| In order to probe the conformational preferences for binding of L-glutamate to various proteins involved in the process of neurotransmission, rigid analogues containing an embedded homoglutamate, glutamate or aspartate moiety have been prepared. S{dollar}sb{lcub}rm N{rcub}2{dollar} substitution of tosylates, derived from trans-4-hydroxy-L-proline and cis-4-hydroxy-D-proline, with cyanide constituted the crucial steps in the synthesis of all four possible enantiomerically pure stereoisomers of 4-carboxyproline. By using NaCH(CO{dollar}sb2{dollar}Et){dollar}sb2{dollar} as the nucleophile instead of NaCN, this same displacement strategy provided access to trans-4-carboxymethyl-L-proline 18. The 3-carboxyprolines were prepared via intramolecular alkylation of tosyloxy urethanes or intramolecular reductive amination of amino-aldehydes derived from {dollar}beta{dollar}-allylated cyanoalanines. Since partial racemization had occurred during reduction of N-CBZ-cyanoalanine methyl ester, a different route to enantiomerically pure N-CBZ-cyanoalaninols, starting from serine, was developed. Cationic cyclization of epoxy urethanes, derived from deoxy-D-ribose, provided an alternate strategy for C-3 substituted proline synthesis. The protected 3-hydroxyproline products of this cyclization were tosylated and treated with either NaCN or NaCH(CO{dollar}sb2{dollar}Et){dollar}sb2{dollar} to provide, via S{dollar}sb{lcub}rm N{rcub}2{dollar} substitution, intermediates which were transformed into enantiomerically pure 3-carboxyprolines or trans-3-carboxymethyl-L-proline 22 respectively. Each of the conformer mimics were tested for their ability to inhibit the high affinity transport of {dollar}sp3{dollar}H-L-glutamate into synaptosomes and to block the binding of radioligands to the NMDA, KA and QA glutamate neurotransmitter receptor sites. While none of the four 4-carboxyprolines exhibited potent binding to the excitatory receptors, the L-trans isomer 10 is a potent and selective inhibitor of L-glutamate transport. These results delineate a specific structural/conformational preference for binding to the uptake system that is distinct from that required for binding to the NMDA, KA and QA receptors. In the 3-carboxyproline series, all of the four stereoisomers were found to potently inhibit the binding of {dollar}sp3{dollar}H-L-glutamate to the NMDA receptor. The L-cis- 13, D-cis- 15 and D-trans- 16 3-carboxyprolines displayed no binding to either the QA or KA receptor. However, the L-trans isomer 14 also potently inhibited binding to the KA receptor. Not surprisingly, the trans-3-carboxymethyl-L-proline 22 inhibited binding to the KA receptor, showing conclusively that the isopropenyl group of KA is not required for binding. |
