The structure and absolute configuration of (5R)-5,6-anhydro-3-O-benzyl-1,2-O-isopropylidene-5-C-(dimethoxyphosphinoyl)-α-d-xylo-hexofuranose

The absolute configuration of the title compound, C lsH25OsP, a derivative of the antibiotic fosfomycin, has been determined through the anomalous-dispersion effect. The crystals are orthorhombic, space group P212121, with a = 15.658 (3), b = 20.158 (3), c = 6.205 (1) A, V = 1958.5 (5) A 3, z = 4, D x = 1.358, Dm = 1.358 Mg m -3 (flotation in aqueous KI solution), M r = 400.4, m.p. 380-381 K, [a] 24°c = --6.1 ° (c 1.27 gdm -3, CHC13), #(Cu Ka) = 1.62 mm -1. Final R = 0.044 for 2834 reflexions. The furanoid ring takes a C(4)-exo envelope conformation (E4) with P = 44.7 ° and rm = 36"7 °. The benzyloxy group at C(3) is linked axially, and the C(5) atom of the epoxy group is bound equatorially to C(4). The chirality of R at C(5) clarifies the steric course of formation of the a,fl-epoxyphosphinoyl compound.