Efficient syntheses of chiral vicinal diamines derived from (S)-oxo-proline and (S)-proline are described. The novel diastereomerically pure precursor (2R, 5S)-2-trichloromethyl-1-aza-3- oxabicyclo-[3.3. 0]-octan-4, 8-dione 3 and its enantiomer are readily available by reaction of