5) Derivatives of pyridine: nicotinamide and pyridocal.
Pyridine derivatives are also of great biological importance. For example, nicotinic acid is more commonly known as the B-complex vitamin niacin; a nutritionally equivalent form of niacin is nicotinamide, or niacinamide. Pyridoxine is another member of the B complex, vitamin B6. The structures of pyridoxine and nicotinamide are:
Two coenzymes involved in many important metabolic reactions in living cells, nicotinamide adenine dinucleotide (NAD, also called coenzyme I) and nicotine adenine dinucleotide phosphate (NADP, coenzyme II), are derived from nicotinamide, and the coenzyme pyridoxal phosphate (codecarboxylase) is a physiologically active form of pyridoxine. Many alkaloids contain a pyridine or piperidine ring structure, among them nicotine.
Pyridine, which once was extracted commercially from coal tar but now is prepared catalytically from tetrahydrofurfuryl alcohol and ammonia, is an important solvent and intermediate used to make other compounds. Vinylpyridines such as
are important monomer building blocks for plastics, and fully saturated pyridine, piperidine, is used in rubber processing and as a chemical raw material.
Pharmaceutically important pyridines include the tuberculostat isoniazid (isonicotinic acid hydrazide), the anti-AIDS-virus drug nevirapine, the vasodilator nicorandil, is used for treatingangina, the urinary-tract analgesic phenazopyridine, and the anti-inflammatory sulfa drug. 1-(1-phenylcyclohexyl) piperidine (PCP, phencyclidine) was originally used as an anesthetic, but its powerful hallucinogenic properties have led to abuse. Diquat, paraquat, clopyralid, and diflufenican are well-known pyridine derivatives used as herbicides.
Quinoline itself can be used to manufacture nicotinic acid and other compounds such as drugs anddyes. The production of synthetic quinoline far exceeds that from coal tar. Morphine, codeine, and thebaine—all containing partially reduced isoquinoline rings—are alkaloids of the opium poppy and have been used for many centuries as hypnotics and analgesics. The semisynthetic derivative of morphine, heroin, is an even more powerful hypnotic and a highly addictive drug.
Important synthetic derivatives of the benzo- and dibenzopyridines include cyanine dyes, used as sensitizers in silver halide photographic emulsions, and quinophthalone dyes, applied in plastics, polymer textiles, paper, cosmetics, transfer-printing processes, electronic photography, and laser dyes. Other derivatives are useful bacteria-staining agents for microscopy, antiseptics such as the coal-tar dye acriflavine, antimalarial agents such as mepacrine (quinacrine) and chloroquine, antibacterial agents such as ciprofloxacin, trypanocides (drugs that destroy trypanosomes, which are parasitic protozoans responsible for Chagas disease, sleeping sickness, and other serious infectious illnesses) such as ethidium bromide, and the reagent oxine (8-hydroxyquinoline or 8-quinolinol), used in analytical chemistry.