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 (alpha-haloallyl)lithiums with methoxy-9-BBN or Ipc(2)BOMe followed by treatment with BF(3).OEt(2) leads to (Z)-(gamma-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-alpha-halohydrins (de >/= 90%) that are also easily cyclized to cis-vinylepoxides. Extension of this protocol using [(Z)-gamma-chloroallyl]BIpc(2) leads to chiral syn-alpha-chlorohydrins and cis-vinylepoxides in high de (>/=90%) and ee (90-99%). Enantioselectivity of reactions of chiral (Z)-(gamma-chloroallyl)boranes with aldehydes are more sensitive to reaction conditions than enantioselectivity of reactions of other alpha-or gamma-substituted allylboranes. The effects of proportion of BF(3).OEt(2) and the relative efficacies of LiNR(2) bases on diastereo- and enantioselectivity of the chloroallylation are reported.