What was the first demonstration of organic total synthesis?
The first total synthesis was the Finnish chemist Gustav Komppa's total synthesis of camphor.
In 1903, Komppa succeeded in synthesizing camphoric acid from diethyl oxalate and diethyl β,β-dimethylglutarate.
A few years earlier, Haller and Blanc had converted camphoric acid to camphor, so this constituted a total synthesis of camphor.
At the time, camphor was a scarce natural product with a worldwide demand.
Komppa realized this and began industrial production of camphor in 1907.
For the terminally curious, here's a summary of the steps.
diethyl diketocamphorate (I)