1H NMR study of protected and unprotected kainoid amino acids: facile assignment of C-4 stereochemistry

The kainoid amino acids exhibit potent neuroexcitatory activity in the mammalian central nervous system. Around their pyrrolidine ring, a trans disposition between the C-2 and C-3 substituents and a cis relationship between the C-3 and C-4 substituents are crucial for their potent biological activity. During synthetic studies into the kainoids, we have established a straightforward, empirical rule, which allows the facile assignment of C-4 stereochemistry to both protected and unprotected kainoids. When pairs of C-4 epimers are available, the rule indicates that, when their (1)H NMR spectra are compared, one of the methylene protons on the C-3 side chain appears at significantly lower chemical shift in the C-3, C-4 cis isomer than the corresponding signal for the proton in the spectrum for the C-3, C-4 trans isomer. In addition, the rule states that the difference in chemical shift between the two individual protons on the C-3 side chain of the C-3, C-4 cis isomer is significantly greater than the corresponding difference for the C-3, C-4 trans isomer. The rule is demonstrated for kainoids possessing an unsaturated substituent at C-4 and when comparing spectra in D(2)O for pairs of unprotected C-4 epimers, the spectra were recorded at approximately the same pD.