Non-reducing polyketide synthase; part of the gene cluster that mediates the biosynthesis of the bicoumarin desertorin (PubMed:26389790). The non-reducing polyketide synthase desS first catalyzes the formation of the pentaketidic 4,7-dihydroxy-5-methylcoumarin from acetyl coenzyme A and 4 malonyl coenzyme A molecules (PubMed:26389790). Further O-methylation by desB leads to the formation of 7-demethylsiderin (PubMed:26389790). Then, an oxidative phenol coupling catalyzed by the cytochrome P450 monooxygenase desC forms the 6,8'-dimer M-desertorin A via dimerization the monomeric precursor, 7-demethylsiderin (PubMed:26389790). M-desertorin A is further converted to M-desertorin C (PubMed:26389790). 2 7-demethylsiderin + NADPH + O2 = desertorin A + 2 H2O + NADP(+) Secondary metabolite biosynthesis. Belongs to the cytochrome P450 family.