['CCO', 'Cc1cc([N+](=O)[O-])cc2c(=O)c(C=O)c[nH]c12', 'CC', 'Cc1cc([N+](=O)[O-])cc2c(=O)c(C(=O)O)c[nH]c12', 'CCOC=O', 'Cc1cc([N+](=O)[O-])cc2c(=O)cc[nH]c12']
The Murcko scaffold (minmal number of connected rings) of <smiles></smiles> is O=c1cc[nH]c2ccccc12.