Begin with a mapped reaction. Must be completely mapped pairwise,
all atoms must be shown on both sides of the reaction. Typically, few
reactions (10%) are completely mapped in the database. Fortunately,
most can be easily deduced.
See an example.
Identify the reacting center. These are atoms
an bonds which change during the reaction.
Identify the environment around the reacting
One atom shell,
additional conjugation, aromaticity.
Throw away all other atoms and bonds.
Generate a transformation string, restrict the language to valid
SMILES (which are all valid SMIRKS).