Design and syntheses of C‐4 4‐(1‐methoxycarbonyl‐1,2‐dihydropyridyl)‐, and 3‐[1‐methoxycarbonyl‐1,2‐(and 1,6‐)dihydropyridyl]‐, derivatives of alkyl 1,4‐dihydro‐2,6‐dimethyl‐3‐nitro‐5‐pyridinecarboxylates with cardiospecific calcium channel agonist activity