Starting from methyl 2,3-O-isopropylidene-α-D-mannofuranoside (5), methyl 6-O-benzyl-2,3-O-isopropylidene-α-D-lyxo-hexofuranosid-5-ulose (12) was prepared in three steps. The addition reaction of dimethyl phosphonate to 12, followed by deoxygenation of 5-OH group, provided the 5-deoxy-5-dimethoxyphosphinyl-α-D-mannofuranoside derivative 15a and the β-L-gulofuranoside isomer 15b. Reduction of 15a and 15b with sodium dihydrobis(2-methoxyethoxy)aluminate, followed by the action of HCl and then H₂O₂, afforded the D-mannopyranose (17) and L-gulopyranose analog 21, each having a phosphinyl group in the hemiacetal ring. These were converted to the corresponding 1,2,3,4,6-penta-O-acetyl-5-methoxyphosphinyl derivatives 19 and 23, respectively, structures and conformations (⁴C₁ or ¹C₄, resp.) of which were established by ¹H-NMR spectroscopy.