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A small-molecule natural product, euodenine A (1), was identified as an agonist of the human TLR4 receptor. Euodenine A was isolated from the leaves of Euodia asteridula (Rutaceae) found in Papua New Guinea and has an unusual U-shaped structure. It was synthesized along with a series of analogues that exhibit potent and selective agonism of the TLR4 receptor. SAR development around the cyclobutane ring resulted in a 10-fold increase in potency. The natural product demonstrated an extracellular site of action, which requires the extracellular domain of TLR4 to stimulate a NF-κB reporter response. 1 is a human-selective agonist that is CD14-independent, and it requires both TLR4 and MD-2 for full efficacy. Testing for immunomodulation in PBMC cells shows the induction of the cytokines IL-8, IL-10, TNF-α, and IL-12p40 as well as suppression of IL-5 from activated PBMCs, indicating that compounds like 1 could modulate the Th2 immune response without causing lung damage.

Fragment-based screening is commonly used to identify compounds with relatively weak but efficient localized binding to protein surfaces. We used mass spectrometry to study fragment-sized three-dimensional natural products. We identified seven securinine-related compounds binding to Plasmodium falciparum 2'-deoxyuridine 5'-triphosphate nucleotidohydrolase (PfdUTPase). Securinine bound allosterically to PfdUTPase, enhancing enzyme activity and inhibiting viability of both P. falciparum gametocyte (sexual) and blood (asexual) stage parasites. Our results provide a new insight into mechanisms that may be applicable to transmission-blocking agents.

Treatment of triphenylphosphine (Ph(3)P) with an excess of diisopropyl azodicarboxylate at 0-25 °C resulted in the formation of a symmetrical tetraalkyl tetrazetidinetetracarboxylate radical cation, containing the elusive cyclic N(4) ring system. Electron paramagnetic resonance (EPR) spectroscopy revealed a 9-line spectrum, with hyperfine coupling constants indicative of four almost magnetically equivalent nitrogen atoms. The radical species was surprisingly long-lived, and could still be observed several hours after generation and standing at 25 °C. Expansion of the central resonance revealed further splitting into a pentet (hyperfine coupling to the four methine protons). Three mechanistically plausible structures containing the tetrazetidine substructure were proposed based on the 9-line EPR spectrum. Following DFT calculations, the predicted hyperfine coupling constants were used to simulate the EPR spectra for the three candidate structures. The combined calculations and simulations were consistent with a radical cation species, but not a radical anion or radical-carbenoid structure. The lowest energy conformation of the N(4) ring was slightly puckered, with the alkyl carboxylate groups all trans and the four carbonyl groups aligned in a pinwheel arrangement around the ring. Analogous results were obtained with the original Mitsunobu reagents, Ph(3)P and diethyl azodicarboxylate, but not with Ph(3)P and di-tert-butyl azodicarboxylate. A mechanism is proposed based on a radical version of the Rauhut-Currier or Morita-Baylis-Hillman reactions.