Complexity of synthetic routes: Linear, convergent and reflexive syntheses1Electronic supplementary information (ESI) available: values of indices used to calculate analytical functions for routes 1–3. See http://www.rsc.org/suppdata/nj/b2/b210844p/

Complexity and diversity indices can be used to evaluate the structural changes that take place during the course of a synthesis. From the generalized 'complexity versus step' plot and the definition of the 'ideal synthesis' we derive the concept of excess complexity and the heuristic, minimization of excess complexity. Molecular complexity and synthetic complexity are differentiated by identifying them with intrinsic and extrinsic complexity, respectively, and the effect of chirality on each is clarified. The role of symmetry in synthetic analysis is delineated by considering symmetry in molecules, their molecular graphs and the synthesis digraphs for their preparation, which leads to the concept of reflexivity and the heuristic, maximization of reflexivity. Vulnerability and robustness are introduced in connection with linear and convergent syntheses, which are also considered in relation to reflexivity.