Unlocking the Secrets of Roman Chamomile (Anthemis nobilis L.) Essential Oil: Structural Elucidation and Acute Toxicity of New Esters

To address gaps in the characterization of Roman chamomile (<i>Anthemis nobilis</i> L., Asteraceae)-an ethnobotanically and commercially important species-we profiled its essential oil (EO), focusing on esters that are incompletely characterized or unreported. Comprehensive GC-MS of two commercial EOs and their chromatographic fractions, combined with synthesis and co-injection of reference compounds, enabled the identification of 190 constituents. We uncovered a homologous series of angelates, tiglates, and senecioates by partial-ion-current (PIC) screening (<i>m</i>/<i>z</i> 55, 83, 100, 101), augmented by co-injection and NMR confirmation. Among these EO constituents, four esters, methallyl 3-methylbutanoate (<b>6h</b>), methallyl senecioate (<b>3h</b>), 3-methylpentyl 2-methylbutanoate (<b>5c</b>), and 5-methylhexyl angelate (<b>2g</b>) are reported here as new natural products and previously unreported compounds in the literature. Selected methacrylates and related α,β-unsaturated esters underwent model Michael additions to methanethiol (generated in situ from dimethyl disulfide and NaBH₄), confirming their thiol-acceptor reactivity. In an <i>Artemia salina</i> assay, the EO and most esters were non-toxic; methacrylates showed only low toxicity at the highest concentrations. These results refine the chemical map of <i>A. nobilis</i> EO and highlight specific ester families for future mechanistic and biological evaluation.