 When X is a carbanion and Y an alkoxide, the rearrangement is called the [2,3]-Wittig rearrangement and the products are pent-1-en-5-ols.The [1,2]-Wittig rearrangement, which produces isomeric pent-5-en-1-ols, is a competitive process that takes place at high temperatures.Because of the high atom economy and stereoselectivity of the [2,3]-rearrangement, it has gained considerable synthetic utility.