The [2,3] Wittig rearrangements of the lithio enolates of the cisconfigurated allylic ethers 5a -d are stereoselective. The tert-butyl ester 5d gives 79% of a single rearrangement product 6d. The chiral center of the dioxolane controls the configuration at one of the newly formed stereogenic centers through asymmetric induction. The size of the dioxolane is responsible for the concomitant high syn selectivity. The [2,3] Wittig rearrangements of the trans-configurated ally1 esters 20a -d exhibit moderate stereocontrol through asymmetric induction; the ratio of syn ( 6) and anti products (21) can be tuned from 2: 1 as in the case of the tertbutyl ester, to 1 : 3 by choosing the methyl ester.
The [2,3] Wittig rearrangement 1 --f 3 of metalated allylic ethers was discovered in 1960'). However, applications in synthesis remained scarce. It was not until two decades later, that studies by Still2"' and NakaiZb) spearheaded a revival of this reaction. The Wittig rearrangement became appreciated increasingly as a means of achieving acyclic diastereocontrol in stereoselective synthesis3! Control of the relative configurations of the newly formed stereogenic centers ("stereocenters") C-1 and C-2 in the rearrangement product 3 highlights the earlier stages of this development.