S - - -5- a-Phenylethyl semioxamazide

Elimination of Acetals. Treatment of 2,4-pentanediol acetals of meso ketones with Triisobutylaluminum gives enol ethers with high diastereoselectivities (eq 3).  

Acetals as Chiral Auxiliaries. There have been many applications of acetals of 2,4-pentanediol as chiral auxiliaries to control the diastereoselectivity of reactions on another functional group. Examples include cyclopropanation of alkenyl dioxanes, lithium amide-mediated isomerization of epoxides to allylic alcohols, and addition of dioxane-substituted Grignard reagents or organolithiums to aldehydes. 

Other Uses. Acetals of 2,4-pentanediol have also been prepared in order to determine the enantiomeric purity of aldehydes and ketones by analysis of diastereomers by GC or NMR. 2,i-ButanedioP is more commonly used for this purpose but has been shown to be less effective in some cases. 

(a) Basile, T Thgliavini, E. Trombini, C. Umani-Ronchi, A. Chem. Commun./J. Chem. Soc., Chem. Commun. 1989, 596. (b) Basile, X Tagiavini, E. Xrombini, C. Umani-Ronchi, A. Synthesis 1990, 305. 

Michael Chong University of Waterloo, Ontario, Canada 

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