Enantioselective Wittig-Homer Reaction

Only three enantioselective Wittig-Homer reactions using optically active reagents have been reported [17J. In comparison with these, the solid state reaction is much more simple and successful. 

Asymmetric Wittig-Homer reactions Wittig-Horner reactions have usually involved phosphine oxides, phosphonates, or phosphonamides. Highly enantioselective ole-finations have now been achieved by use of this chiral phosphonamidate 1. Thus the anion (/-BuLi) reacts with 4-/-butylcyclohexane to form 2 in >98% de. Elimination with trityl triflate/2,6-lutidine provides the alkylidene 3 with essentially complete stereospecificity. The reaction was shown to be applicable to a variety of 4-aIkylcyclohexanones. 

One of the limitations of the Warren s adaptation of Homer-Wittig olefina-tion, the failure of the (Z)-selective route when the alkene has a branched chain substituent, has now been overcome. Reduction of the p-ketophosphonates carrying a-branches, e.g. (112) and (113), with sodium borohydride and cerium chloride gives excellent a / -stereoselectivity and hence (Z)-alkene on base-induced elimination. Enantioselective synthesis of both jy -(115) and anti- ll) P-hydroxy-phosphine oxides has been achieved with up to 90% e.e. through two separate approaches. The jyn-isomer was obtained by reduction of the corresponding ketone (114), while the anti-isomer is the product of the reaction of the oxazolidine substituted aldehyde (116) with lithiated diphenylmethyl-phosphine oxide (Scheme 10). A new, highly stereoselective approach to trisubstituted alkenes has been reported. Cerium(III) chloride-promoted... 

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