 The most successful reagents employed in a stoichiometric fashion are shown below . The first is a <unk> <unk> that has been employed in the synthesis of the β @-@ <unk> compound <unk> ( <unk> ) but is limited by the availability of only one <unk> of the reagent . The synthesis of the axial <unk> is <unk> via the 1 @,@ 3 @-@ <unk> effect which reduces the <unk> of the equatorial lone pair . The conformation of the ylide is limited by <unk> strain and approach of the aldehyde is limited to one face of the ylide by <unk> interactions with the methyl <unk> . 
