclopentenones. formation

Transition-metal-promoted cycloaddition is of much interest as a powerful tool for synthesis of carbocyclic stmcture in a single step. Utilization of carbon monoxide as a component of the cycloaddition reaction is now widely known as the Pauson-Khand reaction, which results in cyclopentenone formation starting from an alkyne, an alkene, and carbon monoxide mediated by cobalt catalyst. Although mechanistic understanding is limited, a commonly accepted mechanism is shown in Scheme 4.16. Formation of dicobalt-alkyne complex followed by alkene... 

Selectivity for the cyclopentenone formation is improved by an intramolecular version. For example, 1,6-hepta-diyne (256, X = CH2) reacts in benzene with 2molarequiv. of Bu Me2SiH to give two bicyclo[3.3.0]octenones, 257 and 258, in 14% and 63% yield, respectively, at 95 °G under GO (20 atm) pressure.When benzene is replaced by acetonitrile, 258 becomes a sole product (60% yield). On the other hand, the reaction at 25 °G gives normal silylformylation product 259 as a major component (59%) with the concomitant formation of 258 (20%) (Scheme 12). ... 

Scheme 23 Cyclopentenone formation in the total synthesis of ( )-asteriscanolide.

In a novel combination of Pauson-Khand cycloaddition with vinylcyclopropane chemistry, de Meijere has described an entry to linearly fused triquinanes beginning with cyclopropylalkynes. Cyclopentenone formation has been carried out with a variety of substitution patterns on the cyclopropane, and moderate yields achieved with both norbomene and cyclopentene as substrates. Thermal vinylcyclopropane-cy-clopentene rearrangement of the cycloaddition products leads to the final tricyclic system (Scheme... 

Intramolecular Pauson-Khand reaction. The commercially available CpjTi(CO), catalyzes cyclopentenone formation in a moderate pressure (18 psig) of CO. The synthesis of a dienone from an allenyl alkyne is interesting. 

The Nazarov cyclization is a four-electron cyclization and occurs thermally by a conrotatory process. The stereochemical outcome across the new carbon-carbon bond is often obscured by the loss of a proton at one of these centres during the cyclopentenone formation. If, however, the proton loss occurs exo to the five-membered ring or if the aUyl cation is quenched by a nucleophile, then the stereochemistry can be observed. For example, trapping the allyl cation by reduction with... 

The use of DMSO and other highly polar liquids as solvents for the reaction of [Co2(CO)6( i-alkyne)] with alkenes was shown S to promote both inter- and intramolecular cyclopentenone formation. The use of such solvents was shown to lead to significant changes in regio- and stereoselectivity when compared with other reaction conditions. 

Cyclopentenone formation by the isomerization of 3-acetoxy-1,4-enynes is catalyzed by Pd(PhCN)2Cl2 (eq 60). ... 

Cyclopentenones could be also prepared from the BuLi-mediated reaction of zirconacyclopentadienes with CO (Eq. 47) [49]. The work reported by Taka-hashi represents the first example of cyclopentenone formation from two alkynes and CO mediated by metallocenes. 

The few examples of catalytic cyclopentenone formation from alkyne, alkene, and CO are discussed below, as is the bifuryli-denedione synthesis from alkyne and CO. It should also be noted that Co4(CO)i2, obtained by heating the dinuclear carbonyl, can probably replace the latter in most of the above catalytic reactions, but without obvious advantage. 

Al-Methylmorpholine iV-oxide, preferably as monohydrate, as well as MesNO not only induces cyclopentenone formation under very mild conditions (0-20 °C) and in frequently excellent yields, but also removes some earlier limitations. The reductive processes (eqs 58 and 59) do not occur when these promoters or DMSO are used under oxygen. Hence two efficient syntheses of (—)-kainic acid have employed cyclizations of allylpropar-gylamine derivatives as the key step in one of these (eq 62), as well as in a synthesis of the dendrobine skeleton, use of a chiral precursor was shown to result in enantiospecific bicyclization. ... 

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