Wittig Rearrangements of Boron-Based Oxazolidinone Enolates

[2,3]-Sigmatropic rearrangements (Wittig rearrangements) of alpha-alkoxy oxazolidinone enolates are described. Whereas alkali metal enolates fail, owing to facile deacylation, boron enolates generated from di-n-butylboron triflate and triethylamine rearranged in good yields and high selectivities with exceptions noted. IR and NMR spectroscopies show the boron is chelated by the alpha-alkoxy group rather than the more distal oxazolidinone carbonyl in the complex and enolate. The rearrangement product contains a boron alkoxide that remains unchelated by either carbonyl. Optimization was guided by density functional theory computations, suggesting that valine-derived oxazolidinones would be superior to the phenylalanine-derived analogues.