Emerging chemical and genetic evidence suggests that separate biochemical solutions have evolved to synthesize the four known classes of beta-lactam antibiotics. One of these classes contains clavulanic acid (1) and a family of structurally related but antipodal clavam metabolites 2-5, 7, and 8 which lack a carboxylate at C-3, have a different oxidation state, and exhibit stereochemical features at C-2. Previous work has demonstrated the incorporation of omithine/arginine in the identical regiochemical sense in all of these natural products, and has established the common intermediacy of the monocyclic beta-lactam proclavaminic acid (12) as well. In this paper the quite advanced bicyclic intermediate clavaminic acid (14) has been synthesized in doubly C-13-labeled form by preparative incubation of recombinant clavaminate synthase. The intact and equally efficient incorporation of 14 into valclavam (7) and 2-(2-hydroxyethyl)clavam (8), together with O-18(2)-incorperation experiments, has been interpreted to define clavaminic acid as the final intermediate shared in the biosynthesis of clavulanic acid and the antipodal clavams. A mechanistic rationale of this interrelationship and the late stages of the respective biosyntheses is proposed.