Synthesis of β-cyanoalanine and enantiomerically enriched aspartate derivatives via the Zn- or In-mediated nucleophilic addition to α-imino esters

Abstract

We report the synthesis of β-cyanoalanine derivatives and enantiomerically enriched aspartates. The zinc-mediated addition of α-bromoacetonitrile to α-imino esters afforded various β-cyanoalanine derivatives 3a-l and 5a-d in satisfactory to good yields. The zinc- or indium-mediated addition of α-bromoacetonitrile or ethyl 2-bromoacetate to enantiopure N-tert-butanesulfinyl α-imino esters gave the corresponding products 3m-p in satisfactory yields and diastereoselectivity. Notably, the indium-mediated addition of alkyl 4-bromocrotonates to enantiopure N-tert-butanesulfinyl α-imino esters gave the corresponding enantiomerically enriched β-vinyl aspartates 10c-e (aspartic acid derivatives) with high diastereoselectivity (syn isomers). The stereochemistry of the vicinal stereocenters of the compounds 10c-e (major isomers) were assigned based on the X-ray structure of a derivative 11d which was obtained from 10c. To show the utility of β-cyanoalanine and β-vinyl aspartate derivatives, representative β-cyanoalanine compounds 3a/3b, 5d and aspartic acid derivative 10c were subjected to selected synthetic transformations.