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 i n stereoselective synthesis was prepared conveniently by a new three-step s ynthetic method from natural chiral resource of L-ascorbic acid 1. The chir al 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 kete ne 8 which was obtained from phthalimidoacetyl chloride and triethylamine u nder mild conditions, and produce the title compound, N-benzyl-3(S) -phthal imido-(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 attacki ng with ketene in unblocked face of the imine, the optical purity of the fi nal product is quite satisfactory.