Stereocontrolled total syntheses of penicillanic acid S,S-dioxide (10) and 6-aminopenicillanic acid (26) from (S)-aspartic acid and (r,R)-tartaric acid, respectively, are described. The key steps involve the preparation and cyclization of the nitroalkenes 8 and 23. Reaction of 8 and 23 with tetrabutylammonium fluoride followed by ozone and DBU gave the bicyclic 0-lactams 9 and 24. These substances were readily transformed into the target penicillanic acid derivatives 10 and 26. © 1990, American Chemical Society. All rights reserved.