3,4,5-Trimethoxyphenylacetic acid gives us a beautiful example. Compare it with mescaline. Mescaline contains: (trimethoxyphenyl)-CH2-CH2-NH2 Its metabolite can contain: (trimethoxyphenyl)-CH2-CO2H Look at what stayed.
The aromatic ring stayed. All three methoxy groups stayed. Even much of the carbon skeleton is still recognizable. But the end of the side chain has been chemically rewritten.
Global Chemical Database