Synthesis of N-benzyl-3(S)-phthalimido-4(S)-[4(S)-2,2-dimethyl-1,3-dioxolan-4-yl]-2-oxo-azetidine

(S)-Glyceraldehyde acetonide 4 which is a very useful chiral intermidiate in stereoselective synthesis was prepared conveniently by a new three-step synthetic method from natural chiral resource of L-ascorbic acid 1. The chiral aldehyde condensed with benzyl amine to produce adapted chiral imine 5, The imine is unstable, but it can carry a [2 + 2] cyclic addition with ketene 8 which was obtained from phthalimidoacetyl chloride and triethylamine under mild conditions, and produce the title compound, N-benzyl-3(S) -phthalimido-(S)-[4(S)-2, 8-dimethyl-1, 3-dioxolan-4-yl]-2-oxo-azetidine. Because of the high facial stereoselection in cyclic addition mechanism and attacking with ketene in unblocked face of the imine, the optical purity of the final product is quite satisfactory.