Ketones, 2-allyloxy Wittig rearrangement

It has been demonstrated that the oxygen anion of initially formed product (36) effectively catalysed the [2,3]-Wittig rearrangement as a Lewis base. Other Lewis-base catalysts such as lithio or sodio 2-pyrrolidone promote the same [2,3]-Wittig rearrangement of silyl enolates generated from a-allyloxy ketones, whereas rearrangements of enolates from a-allyloxy esters were efficiently catalysed by ammonium 4-methoxybenzoate.24... 

Silyl enolates generated from a-allyloxy ketones undergo the [2,3]-Wittig rearrangement in the presence of a catalytic Lewis base such as lithium 2-pyrrolidone, lithium acetamide, or lithium hexamethyldisilazide (Scheme ll).15... 

These authors found that pyrrolidine was indeed able to catalyze the 2,3-Wittig rearrangement of several a-allyloxy methyl ketones with good yields and moderate to good diastereoselectivities (Scheme 40.35). The fact that the major diastereomer had a 1,2-syn relative configuration was explained by assuming that the rearrangement proceeded via an envelope syn transition state. 

Scheme 40.35 Racemic organocatalytic 2,3-Wittig rearrangement of a-allyloxy methyl ketones.

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