Alkaloids constitute a class of nitrogen-containing specialised metabolites that are biosynthesised by plants, fungi, bacteria, and certain animals and are characterised by the presence of at least one nitrogen atom within their molecular framework ^[1][2]. This nitrogen atom is typically basic and, in the majority of alkaloids, is incorporated into a heterocyclic ring system, a structural feature that confers the alkaline character from which the class derives its name ^[3][4]. Alkaloids are further organised according to their biosynthetic origin into three structural categories: true alkaloids, in which the nitrogen atom is derived from an amino acid and resides within a heterocycle; proto-alkaloids, in which the nitrogen is amino-acid-derived but lies outside a heterocyclic ring; and pseudo-alkaloids, in which the nitrogen is introduced at a late biosynthetic stage and is not amino-acid-derived ^[3][5]. The class is defined by pronounced structural diversity, with more than 27,000 structures reported and approximately 21,000 of these isolated from plants, and by a correspondingly broad range of biological activities that include toxicity, antimicrobial action, and pharmacological activity in humans and other animals ^[1][2]. Within the taxonomy of alkaloid synthesis and bioactivity, the term designates this nitrogen-containing, structurally heterogeneous group of specialised metabolites together with the biosynthetic pathways—rooted in amino acid metabolism and centred on iminium cation chemistry—through which their characteristic nitrogen heterocyclic scaffolds are constructed ^[1][3].