Creating molecular complexity in the chemoenzymatic synthesis of chlorothricin analogues using tandem Diels–Alderases

Abstract

Chlorothricin is a polyketide-derived natural product isolated from Streptomyces antibioticus. It possesses an elaborate pentacyclic aglycone core which incorporates a spirotetronic acid moiety, linked to a trans-decalin system, embedded within a macrocycle. Using synthetic substrate analogues and purified recombinant proteins, here we demonstrate that assembly of this scaffold proceeds via sequential biocatalytic Diels–Alder reactions, promoted by the enzymes ChlE3 and ChlL. Both Diels–Alderases exhibit sufficiently relaxed substrate selectivity to facilitate access to non-natural chlorothricin analogues via biotransformations. The X-ray crystal structure of ChlE3 reveals the molecular basis of decalin formation by this enzyme. Harnessing this enzymatic cascade in biocatalysis could provide a valuable biomimetic route to both natural and non-natural spirotetronates, and the work described herein lays the foundation for application of these enzymes in chemoenzymatic syntheses of complex products.