Enantioselective Synthesis of (1R,2S)-1-Amino-2-vinylcyclopropanecarboxylic Acid Ethyl Ester (Vinyl-ACCA-0Et) by Asymmetric Phase-Transfer Catalyzed Cyclopropanation of (E)-N-Phenylmethyleneglycine Ethyl Ester

A concise asymmetric synthesis of (1R,2S)-1-amino-2-vinylcyclopropanecarboxylic acid ethyl ester, a key intermediate in the preparation of many hepatitis C virus inhibitors, is described. Stereoselective cyclopropanation of (E)-N-phenylmethyleneglycine ethyl ester was effected by treatment with trans-1,4-dibromo-2-butene in the presence of a catalytic amount of a chiral phase-transfer catalyst. Microscale high-throughput experimentation techniques were successfully used to identify a cinchonidine-derived catalyst that provided (1R,2S)-1-(E)-N-phenylmethylene-amino-2-vinylcyclopropanecarboxylic acid ethyl ester in up to 84% cc. This was translated to a lab scale process to attain 78% yield and 77.4% cc. Chiral purity upgrade and isolation of the ester was accomplished via preparatory supercritical fluid chromatography followed by crystallization of the ester as its tosylate salt. The improved synthesis described herein represents a potentially more economical preparation of this valuable intermediate.