Tropinone biosynthesis

Also, a large number of indole alkaloids are formed by Mannich reactions involving tryptophan or its decarboxylation product tryptamine with various aldehydes. Both the a- and p-position of the indole nucleus are electronegative, and a Mannich reaction with tryptamine can yield a p-carboline derivative or a 3,3-spiroindolenine [30]. A further example is provided by the tropinone biosynthesis (Figure 1.29) [31 ]. 

Figure 1.29 Examples of Mannich reactions, (a) Biosynthesis of alkaloids bearing the 1,2,3,4-tetrahydroisoqumoline ring, (h) biosynthesis of indole alkaloids (P-carboline, 3,3-spiroindolenine), and (c) tropinone biosynthesis.

Mannich reactions, or a mechanistic analog, are important in the biosynthesis of many nitrogen-containing natural products. As a result, the Mannich reaction has played an important role in the synthesis of such compounds, especially in syntheses patterned after the biosynthesis, i.e., biomimetic synthesis. The earliest example of the use of the Mannich reaction in this way was Sir Robert Robinson s successful synthesis of tropinone, a derivative of the alkaloid tropine, in 1917. 

The domino approach is also used by Nature for the synthesis of several alkaloids, the most prominent example being the biosynthesis of tropinone (0-16). In this case, a biomimetic synthesis was developed before the biosynthesis had been disclosed. Shortly after the publication of a more than 20-step synthesis of tropinone by Willstatter [14], Robinson [15] described a domino process (which was later improved by Schopf [16]) using succinaldehyde (0-13), methylamine (0-14) and acetonedicarboxylic acid (0-15) to give tropinone (0-16) in excellent yield without isolating any intermediates (Scheme 0.5). 

NAKAJIMA, K., HASHIMOTO, T., Two tropinone reductases, that catalyze opposite stereospecific reductions in tropane alkaloid biosynthesis, are localized in plant root with different cell-specific patterns. Plant Cel Physiol., 1999,40, 1099-1107. 

This complex route to tropinone was imitated as long ago as 1917 in one of the most celebrated reactions of all time, Robinson s tropinone synthesis. Robinson argued on purely chemical grounds that the sequence of imine salts and enols, which later (1970) turned out to be Nature s route, could be produced under natural conditions (aqueous solution at pH 7) from a C4 dialdehyde, MeNH2 and acetone dicarboxylic acid. It worked and the intermediates must be very similar to those in the biosynthesis. 

Figure 2.3 Biosynthesis of the tropane alkaloids. PMT, putrescine N-methyltransferase DAO, diamine oxidase MPO, N-methylputrescine oxidase TR I and II, tropinone reductase H6H, hyoascyamine 6-hydroxylase.

Robins, R.J., Abraham, T., Parr, A.J., Eagles, J. and Walton, N.J. (1997) The biosynthesis of tropane alkaloids Datura stramonium the identity of the intermediate between N-methylpyrrolinium salt and tropinone.. Am. Chem. Soc., 119,10929-34. 

Figure 3 (a) Representative tropane and nicotine alkaloids, (b) Tropane biosynthesis. ODC, ornithine decarboxylase PMT, putrescine N-methyltransferase MPO, diamine oxidase TRl, tropinone reductase 1 H6H, hyocyamine 6b-hydroxylase. 

The overexpression of both trl and h6h from H. niger in N. tabacum plants have been reported [160]. Here, transgenic and control tobacco plants were fed with the tropane intermediate tropinone. Thus, tropine, the TRI-reaction product, was detected only in leaves of transgenic plants, with no correlation with trl transcript level and tropine amounts. Surprisingly, transgenic tobacco plants contained 3 to 13-fold more nicotine than wild type plants. Also, the presence of considerable amounts of nornicotine, myosmine, anabasine and anatabine contrasted with low levels in wild-type plants, indicating that the overexpression of trl and h6h perturb the normal nicotine biosynthesis when these new genes taken from a different metabolic pathway are introduced in tobacco [160]. 

Proline and ornithine would also appear to be the most probable amino acid precursors of 1-hyoscyamine, and, as such, the source of the nitrogen of the tropane nucleus. The biosynthesis of the tropic acid half of the molecule has not yet received attention. Such investigations as have been made up to the present have been based upon Robinson s (113) proposed tropinone synthesis from ornithine plus acetone, and have been principally concerned with the identification of the original nitrogenous precursor. 

As with tropinone, it is possible to make benzyl isoquinoline alkaloids very simply under mild conditions in the laboratory, providing that we use an aldehyde as the carbonyl component. The reaction (sometimes known as the Pictet-Spengler reaction) gives a reduced heterocyclic ring, as does the biosynthesis, but chemical oxidation can be used to give the isoquinoline. 

A key branch point in tropane alkaloid biosynthesis is the conversion of tropinone into either tropine or pseudotropine (or i/ -tropine), which possess opposite stereochemistry at the 3-hydroxyl position. Two different NADPH-dependent enzymes catalyze the stereospecilic reduction of tropinone the 3-carbonyl of tropinone is reduced to the 3a-hydroxy group of tropine by tropinone reductase I (TR-I) and to the 3jS-hydroxy group of pseudotropine by tropinone reductase II (TR-II). Genes encoding both TR-I and TR-II have been identified in the tropinone... 

Scheme 3. Biosynthesis of the tropane alkaloids hyoscyamine and scopolamine, the ca-lystegines, and nicotine. Molecular clones have been isolated for the enzymes shown. Abbreviations CYP82E4, nicotine A-demethylase H6H, hyoscyamine 6jS-hydroxylase ODC, ornithine decarboxylase PMT, putrescine A-methyltransferase TR-I, tropinone reductase-I TR-II, tropinone reductase-II.

In E. coca (Erythroxylaceae), the stereospecific reduction of the 3-keto function of tropane alkaloids is catalyzed by methylecgonone reductase, which converts methylecgonone (2-carbomethoxy-3-tropinone) to methylecgonine (2-carbomethoxy-3p-tropme), the penultimate step in cocaine biosynthesis. The subsequent step is the benzoylation via benzoyl-CoA to yield cocaine [50]. 

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. 

Another well-known example is the biosynthesis ofatropine within the formation of the central skeleton tropinone. Using a twofold Mannich reaction, tropinone has been prepared in a single process [8] (Scheme 5). 

Tropinone. tn an-3-one a possible biosynthetic precursor of the Tropane alkaloids (see), present in members of the Solanaceae. M, 139.19, m.p. 42 "C, b.p. 224-225 °C Robinson s lalmratory synthesis of T. (Fig.) from succindialdehyde, methylamine and the calcium salt of acetone-dicarboxylic acid in aqueous solution at ordinary temperatures (yield 42%, 2 molecules of COj are readily lost on subsequent treatment with acid), i.e. under mild or apparently physiological conditions, gave impetus to modem studies on alkaloid biosynthesis... 

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