Fig. 4
From: Molecular structures enumeration and virtual screening in the chemical space with RetroPath2.0
Rules before solution space reduction due to valence and structure considerations. Reactions in green move bonds around without creating or deleting cycles. Reactions in blue change bond order by creating or deleting at least one cycle. Reactions in purple were deleted because valence is limited to 4, and reactions in black were deleted because there cannot be two double bonds on the same atom in a 3 or 4 membered ring