The title compound (7) was isolated as a minor product of successive tosylation, acetylation and iodide displacement of methyl α-D-mannopyranoside (1). Crystals of the title compound are orthorhombic, space group P212121, a= 9.293(2), b= 14.220(6). c= 16.969(6)à Z= 4. The structure was solved from diffractometer data by heavy-atom techniques, and refined by block-diagonal least-squares methods to R 0.052 for 1 802 independent reflections. The results supply the constitution and absolute configuration (8). The conformation of the pyranoside ring is ⁴ C 1(.tbd6;C1) in the solid state and ¹H n.m.r. studies showed this to be the predominant conformation in solution also.