Carbon-oxygen bonds diene conjugation, allylic intermediates

Intramolecular reactions of allylic acetates with conjugated dienes catalyzed by Pd(0) lead to a 1,4-addition of a carbon and an oxygen nucleophile to the diene. The reaction, which is formally an isomerization, involves tw different yr-allyl complexes (Scheme 8-4) [44]. Reaction of 22 in the presence of the Pd(0) catalyst Pd2(dba)3-CHCl3 (dba = dibenzyl-ideneacetone) and LiOAc/HOAc in acetonitrile at reflux produces the cyclized isomer 25 in 62% yield. The double bond was exclusively of E stereochemistry, while the ring stereochemistry was a mixture of cis and tram isomers. Oxidative addition of the Pd(0) to the allylic acetate gives the intermediate jr-allyl complex 23. Subsequent insertion of a diene double bond into the allyl-palladium bond produces another jr-allyl intermediate (24), which is subsequendy attacked by acetate to give the product 25. 

Hydroperoxides have geometric as well as positional isomers on lipid chains. When the hydrogen is abstracted at an allylic carbon, the double bond shifts one carbon to a position (3 to the abstraction site, and it reforms in the trans rather than cis configuration. The fran5,c/i-conJugated diene structure is retained whether oxygen adds or not, and provides the first detectable intermediate in lipids during autoxidation (238). 

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