Chiral ligands 2.3 -Wittig rearrangement

The bis(oxazoline) S, 5)-(115) has been used as an external chiral ligand to induce asymmetric diastereoselective lithiation by r-BuLi during [2,3]-Wittig rearrangement of achiral substrates, (fj-crotyl propargylic ethers.It is believed that the enantios-electivity is determined predominantly at the lithiation step. 

The mechanism of the Stevens rearrangement has been reviewed. The key reaction in the total synthesis of ( )- and (+)-eupomatilone was an asymmetric [2,3]-Wittig rearrangement employing a bis(oxazoline) chiral ligand (Scheme 101). ... 

A single report on enantioselective [1,2]-Wittig rearrangements was published by Tomooka and Nakai in 1999. A chiral bis-oxazoline ligand was employed to facilitate asymmetric lithiation/rearrangement of dibenzyl ether and benzyI(propargyl) ether derivatives with enantioselectivities ranging from 40-65% ee. In a representative transformation, treatment of 61 with -BuLi and bis-oxazoline 62 led to the formation of 63 in 86% yield and 60% ee. 

Another protocol to achieve an enantioselective [2,3]-Wittig rearrangement is by the use of -BuLi with sparteine 20 as the chiral ligand. This has been demonstrated by Kimachi and Kawasaki with allyl ortho-substituted benzyl ethers 18 and ort/to-substituted benzyl prenyl ethers as... 

A study of chiral induction in the asymmetric [2,3]-Wittig rearrangement of allyl benzyl ether in the presence of a bis(oxazoline) chiral ligand [(5,5)-Box-r-Bu] has been reported (Scheme 45). °... 

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