Addition reaction of benzylbenzylidenenamine to lithium enolates of 1,3-dioxolan-4-one: synthesis of 2-phenylisoserines

The synthesis of two new 2-phenylisoserines, each bearing a quaternary stereocenter is described. These compounds, which are analogs of the amino acid side chain found in taxol and taxotere, were obtained by addition of the lithium enolates of (2S,5S)-2-isopropyl-5-phenyl-1,3-dioxolan-4-one and (2S,5S)-2-tert-butyl-2-methyl-5-phenyl-1,3-dioxolan-4-one to benzylbenzylideneamine in the presence of BF3 .O(Et)2. The diastereomeric mixtures were separated in each case and the absolute configurations thereof were determined by X-ray analysis.