Diels-Alderase; part of the gene cluster that mediates the biosynthesis of myceliothermophins, mycotoxins that contain a trans-fused decalin ring system connected to a conjugated 3-pyrrolin-2-one moiety and that have potential anti-tumor properties (PubMed:27960349). The polyketide synthase module (PKS) of the PKS-NRPS mycA is responsible for the synthesis of the octaketide backbone (PubMed:27960349). The downstream nonribosomal peptide synthetase (NRPS) module then amidates the carboxyl end of the octaketide with a leucine (PubMed:27960349). A reductase-like domain (R) at the C-terminus catalyzes the reductive release of the polyketide-amino acid intermediate (PubMed:27960349). Because mycA lacks a designated enoylreductase (ER) domain, the required activity is provided the enoyl reductase mycC (PubMed:27960349). Following mycA-catalyzed construction and release of aminoacyl polyketide aldehyde, Knoevenagel condensation yields the expected ketone (PubMed:27960349). This C18 keto acyclic precursor is the substrate of the Diels-Alderase mycB, that catalyzes the Diels-Alder cycloaddition to produce myceliothermophin E (PubMed:27960349). A yet unknown oxygenase involved in the production of myceliothermophin A, via substitution with a hydroxyl group at the C21, has still to be identified (PubMed:27960349). (5S)-5-(2-methylpropyl)-3-[(2E,6R,8E,10E,12E)-6,8,10,12-tetramethyltetradeca-2,8,10,12-tetraenoyl]-2,5-dihydro-1H-pyrrol-2-one = (5S)-3-[(1S,2R,4aR,6R,8aS)-2-(but-2-en-2-yl)-3,4a,6-trimethyl-1,2,4a,5,6,7,8,8a-octahydronaphthalene-1-carbonyl]-5-(2-methylpropyl)-2,5-dihydro-1H-pyrrol-2-one (5Z)-5-(2-methylpropylidene)-3-[(2E,6R,8E,10E,12E)-6,8,10,12-tetramethyltetradeca-2,8,10,12-tetraenoyl]-2,5-dihydro-1H-pyrrol-2-one = myceliothermophin E Mycotoxin biosynthesis. Completely abolishes the production of myceliothermophin E, but leads to the accumulation of its C18 keto acyclic precursor. Belongs to the Diels-Alderase family.