"Space", as Douglas Adams famously said, "is big. You just won't believe how vastly, hugely, mind-bogglingly big it is." Change 'space' to 'chemical space', and his statement has similar resonance: the total number of possible small organic molecules that populate 'chemical space' has been estimated to exceed 1060 - an amount so vast when compared to the number of such molecules we have made, or indeed could ever hope to make, that it might as well be infinite. So, it is not surprising that our exploration of chemical space has so far been extremely limited.
