Cinchonidine → (R)-1-(2-methoxyethoxy)methyl-2,6-dimethyl-3-carbomethoxy-4-(3-nitrophenyl)-5-carboxy-1,4-dihydropyridine

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The racemic N-protected mono-ester of formula 18 is then dissolved in a suitable lower alkanol solvent at about the solvent reflux temperature along with a suitable optically active base, and the resulting salt allowed to crystallize as the solution cools to provide the optically resolved salt of either the (R)-18 or (S)-18. (It is preferred to recrystallize the resulting optically active salt to obtain a higher degree of purity.) For example, (RS)-1-(2-methoxyethoxy)methyl-2,6-dimethyl-3-carbomethoxy-4-(3-nitrophenyl)-5-carboxy-1,4-dihydropyridine is dissolved in EtOH at reflux temperature along with one molar equivalent of cinchonidine, and the mixture allowed to stand at room temperature overnight. The resulting crystals are recrystallized twice from EtOH to yield the pure (R)-18 salt, (R)-1-(2-methoxyethoxy)methyl-2,6-dimethyl-3-carbomethoxy-4-(3-nitrophenyl)-5-carboxy-1,4-dihydropyridine.cinchonidine. (Scheme VI, Step 3)

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