Virtual libraries can significantly enlarge the part of chemistry space amenable to in silico screening. Prominent examples of very large libraries of enumerated compounds are the GDB databases, in particular GDB-17 of 166 billion enumerated organic small molecules of up to 17 heavy atoms of C, N, O, S, and halogens . However, such automatically enumerated databasesas well as in principle any purely de novo designed moleculesuffer from the significant drawback that no practical synthetic route is a priori attached to these structures, and that therefore, in general, (a) manualand thus expensiveinvestigation of possible synthetic routes is necessary, (b) resulting routes may be complicated, multi-step syntheses, and (c) synthesis of the molecule may in the end prove altogether unsuccessful (or untenably expensive) even after significant effort.