Nuclear magnetic resonance studies of 5,6‐dicarboxy‐2‐norbornene derivatives

A detailed study has been made on the 1 H nuclear magnetic resonance (NMR) spectra of 5,6‐dicarboxy‐2‐norbornene derivatives, whose chemical shifts and geminal, vicinal and long‐range spin‐spin coupling constants were determined. Each signal of the spectra of these derivatives shows a slight low‐field shifts compared with norbornene itself. In the case of five methyl esters studied in this work, the signal of the 7 s ‐proton remains at approximately the same position in all the compounds, but that of the 7 a ‐proton is more variable. For the chemical shifts of the 7 a ‐proton of the five methyl esters, an additive rule seems to hold: that the magnitude of the low‐field shift, from the 7 a ‐signal of norbornene itself, is obtained by adding each of the effects of the carboxyl and the carbomethoxy groups substituted at the endo ‐5‐(or 6‐) or the exo ‐5‐(or 6‐) positions of the norbornene ring. In most of the derivatives considered in this work, the spin‐spin coupling constants, except those between bridgehead and bridged methylene protons, are about 1 to 2 Hz smaller than those of many other norbornene derivatives already studied. This fact seems to suggest that norbornenes, which have two bulky adjacent substituents at 5‐ and 6‐positions, may suffer distortions in the norbornene ring.