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Prototypical basic structure of a toxic 1,2-unsaturated pyrrolizidine alkaloid; a sketeletal formula of retronecine, a PA found in the Common groundsel (Senecio vulgaris) and comfrey (Symphytum spp.). R illustrates the position of the different necic acids. Of note, the chemical structure of non-toxic PAs is similar to the structure shown above except that the double bond between C1 and C2 is missing. Structure was composed from various publications [1,2,3,4,5,6,21,22,23,24].
