A stereocontrolled and enantioselective synthesis of the branched 6-amino-4,6-deoxyheptopyranuronic acid component of amipurimycin is reported. Key stages in the synthesis include the stereodivergent assembly of the dihydropyrones 12 and 14 from serinal derivatives (S)-10 and (R)-10, elaboration of the tetrahydropyran ring to give 26 and 31, and finally, introduction of the cis-2-aminocyclopentanecarboxylic acid moiety to produce the diastereomeric peptides 2829 and 3233.