We first synthesized both the Rp and Sp isomers of the phosphorothioate analogs of the N-acetyl-D-mannosamine-alpha-1-phosphate structure, which is found in the capsular saccharides of serogroup A Neisseria meningitidis (MenA). The stereoselective synthesis of P-modified alpha-glycosyl phosphates was conducted using the oxazaphospholidine approach. In this study, we used N-trichloroacetyl mannosamine derivatives to improve the troublesome instability of synthetic intermediates resulting from the strong neighboring group participation of the acetamido group, and the trichloroacetyl group was directly converted to an acetyl group by reductive hydro-dechlorination.