Enone Formation from Ketones and Cycloalkenylation

Preparation of enones from saturated ketones by Pd(II)-promoted dehydrosilyla-tion via silyl enol ethers was reported by Ito. Transmetallation of the silyl enol ether of cyclohexanone 519 with Pd(OAe)2 gives the oxo-TT-allylpalladium complex 520 (Pd enolate), which undergoes -H elimination to afford cyclohexenone. BQ is used as an oxidant of Pd(0) [211], However, the enone formation can be carried out using a catalytic amount of Pd(OAc)2 in DMSO under oxygen without other oxidants at room temperature. Also aldehyde 521 is converted to unsaturated aldehyde 522 via silyl ether in DMSO [4], 

The enone formation has been applied to a number of natural product syntheses. The enone 524 was prepared from the complex molecule 523 and successfully applied to the total synthesis of pallescensin [212], Even the phenolic OH in 525 was converted to the conjugated ketone. The reaction was utilized as a key step in hypoxyxylerone synthesis [213]. In the total synthesis of galbulimima alkaloid GB 13, Mander converted a cyclohexanone in the complicated molecule 526 to the corresponding cyclohexenone via silyl enol ether in 82% yield [214]. 

In the presence of a double bond, its intramolecular insertion to Pd enolate in 528 takes place. For example, the silyl enol ether 527 undergoes transmetallation with Pd(OAc)2 to give the Pd enolate 528, or the oxy-7r-allylpalladium, which undergoes 6-exo cyclization to generate 529. The subsequent jS-H elimination gives the 3-methylcyclohexenone (530) [215]. The reaction is called cycloalkeny-lation [216]. 

Five- and six-membered rings can be prepared by this reaction [217], and the reaction has been plied to syntheses of natural products. Now the cycloalkeny-lation can be carried out with a catalytic amount of Pd(OAc)2 in DMSO under O2. The bicyclo[3.2.1]octenone 532, a partial skeleton of gibberellin, was obtained in 81 % yield by the cycloalkenylation of the silyl dienol ether 531 in DMSO using 

Accounts S. Uchiumi, K. Ataka, and T. Matsuzaki, J. Organomet. Chem., 576, 

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