Absolute configuration assignment of stigmasterol oxiranes

Diastereoisomeric stigmasterol oxiranes 4, 5, 8, and 9 are known phytosterol oxidation products (POPs) that have been evaluated for their cytotoxicity, although the results are of limited significance since, in most cases, they were evaluated as mixtures. Consequently, to establish biological activity hierarchy of these oxides, it is critical to evaluate individual pure POPs. Therefore, we now describe the obtention of individual molecules and their absolute configuration (AC) determination. The two acetylated C-5−C-6 oxiranes 6 and 7; the two acetylated C-22−C-23 oxides 10 and 11, obtained by means of Δ⁵ double bond protection-deprotection; and the four C-5−C-6, C-22−C-23 diepoxystigmasteryl acetates 19–22 were now individually gained and their AC determined by vibrational circular dichroism. Vibrational modes associated with the C-5−C-6 and the C-22−C-23 bonds were identified in dioxiranes 19–22 and used to assign the AC of monoepoxides 6, 7, 10, and 11. The AC of biological active non-acetylated molecules follows immediately. Due to the scarce spectroscopic information available for these POPs, the ¹H and ¹³C NMR chemical shifts of 3–22 were assigned using 1D- and 2D-NMR experiments.