Conjugated unsaturated systems allylic substitution

The presence of /3-unsaturation normally leads to the formation of the corresponding conjugated system. The usual order of reactivity of a /3 proton is thus benzylic, allylic > CH3 > CH2 > CH. However, this preference can be offset by additional substitution at the more activated position competition between the /3-elimination modes leads to a mixture of regio-isomers as shown in Scheme 20. 

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