Diels-Alderase; part of the gene cluster that mediates the biosynthesis of the HIV-1 integrase inhibitor equisetin and of fusarisetin A, both trans-fused decalin-containing tetramic acids showing also antimicrobial activity (PubMed:25770422). The PKS module of fsa1 together with the enoylreductase fsa3 catalyze the formation of the polyketide unit which is then conjugated to L-serine by the condensation domain of the fsa1 NRPS module (PubMed:25770422). Activity of the Dieckmann cyclase domain (RED) results in release of the Dieckmann product intermediate (PubMed:25770422). Diels-Alderase fsa2 is involved in endo-selective Diels-Alder cycloaddition to form the decalin ring, leading to the production of N-desmethylequisetin also called trichosetin (PubMed:25770422, PubMed:28401214, PubMed:29972614, PubMed:34121297). Subsequent N-methylation is carried out by fsa4 to give equisetin (PubMed:25770422). The enzymatic gene responsible for the conversion of equisetin to fusarisetin A has not been identified yet and is probably located outside of the fsa cluster (PubMed:28401214). (5S)-3-[(2E,6R,8E,10E,12E)-2,6-dimethyltetradeca-2,8,10,12-tetraenoyl]-5-(hydroxymethyl)pyrrolidine-2,4-dione = trichosetin Optimum pH is 7. Optimum temperature is 30 degrees Celsius. Mycotoxin biosynthesis. Results in the loss of production of equisetin and fusarisetin A (PubMed:25770422). Belongs to the Diels-Alderase family.