Thermal intramolecular oxime olefin cycloaddition of a-allylamino aldoximes and ketoximes 4 led stereospecifically to formation of oxadiazabicyclo [3.3. 0] octanes 6. The presence of heteroatoms in these bicyclic fused 5-membered rings permits for the first time an evaluation of the conformation of this system by means of NMR. We found that some substituents in 6 restrict the conformational mobility of these five-membered rings to the extent that only one ...