LHASA (Logic and Heuristics Applied to Synthetic Analysis) is a program for synthesis planning, an expert system to assist chemists in designing efficient routes to target molecules for organic synthesis. Operating differently from reaction retrieval systems, LHASA searches its own way in synthesizing known and unknown compounds, using a chemical knowledge base (rather than a database of literature examples). Since LHASA operates in a rigorously retrosynthetic fashion, the knowledge base contains information about retro-reactions (or transforms) rather than reactions. The current version of LHASA contains 2239 transforms and 493 so-called tactical combinations.
The program is very user-friendly and almost all communication proceeds via the graphics interface. Consequently, using LHASA does not require prior computer experience. The user draws in a target molecule and indicates a strategy for the retrosynthetic analysis. The program then searches the transform knowledge base for those that satisfy the strategy selected, decides which transforms are appropriate for the particular target structure, and displays the resulting precursors to the chemist. The chemist may then select a precursor for further analysis and choose another strategy option, upon which the program returns a second level of precursors in the same way. Processing continues in this manner until the chemist is satisfied that one or more of the precursors correspond to a reasonable starting point for a synthesis.
The program is being continuously developed at Harvard University, in cooperation with the collaborative groups of LHASA Ltd (at Leeds University) and the CMBI (at the University of Nijmegen).