Resolution of carboxylic acids using copper(I)-promoted removal of propargylic esters under neutral conditions

Improved synthetic route of exo-16,17-dihydro-gibberellin A5-13-acetate and the bioactivity of its derivatives towards Arabidopsis thaliana

BACKGROUND

The use of exo-16,17-dihydro-gibberellin A5-13-acetate (DHGA₅ ) in agriculture has been limited by its low synthetic yield. This study was aimed at optimizing the synthetic route of DHGA₅ , designing and synthesizing new derivatives with strong plant growth inhibitory activities.

RESULTS

Previous synthetic methods were replaced with a shorter, milder and faster reaction route with higher yield (76.3%) of DHGA₅ . Based on this novel route, a series of new derivatives were designed and synthesized using DHGA₅ as a lead compound and characterized and evaluated for biological activities in Arabidopsis thaliana. Among the 15 tested derivatives, compound 14j showed a lower medium inhibition concentration (IC₅₀ , 73 μm) in Arabidopsis than that of DHGA₅ (91 μm). Gibberellin deficient mutant assay further revealed that 14j had very different activities compared to DHGA₅ as it specifically inhibits gibberellin biosynthetic pathways. In addition, 14j does not influence the interaction between gibberellin receptors (GID1) and the master growth repressor (RGA) based on yeast two-hybrid assay.

CONCLUSION

The optimized synthetic route provides a promising method for large-scale preparation of DHGA₅ . Our biological assays indicate that 14j likely acts on gibberellin signaling elements other than GID1. These results indicate that novel plant growth regulators can be developed. © 2019 Society of Chemical Industry.

Radiosynthesis of (R,S)-[18 F]GE387: A Potential PET Radiotracer for Imaging Translocator Protein 18 kDa (TSPO) with Low Binding Sensitivity to the Human Gene Polymorphism rs6971

Translocator protein (TSPO) is a biomarker of neuroinflammation, which is a hallmark of many neurodegenerative diseases and has been exploited as a positron emission tomography (PET) target. Carbon-11-labelled PK11195 remains the most applied agent for imaging TSPO, despite its short-lived isotope and low brain permeability. Second-generation radiotracers show variance in affinity amongst subjects (low-, mixed-, and high-affinity binders) caused by the genetic polymorphism (rs6971) of the TSPO gene. To overcome these limitations, a new structural scaffold was explored based on the TSPO pharmacophore, and the analogue with a low-affinity binder/high-affinity binder (LAB/HAB) ratio similar (1.2 vs. 1.3) to that of (R)-[¹¹ C]PK11195 was investigated. The synthesis of the reference compound was accomplished in six steps and 9 % overall yield, and the precursor was prepared in eight steps and 8 % overall yield. The chiral separation of the reference and precursor compounds was performed using supercritical fluid chromatography with >95 % ee. The absolute configuration was determined by circular dichroism. Optimisation of reaction conditions for manual radiolabelling revealed acetonitrile as a preferred solvent at 100 °C. Automation of this radiolabelling method provided R and S enantiomers in respective 21.3±16.7 and 25.6±7.1 % decay-corrected yields and molar activities of 55.8±35.6 and 63.5±39.5 GBq μmol^-1 (n=3). Injection of the racemic analogue into a healthy rat confirmed passage through the blood-brain barrier.

Seeking (and Finding) Biased Ligands of the Kappa Opioid Receptor

The discovery and characterization of two classes of kappa opioid receptor agonists that are biased for G protein over βarrestin signaling are described.

Synthesis of Gibberellic Acid Derivatives and Their Effects on Plant Growth

A series of novel C-3-OH substituted gibberellin derivatives bearing an amide group were designed and synthesized from the natural product gibberellic acid (GA₃). Their activities on the plant growth regulation of rice and Arabidopsis were evaluated in vivo. Among these compounds, 10d and 10f exhibited appreciable inhibitory activities on rice (48.6% at 100 μmol/L) and Arabidopsis (41.4% at 100 μmol/L), respectively. These results provide new insights into the design and synthesis of potential plant growth regulators.