Homer-Wittig-type reagent

The action of a chiral Homer-Wittig type reagent on a racemic aldehyde has been mentioned in Section 2.3.7. In the E/Z mixture of products arising from olefination of 109, the -stereoisomer is mainly formed from one enantiomer of the aldehyde 109 while the Z-isomer is derived from the other enantiomer. Here, each enantiomer gives a defined stereochemistry for the double bond that is created (69). A new stereogenic unit is formed, which is conceptually similar to the formation of a new asymmetric centre in a racemic substrate. 

A second route to epoxides of type 651 can evolve from treatment of the fluorinated Wittig Homer reagent 652 with aldehydes or ketones [253]. 

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