Bromine conjugate enolate trap

Another intermediate for which Die Is-Alder trapping provided convincing evidence is the oxy-allyl cation. This compound can be made from a,oc -dibromoketones on treatment with zinc metal. The first step is the formation of a zinc enolate (compare the Reformatsky reaction), which can be drawn in terms of the attack of zinc on oxygen or bromine. Now the other bromine can leave as an anion. It could not do so before because it was next to an electron-withdrawing carbonyl group. Now it is next to an electron-rich enolate so the cation is stabilized by conjugation. 

Methyltryptophan derivatives have been synthesized stereoselectively using chiral auxiliaries. Conjugate addition of magnesium methylcuprate to the indole acrylic add (392) gives an enolate which can be trapped by bromination to give the product (393), which can then be transformed into the tryptophan (394) by azide displacement, reduction and hydrolysis (Scheme 131) <92TL7491>. 

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