CONICET | Buscador de Institutos y Recursos Humanos

In the nematode Caenorhabditis elegans the DAF-12 receptor regulates the developmental timing, stage specification and longevity.1 Many biological processes are conserved between mammals and C. elegans, using similarity searches the Liver X Receptor (LXR) was identified as one of the human NHRs, the protein sequence of which is most similar to ceDAF-12.2 Once activated, LXR isoforms are involved in many physiological functions being key regulators of lipid homeostasis; as such LXRs have been proposed as key factors affecting human life span.3 The flexibility of the side chain in the natural ligands of DAF-12 and LXR (e.g. 1 and 2) makes an unfavorable entropic contribution upon ligand binding to the receptor. Also restricting the movement of the side chain to certain orientations may have profound effects in the activity profile. In a first attempt to obtain analogues with restricted side chain mobility, we now report the synthesis and in vitro biological activity of the 16,22-epoxysteroids 3 and 4. Compound 5 was prepared from diosgenin following the procedure of Fernández-Herrera et al.4 Removal of the acetates at positions 3 and 16 under basic conditions gave the 16,22-hemiketal that was reduced with NaCNBH3/MeOH-HCl (pH 3) to give 6 (75% yield). Using a one pot procedure for this sequence, compound 6 was obtained in 92% yield. Compound 3 was obtained from 5 by oxidation of the 26-OH with Jones reagent, followed by formation and reduction of the corresponding 16,22-hemiketal as described for 6. Compound 4 activated DAF-12 in a transactivation cell-based luciferase receptor assay, but was less active than the corresponding dafachronic acid 1. On the other hand compound 3 was inactive in a similar assay with LXRa and b. This behavior may be explained considering the different conformation of the side chains of 1 and 2 when bound to DAF-12 and LXR respectively.