a total synthesis is the complete chemical synthesis of complex organic molecules from simple, commercially available (petrochemical) or natural precursors. in a "linear" synthesis—often used for simple structures—several steps are performed one after another until the molecule is complete. the chemical compounds made in each step are usually called "synthetic intermediates". for more complex molecules, a different approach may be preferable: convergent synthesis involves the individual preparation of several "pieces" (key intermediates), which are then combined to form the goal product.