Create combinatorial library of biologically plausible chemical compounds based on library of known compounds
I think your best bet would be to go to the KEGG Reaction Database (http://www.kegg.jp/kegg/reaction/) which is a comprehensive database of known biochemical (enzyme-catalyzed) reactions, plus various associated tools. It includes a module that predicts a likely biosynthetic pathway for any pair of structures. All your pheromones are natural products and are formed via enzymatic reactions, so you should be able to explore out from your starting set by exhaustively applying the reactions in the KEGG database to each one of them. There may be a module that does this for you  automatically, but it is not documented on the KEGG web site.