['CO', 'CCCn1cc(C(=O)O)c(=O)c2ccccc21', 'O=CO', 'CCCn1ccc(=O)c2cc(OC)ccc21', 'CCC', 'COc1ccc2[nH]cc(C(=O)O)c(=O)c2c1']
The Murcko scaffold (minmal number of connected rings) of <smiles></smiles> is O=c1cc[nH]c2ccccc12.