FAD-linked oxidoreductase; part of the gene cluster that mediates the biosynthesis of the phytotoxin stemphyloxin II. The first step of the pathway is the synthesis of dehydroprobetaenone I by the polyketide synthase sthA and the enoyl reductase sthE via condensation of one acetyl-CoA starter unit with 7 malonyl-CoA units and 5 methylations. The C-terminal reductase (R) domain of sthA catalyzes the reductive release of the polyketide chain. Because sthA lacks a designated enoylreductase (ER) domain, the required activity is provided the enoyl reductase sthE. The short-chain dehydrogenase/reductase sthC then catalyzes reduction of dehydroprobetaenone I to probetaenone I. The cytochrome P450 monooxygenase sthF catalyzes successive epoxidation, oxidation (resulting from epoxide opening) and hydroxylation to install a tertiary alcohol in the decaline ring to yield betaenone C from dehydroprobetaenone I and betaenone B from probetaenone I. The FAD-linked oxidoreductase sthB is responsible for the conversion of betaenone C to betaenone A via an intramolecular aldol reaction between C-1 and C-17 to form the bridged tricyclic system in betaenone A. Finally, the cytochrome P450 monooxygenase sthD catalyzes the hydroxylation of C-15 to afford the final metabolite stemphyloxin II