Ethyl (3 S )-3-[(3a R ,5 R ,6 S ,6a R )-6-hy-droxy-2,2-di-methyl-tetra-hydro-furo[4,5- d ][1,3]dioxol-5-yl]-3-{(3 S )-3-[(3a R ,5 R ,6 S ,6a R )-6-hy-droxy-2,2-di-methyl-tetra-hydro-furo[4,5- d ][1,3]dioxol-5-yl]-5-oxoisoxazolidin-2-yl}propano-ate chloro-form monosolvate.

The title compound, C 22 H 33 NO 12 ·CHCl 3 , was obtained as a product of a double aza-Michael addition of hydroxyl-amine on a Chiron with a known absolute configuration. The enanti-opure compound crystallized as a chloro-form solvate, in space group P 1, and diffraction data were collected at room temperature with Ag K α radiation. The Flack parameter refined to x = -0.01 (16); however, the Flack and Watkin 2 AD plot clearly shows that differences between Friedel opposites (the D component of the plot) do not carry any reliable information about resonant scattering of Cl atoms, and are rather dominated by random and systematic errors. The R D factor calculated using 1941 acentric Friedel pairs is R D = 0.995. On the other hand, the 2 A component of the plot, related to average intensities of Friedel pairs, shows that data are of good quality ( R A = 0.069). This example illustrates that while using Ag K α radiation (λ = 0.56083 Å), scatterers heavier than Cl should be present in a chiral crystal in order to determine confidently the absolute structure of the crystal.