eSynth is a graph-based molecular synthesis tool written in C++. The author is Dr. Chris Alvin. The source code is shared here with the permission granted. All credit goes to Dr. Chris Alvin and Computational Systems Biology Group.
If you find this tool is useful to you, please cite this paper:
Naderi, Misagh, Chris Alvin, Yun Ding, Supratik Mukhopadhyay, and Michal Brylinski. "A graph-based approach to construct target-focused libraries for virtual screening." Journal of Cheminformatics 8, no. 1 (2016): 14.