Stereoselective synthesis of (E)-trisubstituted alpha,beta-unsaturated amides and acids

Reference:

Feuillet, F. J. P., Cheeseman, M., Mahon, M. F. and Bull, S. D., 2005. Stereoselective synthesis of (E)-trisubstituted alpha,beta-unsaturated amides and acids. Organic and Biomolecular Chemistry, 3 (16), pp. 2976-2989.

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Abstract

Potassium alkoxides of N-acyl-oxazolidin-2-one-syn-aldols undergo stereoselective elimination reactions to afford a range of trisubstituted (E)-alpha,beta-unsaturated amides in > 95% de, that may be subsequently converted into their corresponding (E)-alpha,beta-unsaturated acids or (E)-alpha,beta-unsaturated oxazolines in good yield. syn-Aldols derived from alpha,beta-unsaturated aldehydes gave their corresponding trisubstituted (E)-alpha,beta-unsaturated-amides with poorer levels of diastereocontrol, whilst there was a similar loss in (E)- selectivity during elimination of syn-aldols derived from chiral aldehydes. These elimination reactions proceed via rearrangement of the potassium alkoxide of the syn-aldol to a 1,3-oxazinane-2,4-dione enolate intermediate that subsequently eliminates carbon dioxide to afford a trisubstituted (E)- alpha,beta-unsaturated amide. The (E)- selectivity observed during the E1cB-type elimination step has been rationalised using a simple conformational model that employs a chair-like transition state to explain the observed stereocontrol.

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