Oxazolidinone, Reformatsky reaction

Analogous asymmetric, samarium Reformatsky reactions of chiral 3-bro-moacetyl-2-oxazolidinones have been described by Fukuzawa.140 For example, reduction of 124 with Sml2 generates a samarium enolate that then reacts with pivalaldehyde to give the a-unbranched (I-hydroxycarboximide 125 in 87% yield and in high diastereoisomeric excess (Scheme 5.89). The reaction is synthetically noteworthy as highly diastereoselective acetate aldol processes are difficult to achieve. Sm(III) ions are likely to play an important role in the... 

L. Wessjohn and co-workers successfully applied the CrCl2-mediated Reformatsky reaction for the synthesis of C1-C6 fragment of epothilones. In their approach, they utilized the Evans (R)-4-benzyl-oxazolidinone chiral auxiliary to control the absolute stereochemistry. The chromium-Re/brmafsAy reaction between the (R)-4-benzyl-3-(2-bromoacetyl)-oxazolidinone and 2,2-dimethyl-3-oxo-pentanal occurred with complete chemoselection providing the product with 63% yield and as a single diastereomer. 

Catalysed aerobic epoxidation of the styrene-cobalt complex, followed by the addition of trimethylsilyl isothiocyanate, yields 4-phenyloxazolidine-2-thione 123 <96TL7315>. The 4-substituted 2-oxazolidinone 124 undergoes a "chromium Reformatsky" reaction with aldehydes RCHO (R = i-Pr or Ph) in the presence of chromium(ll) chloride and lithium iodide to give mainly the unusual anti-aldol products 125 in excellent dia,stereomeric excess and yield <97TL4,387>. 

Guided by the success of the Evans and related auxiliaries, several attempts were made to use enantiomerically pure a-bromoacyl oxazolidinones for stereoselective Reformatsky reactions. Fukuzawa and coworkers developed the reaction of various bromoacetyl oxazolidinones 323 as an alternative to an asymmetric acetate aldol addition. The conversion was mediated by samarium iodide and yielded P-hydroxy carbonyl compounds 325 with high diastereoselectivity in optimal combinations of auxiliary group and aldehyde. Among the different auxiliaries, the geminal dimethyl- and diphenyl-substituted ones performed better than the original Evans oxazolidinones. The stereochemical outcome was rationalized by assuming that an O-bound samarium(III) enolate reacts via a chair-like... 

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