is methylmagnesium bromide expulsing the pyrazole group. An SN2' reaction should explain the outcome of the reaction of an aziridine carrying a methylene bromide group with methyllithium: In this reaction one equivalent of acetylene is lost. Examples of allylic shifts: Ferrier rearrangement Meyer–Schuster rearrangement SN2' reduction In one adaptation called a SN2' reduction a formal organic reduction on an allyl group containing a good leaving group is accompanied by a rearrangement. One example of such reaction is found as part of a taxol total synthesis (ring C): The hydride is lithium aluminium hydride and the leaving group a phosphonium salt. The product contains a new exocyclic double bond. Only when the cyclohexane ring is properly substituted will the proton add in a trans position with respect to the adjacent methyl group. A conceptually related reaction is the Whiting reaction forming dienes. Electrophilic allyl shifts Allyl shifts can also take place with electrophiles. In the example below the carbonyl group in benzaldehyde is activated by diboronic acid prior to reaction with the allyl alcohol (see: Prins reaction): References Rearrangement reactions Reaction mechanisms