Absolute configuration assignment of stigmasterol oxiranes

Abstract

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.

Vibrational Circular Dichroism Similarity-Guided Absolute Configuration Determination of 11-Acyloxytremetones

“…In addition, naturally occurring benzodihydrofurans 1 and 2 were successfully tested 4 as growth inhibitors of the insect species Tenebrio molitor, while the structure of 1 was independently verified 5 in a single-crystal x-ray diffraction (XRD) study. 5 From our three lustrous vibrational circular dichroism (VCD) experience, accumulated since the first study 6 of alkaloids from plants of Chilean origin, through the years during which a few VCD reviews 7,8 were put forward, 9 until current times, 10 it could be expected that the structural attributes of 1 would allow a straightforward AC determination of the molecule. Inspection of the structure of 1 reveals that this C 22 H 20 O 5 compound possesses only 4 sp 3 carbon atoms of which one is a methyl group, a sole stereogenic center which is located at C-2 on a benzodihydrofuran ring system and has an almost rigid p-hydroxycinnamoyl ester group whose phenolic hydroxy group can adopt 2 orientations.…”