α-Spirocyclopentyl- and α-Spirocyclopropyl-γ-butyrolactones: Conformationally Constrained Derivatives of Anticonvulsant and Convulsant αα-Disubstituted γ-Butyrolactones

To further study the putative γ-butyrolactone site of the GABA_A/chloride channel complex, constrained derivatives of convulsant and anticonvulsant α,α-disubstituted γ-butyrolactones (α-spirocyclopropyl- and α-spirocyclopentyl-γ-butyrolactones) were synthesized and evaluated biologically. Most of the spirocyclopropyl agents were anticonvulsants when tested against pentylenetetrazole-induced seizures in mice. These agents effectively displaced ³⁵[S]-tertbutylbicyclophosphorothionate (³⁵[S]-TBPS), a ligand for the picrotoxin binding site of the GABA_A/chloride channel, from rat neuronal membranes and affected the GABA-mediated current in hippocampal neurons. The monomethyl-substituted spirocyclopropyl agent with a methyl group cis to the carbonyl (15) potentiates GABA-induced current whereas the trans derivative (16) blocks the current. The only anticonvulsant in the spirocyclopentyl series was the unsubstituted spirocyclopentyl compound 2. All the other substituted spirocyclopentyl targets were inactive in vivo at the highest dose tested except for convulsant 9, which has a trans 2,5-dimethyl-substituted cyclopentyl ring. All the spirocyclopentyl derivatives displaced ₃₅[S]-TBPS from rat neuronal membranes very effectively, and they also all potentiated GABA-induced chloride current except for convulsant 9 which blocked the current. From the data obtained in this investigation, it appears that when the volume occupied above and below the lactone ring is as large as that occupied by spirocyclopentyl agent 9, convulsant activity is observed. Groups with less volume in these areas either are inactive in the behavioral test or have anticonvulsant activity. When bound to the GABA_A/chloride channel, the larger molecules may stabilize the closed state of the channel whereas the smaller molecules may stabilize the open state.