dihydroquinoline

Dihydroquinoline refers to a family of heterocyclic compounds that are derived from quinoline by adding two hydrogen atoms to the heteroaromatic ring. This partial saturation yields a bicyclic system in which a benzene ring remains fused to a partially reduced pyridine ring. The term covers several structural isomers, with 1,2-dihydroquinoline and 2,3-dihydroquinoline being the most common, differing in the positions of the added hydrogens on the ring system.

Dihydroquinolines are typically produced by catalytic hydrogenation of quinoline, usually over metal catalysts such as palladium on carbon or Raney nickel under hydrogen pressure. Depending on reaction conditions, mixtures of dihydroquinoline isomers can form, and further hydrogenation can lead to tetrahydroquinoline derivatives. They also appear as intermediates in the synthesis of more complex nitrogen-containing heterocycles and can be prepared via selective reductions of quinoline derivatives or oxidative transformations from tetrahydroquinolines.

The nitrogen atom in dihydroquinolines remains basic and can participate in alkylation or acylation reactions. Dihydroquinolines serve as useful building blocks in medicinal chemistry for the construction of piperidine-containing compounds and as intermediates in the synthesis of dyes, agrochemicals, and natural product analogs. Physical properties vary with substitution; unsubstituted dihydroquinolines are typically colorless to pale solids or liquids with moderate volatility.

See also quinoline and tetrahydroquinoline for related structures and transformations.