Zielreaktion

Zielreaktion is a term used in German-language chemistry literature to denote the key bond-forming transformation in a synthetic sequence that leads to the target molecule. It describes the principal step that establishes the core framework of the product, serving as the forward counterpart to the planned retrosynthetic disconnections. The concept emphasizes the step that most determines the overall strategy, selectivity, and practicality of the synthesis.

In retrosynthetic planning, the Zielreaktion guides route selection by highlighting which transformation will most efficiently assemble the essential skeleton from available precursors. The choice of Zielreaktion is influenced by factors such as overall yield, regio- and stereoselectivity, functional-group tolerance, scalability, and the availability of reagents and catalysts. Protecting groups and auxiliary steps are commonly considered in conjunction with the Zielreaktion, but the focus remains on the transformation that delivers the main bond construction or ring formation.

Common examples of Zielreaktionen include carbon–carbon bond-forming processes, such as cross-couplings (for example Suzuki or Negishi) that build key skeletons, and cyclization steps (such as ring-closing metathesis or intramolecular Diels–Alder reactions) that construct rings. However, the concept is not limited to a particular reaction class; any step that achieves the primary bond-forming event or structural feature of the target can be regarded as the Zielreaktion. In practice, a late-stage Zielreaktion is often designed to maximize stereochemical control or to improve scalability.

The notion remains a planning aid rather than a fixed category, evolving with advances in catalysis, selectivity, and synthetic methodology.