Diastereo- and Enantioselective Synthesis of syn-α-Vinylchlorohydrins and cis-Vinylepoxides

A new method to generate chiral syn-vinylchlorohydrins and cis-vinyloxiranes is reported. Reaction of (α-haloallyl)lithiums with methoxy-9-BBN or Ipc2BOMe followed by treatment with BF3·OEt2 leads to (Z)-(γ-haloallyl)boranes which react with aldehydes to yield cis-vinylepoxides (de ≥ 90%) upon oxidative workup. Alternatively, addition of ethanolamine to the allylboration product yields syn-α-halohydrins (de ≥ 90%) that are also easily cyclized to cis-vinylepoxides. Extension of this protocol using [(Z)-γ-chloroallyl]BIpc2 leads to chiral syn-α-chlorohydrins and cis-vinylepoxides in high de (≥90%) and ee (90−99%). Enantioselectivity of reactions of chiral (Z)-(γ-chloroallyl)boranes with aldehydes are more sensitive to reaction conditions than enantioselectivity of reactions of other α-or γ-substituted allylboranes. The effects of proportion of BF3·OEt2 and the relative efficacies of LiNR2 bases on diastereo- and enantioselectivity of the chloroallylation are reported.