4,7-Dichloroquinoline

4,7-Dichloroquinoline is a two-ring heterocyclic compound used as a chemical intermediate to aminoquinoline antimalarial drugs including amodiaquine, chloroquine and hydroxychloroquine.

Synthesis
4,7-Dichloroquinoline was first reported in a patent filed by IG Farben in 1937. However, its synthesis was not investigated in detail until chloroquine was developed as an antimalarial drug. A route to the intermediate starting from 3-chloroaniline was developed by chemists at Winthrop Chemical Co.
 * 4,7-Dichloroquinoline synthesis.svg

The substituted aniline is condensed with the diethyl ester of oxaloacetic acid under mildly acidic conditions, forming an imine, which is cyclised to form the pyridine ring by heating in mineral oil. Hydrolysis and decarboxylation follows, before the hydroxy group in the 4-position is converted into the second chloro group using phosphoryl chloride.

The availability of 4,7-dichloroquinoline allowed alternative structural analogs of the 4-aminoquinoline type to be investigated, leading to the discovery of hydroxychloroquine in 1949. By that time, chloroquine manufacturing processes had been established to allow its widespread use. 4,7-Dichloroquinoline has also been prepared by the Gould–Jacobs reaction using an alternative method of constructing the pyridine ring from 3-chloroaniline.

Reactions
The chlorine atom in the 4-position in the pyridine ring is much more reactive in nucleophilic aromatic substitution reactions than the chlorine in the benzene ring. As a result, it can be replaced selectively to form derivatives at that position. A typical reaction with a specific primary amine gives chloroquine in high yield:
 * Chloroquin Synthese.svg

Uses
Apart from its use in the manufacture of antimalarials already described, 4,7-dichloroquinoline is of continuing interest as an intermediate to new drug candidates.