Diels-Alderase; part of the gene cluster that mediates the biosynthesis of the trans-fused decalin-containing tetramic acid phomasetin, the stereochemical opposite of the HIV-1 integrase inhibitor equisetin (PubMed:29972614). The PKS module of phm1 together with the enoylreductase phm4 catalyze the formation of the polyketide unit which is then conjugated to L-serine by the condensation domain of the phm1 NRPS module (PubMed:29972614). Activity of the Dieckmann cyclase domain (RED) of phm1 results in release of the Dieckmann product intermediate (PubMed:29972614). The Diels-Alderase phm7 then uses the Dieckmann product of phm1 as substrate and catalyzes the Diels-Alder cycloaddition to form the decalin ring of N-desmethylphomasetin (PubMed:29972614, PubMed:34121297). N-desmethylphomasetin is further methylated to phomasetin by the methyltransferase phm5 (PubMed:29972614). 3-aminomethyl-p-menthane which is similar to the phomasetin substructure, dose-dependently inhibits phm7 activity in vitro and production of phomasetin in the fungus. Secondary metabolite biosynthesis. Leads to a significant decrease in the production of phomasetin. Belongs to the Diels-Alderase family.