Experiments and speculations on the role of oxidative cyclization chemistry in natural product biosynthesis

Abstract

Syntheses of [2-²H,3-¹³C]- and [2-²H,3-²H₂]proclavaminic acid, (22) and (29) respectively, are described. These labeled substrates were incubated with clavaminate synthase, a key enzyme acting in the biosynthesis of the lactamase inhibitor clavulanic acid, in the presence of ferrous ion, α-ketoglutaric acid and molecular oxygen. The apparent oxidative cyclization/desaturation chemistry evident in the conversion of proclavaminic acid (8) to clavaminic acid (9) takes place without loss or exchange of label at C-2 and C-3 in the substrate. These observations point to notable similarities to sulfur insertion reactions in natural product biosynthesis and lead to the proposition of a generalized mechanism for oxidative cyclization invoking substrate heteroatom participation. By extension of this mechanistic hypothesis, a new biogenetic speculation is advanced to account for polyether formation, for example, in monensin (52) and brevetoxin A (55). Isotopically-labeled samples of proclavaminic acid (8) have been prepared and converted to clavaminic acid (9) by the enzyme clavaminate synthase (CS). Results of these experiments point to a subset of oxygenase chemistry involving cyclization/desaturation reactions that may play a role in the biosynthesis of other natural product groups, e.g. polyethers as monensin and brevetoxin A.