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

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Thomas E. Smith , David P. Richardson , George A. Truran , Katherine Belecki and Megumi Onishi

Department of Chemistry, Williams College, Williamstown, MA 01267

J. Chem. Educ., 2008, 85 (5), p 695

DOI: 10.1021/ed085p695

Publication Date (Web): May 1, 2008

Abstract

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.

Keywords (Audience):

Upper-Division Undergraduate

Keywords (Domain):

Laboratory Instruction

Keywords (Pedagogy):

Hands-On Learning / Manipulatives

Keywords (Subject):

Alkylation

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