The pyridine ring is a potent pharmacophore in alkaloid natural products. Nonetheless, its biosynthetic pathways are poorly understood. Rubrolones A and B are tropolone alkaloid natural products possessing a unique tetra-substituted pyridine moiety. Here, we report the gene cluster and propose a biosynthetic pathway for rubrolones, identifying a key intermediate that accumulates upon inactivation of sugar biosynthetic genes. Critically, this intermediate was converted to the aglycones of rubrolones by non-enzymatic condensation and cyclization with either ammonia or anthranilic acid to generate the respective pyridine rings. We propose that this non-enzymatic reaction occurs via hydrolysis of the key intermediate, which possesses a 1,5-dione moiety as an amine acceptor capable of cyclization. This study suggests that 1,5-dione moieties may represent a general strategy for pyridine ring biosynthesis, and more broadly highlights the utility of non-enzymatic diversification for exploring and expanding natural product chemical space.

The skeleton of tropane alkaloids is derived from ornithine-derived N -methylpyrrolinium and two malonyl-CoA units. The enzymatic mechanism that connects N -methylpyrrolinium and malonyl-CoA units remains unknown. Here, we report the characterization of three pyrrolidine ketide synthases (PYKS), Aa PYKS, Ds PYKS, and Ab PYKS, from three different hyoscyamine- and scopolamine-producing plants. By examining the crystal structure and biochemical activity of Aa PYKS, we show that the reaction mechanism involves PYKS-mediated malonyl-CoA condensation to generate a 3-oxo-glutaric acid intermediate that can undergo non-enzymatic Mannich-like condensation with N -methylpyrrolinium to yield the racemic 4-(1-methyl-2-pyrrolidinyl)-3-oxobutanoic acid. This study therefore provides a long sought-after biosynthetic mechanism to explain condensation between N -methylpyrrolinium and acetate units and, more importantly, identifies an unusual plant type III polyketide synthase that can only catalyze one round of malonyl-CoA condensation.

Nine new pentacyclic polyketides, fasamycins G−K (1−5) and formicamycins N−Q (6−9), along with 10 known analogues (10−19), were isolated from a rhizospheric soil-derived Streptomyces sp. KIB-1414. Their structures and absolute configurations were elucidated by interpretation of NMR and HRMS data and comparisons of CD data. The compounds were active against methicillin-resistant Staphylococcus aureus (MRSA), Staphylococcus aureus, Bacillus subtilis, and Escherichia coli strains, with MIC values ranging from 0.20 to 50.00 μg/mL.

Tropane alkaloids such as hyoscyamine and cocaine are of importance in medicinal uses. Only recently has the hyoscyamine biosynthetic machinery become complete. However, the cocaine biosynthesis pathway remains only partially elucidated. Here we characterize polyketide synthases required for generating 3-oxo-glutaric acid from malonyl-CoA in cocaine biosynthetic route. Structural analysis shows that these two polyketide synthases adopt distinctly different active site architecture to catalyze the same reaction as pyrrolidine ketide synthase in hyoscyamine biosynthesis, revealing an unusual parallel/convergent evolution of biochemical function in homologous enzymes. Further phylogenetic analysis suggests lineage-specific acquisition of polyketide synthases required for tropane alkaloid biosynthesis in Erythroxylaceae and Solanaceae species, respectively. Overall, our work elucidates not only a key unknown step in cocaine biosynthesis pathway but also, more importantly, structural and biochemical basis for independent recruitment of polyketide synthases in tropane alkaloid biosynthesis, thus broadening the understanding of conservation and innovation of biosynthetic catalysts.

Three unprecedented cytochalasan homodimers, bisaspochalasins A–C (1–3), and two known monomers, aspochalasins B and D (4 and 5), were isolated from an endophytic Aspergillus flavipes. Bisaspochalasin A (1) contains a 13-hydroxy-3,24-dioxatricyclo­[11.10.11,13.02,15]­tetracos-4-one cross-linkage, representing an unprecedented carbon skeleton. Bisaspochalasins B (2) and C (3) share a thioether bridge, while 3 has a peroxy modification at C-7, which may be generated by Schenck-ene photooxygenation. Their structures, including their absolute configurations, were elucidated by HRESIMS, NMR, chemical transformation, and X-ray crystallography. Bisaspochalasin A showed inhibitory activity against human T cell proliferation with an IC50 value of 15.8 μM while maintaining low cytotoxicity to T cells.

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Euphordraculoates A (1) and B (2), featuring tigliane diterpenoids with two new carbon skeletons, were characterized as metabolites of Euphorbia dracunculoides and semisynthetic products, respectively. Their structures were determined by spectroscopic analyses and X-ray crystallography. The respective biosynthetic and chemical formation mechanisms for 1 and 2 from a known tigliane 3 was proposed. The detailed decarbonization mechanism from 3 to 2 was further explored by O-labeling experiment. Compound 2 could inhibit Wnt pathway in a dose- and time-dependent manner.

Rubrolones, isarubrolones, and rubterolones are recently isolated glycosylated tropolonids with notable biological activity.