Synthesis, anti-GABA activity and preferred conformation of bicuculline and norbicuculline enantiomers

Synthesis of erythro-(±)-[1SR,9RS]-norbicuculline and threo-(±)-[1SR,9SR]-noradlumidine from piperonal was performed using Bischler-Napieralski cyclization as a key step. Resolution gave rise to (+)-[1S,9R]-norbicuculline ([1S,9R] norBIC) and (-)-[1R,9S]-norbicuculline ([1R,9S] norBIC) in >99.5% enantiomeric purity. Bicuculline enantiomers were readily obtained by methylation of the latter products. [1S,9R]BIC was about 70 times more potent than [1R,9S] BIC as an inhbitor of GABA(A) receptor binding and was about 100 and 900 times more potent than [1S,9R] norBIC at pH 7.1 and 5.0 respectively. Similarly, [1S,9R] norBIC was much less potent than [1S,9R] BIC as an inhibitor of GABA-specific (36)Cl(-) ion flux. The observed increase of about two orders of magnitude in the in vitro biological activity caused by N2-CH(3) substitution in [1S,9R] norBIC was attributed to different conformations for erythro- and nor-erythro-bicucullines indicated by (1)H nuclear Overhauser enhancements of [1S,9R] BIC and [1S,9R] norBIC.