Molecular Complexity and Retrosynthesis.

An atom-environment complexity measure, CA, to assess local changes in complexity during synthetic transformations is described. The complexity measure is based on applying Shannon's equation to the number and diversity of paths up to two bonds in length emanating from an atom node. The method requires no explicit accounting for bond type, stereochemistry, ring membership, symmetry, or molecular size. CA varies with expectation across a number of basic reaction examples and may identify the key disconnections to guide retrosynthesis.