Vinyllead triacetates

Aryllead triacetates are also good reagents for aryladon of stabilized carbanions, including le anion of nltro ilkanes fEq 5 70 As a related reaction, ct-vinyladon"° or ct-acetyladon" f nitre compounds is possible using vinyllead triacetates or ilky nyllead tr... 

These in situ generated unstable vinyllead triacetates react with soft carbon nucleophiles to give moderate to good yields of C-vinylated products. This method is potentially very useful in organic synthesis. As an example, styryllead triacetates reacted with 4-ethoxycarbonyl-2-phenyl-4,5-dihydrooxazol-5-one (64, R=Ph) to afford, after hydrolysis, the a-styryl-V-benzoylaminoesters in relatively modest yields.82 Better yields were obtained with the 2-mcthyM,5-dihydrooxazol-5-one analog (64, R = Me) (Equation (60)).83... 

In 1988, Ikegami et al. reported an efficient alkenylation reaction involving a vinyllead triacetate reagent containing a protected allylic alcohol (Equation (64)).86 When this reagent was used with a more rigid polycyclic system toward the synthesis of benzocarbacyclines, only a modest yield (29%) of the expected alkenylated product was isolated (Equation (65)).87... 

The possible involvement of vinyllead tricarboxylates has been suggested by Corey and Wollenberg and by Larock et as reactive intermediates. Their existence is supported by NMR studies and by the isolation of cyclopent-l-enyllead triacetate. 2 Vinyllead triacetates are extremely unstable compounds which generally decompose by formation of vinyl cations and afford acetylenes or enol acetates, depending on the precursor and the substitution pattern. 

In the presence of a soft nucleophile, the unstable intermediate vinyllead triacetate (87) behaves as a C-vinylating reagent, whatever the mode of formation of the vinyllead intermediate (metal-metal exchange from divinylmercury or from vinyltin compounds). 

An elegant application of the reaction of vinyllead triacetates with p-ketoesters was described in a convergent synthesis of (+)isocarbacyclin, an analogue of prostacyclin (PGI2). This synthesis involved the vinylation of a benzyl p-ketoester. i l The benzyl ester was later removed under conditions compatible with the other functionalities of the molecule. 

Moloney and co-workers have studied the decomposition of vinyllead triacetates prepared by the reactions of vinylmercury or vinyltin compounds with lead tetraacetate. Vinyllead triacetates generated in chloroform undergo reductive elimination of lead(II) acetate to afford vinyl cations (Scheme 41). The products are those of substitution (acetates) or elimination (alkynes), depending on the reaction... 

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