Résumé : 1‐azabicyclo [2,2,2] octan‐3‐one (3‐quinuclidone) treated with triphenyl‐ethylphosphonium bromide in dimethylsulfoxide and sodium hydride gives pure 3‐ethylidene‐1‐azabicyclo [2,2,2] octane in high yield. Evidence for the existence of cis and trans isomers, whose structures were determined by study of long range coupling, is given by N.M.R. spectroscopy. A correlation with sarpaginelike indole alkaloids has been established and a simple rule for the determination of the configuration of the ethylidene side chain in such alkaloids is given. Copyright © 1965 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim