Synthesis and biological activities of turkesterone 11α-acyl derivatives

Turkesterone is a phytoecdysteroid possessing an 11α-hydroxyl group. It is an analogue of the insect steroid hormone 20-hydroxyecdysone. Previous ecdysteroid QSAR and molecular modelling studies predicted that the cavity of the ligand binding domain of the ecdysteroid receptor would possess space in the vicinity of C-11/C-12 of the ecdysteroid. We report the regioselective synthesis of a series of turkesterone 11α-acyl derivatives in order to explore this possibility. The structures of the analogues have been unambiguously determined by spectroscopic means (NMR and low-resolution mass spectrometry). Purity was verified by HPLC. Biological activities have been determined in Drosophila melanogaster B(II) cell-based bioassay for ecdysteroid agonists and in an in vitro radioligand-displacement assay using bacterially-expressed D. melanogaster EcR/USP receptor proteins. The 11α-acyl derivatives do retain a significant amount of biological activity relative to the parent ecdysteroid. Further, although activity initially drops with the extension of the acyl chain length (C(2) to C(4)), it then increases (C(6) to C(10)), before decreasing again (C(14) and C(20)). The implications of these findings for the interaction of ecdysteroids with the ecdysteroid receptor and potential applications in the generation of affinity-labelled and fluorescently-tagged ecdysteroids are discussed. Abbreviation: / CoMFA: comparative molecular field analysis DCM: dichloromethane DMF: dimethylformamide DMP: 2,2-dimethoxypropane 4D-QSAR: 4-dimensional quantitative structure-activity relationship EcR: ecdysteroid receptor EcRE: ecdysteroid response element HPLC: high-performance liquid chromatography LBD: ligand-binding domain NMR: nuclear magnetic resonance ponA: ponasterone A QSAR: quantitative structure-activity relationship RXR: retinoid X receptor SAR: structure-activity relationship SPE: solid-phase extraction THF: tetrahydrofuran TLC: thin-layer chromatography p-TsOH: para-toluenesulphonic acid USP: ultraspiracle UV-VIS: ultraviolet-visible