Acylation, Diastereoselective Alkylation, and Cleavage of an Oxazolidinone Chiral Auxiliary

An introduction to the concepts and experimental techniques of diastereoselective synthesis using a chiral auxiliary is described. The 4-benzyl-2-oxazolidinone chiral auxiliary developed by Evans is acylated with propionic anhydride under mild conditions using DMAP as an acyl transfer catalyst. Deprotonation with NaN(TMS)₂ at -78 °C leads to a rigidly chelated enolate that is alkylated by allyl iodide preferentially from the least hindered diastereoface with 98:2 selectivity. Chromatographic purification followed by hydrolytic cleavage of the auxiliary using LiOH/H₂O₂ gives 2-methyl-4-pentenoic acid in high enantiomeric purity. This exercise gives students the opportunity to gain proficiency with the techniques of modern organic synthesis, including the manipulation of moisture-sensitive reagents, low temperature reactions, column chromatography, TLC, and NMR analysis. The experiment also provides a context for the discussion of important reaction mechanisms, such as acylation and enolate alkylation, and serves to reinforce the critical conceptual distinction between absolute and relative stereochemical control.