Robert Michael Williams

Fifteen new isopimarane-type diterpenes, taichunins E–S (1–15), and a new 20-nor-isopimarane, taichunin T (16), together with four known compounds were isolated from Aspergillus taichungensis (IBT 19404). The structures of these new compounds were determined by NMR and mass spectroscopy, and their absolute configurations were analyzed by NOESY and TDDFT calculations of ECD spectra. Taichunins G, K, and N (3, 7, and 10) completely inhibited the receptor activator of nuclear factor-κB ligand (RANKL)-induced formation of multinuclear osteoclasts in RAW264 cells at 5 μM, with 3 showing 92% inhibition at a concentration of 0.2 μM.

Fungal bicyclo[2.2.2]diazaoctane indole alkaloids represent an important family of natural products with a wide spectrum of biological activities. Although biomimetic total syntheses of representative compounds have been reported, the details of their biogenesis, especially the mechanisms for the assembly of diastereomerically distinct and enantiomerically antipodal metabolites, have remained largely uncharacterized. Brevianamide A represents a basic form of the subfamily bearing a dioxopiperazine core and a rare 3-spiro-ψ-indoxyl skeleton. In this study, we have identified the brevianamide A biosynthetic gene cluster from Penicillium brevicompactum NRRL 864 and elucidated the metabolic pathway. BvnE was revealed to be an essential isomerase/semipinacolase that specifies the selective production of the natural product. Structural elucidation, molecular modelling and mutational analysis of BvnE as well as quantum chemical calculations have provided mechanistic insights into the diastereoselective formation of the 3-spiro-ψ-indoxyl moiety in brevianamide A. This occurs through a BvnE-controlled semipinacol rearrangement and a subsequent spontaneous intramolecular [4+2] hetero-Diels–Alder cycloaddition.

The fungal indole alkaloids are a unique class of complex molecules that have a characteristic bicyclo[2.2.2]diazaoctane ring and frequently contain a spiro‐oxindole moiety. While various strains produce these compounds, an intriguing case involves the formation of individual antipodes by two unique species of fungi in the generation of the potent anticancer agents (+)‐ and (−)‐notoamide A. NotI and NotI′ have been characterized as flavin‐dependent monooxygenases that catalyze epoxidation and semi‐pinacol rearrangement to form the spiro‐oxindole center within these molecules. This work elucidates a key step in the biosynthesis of the notoamides and provides an evolutionary hypothesis regarding a common ancestor for production of enantiopure notoamides.

A new isoindolinone alkaloid, irpexine (1), was isolated as a racemate, along with a known green pigment, hypoxyxylerone (2), from the coculture of two endophytic fungi, Irpex lacteus and Phaeosphaeria oryzae. Compound 1 was found to be a newly produced metabolite of I. lacteus in the coculture with P. oryzae. Although 2 was produced in a monoculture of I. lacteus, its production was markedly enhanced by the coculture.

New structural classes of antibiotics are rare, structurally-novel broad-spectrum antibiotics exceptionally so. The recently discovered baulamycins constitute a remarkable example of these highly-prized compounds and, as such, have attracted considerable attention in the form of both synthetic efforts and biological studies. For the first time, we report a gram-scale preparation of the common carbon framework of the baulamycin family, as well as the total synthesis of its most potent member, baulamycin A. Our approach employs highly-stereoselective, catalyst-controlled asymmetric conjugate additions to thioesters to set key stereocenters, as well as the first reported use of “dry ozonolysis” to reveal a masked carboxylic acid in the total synthesis of a natural product.

The paraherquamides are potent anthelmintic natural products with complex heptacyclic scaffolds. One key feature of these molecules is the spiro-oxindole moiety that lends a strained three-dimensional architecture to these structures. The flavin monooxygenase PhqK was found to catalyze spirocycle formation through two parallel pathways in the biosynthesis of paraherquamides A and G. Two new paraherquamides (K and L) were isolated from a ΔphqK strain of Penicillium simplicissimum, and subsequent enzymatic reactions with these compounds generated two additional metabolites, paraherquamides M and N. Crystal structures of PhqK in complex with various substrates provided a foundation for mechanistic analyses and computational studies. While it is evident that PhqK can react with various substrates, reaction kinetics and molecular dynamics simulations indicated that the dioxepin-containing paraherquamide L is the favored substrate. Through this effort, we have elucidated a key step in the biosynthesis of the paraherquamides and provided a rationale for the selective spirocyclization of these powerful anthelmintic agents.

Prenylated indole alkaloids such as the calmodulin-inhibitory malbrancheamides and anthelmintic paraherquamides possess great structural diversity and pharmaceutical utility. Here, we report complete elucidation of the malbrancheamide biosynthetic pathway accomplished through complementary approaches. These include a biomimetic total synthesis to access the natural alkaloid and biosynthetic intermediates in racemic form and in vitro enzymatic reconstitution to provide access to the natural antipode (+)-malbrancheamide. Reductive cleavage of an l-Pro–l-Trp dipeptide from the MalG non-ribosomal peptide synthetase (NRPS) followed by reverse prenylation and a cascade of post-NRPS reactions culminates in an intramolecular [4+2] hetero-Diels–Alder (IMDA) cyclization to furnish the bicyclo[2.2.2]diazaoctane scaffold. Enzymatic assembly of optically pure (+)-premalbrancheamide involves an unexpected zwitterionic intermediate where MalC catalyses enantioselective cycloaddition as a bifunctional NADPH-dependent reductase/Diels–Alderase. The crystal structures of substrate and product complexes together with site-directed mutagenesis and molecular dynamics simulations demonstrate how MalC and PhqE (its homologue from the paraherquamide pathway) catalyse diastereo- and enantioselective cyclization in the construction of this important class of secondary metabolites.

Fungal bicyclo[2.2.2]diazaoctane indole alkaloids demonstrate intriguing structures and a wide spectrum of biological activities. Although biomimetic total syntheses have been completed for representative compounds of this structural family, the details of their biogenesis have remained largely uncharacterized. Among them, Brevianamide A represents the most basic form within this class bearing a dioxopiperazine core structure and a rare 3-spiro-psi-indoxyl skeleton. Here, we identified the Brevianamide A biosynthetic gene cluster from Penicillium brevicompacticum NRRL 864 and fully elucidated the metabolic pathway by targeted gene disruption, heterologous expression, precursor incorporation studies, and in vitro biochemical analysis. In particular, we determined that BvnE is a cofactor-independent isomerase that is essential for selective production of Brevianamide A. Based on a high resolution crystal structure of BvnE, molecular modeling, mutational analysis, and computational studies provided new mechanistic insights into the diastereoselective formation of the 3-spiro-psi-indoxyl moiety in Brevianamide A. This occurs through a biocatalyst controlled semi-Pinacol rearrangement and a subsequent spontaneous intramolecular [4+2] hetero-Diels-Alder cycloaddition.