['O=CO', 'Cn1ccc2[nH]c(=O)n(-c3ccccc3)c(=O)c21', 'c1ccccc1', 'Cn1cc(C(=O)O)c2[nH]c(=O)[nH]c(=O)c21']
The Murcko scaffold (minmal number of connected rings) of <smiles></smiles> is O=c1[nH]c2cc[nH]c2c(=O)n1-c1ccccc1.