Hyoscyamine racemization

Upon heating with dilute base such as 1% NaOH for about an hour, hyoscyamine racemizes, and the solution becomes optically inactive (see Box 10.9). At shorter times, racemization is incomplete and the solution will still be optically active. Consider first a very simple situation in which exactly half of the material has racemized. Half of the material is now optically inactive, consisting of equal amounts of each enantiomer, whilst the other half is still unchanged. Since the concentration of the unchanged part is half of the original concentration, the optical rotation will also have dropped to half its original value. The solution will contain 50% laevorotatory isomer and 50% racemate. However, the racemate is itself a 50 50 mixture of the two enantiomers, so the solution actually contains 25% dextrorotatory and 25 + 50% = 75% laevorotatory enantiomers. 

Atropine is a racemic compound but the (S)-enantiomer occurs in henbane (tfyoscyamus niget ) and was given a different name, hyoscyamine, before the structures were known. In fact, hyoscyamine racemizes very easilyjust on heating in water or on treatment with weak base. This is probably what happens in the deadly nightshade plant. 

M.p. 114-116 C. Prepared by racemization of hyoscyamine. It and its salts are used to dilate the pupil of the eye. Given internally they reduce the secretion of saliva and relieve spasmodic pains. 

Atropiae (41), isolated from the deadly nightshade Airopa belladonna L.) is the racemic form, as isolated, of (—)-hyoscyamine [which is not isolated, of course, from the same plant but is typically found ia solanaceous plants such as henbane (HyosQiamus mgerl. )]. Atropiae is used to dilate the pupil of the eye ia ocular inflammations and is available both as a parasympatholytic agent for relaxation of the intestinal tract and to suppress secretions of the saUvary, gastric, and respiratory tracts. In conjunction with other agents it is used as part of an antidote mixture for organophosphorus poisons (see Chemicals in war). 

By extraction of Solanacean drugs, especially Atropa belladonna, Hyoscyamus niger or other species. On careful extraction L-hyoscyamine is obtained first, which can be racemized to atropine by addition of alkali in ethanolic solution. 

Indeed, it was shown that aposcopolamine was not formed by direct dehydration of scopolamine, but via the conjugate scopolamine O-sulfate generated by a sulfotransferase [127]. This explains the species differences observed, and indicates a mechanism of heterolytic C-0 bond cleavage made possible by the electron-withdrawing capacity of the sulfate moiety. The reaction is also facilitated by the acidity of the departing proton carried by the vicinal, stereogenic C-atom. This acidity also accounts for the facile base-catalyzed racemization of scopolamine and hyoscyamine [128]. 

Thus, the anticholinergic activity of the alkaloid hyoscyamine is almost entirely confined to the (—)-isomer, and the (+)-isomer is almost devoid of activity. The racemic ( )-form, atropine, has approximately half the activity of the laevorotatory enantiomer. An anticholinergic drug blocks the action of the neurotransmitter acetylcholine, and thus occupies the same binding site as acetylcholine. The major interaction with the receptor involves that part of the molecule that mimics acetylcholine, namely the appropriately positioned ester and amine groups. The chiral centre is adjacent to the ester, and also influences binding to the receptor. 

This behaviour contrasts with the racemization of hyoscyamine to atropine, which also involves an enolate anion derived from an ester system (see Section 10.8). As the term racemization implies, atropine is a 50 50 mixture of the two enantiomers. It shows how the proportion of each epimer formed can be influenced by other stereochemical factors. 

The tropane alkaloids (—)-hyoscyamine and (—)-hyoscine are found in the toxic plants deadly nightshade (Atropa belladonna) and thornapple (Datura stramonium) and are widely used in medicine. Hyoscyamine, usually in the form of its racemate atropine, is used to dilate the pupil of the eye, and hyoscine is employed to control motion sickness. Both alkaloids are esters of (—)-tropic acid. 

The Mannich reaction was used for the first synthesis of tropine, the parent alcohol of the tropane alkaloids. One of the natural tropane alkaloids used medicinally is hyoscyamine, sometimes in its racemic form atropine. Hyoscyamine is an anticholinergic, competing with acetylcholine for the muscarinic site of the parasympathetic nervous system, and thus prevendng the passage of nerve impulses. 

The base-catalysed racemization of the alkaloid (-)-hy oscy amine to ( )-hyoscyamine (atropine) is an example of enolate anion participation. Alkaloids are normally extracted from plants by using base, thus liberating the free alkaloid bases from salt combinations. (—)-Hyoscyamine is found in belladonna Atropa belladonna) and stramonium Datura stramonium) and is used medicinally as an anticholinergic. It competes with acetylcholine for the muscarinic site of the parasympathetic nervous system, thus preventing the passage of nerve impulses. However, with careless extraction using too much base the product isolated is atropine, which has only half the biological activity of (—)-hyoscyamine, since the enantiomer (+)-hyoscyamine is essentially inactive. 

Additionally, note that base hydrolysis of hyoscyamine gives ( )-tropic acid and tropine, with racemization preceding hydrolysis. Base hydrolysis of littorine gives optically pure phenyl-lactic acid, so we deduce that hydrolysis is a more favourable process than racemization. 

Certain plants of the family Solanaceae, such as Atropa belladonna L., Hyoscyamus niger L., and Datura stramonium L., have been used medicinally for centuries in Europe because they contain tropane-type alkaloids.For example, atropine (1) [a racemic mixture of (+)- and (—)-hyoscyamine (2)] and (-)-hyoscyamine are competitive antagonists at the muscarinic acetylcholine receptor site, leading to antispasmodic and antiallergic effects. Scopolamine [(—)-hyoscine)] (3) is used in a transdermal patch for the prevention of motion sickness. Since these tropane alkaloids penetrate the blood-brain barrier, they also have psychoactive effects. ... 

The best known of the muscarinic blocking drugs are the belladonna alkaloids, atropine (Atropine) and scopolamine (Scopolamine). They are tertiary amines that contain an ester linkage. Atropine is a racemic mixture of DL-hyoscyamine, of which only the levorotatory isomer is pharmacologically active. Atropine and scopolamine are parent compounds for several semisynthetic derivatives, and some synthetic compounds with little structural similarity to the belladonna alkaloids are also in use. All of the antimuscarinic compounds are amino alcohol esters with a tertiary amine or quaternary ammonium group. 

Atropine and its naturally occurring congeners are tertiary amine alkaloid esters of tropic acid (Figure 8-1). Atropine (hyoscyamine) is found in the plant Atropa belladonna, or deadly nightshade, and in Datura stramonium, also known as jimsonweed (Jamestown weed), sacred Datura, or thorn apple. Scopolamine (hyoscine) occurs in Hyoscyamus niger, or henbane, as the /(-) stereoisomer. Naturally occurring atropine is /(-)-hyoscyamine, but the compound readily racemizes, so the commercial material is racemic d,/-hyoscyamine. The /(-) isomers of both alkaloids are at least 100 times more potent than the d(+) isomers. 

Muscarinic agonists such as muscarine and pilocarpine and antagonists such as atropine, the racemic form of natural hyoscyamine, have been known for more than a century. Only recently more selective ligands have been found that will hopefully generate a better understanding of the role mAChR subtypes in physiological processes and especially brain function. 

The alkaloids of this group are derived from a combination of a piperidine and a pyrrolidine ring, designated as tropane (Figure 14.2). The 3-hydroxy derivative of tropane is known as tropine and is the basic component of atropine. When atropine is hydrolyzed, it forms tropine and tropic acid (a-phenyl-p-hydroxy-propionic acid). Atropine is the tropic acid ester of tropine. It has been prepared synthetically. Tropic acid contains an asymmetric carbon atom. The racemic compound (atropine) as obtained naturally or as synthesized may be resolved into its optically active components, d- and /-hyoscyamine. Atropine is racemic hyoscyamine that is, it consists of equal parts of /-hyscyamine and plant cells and also in the process of extraction, so that the relative proportion of the isomers in the plants and in the preparations varies. However, atropine itself does exist in small amounts in the plants, although most of it is formed from the /-hyoscyamine in the process of extraction. 

Hyoscyamine, which, as already noted, is the /-isomer of racemic atropine, exerts the same action as the latter but is approximately twice as potent. It is, however, rarely obtainable in pure form, as it is almost always mixed with atropine, into which it changes when kept in solution. The action of atropine, as has been stated, is compounded by that of natural or levorotary hyoscyamine with that of its dextrorotary isomer. The latter does not exist free in nature and possesses little or no parasympatholytic action. 

The quantitative racemization of hyoscyamine to atropine was achieved16 by refluxing the methanolic or butanolic solution (catalysed by a base) or, even better, by refluxing in diethylamine. The partial racemization, i.e. epimerization at C-2, of (—)-cocaine to (+)-pseudococaine was effected by strong bases in methanol followed by re-benzoylation of C-3 in pseudoecgonine methyl ester. This interconversion17 allows the detection of small amounts of cocaine in forensic medicine. 

Atropine is the racemic mixture of R- and S-hyoscyamine produced during the pharmaceutical plant extraction process. R-hyoscyamine is nearly inactive on MR (distomer) whereas S-hyoscyamine exhibits high affinity (eutomer). Nevertheless, due to economic reasons atropine is typically administered even though only half of the applied dose (S-hyoscyamine) is pharmacologically active on MR. Surprisingly, there is still little information about different pharmacokinetic behaviour of both enantiomers anyhow [46,47],... 

Fig. 3 Chiral separation of atropine (racemic mixture of S- and //-hyoscyamine). Analysis of buffered serum dilution was performed according to John et al. [49] using two consecutively coupled AGP columns (150 mmx2.0 mm I.D., 5 pm) in isocratic mode with solvent A (0.01 M NH4FA, pH 8.0) and solvent B (0.01 M NH4FA in 25 % v/v ACN, pH 8.0) in 85 15 ratio at 300 pi/ min and 40 °C. Detection was done by positive ESI MS/MS in MRM mode...

Spectra for the diastereoisomeric pairs quinine-quinidine, cinchonine-cinchonidine alkaloids are mirror images of each other and mixtures have been determined using CD detection [57]. Spectra for the pilocarpine-isopilocarpine pair were such low quality that they could be used only for qualitative distinction. CD detection combined with UV detection was used to measure enantiomeric excesses in mixtures of L-hyoscyamine and atropine, i.e. racemic hyoscyamine. This subject is returned to in greater depth later. 

Hyoscyamine (duboisine) and the racemate atropine are mACh-R antagonists and a number of atropine derivatives also have this property, namely anisodamine (6P-hydroxyhyoscyamine), 7 (3-hydroxyhyoscyamine, hyoscine (6,7-epoxyhyoscyamine or scopolamine), benzoyltropein (tropine benzoate), littorine (tropine a-hydroxyphenylpropionate), tigloidine (pseudotropane tiglate) and tropacocaine (pseudotropine benzoate). The further derivatives apoatropine (a-dehydrohyoscyamine) and tropine are very toxic. 

Tropane alkaloids are an important class of natural products possessing different and interesting pharmacological activities. Hyoscyamine (atropine in the racemate form), scopolamine, and cocaine are the major representatives of this class. They are commonly found in plant materials, mainly in genera belonging to three families Solanaceae, Erythroxylaceae, and Convolvulaceae. The importance of these compounds requires that there are accurate analytical methods for their determination in plants and in biological matrices. This chapter describes the state-of-the-art of analytical procedures (extraction and analysis) for analyzing tropane alkaloids. 

Atropine is also a racemate (dl-hyoscyamine), and almost all of its antimuscarinic effects are attributable to the 1-isomer alone. It is, however, more stable chemically as the racemate which is the preferred formulation. 

Aaltonen et al. (61) compared RRA and RIA for atropine. These workers obtained preparations of receptor from rat brain and lyophilized them to a stable, dry form, They used the tritium-labeled quinuclidinyl benzilate at 35 Ci/mmol. The affinity constant was 0.48 nM, and by analysis of 25- xL serum samples they could obtain a sensitivity down to 1.25 ng/mL in serum. Nonspecific binding was again quite reasonable (4%) and a filtration-type separation was used. The d isomer of an atropine did not bind, and therefore, the cross-reaction of the d,l compound was 50% that of the ( isomer. For comparison they used RIA developed by the method of Virtanen et al, (37). The immunogen was an /-hyoscyamine-bovine serum albumin conjugate, but the antiserum was sensitive to both d,l and I isomers. Racemic tritium-labeled atropine was used as the radioligand. 

Hyoscyamine is Ihe lero form of Ihe racemie mixture known as atropine. The dextro form does not exist naturally but has been synthesized. Cushny compared the activities of (-)-hyoscyamine. (-) )-hyoseyaminc. and Ihe racemate (atropine) in 1904 and found greater peripheral potency for Ihe (-) isomer and twice Ihe potency of the racemate. All later studies have essentially confirmed that the (-)-) isomer is only weakly active and that Ihe (-) i.somer is. in effect. Ihe active portion of alropinc. Inspection of the relative do.ses of alropinc sulfate and hyo.scyamine sulfate illustrates the differences very nicely. The principal criticism offered against the use of hyoscyamine. sulfate cxclu.sively is that it lends to rdccmizc to atropine sulfate rather easily in solution, so that atropine sulfate llicn becomes the more stable of Ihe two. All of the isomers behave very much the. same in Ihe CNS. 

Atropine is the racemic mixture of l- and o-hyoscya-mine and possesses 50% of the antimuscarinic potency of L-hyoscyamine. Atropine is derived from components of the Belladonna plant and is also present in other plants from the Solanaceae family. Women in ancient times often dripped the plant s juices into their eyes, causing mydriasis and thereby enhancing their beauty. In Italian, Belladonna translates to beautiful lady . In the United States, the atropine autoinjector has been in use since 1973 for the treatment of exposures to chemical warfare nerve agents and insecticides. 

Atropine is an ester, and on hydrolysis yields a basic substance, tropine, and optically inactive tropic acid (1). It has been shown that the alkaloid hyoscyamine, which is also obtained from belladonna and is laevo-rotatory, is the ester of tropine with laevo-tropic acid (2), and therefore atropine appears to be racemic hyoscyamine. This view of the nature of atropine has been confirmed by Ladenburg (3), and dextro-hyoscyamine has also been prepared by the union of tropine with dextro tropic acid (4). 

The pharmacology of these three stereo-isomerides, o -hyoscyamine, Z-hyoscyamine, and the racemic form, atropine, has been investigated by Cushny, (5) using the frog as the subject of the experiments. It was found that all three were alike in certain respects, but that with regard to some aspects of their action dextro-hyoscyamine was the strongest and the levo variety the weakest, while with other effects of the drug exactly the reverse was the case. In all cases the action of atropine was intermediate between that of the two optically active forms, and this fact is explained by Cushny by the assumption that atropine is probably decomposed in solution into its two active components. 

The ethyl ether solution of bases is dehydrated with anhydrous sodium sulfate, filtered and the ether concentrated and cooled to crystallize a mixture of hyoscyamine and atropine. The mixture is mixed with one quarter its weight in chloroform and refluxed at 116° to 120° for 2 hours. The racemization of hyoscyamine produces atropine. 

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