Substitutions Following Primary Rearrangements The Prototropic Routes

Substitutions Following Primary Rearrangements (The Prototropic Routes) 

In systems such as (241) which are capable of prototropic change, basic nucleophiles could cause preliminary rearrangement to (242). The migration of the double bond places the leaving group in the reactive allylic position, and its replacement becomes very facile. The primary substitution product (243) is the allylic one, but further prototropy could form the vinylic substitution product (244) formally derived directly from (241) (equation 24). This route, which requires highly 

A differentiation between those routes has not been made for a-bromo-crotonic acid. However, a-bromoacrylic acid, which is incapable of a prototropic change but should be more reactive in direct substitution, 

The structurally related a-bromocinnamic acid (257) gave only dehydrobromination with alkali (Owen and Sultanbawa, 1949a). 

Direct evidence for the prototropic route in a substituted crotonic acid was obtained from reactions of j8,j8-dimethylacrylic acid (258) (Owen and Sultanbawa, 1949a). With alkoxides, both unsaturated alkoxy acids (259) and (260) were isolated, and differentiation between the substitution-rearrangement route (258) —(259) - (260) and the rearrangement - substitution - rearrangement route (258) (261) -  

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