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Table of Contents:

Abstract

Introduction

Historical Usage of Scopolamine

Scopolamine in the 20th Century

Ladenburg's First Isolation of Scopolamine

Fodor's Laboratory Synthesis of Scopolamine from tropane-3a, 6b-diol

Chemistry of Scopolamine

Biosynthesis of Scopolamine

Conclusions

Works Cited

Introduction

Scopolamine's uses have roots in ancient times and have pervaded into the present. This heterocyclic tropane alkaloid is naturally found in Solanaceas plants and can be prepared in the laboratory from various precursors. First isolated in the late nineteenth century, it has found various uses in the modern world. In humans, scopolamine is therapeutically employed in opthamallogical procedures to cause mydriosis, prolonged dialation of the iris, and is used to prevent and treat motion sickness. Because it depresses the central nervous system, it has been used as an amnesiac for birthing mothers, producing "Twlight Sleep." It does not have great chemical utility and decomposes on standing or heating; therefore it is stored in hydrated forms or as salts with HCl or HBr.

Historical Usage of Scopolamine

During the Middle Ages, drug addicts used potions derived from compounds such as the tropane alkaloids, which are found in solanaceous plants. The sensation of flying, in addition to hallucinations were attributed to the application of such ointments. These early drug addicts were often victims of witch-hunts. In addition, scopolamine was used as an anaesthetic during surgery, until physicians were accused of sorcery upon the disclosure of their patients' odd dreams.

Plant-derived tropane alkaloids were used for cosmetic reasons: until the Renaissance, women applied it to their eyes, causing mydriasis, which was considered attractive (1).

However, the use of these compounds (including scopolamine), has been documented to as far back as 3000 BC among the ancient Hindus and American Indians, who knew of their effects. Alkaloid-containing plants were smoked, as well as employed in ritualistic ceremonies for young men's coming of age. Adolescent boys were administered doses on a daily basis for weeks, and following the extended drug-induced state, had little recollection of their childhood (2).

Over the course of history, the solanaceous plants, especially those from the genus Datura, have been used as treatments for mental illness, tumours, infections and even as aphrodisiacs. Datura plants are the main natural producers of scopolamine and its enantiomer hyoscine. Datura are paradoxical; on one hand, the genus contains the Deadly Nightshade, which is poisonous due to its high alkaloid content. At the other extreme, the genus contains edible plants, including tomatoes, potatoes, eggplant and peppers (2).

Scopolamine in the 20^th Century

In the twentieth century, the anticholinergic effects of scopolamine have been used to treat patients experiencing a variety of conditions including labor and motion sickness. Scopolamine has also been used as an anesthetic and more dubiously in narcoanalysis (as a truth serum) and poisoning.

Starting early in the century, and for the next sixty years, scopolamine was administered in conjunction with morphine just prior to child birth for the purpose of producing "Twilight Sleep", a temporary condition where the expectant mother would remain semi-conscious during but forget the birthing process (3). This procedure, which came to be known as the Gauss, Kronig and Freiburg techniques, was pioneered by German doktor Von Steinbuchtel of Graz (3) and was refined by his contemporaries. The process whereby successive doses of scopolamine and morphine were given according to a precise protocol was reputed to reduce the pain endured by the mother. Later research showed that scopolamine, when used with morphine, did not decrease the actual pain of labor but caused the mother to forget the pain and the birth. This particular application of scopolamine, however, often had undesirable consequences. Neonatal depression, the transfer of the drug to the foetus through the placenta, causing depressed respiration and cardiac function or teratogenic effects, and in rare cases, death of the mother and or child, were encountered (3). After the birth, scopolamine could also be introduced to the neonate through breast milk.

American practitioners, who often had excessive case loads, frequently delegated the procedure to improperly trained assistants who did not always follow the protocol. As a result, the occurrence of unintended consequences in America was greater than reported in Germany and thus the procedure was deemed unsafe (3).

Scopolamine's relative toxicity, resulting in death with the use of as little as 0.6g (4), was reportedly exploited for a brief period around the beginning of the Second World War by Germany as a part of its T-4 program for the euthanasia of the terminally ill (5).

As recently as the 1970's, the action of scopolamine in reducing inhibitory impulses was investigated for use as a possible truth serum (4). Scopolamine, an antimuscarinic agent, acts as a competitive inhibitor, mimicking acetylcholine at the neural synapses and depressing the central nervous system. Its most potent activity is manifested at the iris, ciliary body, and secretory (salivary, bronchial and sweat) glands. The mechanism of action for the prevention and treatment of motion sickness is as yet unclear but is suspected to include direct action on the vomiting center or anticholinergenic effects. These inhibit vestibular input to the CNS and thus the inhibition of the vomiting reflex (6).

Scopolamine and one of its salts, scopolamine hydrogen-bromide, have found wide use and acceptance for use as anti-motion-sickness agents, a US$100 million industry (6, 7). Transderm Scop, produced by ALZA Cooperation and Novartis Consumer Health Inc., delivers 1.0 mg of scopolamine over a period of 3 days through an adhesive patch placed behind the ear. It is a cost-effective treatment for motion sickness at approximately US$5 to US$10 per patch (8). While Transderm Scop is approved for preventing nausea if applied prior to travel, a new formulation delivered inranasally has been shown to be effective in the treatment and prevention of nausea and will be available to consumers pending approval by thr US Food and Drug Administration (9).

The drug itself may be ingested, injected intravenously or applied topically as a patch behind the ear. Typically, a dose of 0.3 to 0.6 mg three to four times a day is given to adults while a smaller dose of 0.006 mg/kg is administered to children. After rapid absorption, scopolamine is excreted via urine after nearly complete metabolism in the liver.

Side effects of scopolamine usage or overdose, resulting in CNS sedation, can include dry mouth, restlessness, giddiness, disorientation, memory disturbances, hallucinations, delirium, constipation and confusion (6). Discontinuation after prolonged use of scopolamine may result in withdrawal symptoms including dizziness, nausea, vomiting, headache and disturbances of equilibrium (7).

Ladenburg's First Isolation of Scopolamine

The first reported isolation of scopolamine, in a racemic mixture with hyoscine, was by Albert Ladenburg, a German chemist who was investigating naturally occurring plant alkaloids in the late 1800's (10). While Ladenburg accomplished the first isolation of scopolamine, O. Hesse and E. Schmidt also contributed to the characterization of scopolamine and hyoscine at later dates (12).

During the crystallization of hyoscyamine from mother liquor extracts of the plant Hyoseyamus, Ladenburg noted the presence of a previously unisolated alkaloid which remained in the solution. Scopolamine, which had been dubbed 'amorphous hyoscyamine' (Figure 1), was considered to be a waste product by previous researchers including Merck (10). The procedure used to obtain hyoscine from the byproducts of hyoscyamine isolation involved acidification of the solution with hydorchloric acid, followed by salt-resin formation with Ag from AgCl. Since resins form with other materials in the solution such as hyoscyamine, purification was required. After the formation of the desired product, purification was accomplished by repeated dissolution and recrystallisation of the hyoscine resin product. As a part of his research, Ladenburg determined that the melting points of the gold salts of the two compounds differed by approximately 38 to 40 ^o C. The hyoscine salt melted and decomposed to a froth at 196 to 198 ^o C while the hyoscyamine salt melted at 160 ^o C.

Fodor's Laboratory Synthesis of Scopolamine from tropane-3a, 6b-diol

A modern synthesis of scopolamine was achieved by Fodor in 1957 (11) from 3a-acetoxytrop-6-ene, obtained via 3 routes, through several alkaloid intermediates.

In the first route (Figure 2), 6b-phenylcarbamoyloxytropanone was converted to 6bphenylcarbamoyloxytropan-3a-ol. The latter, upon esterification with acetyl chloride produced 3a-acetoxy-6b-phenylcarbamoyloxytropan-3a-ol. Purification by in vaccuo distillation in in addition to pyrrolysation at 250 ^o C yielded 3a-acetoxytropna-6b-ol.

Figure 2. Synthesis of 3a-acetoxytrop-6-ene via Routes 1 and 2.

A second more efficient route to 3a-acetoxytropna-6b-ol involved the conversion of 3a, 6b-diacetoxytrpoane by Kunz hydrolysis which removed the 6b-acetyl group (Figure 2) (13).

In both cases the 3a-acetoxytropan-6b-ol was esterified with toluene-p-sulpohnate ester and treated with collidine and heat, which gave 3a-acetoxytrop-6-ene via elimination.

In the third route to 3a-acetoxytrop-6-ene (Figure 3), a dehydration reaction of racemic tropane-3a-6b-diol using Ts₂O or PCl₃ formed an expoxide between carbons 3 and 6 on the tropane ring via an S_N2i reaction. Acetyl bromide in collidine and diethyl aniline was added to cleave the epoxide bridge, yielding (+/-)-3a-acetoxy-6b-bromotropane. Dehydrobromination and deracemization produced 3a-acetoxytrop-6-ene.

Exposure of 3a-acetoxytrop-6-ene to trifloroperacetic acid (CF₃CO₃H) formic acid and 80% HOOH cleaved the double bond between C6 and C7 of the tropane ring and formed O-acetylscopine (Figure 4). Conversion to scopine followed by the removal of the Ac group via Kunz hydrolysis. Treatment with HCl and nitrobenzene at 65 ^o C yielded O-acetylscopolamine, which was purified by partition chromatography eluted with butanol-N-HCl. Acid hydrolysis resulted in the removal of the acetyl group and conversion to (+)-scopolamine.

Chemistry of Scopolamine

(+)-Scopolamine, a heterocyclic compound with a single chiral centre to which a MeOH group is bonded, exists as an enantiomer of hyoscine ((-)-scopolamine) (14). Biologically, both forms of scopolamine have similar function and activity although they are chemically distinct. The two chemical forms may be separated by chromatography (11), or based on melting points of crystallized salts and show the same ¹³C-NMR (Figure 5) and H-NMR (Figure 6).

Because of its many exposed functional groups, scopolamine is readily subject to chemical and biological decomposition and is thus stored and administered as a hydrated hydro-halogen (typically HCl or HBr) salt for stability. The molecule shows weak acidic properties at its tropic acid group while the methylated nitrogen on the tropane ring readily accepts a hydrogen. Synthetically and naturally, the tropic acid group is added as a complete unit in a state very similar to its final form due to the relative ease of esterification of tropic carbon 3, and due to the stability of the heterocyclic tropane system.

Scopolamine has other characteristics as shown in Table 1.

Biosynthesis of Scopolamine

The sites of synthesis and accumulation of scopolamine differ from plant to plant within the genus Datura. In general, scopolamine is synthesized in the roots of young plants and accumulates in the aerial parts (16). Using carbon and hydrogen isotopes, Romeik and Aurich were among those who discovered (2) the method by which scopolamine is synthesized (Figure 7). For instance, it was found that incorporation of tropine and sodium acetate into cell cultures led to the formation of radioactive acetyl tropine. Therefore, it was postulated that tropine is esterified within the plant.

Biosynthesis is regulated by numerous enzymes in the plant, including tropinone reductase I and hyoscyamine 6b -hydroxylase. The actual synthesis of scopolamine follows a tropinone pathway, which also generates many other products. The amino acid ornithine is converted to N-methylornithine, which is a precursor to N-methylputrescine, via putrescine N-methyl transferase (17).

Next, N-methylpyrrolinium is generated by an oxidative deamination. The a-amino group from the ornithine precursor is lost here. This compound is converted to hygrine by way of condensation with acetoacetyl-CoA. Conversion to tropinone, due to the cyclisation of hygrine and its reduction to tropine follows, catalyzed by the enzyme tropinone reductase I. Esterification of tropine with 3-phenyllacetic acid (a derivative of phenylalanine) forms littorine (1). This part of the synthesis corresponds to the research findings mentioned above. Next, a rearrangement in the phenyllactoyl group of littorine forms hyoscyamine. Finally, conversion to scopolamine is catalyzed by hyoscyamine 6b-hydroxylase, which adds a hydroxyl group to the 7b position. Oxidation to form the epoxide at the 6- and 7- b carbons follows.

Conclusions

Although scopolamine has been employed therapeutically for thousands of years, only recently has it been purified and studied. Syntheses are complex involving natural precursors in plants and synthetic raw materials in laboratories which require specific conditions. Scopolamine's innate instability is overcome by the formation of salts and/or hydrates which are found in such products as the Transderm Scop ^TM patch. Although it's potential applications are not fully understood, there is no doubt that it will have a significant role in future studies of the central nervous system. As was previously discussed, scopolamine is itself paradoxical in nature. On one hand, medicinal benefits can be obtained from the drug if used correctly, however, its misuse has resulted in unforseen effects. Unethical deployment of the drug in the past for subversive purposes require that any future use be carefully studied and considered.

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