Chiral Resolution and Stereospecificity of 6-Phenyl-4-phenylethynyl- 1,4-dihydropyridines as Selective A3 Adenosine Receptor Antagonists

Abstract

Racemic 6-phenyl-4-phenylethynyl-1,4-dihydropyridine derivatives have been shown to be highly selective A₃ adenosine receptor antagonists (Jiang et al. J. Med. Chem. 1997, 40, 2596−2608). Methods for resolving the optical isomers at the C4 position, involving selective crystallization or chromatographic separation of diastereomeric ester derivatives, have been developed. Optically pure glycerol and threitol derivatives were used as chiral auxiliary groups for ester formation at the 3-position, resulting in diastereomeric mixtures of dihydropyridines. Esterification of a 6-phenyl-4-phenylethynyl-1,4-dihydropyridine derivative at the 3-position with a chiral, protected glycerol moiety, (S)-(+)-2,2-dimethyl-1,3-dioxolane-4-methanol, allowed the selective crystallization of a pure diastereomer, 9. The ¹H NMR spectrum of 9 using the lanthanide shift reagent Eu(fod)₃ indicated optical purity, and the (4S,2‘R)-configuration was assigned using X-ray crystallography. The noncrystalline (4R,2‘R)-isomer 10 was also isolated and shown to be 3-fold more potent than the (4S,2‘R)-isomer in binding to A₃ receptors. The 2,2-dimethyl-1,3-dioxolane moiety also served as a protected form of a diol, which showed selective reactivity versus a 5-ethyl ester in basic transesterification reactions. A racemic 5-carboxylic acid derivative could not be resolved through crystallization of diastereomeric salts. Enantiomers of 5-benzyl 3-ethyl 2-methyl-6-phenyl-4-phenylethynyl-1,4-dihydropyridine-3,5-dicarboxylate (2) were obtained via an ester derived from (4R,5R)-(−)-2,3-O-isopropylidene-d-threitol at the 3-position, which was resolved using HPLC, and each diastereomer was subsequently deprotected in acidic conditions. The resulting diols were exchanged for ethyl ester groups by base-catalyzed transesterification. The binding of pure enantiomers of 2 at A₃ adenosine receptors indicated a 35-fold stereoselectivity for the (4S)-isomer 21. A receptor docking hypothesis, using a previously derived human A₃ receptor model, shows the bulkier of the two ester groups (5-Bn) of 21 oriented toward the exofacial side and the 4-position phenylethynyl group situated between transmembrane helical domain TM6 and TM7.