Ethyl (3S)-3-[(3aR,5R,6S,6aR)-6-hydroxy-2,2-dimethyltetrahydrofuro[4,5-d][1,3]dioxol-5-yl]-3-{(3S)-3-[(3aR,5R,6S,6aR)-6-hydroxy-2,2-dimethyltetrahydrofuro[4,5-d][1,3]dioxol-5-yl]-5-oxoisoxazolidin-2-yl}propanoate chloroform monosolvate

The Flack and Watkin 2AD plot based on 1941 acentric Friedel pairs for the title chloro­form solvate shows that the observed intensity differences for Friedel opposites are dominated by random and systematic errors, erasing information about resonant scattering.

The title compound, C₂₂H₃₃NO₁₂·CHCl₃, was obtained as a product of a double aza-Michael addition of hydroxyl­amine on a Chiron with a known absolute configuration. The enanti­opure compound crystallized as a chloro­form solvate, in space group P1, and diffraction data were collected at room temperature with Ag Kα radiation. The Flack parameter refined to x = −0.01 (16); however, the Flack and Watkin 2AD 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 2A 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 Kα 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.