A total synthesis is the complete chemical synthesis of complex organic molecules from simple, commercially available (petrochemical) precursors. Total syntheses are often used as a playground for the development of new chemical reactions and routes.
Classical examples are the total syntheses of quinine, cholesterol, cortisone, strychnine, lysergic acid, reserpine, chlorophyll, colchicine, vitamin B12, and prostaglandin F-2a by the Nobel Prize winner Robert B. Woodward between 1945 and 1976.
Today the group of Kyriacos Costa Nicolaou at the Scripps Research Institute is famous for their total syntheses.
References
Angew. Chem. Int. Ed. 39: 44-122 (2000) (Review, pdf)