Highly Stereoselective Synthesis of α-d-Mannopyranosyl Phosphosugars

α-Mannopyranosyl phosphosugars are obtained in 61−90% yields from 4,6-O-benzylidene-protected mannosyl thioglycosides bearing ester functionality in the 3-O-position by coupling reactions with ammonium salts of phosphosugars on activation with 1-benzenesulfinyl piperidine, 2,4,6-tri-tert-butylpyrimidine, and trifluoromethanesulfonic anhydride. Due to the presence of the disarming ester group, only the formation of the α-isomer was observed.