Image bits of note: the active ingredient of Willow bark extract, (the guy in the bottom right corner, Salicin) is converted into Salicylic Acid (the guy to the left of the arrow to the left of Salicin) which may be familiar to you if you were a greasy and or pizza-faced teenager. Haha. Anyhow, in order to avoid the pooping blood and stomach pains of trying to use that stuff for pain relief, it's esterified to make that happy camper in the center there which makes it a little more pharmakokinetically gentle. Compress the compound into tablets and you've got yourself one of the most important commercial drugs in the last 100 years. Hooray for natural product mods.
Fun fact: a different synthetic step from the Salicylic Acid can turn get you to Bismuth Salicylate, C7H5BiO4, which is the only
active ingredient in an over the counter medicine that'll leave a shiny piece of metal slag
behind when burnt with a blow torch. Thanks, Pepto Bismol!
Obligatory links to the other small MW compound images: