Fifteen isoquinolone alkaloids are presently known. They may be found either in the fully aromatic form A, or else in the partially reduced form B. The numbering system is as indicated in expression A. The R substituent on the nitrogen atom may be either hydrogen or methyl, and carbons 6 and 7 usually possess oxygenated substituents. Three dimeric isoquinolone alkaloids are known; these are baluchistanamine (l), revolutinone (9), and punjabine (15). Isoquinolone alkaloids have been found among the Ranunculaceae, Menispermaceae, Berberidaceae, Papaveraceae, Hernandiaceae, Fumariaceae, Lauraceae, and Jlonimiaceae. The alkaloid siamine (ll), found in a leguminaceous plant, has an exceptional oxygenation pattern and also bears a methyl substituent at C-3. It, thus, differs markedly from the other naturally occurring isoquinolones.Isoquinolones may originate in nature from the oxidation of benzylisoquinolines. It is possible, however, that they may also be formed from the in civo oxidation of protoberberines, phthalideisoquinolines, or even spirobenzylisoquinolines. The biogenesis of siamine (11) is clearly different from that of the fourteen other isoquinolone alkaloids.