['CO', 'CCN1C(=O)c2ccccc2[C@H](C(=O)O)C12CCCC2', 'CCOC', 'O=C1NC2(CCCC2)[C@@H](C(=O)O)c2ccccc21']
The Murcko scaffold (minmal number of connected rings) of <smiles></smiles> is O=C1NC2(CCCC2)Cc2ccccc21.