They're cousins of both flavonoids and humanoids.
Or perhaps they aren't, but they're all bases. That is, the opposite of acidic.
And that means that vinegar mops them up quite admirably - but so will water (as teas) and alcohol+water (as tinctures). And thus the very few "official" vinegars are all of plants with active alkaloids.
Mix alkaloids with tannins and watch the alkaloids' action disappear.
Alkaloids either have a very strong action, or none at all. Although I'm sure that there are a few in-betweens ... here's the short list, off the top of my head: nicotine, caffeine (and the other xanthines: theine, theophylline, theobromine etc.), codeine, morphine, papaverine, cocaine ... and pyrrolizidine alkaloids, some of which will do your liver in, in very small doses.
Pretty drastic compounds, no? A quick search gives more: scopolamine, hyoscyamine, and atropine (deadly, in Scopola, Hyoscyamus, Atropa, Datura and others), ephedrine (in Ephedra), solanine (in potatoes and other nightshades), strychnine (deadly, from nux vomica (Strychnos nux-vomica)), coniine (deadly, from water hemlock (Conium maculatum)), and so on.
And berberine, in Berberis, Mahonia, and others: that's a simple bitter, strangely enough.
Cats can't take chocolate (which contains theobromine), cos they're not equipped to handle plant-defense substances: cats are meat-eaters, exclusively. Dogs can't take salicylates (which aren't alkaloids), even though dogs do eat some plants. Puzzling, that.
Of the alkaloid-containing herbs: I give California poppy (Eschscholzia) to people who want to get rid of their opium alkaloid habit(s), as California poppy contains alkaloids which are close enough to opium alkaloids to calm the cravings, without either the addiction or the mindblowing effects.
I use coffee, chocolate and tea in my everyday life.
And I use Berberis and Mahonia to help kick a lazy liver into gear.