Isotopic labelling indole

In times past it was thought that indoles already bearing an alkyl substituent at C-3 were further alkylated by direct attack at C-2. However, although 2,3-dialkylindoles are readily formed the reaction still involves attack at C-3. This can be demonstrated by the example in Scheme 7.3, where 3-(4 -hydroxybutyl)indole, containing an isotopic label located at C-T, is treated with boron trifluoride in diethyl ether. Two 1,2,3,4-tetrahy-drocarbazoles (l,2,3,4-tetrahydrodibenzo[6,J)pyrroles) are formed in a ratio of 1 1. These differ only in the position of the label. This result indicates that a 3,3-spiroindoleninium intermediate is formed first, and this then undergoes rearrangement of either bond a or bond b to C-2. As the two bonds a and b are identical, equal amounts of the tetrahydrocarbazoles... 

There are recognized at present three naturally occurring members of this group, cinchonamine, quinamine, and conquinamine, all minor alkaloids of certain Cinchona and Memijia species. The elucidation of their structures led to the suggestion that the quinoline moiety of the major bases, e.g., cinchonine and quinine, of these plants was probably derived from tryptophan via an indolic precursor. It has since been demonstrated from the results of feeding experiments with isotopically labeled tryptophan that this amino acid really can serve as a precursor of various indole alkaloids (1) as well as of quinine (2). The details of these processes are not yet known but probably involve an intermediate(s) related to cinchonamine (2, 3, 6). 

The mechanism of the PLP-dependent P-reaction involves a number of different chemical transformations (scheme 1B). The reaction requires the forma-tion/scission of C-C, C-O, C-N, C-H, N-H, and O-H bonds and the pathway for the synthesis of i-Trp from i-Ser and indole involves a minimum of at least eight distinct PLP-intermediates. RSSF spectroscopy allows direct detection and spectral characterization of the various catalytic intermediates, which accumulate during the course of the reaction (85,86). Information from RSSF spectroscopic investigations is greatly enhanced by the use of both isotopically labeled substrates (85) and substrate analogs (82), which alter the accumulation of intermediates during the presteady state phase of the reaction. Direct comparison of RSSF spectra for deuterium labeled substrates with the isotopically normal compounds is a powerful tool for the identification and assignment of chromophoric reaction intermediates (85). Finally, structure-function relationships within the bienzyme complex may be addressed by careful comparison of the time-re-solved RSSF spectra for reactions of native and mutant enzyme species (87-89). 

Amino Acids and Peptides. - Wasserman s method of one-carbon homologation of carboxylic acids to give a-ketocarboxylates involves reaction with cyanomethylenetriphenyl-phosphorane followed by ozone (Scheme 24) and has been used as a key step in a chemo-enzymatic synthesis of isotopically labelled L-valine, L-isoleucine, and o/fo-isoleucine. Alkylation of the carbanion derived from the imino-substituted methylphosphonate diphenyl ester (186) with indol-3-ylmethyl bromide followed by appropriate deprotection has been used to prepare the phosphonate analogue (187) of tryptophan (Scheme 25). The deprotected analogue (188) and derived peptides show activity as inhibitors of chymotrypsin. Two approaches to solid phase Wadsworth-Enunons reactions which have applications in combinatorial chemistry have been reported. In one diethylphosphonoacetamide is bound to PEG-PAL resin via a peptide link, while... 

D. Hendry, N.S. Nixon, B.S. Roughley, et al (2004) The synthesis of carbon-14 labelled indoles, in Synthesis and Applications of Isotopically Labelled Compounds, vol. 8 (eds D.C. Dean, C.N. Filer, and KE. McCarthy), John Wiley Sons, Ltd, New York, pp. 25-28. 

D-tryptophan was a direct precursor. The use of various isotopically labelled -tryptophans had demonstrated that the carbon C-2 of the indole ring of this precursor is converted into the carbon C-2 of the pyrrole ring in the product, as the amino-nitrogen is then oxidized to the nitro-group. Tritium from C-2 of the side-chain of L-tryptophan, but not D-tryptophan, is retained during the biosynthesis. ... 

Biosynthesis of some classes of terpene indole alkaloids is well understood. In certain cases, many of the enzymes that are responsible for biosynthesis have been cloned and mechanistically studied. In other cases, biosynthesis pathway is only proposed based on the results of feeding studies with isotopically labeled substtates and from the structures of isolated biosynthetic intermediates. All terpene indole alkaloids are derived from tryptophan and the iridoid terpene secologanin (Fig. 14.11). Tryptophan decarboxylase, a pyridoxal-dependent enzyme [29], converts tryptophan to tryptamine [30]. The following strictosidine synthase-catalyzed Mannich reaction connects ttyptamine and secologanin to yield strictosidine [31]. The Apocynaceae, Loganiaceae, Rubiaceae, and Nyssaceae families of plants each produce terpene indole alkaloids with dramatically diverse structures [32-34]. The mechanisms and control of... 

We chose the microwave-enhanced Raney Nickel catalyzed hydrogen isotope exchange of indole and N-methylindole as our substrates and D20, CD3COCD3, CD3OD and CDC13 as the solvents. The thermal reaction had already been the subject of a recent study [44], The microwave-enhanced method was some 500-fold faster than the corresponding thermal reaction (at 40 °C). Furthermore the pattern of labeling (Scheme 13.3) varied with the choice of solvent. Thus in the case of indole it-... 

Incorporation studies with isotopes showed that when anthranijate was converted to tryptophan, the carboxyl group df anthranilate was lost as carbon dioxide, but the nitrogen was retained. Because the enzymes in the tryptophan biosynthetic pathway have only a limited specificity, it was possible to substitute 4-methyl-anthranilate in E. coli extracts that could convert anthranilate to indole. This nonisotope label was conserved during the conversion to yield 6-methyl indole. 

Isotopic experiments (763) with tryptophan labeled with N and deuterium in the indole ring have shown that quinolinic acid nitrogen is probably entirely derived from the indole nitrogen of tryptophan, and that scission of the benzene ring probably occurs between carbons 3 and 4. Presumably, therefore, the hydroxyanthranilic acid is converted to intermediate A without participation of a catechol-type intermediate, and it is possible that the phosphate-bond energy of hydroxyanthranilic acid phosphate (if this is in fact an intermediate) may contribute to the transformation. It is known... 

Jackson and Lynch" have used deuterium labelling and deuterium kinetic isotope effects to investigate the azo coupling reactions of diazonium ions with 3-substituted indoles in acetonitrile. No primary kinetic isotope effect was observed when 3-deuteroindole reacted with / -nitrobenzenediazonium tetrafluoroborate and it was concluded that the slow step of the reaction was attack of the diazonium ion at the three position of the indole ring... 

The biosynthesis of camptothecin (102) follows that of terpenoid indole alkaloids through strictosidine (79) The lactam (101) derived from (79), and not the C-3 epimer, is the next intermediate.Several unknown steps follow, included in which is dehydrogenation of ring d [see (101)]. In this process a proton is lost from C-14. Tritium label at this site in (101) is retained by a primary isotope effect, the result of non-stereospecific deprotonation.This indicates that removal of the proton is not enzyme-controlled (c/. papaverine biosynthesis ref. 8, p. 19). 

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