NEW APPROACHES TO THE USE OF AMINO-ACIDS AS CHIRAL BUILDING-BLOCKS IN ORGANIC-SYNTHESIS

Alpha-Amino acids protected at nitrogen in quite different ways can be transformed without racemization into the corresponding alpha-amino aldehydes. Provided one chooses the right protecting groups (e.g., two benzyl residues on nitrogen) it is possible for the first time to carry out Grignard-like, aldol, and Me3SiCN additions, and hetero-Diels-Alder reactions with a high degree of nonchelation control. If the reactions of classical carbanions turn out to be nonselective, transmetalation, for example into organotitanium reagents, constitutes an important tool in controlling stereoselectivity. Diastereoselectivity can be reversed by specific variation of the protecting group and reagent. ''Protecting-group tuning'', ''metal tuning'', and ''ligand tuning'' are very useful in reactions of alpha-amino aldimines as well. The alpha-amino aldehydes can also be converted by Wittig reactions into electron-poor gamma-amino olefins, which in turn undergo stereoselective cuprate, Michael, and cycloadditions.