Figure : A representation of the chemical space investigated using machine learning, showing that the NBD/QC motif can be functionalized on four positions with three different groups ("a", "b", and "c"). The group "a" includes electron withdrawing groups (EWGs) and electron donating groups (EDGs) as well as hydrogen, and accounts for a total of 18 different substituents. There are roughly 18 + 2( 1 2 18 5 ) = 1.9M substituents and 1 4 (1.9M) 4 = 10 25 different NBD systems.