2. Glycerin to glutaric acid (Berthelot and De Luca, Cahours and Hofmann, Erlenmeyer, Lermantoff, and Markownikoff).
3. Glutaric acid to suberone (Brown and Walker, Boussingault).
4. Suberone to tropidine (Willstätter).
5. Tropidine to tropine (Willstätter, Ladenburg).
6. Synthesis of tropic acid (Berthelot, Fittig and Tollens, Friedel, Ladenburg, and Rügheimer).
7. Tropine and tropic acid: atropine (Ladenburg).
The alkaloid cocaïne, contained in the leaves of erythroxylon coca and now employed as a local anæsthetic, was discovered by Niemann in 1860. It has been shown to be closely related to atropine in constitution, and has now been synthetically prepared in the dextro-modification.
The alkaloids papaverine, narcotine, narceïne, contained in opium, are derivatives of isoquinoline, as also is berberine, found in the common barberry (berberis vulgaris). Papaverine, which occurs in opium to the extent of about one per cent., was first isolated by Merck in 1848. Its constitution has been established by Goldschmidt. Narcotine is, next to morphine, the most abundant constituent of opium. The study of the products of its hydrolysis and oxidation—viz., opianic acid and cotarnine, both of which substances have long been known—has indicated its probable structure. Narceïne is closely allied to narcotine, and can, indeed, be obtained from it by combining the latter alkaloid with methyl iodide and treating the compound with caustic potash. The constitution of berberine, which is one of the few coloured vegeto-alkaloids known, has been worked out by Perkin. As yet nothing definite is known concerning the structure of the most important and largest constituent of opium—viz., morphine; or of its congeners codeïne and thebaine. Grimaux, however, in 1881 converted morphine into codeïne by treatment with methyl iodide and potash; hence the two alkaloids stand in a relation somewhat similar to that in which narceïne stands to narcotine. There is very little doubt that the three alkaloids are very closely related, and that a knowledge of the constitution of one of them would immediately elucidate the structure of the others. They are probably phenanthrene derivatives.
Quinine and cinchonine, the most important of the cinchona alkaloids, are quinoline compounds, and are closely related in constitution. But the many attempts to unravel their structure have yielded no definite results up to the present.