Tricarbocyclic core formation of tyrosine-decahydrofluorenes implies a three-enzyme cascade with XenF-mediated sigmatropic rearrangement as a prerequisite.

Tyrosine-decahydrofluorene derivatives feature a fused [6.5.6] tricarbocyclic core and a 13-membered -cyclophane ether. Herein, we identified new xenoacremones A, B, and C (-) from the fungal strain ML-31 and elucidated their biosynthetic pathway using gene deletion in the native strain and heterologous expression in . The hybrid polyketide synthase-nonribosomal peptide synthetase (PKS-NRPS) XenE together with enoyl reductase XenG were confirmed to be responsible for the formation of the tyrosine-nonaketide skeleton. This skeleton was subsequently dehydrated by XenA to afford a pyrrolidinone moiety. XenF catalyzed a novel sigmatropic rearrangement to yield a key cyclohexane intermediate as a prerequisite for the formation of the multi-ring system. Subsequent oxidation catalyzed by XenD supplied the substrate for XenC to link the -cyclophane ether, which underwent subsequent spontaneous Diels-Alder reaction to give the end products. Thus, the results indicated that three novel enzymes XenF, XenD, and XenC coordinate to assemble the [6.5.6] tricarbocyclic ring and -cyclophane ether during biosynthesis of complex tyrosine-decahydrofluorene derivatives.