Isoquinoline, isomeric with quinoline, was first discovered in coal-tar in 1885 by S.
A-pyrone condenses with the benzene ring to form coumarin and isocoumarin; benzo-'y-pyrone constitutes the nucleus of several vegetable colouring matters (chrysin, fisetin, quercetin, &c., which are derivatives of flavone or phenyl benzo-y-pyrone); dibenzo--ypyrone is known as xanthone; related to this substance are fluorane (and fluorescein), fluorone, fluorime, pyronine, &c. The pyridine ring condenses with the benzene ring to form quinoline and isoquinoline; acridine and phenanthridine are dibenzo-pyridines; naphthalene gives rise to a-and /3-naphthoquinolines and the anthrapyridines; anthracene gives anthraquinoline; while two pyridine nuclei connected by an intermediate benzene nucleus give the phenanthrolines.
This view has been fairly well supported by later discoveries; but, in addition to pyridine and quinoline nuclei, alkaloids derived from isoquinoline are known.
(4) Isoquinoline group. The opium alkaloids: morphine, codeine, thebaine, papaverine, narcotine, narceine, &c.; and the complicated substances hydrastine and berberine.
The alkaloids fall into two chemical groups: (i) derivatives of isoquinoline, including papaverine, narcotine, gnoscopine (racemic narcotine), narceine, laudanosine, laudanine, cotarnine, hydrocotarnine (the last two do not occur in opium), and (2) derivatives of phenanthrene, including morphine, codeine, thebaine.
It may be separated from the quinoline which accompanies it by means of the difference in the solubility of the sulphates of the two compounds, isoquinoline sulphate being much less soluble than quinoline sulphate.
Numerous derivatives of isoquinoline are obtained in the decomposition of various vegetable alkaloids.