Ketones, diallyl synthesis

Ketones can be prepared by trapping (transmetallation) the acyl palladium intermediate 402 with organometallic reagents. The allylic chloride 400 is car-bonylated to give the mixed diallylic ketone 403 in the presence of allyltri-butylstannane (401) in moderate yields[256]. Alkenyl- and arylstannanes are also used for ketone synthesis from allylic chlorides[257,258]. Total syntheses of dendrolasin (404)f258] and manoalide[259] have been carried out employing this reaction. Similarly, formation of the ketone 406 takes place with the alkylzinc reagent 405[260],... 

The slow oxidation of primary alcohols, particularly MeOH, is utilized for the oxidation of allylic or secondary alcohols with allyl methyl carbonate without forming carbonates of the alcohols to be oxidized. Allyl methyl carbonate (564) forms 7r-allylpalladium methoxide, then exchange of the methoxide with a secondary or allylic alcohol 563 present in the reaction medium takes place to form the 7r-allylpalladium alkoxide 565, which undergoes elimination of j3-hydrogen to give the ketone or aldehyde 566. The lactol 567 was oxidized selectively with diallyl carbonate to the lactone 568 without attacking the secondary alcohol in the synthesis of echinosporin[360]. 

The preparation of a-selenoketones, esters, nitriles and related compounds can easily be performed via alkylation of the corresponding enolates or stabilized carbanions [21]. These compounds have found many synthetic applications in radical chemistry. In Eq. (9), a typical example involving a ketone is depicted [22]. The stability of a-selenoketones such as 41 is remarkable. Similar reactions with lactones have been performed. For instance, this approach has been applied to the stereoselective synthesis of oxygen-containing rings to either faces of a bicyclic structure [23]. The approach based on a-selenenylation/radical allyla-tion compares favorably with classical enolate allylation procedures, which usually leads to mixture of mono- and diallylated compounds. Furthermore, this strategy is excellent for the preparation of quaternary carbon centers [24] as shown by the conversion of 43 to 45, a key intermediate for the synthesis of fredericamycin A, [Eq. (10)] [25]. Similar reactions with sulfoxides [26] and phosphonates [27] have also been reported. 

Meerifield, J. H., Godschal, J. P., Stille, J. K. Synthesis of unsymmetrical diallyl ketones the palladium-catalyzed coupling of allyl halides with allyltin reagents in the presence of carbon monoxide. Organometallics 3, 1108-1112. 

H elimination occurs by the treatment of jr-allylpalladium enolates in boiling MeCN to give a.jS-unsaturated ketones and aldehydes. Shibasaki and co-workers used the enone formation in the total synthesis of strychnine [204], The silyl enol ether 555 was treated with diallyl carbonate 556 in MeCN and the cyclohexanone 557 was obtained in high yield. 

Other Methods. - One of the most important and flexible approaches to ketone synthesis involves the manipulation of 6-keto-esters. Tsuji et al. have reported that the use of allyl keto-esters, which are prepared from ketones and diallyl carbonate or allyl chloroformate, offer advantages over more common esters in that the ester hydrolysis/decarboxylation step... 

Further details of the monoallylation do not appear to have been systanatically investigated. So, its merits relative to those described in SecL B are not clear. However, the reaction may be prone to diallylation as indicated by the results shown in Scheme 15. It should be noted that, unlike the reaction of metal enolates, the enamine reaction must produce iminium salts, which can readily be converted to the corresponding enamine for the second allylation under the reaction conditions. Indeed, this tendency to undergo diallylation has been exploited in the synthesis of bridged bicycUc ketones and further expanded in subsequent investigationsf -f (Scheme 16). 

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