['O=CO', 'O=C(O)c1ccc(-c2ccncc2)cc1', 'O=C(O)c1cc(-c2ccccc2)ccn1', 'O=C(O)c1ccccc1', 'O=C(O)c1ccccn1']
The Murcko scaffold (minmal number of connected rings) of <smiles></smiles> is c1ccc(-c2ccncc2)cc1.