Synthesis of C-Protected α-Amino Aldehydes of High Enantiomeric Excess from Highly Epimerizable N-Protected α-Amino Aldehydes

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Abstract
A new procedure for the preparation of C-protected α-amino aldehydes of high enantiomeric excess is illustrated using five differently substituted α-(N-Fmoc)amino aldehydes as starting materials. Highly epimerization-prone substrates were converted to the corresponding morpholino nitrile-protected α-amino aldehydes with minimal racemization (products ≥ 89% ee). Morpholino nitrile derivatives of phenylglycinal were crystallized and subjected to X-ray structural analysis, allowing for definitive determination of the stereochemistry of amino nitrile formation. A rationale for the stereoselectivity of amino nitrile formation is presented.