Structure–affinity relationship studies of non-competitive NMDA receptor antagonists derived from dexoxadrol and etoxadrol

The synthesis and NMDA receptor affinity of ring and side-chain homologues of etoxadrol and dexoxadrol are described. For the regioselective synthesis of etoxadrol homologues, the regioisomeric 4-azidobutanediols (±)-9 and (±)-14 were employed. A synthesis of the enantiomerically pure azidobutanediols (S)-, (R)-9 and (S)-, (R)-14 was developed and the homochiral building blocks were used for the synthesis of enantiomerically pure etoxadrol and dexoxadrol homologues. The affinity of the racemic and enantiomerically pure primary amines toward the phencyclidine binding site of the NMDA receptor was investigated in receptor binding studies with tritium labeled [3H]-(+)-MK-801 as radioligand. Benzaldehyde derivatives (±)-12a, (±)-13a, and (±)-16a bearing a proton at the acetalic position do not interact significantly with the NMDA receptor. An enantioselective NMDA receptor binding was observed for the trans-configured 2-(2-ethyl-2-phenyl-1,3-dioxolan-4-yl)ethanamine 13b, the (2-ethyl-2-phenyl-1,3-dioxan-4-yl)methanamine 16b, and the (2,2-diphenyl-1,3-dioxan-4-yl)methanamine 16c. The NMDA receptor affinity of these compounds resides almost exclusively in the (S)-configured enantiomers (2S,4S)-13b, (2S,4S)-16b, and (4S)-16c. The lowest Ki-value in this series was found for the (2S,4S)-configured 1,3-dioxolane (2S,4S)-13b (Ki = 69 nM), which is in the range of the Ki-value of the lead compounds etoxadrol and dexoxadrol, indicating that the 2-aminoethyl and the piperidin-2-yl substituents lead to similar NMDA receptor interactions.

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