Trapping of Allylpotassium Reagents with Carbon Electrophiles

1 Trapping of Allylpotassium Reagents with Carbon Electrophiles 

Many stereocontrolled carbon-carbon linking reactions have been described illustrating these principles. Natural insect pheromones have attracted considerable efforts19-21 as shown by the synthesis of compounds listed in Table 4, all obtained as pure Z-isomer through a metallation-equilibration process. 

The superbase metallation of alkenes in the allylic position followed by reaction with electrophiles is one of the best methods to access Z-alkenes with new functional groups. 

As an example, when phenylpotassium is allowed to react with 8-bromonortricyclene, a-santalene is produced which is then submitted again to metallation with LIC-KOR followed by borylation and oxidation.76 The allowance for torsional equilibration produces the (Z)-a-santalol, one of the main constituents of the highly prized sandalwood oil, with no trace of the (A l-isomcr. The entire sequence can also be contracted to a one-flask protocol still giving quite acceptable yields.76 

A more recent example of alkene metallation by superbase followed by reaction with electrophiles is the functionalization of isopulegol which can be performed without protection of the hydroxy group.81 

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