Hyoscyamine 5 -tropic acid moiety

Another area of uncertainty concerned the route by which the tropic acid moiety is incorporated. Although early reports claimed to synthesize hyoscyamine from tropine and tropic acid or tropoyl-CoA, these findings were not readily substantiated (Robins and Walton, 1993). A series of experiments in which labelled phenyllactic acids were fed to plants of D. inoxia or root cultures of D. stramonium confirmed unequivocally that this compound was an intermediate of the pathway (Ansarin and Woolley, 1993, 1994 Chesters et al, 1994, 1995a,b Robins et al, 1994a). A recent reappraisal (Chesters et al, 1996) suggested that it is the S-isomer that is incorporated. 

The failure to find C02 In the expired air of the mice Indicates that the tropic acid moiety in the atropine molecule Is not metabollzed ln accord with the finding that labeled tropic acid Itself was excreted without loss In the urine. The finding of atropine but no tropic acid In the urine of the mice Indicates that hydrolysis of the ester did not occur rapidly. The latter finding was somewhat unexpected because an esterase capable of hydrolysing atropine is present In the livers of many vertebrate species, although It or a similar enzyme appears In the sera of only a few species (93-95). This enzyme Is also able to hydrolyze L hyoscyamine, troplnyl benzoate, and caramlphen (95). 

Tropane Alkaloids.—It is clearly established that the tropic acid moiety (19), found in alkaloids such as hyoscyamine (20), is derived from all the carbon atoms of... 

Both alkaloids have (+) and (-) forms but only the (-) hyoscyamine and (-) scopolamine are active. The biosynthetic pathway of tropane alkaloids, Fig. (1) is not totally understood, especially at the enzymatic level. Edward Leete has pioneered the biosynthetic studies of tropane alkaloid since 1950"s using whole plants and isotope labels [85-86]. The tropane alkaloid hyoscyamine is bioconverted by the enzyme H6H (hyoscyamine 6p-hydroxylase, EC 1.14.11.11) to scopolamine via 6p-hydroxyhyoscyamine. Hyoscyamine is the ester of tropine and (S)-tropic acid. The (S)-tropic acid moiety derives from the amino acid L-phenylalanine, while the bicyclic tropane ring derives from L-omithine primarily or L-arginine via tropinone. Tropinone is stereospecifically reduced to form either, tropine which is incorporated into hyoscyamine, or on the other hand into pseudotropine which proceeds to calystegines, a group of nortropane derivates that were first found in the Convolvulaceae family [87]. 

Fig. (6). Origin of the tropic acid moiety involved in hyoscyamine.

The (S)-tropic acid, the acid moiety found in hyoscyamine (27) and scopolamine (89), is formed from phenylalanine by an intramolecular 2,3-carbonyl shift (5,124). Feeding of the four possible stereoisomers of [l-14C,3-3H]phenylalanine to Datura innoxia and D. stramonium was used to prove that during the 2,3-carbonyl shift a 3,2-hydrogen shift takes place as well (725)(Scheme 23). With D. innoxia, the 2,3-carbonyl shift was shown to... 

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. 

Chesters, N.C.J.E., O Hagan, D. and Robins, R.J. (1995a) The biosynthesis of tropic acid the (R)-d-phenyllactyl moiety is processed by the mutase involved in hyoscyamine biosynthesis in Datura stramonium.. Chem. Soc. Chem. Commun., 127-8. 

Both (— )-hyoscine and (— )-hyoscyamine are esters of levorotatory tropic acid, which represents the asymmetric moiety of these alkaloids. Freudenberg et al. 32) found that a series of derivatives of (— )-tropic acid showed a trend similar to derivatives of l-( + )-lactic acid, the Lg configuration XXIII being tentatively assigned on the basis of molecular rotation. 

Cinnamic acid (24), a metabolite of phenylalanine, has been linked with tropic acid biosynthesis via its epoxide on chemical grounds but neither of these compounds have previously been found to act as a tropic acid precursor. Results with cinnamoyltropine were similarly negative. " More recently examination of [2- C]cinnamic acid [as (24)] as a precursor for the acid moieties of hyoscyamine... 

The origin of the phenyl-lactic acid moiety of littorine (33) in Datura sanguinea has been shown to be from phenylalanine." A specific incorporation of [1- " C]-and [3- C]-phenylalanine was observed into carbons 1 and 3 respectively of the side chain of the phenyl-lactic acid moiety in littorine. That phenylalanine was a better precursor for littorine than for hyoscyamine (30) and hyoscine suggests perhaps that phenylalanine is a more immediate precursor of phenyl-lactic acid than of tropic acid. 

The term anticholinergic as used in the context of this discussion refers more specifically to compounds that selectively block the brain s muscarinic receptor (now known to consist of several sub-types). Atropine (hyoscyamine) and scopolamine (hyoscine) are the most familiar medicinal anticholinergics. Historically, they were obtained from of the botanical family Solanaceae, which includes Jimson (or loco) weed, mandrake root, henbane, belladonna, and nightshade. Atropine and scopolamine are esters of tropic acid and contain a tertiary nitrogen moiety. This gives them the ability to cross the blood-brain barrier and block central muscarinic cholinergic receptors by competitive inhibition with acetylcholine, the natural neurotransmitter at these sites. 

The leaves and roots of Atropa belladonna (Solanaceae) are known as Belladonna leaves and Belladonna roots, and are used as medicine in the form of extracts. These materials are also used as materials for the manufacture of atropine sulfate. Other alkaloids isolated from belladonna include (—)-hyoscyainine and (—)-scopolamine. Atropine is derived from (—)-hyoscyamine by racemization of its tropic acid ester moiety. 

Occurrence in the Solanaceae. p-Coumaric acid, caffeic acid, methyl caffeate, and methyl ferulate as well as certain of their 2,3-dihydro derivatives have been identified as constituents of the leaves of Cestrum parqui L Herit. with good phytotoxic activity against different species (D Abrosca et al. 2004). Family-specific phenylpropanoid acids like tropic acid or 2-hydroxytropic acid as acyl moieties of tropane alkaloids are synthesized via phenylalanine -> phenylpyruvic acid (l )-3-phenyllactic acid (Fig. 3.14 Table 3.1 (T5-T7-B)]. Tropic acid may occur as a metabolite of, e.g., hyoscyamine, but the free acid is not synthesized as such (for details see Sect. 3.4). 

Phenylalanine as a Precursor.— Tropic acid is found in Nature as the add moiety of the ester alkaloids hyoscyamine and hyosdne. A large amount of information, some conflicting, has been published in the past decade on the origin of this relatively simple molecule. We will review these data critically in the hope that it will aid work on the currently unsolved problem of its biosynthesis. It was discovered in 1960 that the administration of [3- C]phenylalanine to intact Datura stramonium plants yielded tropic acid having essentially all its activity at C-2. Later workers confirmed this result in D. stramonium (intact plants) and D. metel (sterile root cultures). It was then established that the other carbons of the phenylalanine side-chain were used for the production of the side-chain of tropic acid. The pattern of labelling found in the tropic... 

Troplc acid is found in nature as the acid moiety of the anticholinergic ester alkaloids hyoscyamine (Scheme 57.4) and scopolamine. These tropane alkaloids can be extracted from plants of the Solanaceae family. Once proven that racemic tropic acid ethyl ester rac-8, which bears a primary hydroxyl group, is enantioselectively acylated with vinyl acetate in the presence of Celite-supported PS lipase and that the product thus formed has the required S configuration, Hossain and Atuu attempted the enzymatic acylation... 

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