Willow bark contains salicin. Things made from willow bark have been used for a very long time as herbal remedies. In the early 1800s, people figured out how to isolate it and also break it down and oxidise it to make salicylic acid. They tried using what they'd extracted as medicine as they knew it had an effect on the body, and that was the height of the bar back then. It generally did much more harm than good, but eventually some things were discovered that it genuinely helped treat. In the late 1800s, people had figured out that if you tried subjecting known bioactive compounds to chemical reactions, sometimes you ended up with a new bioactive compound - that was how diamorphine (heroin) was first synthesised from morphine, for example. Someone tried an esterification reaction with salicylic acid, and got acetylsalicylic acid, and eventually Bayer managed to purify and manufacture it at scale and start selling it as Aspirin once they'd fed it to people and determined it worked as a painkiller.

It's a pretty standard 1800s try extracting compounds from herbal remedies, then kill some people with them, then apply basic chemical reactions to create novel compounds, then get lucky and produce a real medicine story. It doesn't happen anymore because we've run out of things to try and you can't just create new compounds and feed them to people and see what happens anymore - you've got to demonstrate that there's a plausible mode of action against a specific condition before starting human trials.