Synthesis and GABAA receptor activity of oxygen-bridged neurosteroid analogues

VELEIRO, ADRIANA S.; ALVAREZ, LAUTARO D.; EDELSZTEIN, VALERIA C.; BAGGIO, RICARDO F.; GARLAND, MARÍA T.; COIRINI, HÉCTOR; BURTON, GERARDO

San Petersburgo, Rusia

Congreso; International Symposium on Advances in Synthetic and Medicinal Chemistry” ASMC07; 2007

European Federation for Medicinal Chemistry

 g-Aminobutyric acid (GABA) is one of the most prevalent neurotransmitters in the mammalian central nervous system. The naturally occurring neurosteroid allopregnanolone (1) and its 5b-isomer pregnanolone (2), are among the most potent positive allosteric modulators of the GABAA receptor. The structure/activity requirements for neuroactive steroids interacting with the neurosteroid binding site on the GABAA receptor have been studied, and although chemical modification of some positions of the steroid nucleus are well tolerated, the 3a-hydroxyl configuration is required for binding and for activity in all assays. In the search for conformationally restricted analogues that could mimick the molecular shape of neuroactive steroids, we have previously prepared a 6,19-epoxy analogue, which at 0.1 mM concentration significantly increased GABA induced 36Cl- influx in hamster cerebral cortex synaptoneurosomes and had anticonvulsant activity in vivo. We now report the synthesis of compounds 3, 4 and 5, in which the oxygen bridges give a constrained conformation and decrease lipophilicity. Compound 3 was prepared from 11,19-oxidoprogesterone in 30% yield, by reduction to the 3b-hydroxysteroid (NaBH4), conversion to the 3a-formate (DEAD/Ph3P/HCO2H), oxidation to the 20-ketone (PCC) and mild hydrolysis with KHCO3/MeOH. Its 3D structure was obtained by single crystal X ray diffraction. The C1–C11 oxygen bridges were introduced by an intramolecular remote functionalization reaction involving the photolysis of 11a-hydroxysteroids in the presence of diacetoxyiodobenzene and iodine, recently developed by us. Compound 4 was prepared from 1b,11a-epoxy-5a-pregnane-3,20-dione in 27% yield, while 5 was obtained from 11a-hydroxy-5b-pregnane-3,20-dione in 20% yield. Their 3D structures were obtained by ab intio calculations. Biological activity of the synthetic steroids was assayed in vitro using t-[3H]butylbicycloorthobenzoate ([3H]TBOB) as radiolabelled ligand for the GABAA receptor. The activity of compound 3 was similar to that of allopregnanolone (1). Compound 4, conformationally related to 1 and 3, was less active than 1. Compound 5, conformationally related to pregnanolone (2) was more active than 2.