Torreyanic acid is a dimeric quinone first isolated and by Lee et al. in 1996 from an endophyte, Pestalotiopsis microspora. This endophyte is likely the cause of the decline of Florida torreya (Torreya taxifolia), an endangered species that is related to the taxol-producing Taxus brevifolia. The natural product was found to be cytotoxic against 25 different human cancer cell lines with an average IC50 value of 9.4 µg/mL, ranging from 3.5 (NEC) to 45 (A549) µg/mL. Torreyanic acid was found to be 5-10 times more potent in cell lines sensitive to protein kinase C (PKC) agonists, 12-o-tetradecanoyl phorbol-13-acetate (TPA), and was shown to cause cell death via apoptosis. Torreyanic acid also promoted G1 arrest of G0 synchronized cells at 1-5 µg/mL levels, depending on the cell line. It has been proposed that the eukaryotic translation initiation factor EIF-4a is a potential biochemical target for the natural compound. Biosynthesis There are over 150 natural products that are presumed to undergo a [4+2] Diels–Alder type cycloaddition, belonging to classes such as: polyketides, terpenoids, phenylpropanoids, and alkaloids. The Diels–Alder cycloaddition involves the overlap of the p-orbitals of two unsaturated systems: a 1,3-diene and dienophile. The conjugated diene reacts with the dienophile to form a cyclic product in a concerted fashion. This reaction is widely used in synthesis due to its facile nature and reio- and stereoselectivity under mild conditions. This reaction is very useful for forming carbon-carbon bonds, four-chiral centers, and quaternary stereogenic centers.[4,5] Natural products that are constructed biosynthetically via a Diels–Alder reaction occur both uncatalyzed and catalyzed by enzymes such as Diels–Alderase and RNA Diels-Alderase. In their report of the isolation and structural characterization of the natural product, Lee and co-worker proposed that the biosynthesis of torreyanic acid proceeded via an endo-selective [4+2] cycloaddition with a Diels–Alder dimerization of 2H-pyran monomers 2a and 2b. Key observations that indicate a natural product is biosynthesized via a Diels–Alder reaction include: (a) isolation of an adduct with its corresponding precursor, (b) presence of adducts and their regio- and diastereoisomers, (c) a non-enzymatic feasibility of a likely cycloaddition and (d) chirality of the adducts. The proposed biosynthetic pathway is thought to involve: (a) an electrocyclic ring closure of 3, followed by (b) an enzymatic oxidation to furnish diastereomers 2a and 2b, and finally (c) a [4+2] cyclodimerization to generate torreyanic acid 1. The biosynthesis of torreyanic acid was studied extensively by Poroco et al. in