Synthesis and calcium channel antagonist activity of nifedipine analogues containing 1,2-dihydropyridyl in place of the 1,4-dihydropyridyl moiety.

Calcium channel antagonist activities of the 1,2-dihydropyridines (9) were determined using the muscarinic-receptor mediated Ca2+ dependent contraction of guinea pig ileal longitudinal smooth muscle. The relative potency order for 4-substituted analogs 9 was phenyl greater than 3-trifluoromethyl(nitro)phenyl greater than 4- and 2-trifluoromethyl(nitro)phenyl. Increasing the size of the alkyl ester substituents (9h) enhanced activity. The test results indicate that the 1,2-dihydropyridyl ring system (9) is partially bioisosteric with the 1,4-dihydropyridyl ring system (10). Competitive [3H]-nitrendipine binding studies indicated that the affinities of the 1,2-dihydropyridine analogues were much lower than expected from their ID50 activities, suggesting that these analogues inhibit calcium channels by a different receptor mechanism. In the synthetic work, Hantzsch condensation of aldehydes (3) with alkyl acetoacetates (4) afforded 3,5-dialkyl 2,6-dimethyl-4-(substituted-phenyl)-1,4-dihydropyridine-3,5-dicarboxylat es (5). Oxidation of 5 gave the aromatic pyridines (6) which were elaborated to the l-methylpyridinium methyl sulphates (7) and then to the perchlorates (8). Sodium borohydride reduction of 8 in aqueous ethanol gave 3,5-dialkyl 1,2,6-trimethyl-4-(substituted-phenyl)-1,2-dihydropyridine-3, 5-dicarboxylates (9).