Scopolamine from phenylalanine

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... 

Decarboxylation is followed by oxidative cycliza-tion to tropinone, followed by stereospecific reduction to the a-Hydroxy group, which is esterified to hyoscya-mine. The esterifying ester, tropic acid, is an intramo-lecularly rearranged phenyllactic acid (derived from phenylalanine). Further elaboration of hyoscyamine yields scopolamine (Fig. 23). The enzymes for this transformation are known. 

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]. 

In addition, the supplementation with precursors may improve product formation because the flux through the biosynthetic pathway may be limited by the availability of some intermediates [49]. Methylputrescine, phenylalanine and tropic acid were able to effectively increase hyoscyamine and scopolamine production when they were added from the beginning of the incubation [49], It is worth to consider that the precursor should be easily available and not expensive. The concentration, time of incorporation and the relationship with the product to be synthesized should be considered. 

Through the incorporation studies using D. innoxia, it was found that a P-ketothioester was the intermediate in the biosynthesis of (-)-hyoscyamine and (-)-scopolamine [3-7].Tropinone is formed from the P-ketothioester, and is stereoselectively reduced by a NADPH-dependent oxidoreductase TR-1 to give tropine.Then littorine is formed by adding a phenyllactic acid moiety derived from phenylalanine via phenylpyruvic acid. Regarding the incorporation of phenyllactic acid into tropine and its transformation, it was found that [1,3A C2](—)-hyoscyamine was obtained when [1,3- C2] phenyllactic acid was incorporated into D. innoxia [8]. A transformation therefore occurred on littorine to form (—)-hyoscyamine, and scopolamine was biosynthesized from 6P-hydroxyhyoscyamine by oxidation as described below. 

The general biogenetic route of cocaine and other related alkaloids is shown in Fig. 5.2.1. The fundamental skeleton is formed by the combination of lysine-ornithine derived pyrrolidine and the three- or four-carbon moiety of ace-tate-malonate origin. The acyl part of scopolamine has been demonstrated to be formed from phenylalanine through deamination and skeletal rearrangement (63). 

S)-Hyoscyamine and (6)-scopolamine are esters of the amino alcohols tropine and scopine with ( S)-tropic acid, which is derived from phenylalanine (Fig. 3). The two alkaloids occur exclusively in the Solanaceae family. They act as antagonists of muscarinic acetylcholine receptors (parasympatholytics) and lead to an increase in pulse rate, relaxation of smooth muscles, e.g., in the gastrointestine and the bronchial tract, reduction of salivary, bronchial, gastric, and sweat gland secretion. While hyoscyamine is a central stimulant, scopolamine depresses the central nervous system. 

The biosynthesis starts from the amino acid ornithine which is converted into the pyrrolidine system by the two paths sketched above. The 3 C or 4 C chain must now be built on. The starting material fdr this is, in both cases, acetate. It is probable that two units of acetate first form acetoacetate which can then be linked with the pyrrolidine ring to give variant 2 of the tropane skeleton. Variant 2 can then be converted to cocaine. However, decarboxylation of variant 2 gives variant 1 of the tropane skeleton which can then be transformed into hyoscyamine or scopolamine. For this tropic acid, which is derived from phenylalanine, is necessary (Fig. 125). 

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