Pharmacological discrimination between enantiomeric germanes by muscarinic receptors: A study on germanium/silicon bioisosterism

The (hydroxymethyl)diorgano(2-piperidinoethyl)germanes rac-Ph(c-Hex)Ge(CH2OH)CH(2)CH(2)NR(2) (rac-1a), Ph(2)Ge(CH2OH)CH2-CH(2)NR(2) (3a) and (c-Hex)(2)Ge(CH2OH)CH(2)CH(2)NR(2) (5a) (NR(2) = piperidino) were synthesized starting from Cl3GeCH2Cl. The (R)- and (S)-enantiomer of 1a were obtained by resolution of rac-1a using the antipodes of O,O'-di-p-toluoyltartaric acid as resolving agents (resolution by fractional crystallization of diastereomeric salts). The enantiomeric purities of the resolved antipodes of la were shown to be at least 98 (H-1 NMR) and 97% ee (C-13 NMR) respectively (NMR studies using a chiral shift reagent). Reaction of rac-1a, (R)-1a, (S)-1a, 3a and 5a with methyl iodide gave the corresponding methiodides rac-2a, (R)-2a, (S)-2a, 4a and 6a (1a --> 2a, 3a --> 4a, 5a --> 6a). The absolute configuration of (R)-2a was determined by single-crystal X-ray diffraction. On the basis of the experimentally established absolute configuration of (R)-2a, the absolute configurations of all the other aforementioned optically active germanium compounds were assigned by chemical and optical correlations. The enantiomerically pure germanium compounds (R)-1a, (S)-1a, (R)-2a and (S)-2a and their achiral derivatives 3a-6a were studied for their affinities for muscarinic M1, M2, M3 and M4 receptors by functional pharmacological experiments (M1, rabbit vas deferens; M2, guinea-pig atria; M3, guinea-pig ileum) and radioligand binding experiments (M1, human NB-OK 1 cells; M2, rat heart; M3, rat pancreas; M4, rat striatum). The receptor affinities obtained in these studies were compared with those of the related silicon analogues, the (hydroxymethyl)diorgano(2-piperidinoethyl)silanes(R)- and (S)-Ph(c-Hex)Si(CH2OH)CH(2)CH(2)NR(2) [(R)-1b and (S)-1b], Ph(2)Si(CH2OH)CH(2)CH(2)NR(2) (3b) and (c-Hex)(2)Si(CH2OH)CH(2)CH(2)NR(2) (5b) (NR(2) = piperidino) and their corresponding methiodides (R)-2b, (S)-2b, 4b and 6b (a --> b: Ge --> Si; studies on Ge/Si bioisosterism). According to these studies, all the germanes and the related silicon analogues behaved as simple competitive antagonists at muscarinic M1-M4 receptors, The pK(i) values obtained in binding studies at M1-M3 receptors were similar to the corresponding functional affinities (pA(2) values). The receptor affinities of the respective Ge/Si analogues were found to be very similar, indicating a strongly pronounced Ge/Si bioisosterism. The (R)-enantiomers (eutomers) of the Ge/Si pairs 1a/1b and 2a/2b exhibited higher affinities (up to 26-fold) for M1-M4 receptors than their corresponding (S)-antipodes (distomers), the stereoselectivity ratios being higher at M1, M3 and M4 than at M2 receptors. In most cases, the diphenyl (3a/3b and 4a/4b) and dicyclohexyl (5a/5b and 6a/6b) compounds displayed lower affinities to M1-M4 receptors than the related (R)-enantiomers of 1a/1b and 2a/2b, and the sums of the respective affinity differences were very similar to the experimentally established stereoselectivity ratios [(R)/(S)]. Thus, the stereoselective interaction of the enantiomers of 1a/1b and 2a/2b with muscarinic receptors is best explained in terms of opposite and weaker binding of the phenyl and cyclohexyl ring of the (S)-antipodes. The highest receptor selectivity was observed for compound (R)-1b at M1/M2 receptors (25-fold in binding studies).