13C NMR spectroscopy of aristolochic acids and aristololactams

The 13C NMR spectra of more than 30 phenanthrene derivatives were recorded and their carbon shifts assigned. The compounds consisted of representative aristolochic acids and aristololactams, the phenanthrene alkaloid argen‐tinine and some derivatives. Assignments of the carbon resonances were carried out from chemical shifts, one‐bond and long‐range carbon‐hydrogen couplings and selective comparison of the phenanthrene derivatives with each other and with model compounds. The chemical shifts of the aromatic carbons were correlated with substituent effects. It was found that the shift changes on introduction of oxy functions in phenanthrenes parallel those reported for monosubstituted naphthalenes. Steric compression shifts observed in 3,4‐disubstituted and ring A polysubstituted phenanthrenes are discussed. Further evidence about the structures of the aristololactams CII, CIII, DII, DIII, enterocarpam‐I and enterocarpam‐II is provided, based on 13C NMR data. The substituent parameters found for phenanthrene derivatives proved to be additive and their use in signal assignments is described.