The reaction of (2S)-N-benzoyl-2-tert-butyl-4-methylene-1,3-oxazolidin-5-one (9) with isopropylidene-triphenylphosphorane at room temperature gives the corresponding cis and trans spirocyclopropane derivatives 10 in ca. 1:1 ratio. After separation of both diastereomers, by recrystallization and flash chromatography, and further acid hydrolysis, enantiomerically pure (R)- and (S)-2,3-methanovaline 11