Asymmetric formation of alkene double bonds

So far, all the chiral compounds prepared have been of the conventional type, i.e. with tetrahedral stereogenic centres. The compounds whose asymmetric synthesis is discussed below do not 

A second generation route to this class of compound using sulphoxide chiral auxiliaries has already been described in section 5.3.5. The third-generation method here is an asymmetric variant of the Wittig reaction.ti l The asymmetric reagent is derived from (5, 5)- or (R, / )- ,2-diaminocyclohexane (note the C2 symmetry). Deprotonation and reaction with a 4-substituted cyclohexanone leads to one or other enantiomer of the axially chiral alkylidenecyclohexane (43) with a good e.e. of 70-90%. The origin of this selectivity is not yet fully understood. 

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