Simple methods for determining relative stereochemistry of kainoid amino acids by H-1 NMR chemical shifts

The kainoid amino acids are biologically important compounds because they show remarkable neuroexcitatory and excitotoxic activities. For exhibiting potent activity, the stereochemical relationship of the substituents on the pyrrolidine ring is crucial. We found simple methods for determining the relative stereochemistry of these compounds on the basis of the H-1 NMR chemical shifts of H-2 and H-4 in D2O solution. The signals of H-2 appear at fields higher than 4.2 ppm when the compounds have 2,3-trans stereochemistry whereas, in the 2,3-cis compounds, they appear lower than 4.2 ppm, irrespective of the C-4 substituent. This criterion holds when the solution is in the range of pD 3-8. Moreover, when an epimeric pair at C-2 is available and the spectra are recorded at the same or nearly equal pD, the H-2 chemical shift of the 2,3-trans isomer is higher than that of the corresponding 2,3-cis isomer. Similarly, the relative stereochemistry between C-3 and C-4 can be determined from the chemical shift of H-4. The signals of H-4 of the 3,4-cis isomers appear at lower fields than those of the corresponding 3,4-trans isomers in each pair of C-4 epimers when the spectra are recorded at the same or nearly equal pD. This holds for the compounds bearing an unsaturated substituent at C-4. All these phenomena can be rationalized by the anisotropic effect of the pi-electron system in the C-2 and C-4 substituents.