No abstract

Liquid‐crystalline Heterocycloalkanes. I. Synthesis and Properties of Substituted 1,3‐Dioxanes The synthesis of 2,5‐disubstituted 1,3‐dioxanes is described, their melting and clearing points and their physical properties (dielectric anisotropy, viscosity, transition enthalpies, optical birefringence) are investigated. The cyano substituted 1,3‐dioxans 5 and 7 are valuable materials for application in displays.

Summary. We report the discovery of the first antiecdysteroids which belong to a comparatively new class of plant growth regulators, the brassinosteroids. These compounds bind competitively to ecdysteroid receptors partially purified from larvae of the blowfly Calliphora vicina and inhibit biological responses to 20-hydroxyecdysone, the active form of the molting hormone.Key words. Ecdysteroid; brassinosteroid; hormone antagonist.Steroids with signal functions are found in all three kingdoms of eucaryotic organisms -fungi, plants and animals 2. The discovery that plants possess growth regulators with a steroidal structure has only been made recently 3 and since then a series of brassinosteroids have been isolated. They possess striking structural similarities to the ecdysteroids 4, 5 ( fig. 1). These similarities suggest that brassinosteroids and ecdysteroids share a common progenitor hormone which might be the ancestor of all steroid hormones existing today 6. To see whether the analogy between brassinosteroids and ecdysteroids even extends to the recognition by ecdysteroid receptors we investigated the binding of two brassinosteroids and their synthetic precursors to ecdysteroid receptors from the blowfly, Calliphora vicina. Results and discussion. Isolation, purification, and characterization of nuclear ecdysteroid receptors has been described in detail recently 7. Binding of the brassinosteroids was studied by means of a competitive binding assay using the radiolabeled ecdysteroid ponasterone A as a high affinity receptor ligand. The synthetic brassinosteroids 22S,23S-homobrassinolide and 22S,23S-homocastasterone 5 ( fig. 1) as well as the tetra-acetates of these compounds, which served as synthetic precursors, clearly competed with (3H)ponasterone A for the binding to ecdysteroid receptors. Figure 2 shows competition curves for homocastasterone, homobrassinolide and the ecdysteroids 20-hydroxyecdysone and ecdysone. An equilibrium dissociation constant (KD) of 5 x 10-6 M for the homocastasterone-receptor complex could be estimated from the competition curves. This K o is of the same order of magnitude as the Ko for the molting hormone secreted by the endocrine gland, ecdysone, which is 2 x 10 -6 M. Brassinosteroids bound to ecdysteroid receptors could be displaced again by an elevation of the (3H)ponasterone A concentration, demonstrating the competitive nature of the binding. It has been independently shown by two research groups that 22S,23S-homobrassinolide inhibits the evagination ofimagihal discs of the dipteran, Phormia terra-novae, in vitro 8 and that it causes a delay in the imaginal molt when fed to the last larval instar of the cockroach, Periplaneta americana 9. Our results show that these effects can be explained by the competitive displacement of 20-hydroxyecdysone from its receptor binding site. 22S,23S-homocastasterone, however, which was the most effective competitor in the binding assay, did not disturb the imaginal molt when fed to cockroaches 9. We suppose that the lack of effect in the in ...

No abstract