S-adenosyl-l-methionine-dependent Diels-Alderase; part of the gene cluster that mediates the biosynthesis of ilicicolin H, a 4-hydroxy-2-pyridonealkaloid that has potent and broad antifungal activities by inhibiting the mitochondrial respiration chain (PubMed:34947016). IliD catalyzes the Diels-Alder reaction that converts the acyclic 2-pyridone intermediate to 8-epi-ilicicolin H (PubMed:34947016). The biosynthesis of ilicicolin H starts with formation of the tetramic acid by the hybrid PKS-NRPS synthetase iliA with the partnering trans-enoyl reductase iliB since iliA lacks a designated enoylreductase (ER) domain. The cytochrome P450 monooxygenase iliC then catalyzes the ring expansion of the tetramate to the acyclic 2-pyridone. The pericyclase iliD further converts the acyclic 2-pyridone into 8-epi-ilicicolin H. 8-epi-ilicicolin H might then spontaneously convert to ilicicolin H since ilicicolin H is produced in the absence of the epimerase iliE, in contrast to what was observed for the Talaromyces variabilis ilicolin H biosynthetic pathway (PubMed:34947016) (Probable). 3-[(2E,4E,8S,10E,12Z)-4,8-dimethyltetradeca-2,4,10,12-tetraenoyl]-4-hydroxy-5-(4-hydroxyphenyl)-1,2-dihydropyridin-2-one = ilicicolin H Mycotoxin biosynthesis. Belongs to the class I-like SAM-binding methyltransferase superfamily. Erg6/SMT family.