Synthesis of Guaianolide Analogues with a Tunable α‑Methylene−γ-lactam Electrophile and Correlating Bioactivity with Thiol Reactivity

α-Methylene−γ-lactones are present in ∼3% of known natural products, and compounds comprising this motif display a range of biological activities. However, this reactive lactone limits informed structure–activity relationships for these bioactive molecules. Herein, we describe chemically tuning the electrophilicity of the α-methylene−γ-lactone by replacement with an α-methylene−γ-lactam. Guaianolide analogues having α-methylene−γ-lactams are synthesized using the allenic Pauson–Khand reaction. Substitution of the lactam nitrogen with electronically different groups affords diverse thiol reactivity. Cellular NF-κB inhibition assays for these lactams were benchmarked against parthenolide and a synthetic α-methylene−γ-lactone showing a positive correlation between thiol reactivity and bioactivity. Cytotoxicity assays show good correlation at the outer limits of thiol reactivity but less so for compounds with intermediate reactivity. A La assay to detect reactive molecules by nuclear magnetic resonance and mass spectrometry peptide sequencing assays with the La antigen protein demonstrate that lactam analogues with muted nonspecific thiol reactivities constitute a better electrophile for rational chemical probe and therapeutic molecule design.