Stereocontrol of Chiral Carbanions in Tandem Reactions

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(1) We have found that the extent of chirality transfer in [2,3]-Wittig rearrangement employing a chiral β-substituted allyl benzyl carbanion can be used as a tool for evalulation of the effect of substituents on the configurational stability of a chiral carbanion through a double bond.(2) Treatment of (R, Z)-3-(tert-butyldimethylsilyl)-1-cyano-3-hydroxyprop-1-enyl carbamate with a catalytic amount of a base afforded (S, E)-3-(tert-butyldimethylsilyloxy)-1-cyanoallyl diisopropyl- carbamate, showing that S_E2'-type reaction of allylsilicates proceeds in an anti-mode fashion. The overall process is equivalent to trapping of an enantioenriched C-chiral carbanion at α-position of nitrile group in up to 77% ee.