Hemlock Alkaloids Research Articles

Mass Spectrometry Imaging of Coniine and Other Hemlock Alkaloids after On-Tissue Derivatization Reveals Distinct Alkaloid Distributions in the Plant.

Specialized metabolites play important roles in plants and can, for example, protect plants from predators or pathogens. Alkaloids, due to their pronounced biological activity on higher animals, are one of the most intriguing groups of specialized metabolites, and many of them are known as plant defense compounds. Poison hemlock, Conium maculatum, is well-known for its high content of piperidine alkaloids, of which coniine is the most famous. The distribution, localization, and diversity of these compounds in C. maculatum tissues have not yet been studied in detail. The hemlock alkaloids are low molecular weight compounds with relatively high volatility. They are thus difficult to analyze on-tissue by MALDI mass spectrometry imaging due to delocalization, which occurs even when using an atmospheric pressure ion source. In this manuscript, we describe an on-tissue derivatization method that allows the subsequent determination of the spatial distribution of hemlock alkaloids in different plant tissues by mass spectrometry imaging. Coniferyl aldehyde was found to be a suitable reagent for derivatization of the secondary amine alkaloids. The imaging analysis revealed that even chemically closely related hemlock alkaloids are discretely distributed in different plant tissues. Additionally, we detected a yet undescribed hemlock alkaloid in Conium maculatum seeds.

Diastereodivergent Synthesis of (+)‐α‐Conhydrine and (−)‐β‐Conhydrine from a η,γ‐Dibromo‐β‐ketosulfoxide

AbstractDiastereodivergent syntheses of the enantiopure hemlock alkaloid (+)‐α‐conhydrine and its diastereomer (−)‐β‐conhydrine are reported. Our approach relies on efficient Crystallization‐Induced Diastereomer Transformation (CIDT) of an epimeric mixture of a η,γ‐dibromo‐β‐ketosulfoxide, optically pure only at the sulfur atom, that provided a stable α‐bromoketone, and diastereoselective β‐ketosulfoxide reduction that led to the corresponding anti and syn‐bromohydrins with high level of asymmetric induction.

Crystallization-Induced Diastereomer Transformation of α-Bromo α'-Sulfinyl Ketones. Diastereodivergent Synthesis of (+)-α-Conhydrine and (-)-β-Conhydrine.

Crystallization-Induced Diastereomer Transformation (CIDT) of α-bromo-α'-(R)-sulfinylketones is reported. This process provides not readily accessible enantiopure stereolabile α-bromoketones, which after diastereoselective carbonyl group reduction lead to the corresponding highly value-added anti and syn-bromohydrins with excellent diastereoselectivities. As an application, a diastereodivergent synthesis of enantiopure hemlock alkaloid (+)-α-conhydrine and its diastereomer (-)-β-conhydrine is also described.

The killer of Socrates: Coniine and Related Alkaloids in the Plant Kingdom.

Coniine, a polyketide-derived alkaloid, is poisonous to humans and animals. It is a nicotinic acetylcholine receptor antagonist, which leads to inhibition of the nervous system, eventually causing death by suffocation in mammals. Coniine’s most famous victim is Socrates who was sentenced to death by poison chalice containing poison hemlock in 399 BC. In chemistry, coniine holds two historical records: It is the first alkaloid the chemical structure of which was established (in 1881), and that was chemically synthesized (in 1886). In plants, coniine and twelve closely related alkaloids are known from poison hemlock (Conium maculatum L.), and several Sarracenia and Aloe species. Recent work confirmed its biosynthetic polyketide origin. Biosynthesis commences by carbon backbone formation from butyryl-CoA and two malonyl-CoA building blocks catalyzed by polyketide synthase. A transamination reaction incorporates nitrogen from l-alanine and non-enzymatic cyclization leads to γ-coniceine, the first hemlock alkaloid in the pathway. Ultimately, reduction of γ-coniceine to coniine is facilitated by NADPH-dependent γ-coniceine reductase. Although coniine is notorious for its toxicity, there is no consensus on its ecological roles, especially in the carnivorous pitcher plants where it occurs. Lately there has been renewed interest in coniine’s medical uses particularly for pain relief without an addictive side effect.

N-tert-Butanesulfinyl Imines in Alkaloid Synthesis: Stereoselective Synthesis of (R)-Coniine and Formal Synthesis of (S)-δ-Coniceine¹

A stereoselective synthesis of the hemlock alkaloid (R)-coniine and a formal synthesis of the indolizidine alkaloid (S)-δ-coniceine were achieved by means of a highly diastereoselective Barbier-type indium-mediated allylation strategy involving (S)-(N-tert-butanesulfinyl)imines.

Pharmacological actions of hemlock (Conium maculatum) alkaloids.

Abstract The actions of three hemlock alkaloids—coniine, N-methylconiine and α-coniceine—have been examined on isolated tissues, on anaesthetised cats and hens and on conscious mice and young chicks. The most pronounced action of the alkaloids was to block spinal reflexes by an action exerted in the spinal cord. Their peripheral actions on autonomically innervated structures were mainly a consequence of an initial stimulant and a secondary depressant action on autonomic ganglia. Large doses of the alkaloids stimulated skeletal muscle and subsequently caused neuromuscular block. This blocking action differed in many respects from that produced by decamethonium or tubocurarine.

Hemlock alkaloids from Socrates to poison aloes

Hemlock ( Conium maculatum L. Umbelliferae) has long been known as a poisonous plant. Toxicity is due to a group of piperidine alkaloids of which the representative members are coniine and γ-coniceine. The latter is the more toxic and is the first formed biosynthetically. Its levels in relation to coniine vary widely according to environmental conditions and to provenance of the plants. Surprisingly, these piperidine alkaloids have turned up in quite unrelated species in the monocotyledons as well as the dicotyledons. Aloes, for instance, important medicinal plants, are not regarded as poisonous although some species are very bitter. Nevertheless a small number of mostly local species contain the alkaloids, especially γ-coniceine and there have been records of human poisoning. The compounds are recognized by their characteristic mousy smell. Both acute and chronic symptoms have been described. The compounds are neurotoxins and death results form respiratory failure, recalling the effects of curare. Chronic non-lethal ingestion by pregnant livestock leads to foetal malformation. Both acute and chronic toxicity are seen with stock in damp meadows and have been recorded as problems especially in North America. The alkaloids derive biosynthetically from acetate units via the polyketide pathway in contrast to other piperidine alkaloids which derive from lysine.

Efficient Asymmetric Hydrogenation of Pyridines.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Cover Picture: Efficient Asymmetric Hydrogenation of Pyridines (Angew. Chem. Int. Ed. 21/2004)

The stereoselective hydrogenation of oxazolidinone‐substituted pyridines shown in the cover picture gives piperidines containing up to four new stereocenters. One of the biologically active compounds readily accessible by this method is (S)‐coniine, the poisonous hemlock alkaloid used in ancient Greece for executions. The famous painting by David shows Socrates shortly before drinking from the poison chalice. F. Glorius et al. describe this synthetic method on pages 2850 ff.

Efficient asymmetric hydrogenation of pyridines.

available and the reaction has the potential for the simultaneous creation of multiple stereocenters. Here we describe an efficient and unprecedented auxiliary-based method for the asymmetric hydrogenation of substituted pyridines (YR= N), which enables the stereoselective formation of piperidines with up to four new chiral centers in a single operation (Scheme 2a). The heterogeneous catalytic hydrogenation of pyridines is usually performed in acidic media. Protonation not only activates the pyridines for hydrogenation, it also suppresses catalyst poisoning by the resulting piperidines. We reasoned that single-point attachment of chiral oxazolidinones 4 for ease of introduction of the auxiliary in the 2-position of the pyridine would be ideal (Scheme 2a). Moreover, it occurred to us that whereas conformation 2 should be strongly preferred for unprotonated pyridines due to dipole-moment minimization, upon protonation hydrogen bonding between the pyridinium and the oxazolidinone moiety would favor conformation 5, in which the auxiliary is oriented coplanar with the pyridine ring but rotated by 1808. Indeed, on hydrogenation the iPr substituent shields one of the diastereotopic p-faces and selective hydrogen transfer to the opposite side leads to aminal 6 (Scheme 2b). Substrates 2 can be readily synthesized from oxazolidinones and the corresponding 2-bromoor chloropyridines 1 by copper catalysis (Scheme 2a). Gratifyingly, hydrogenation of pyridine 2d in acetic acid under a hydrogen atmosphere of 100 bar with PtO2 as the catalyst led to the formation of (S)-3-methyl piperidine (3d) in 85% ee. Different catalysts were screened, and Pd(OH)2/C was identified as the optimum catalyst, providing 3d in 98% ee (Table 1, entry 4). Importantly, the reaction does not stop at aminal 6d, but leads directly to piperidine 3d and oxazolidinone 4. Evidently traceless cleavage of the auxiliary occurs under the reaction conditions, thereby combining chirality transfer from and release of the auxiliary into a single operation. We were pleased to find that after treatment of the crude reaction mixture with hydrochloric acid, separation and purification of the less soluble piperidine hydrochloride 3d and the more soluble auxiliary could be achieved efficiently by simple extraction with ether/hexanes mixtures. The piperidinium hydrochloride 3d was obtained in 90% yield (98% ee) and 4 was recovered unchanged (93% yield, > 99% ee), allowing the recycling of the auxiliary. This method for the stereoselective synthesis of piperidines has been applied successfully to a large number of substrates, whereby the oxazolidinone with a tBu group often resulted in slightly improved ee values relative to those obtained with the iPr group (Table 1). Substituents in the 4, 5or 6-position of the 2-oxazolidinone-substituted pyridine can be used to create stereocenters at the corresponding positions (entries 1–7). Even multiple stereocenters can be generated, as exemplified by the stereoselective formation of diand trisubstituted piperidines in near-quantitative yield and excellent enantioselectivities (entries 9, 10). As far as we know this is the first highly asymmetric hydrogenation of an aromatic compound that selectively generates three stereocenters. Under milder conditions we even succeeded in the stereoselective synthesis (> 95:5) of aminal 6j, which bears four new chiral centers (entry 11). Furthermore, functional groups on the pyridine ring are well tolerated (entries 3, 5–6). The versatility of the process can be increased still further, since hydrogenation of 2d in the presence of acetaldehyde or acetic anhydride results in the formation of the corresponding (S)-N-ethylpiperidine 7 (entries 12, 13). The only present limitation concerns 3-substitution of the pyridine ring. A methyl substituent in the 3-position leads to a less reactive substrate, presumably because the oxazolidinone is rotated out of the plane of the pyridine ring thereby shielding both pfaces. Hydrogenation of 2h results in a nearly racemic product (entry 8). Finally, we were pleased to find that our method gives easy access to coniine (3b), the poisonous hemlock alkaloid, in excellent yield and enantiomeric excess (entry 2). Scheme 1. Stereoselective hydrogenation of (hetero)aromatic compounds (YR=CR, N).

Sequential two electron photooxidation of t-amines: generation of a regiospecific iminium cation and its application in organic synthesis

PET activation of cyclic t-amines, utilizing 9, 10-dicyanonaphthalene (DCN) as light absorbing electron acceptor in aqueous acetonitrile solution, leads to the generation of iminium cation intermediate involving electron – proton – electron (E-P-E) transfer sequence. Iminium cation generation is found to be highly regiospecific and depends upon the kinetic acidity which is subject to the stereoelectronic factor of the α –C-H proton of the unymmetrical t-amines. Tetrahydro-1, 3-oxazines (6) are synthesized in complete regio-and stereoselective manner from the PET activation of the substrates of type 4. Nucleophilic alkylation of 6 by alkyl Grignard reagents provides cis- α, α’-dialkyl cyclic amines (8). Similarly, chiral perhydropyrido[2,1-b][1, 3, 4]-oxadiazinone (11) is synthesized as a precursor for the synthesis of optically active α-alkyl piperidines. Both enantiomers of hemlock alkaloid coniine (13) are also synthesized. Furthermore, to broaden the scope of these reactions, precursor 16 is designed for the synthesis of various α –amino acids and their N-alkyl derivatives.

A New Asymmetric Synthesis of both Enantiomers of Coniine by 1,2‐Addition of RLi/YbCl3 to Aldehyde SAMP Hydrazones

AbstractBoth enantiomers of the hemlock alkaloid coniine 8 are prepared by asymmetric synthesis in 98% ee using only one enantiomer of the auxiliary. The key step of the synthesis consists in the 1,2‐addition of organoytterbium reagents to aldehyde SAMP hydrazones with virtually complete asymmetric induction, followed by N–N bond cleavage and cyclization.

Phenylethylamine and Piperidine Alkaloids inAloeSpecies

Of the approximately 300 species of ALOE (Liliaceae) native to Africa and Arabia, leaf extracts of 224 species have been examined chromatographically for alkaloids using ninhydrin, Dragendorff's reagent, nitroprusside, and iodoplatinate as revealing agents. From these, 48 (21%) species contained compounds giving a strong colorimetric reaction with at least one of these reagents. Tyramine derivatives were identified in 18 species and piperidine derivatives in a further 6 species. The other coloured zones remain unidentified but often appeared to represent common amino acids. Some taxonomic correlations are suggested. The presence of the toxic hemlock alkaloids in aloes readily available for potential medicinal use by local inhabitants sounds a note of caution against the unthinking use of these otherwise useful plants.

ChemInform Abstract: A CHIRAL SYNTHESIS OF (+)‐PSEUDOCONHYDRINE

AbstractEntsprechend dem Formelschema wird aus dem Desoxyzucker (I) das im Titel genannte Alkaloid (VIb), das auch in Form der Derivate (VIa) und (VIc) charakterisiert wird, mit einer Gesamtausbeute von 32 % synthetisiert.

A Chiral Synthesis of (+)-Pseudoconhydrine

Abstract (+)-Pseudoconhydrine, one of hemlock alkaloids, has been synthesized from methyl 2-acetamido-2,3,4-trideoxy-α-D-erythro-hexopyranoside by an eleven step reaction sequence. An intramolecular nucleophilic attack of the nitrogen atom on 2-acetamido-5-0-benzyl-2,3,4-trideoxy-6-0-tosyl-D-reythro-hexose diethyl di-thioacetal to the terminal tosyloxymethylene group proceeded smoothly and gave the desired disubstituted piperidine compound. A conversion of the thioacetal group into an aldehyde, a two-carbon elongation by Wittig olefination, and a successive hydro-genation completed the chiral synthesis of the title alkaloid.

Hemlock Alkaloids in Aloes. Occurrence and Distribution of γ-Coniceine

The hemlock alkaloid gamma-coniceine was identified in a number of ALOE species, namely A. GILLILANDII, Reynolds A. BALLYI Reynolds, A. RUSPOLIANA Baker, A. IBITIENSIS Perrier and A. DELTOIDEODONTA Baker. Coniine was identified in A. VIGUIERI Perrier. The levels of gamma-coniceine are higher than those found in CONIUM MACULATUM L. Some species also contained trace amounts of conhydrinone and pseudoconhydrin. Three of the species are Madagascan endemics, one is restricted to Arabia, while the rest are remote from each other in East Africa. Some of the species are loosely related but there is no overall taxonomic affinity between them.

Properties and Subcellular Localization of ʟ-Alanine: Aldehyde Aminotransferase: Concept of an Ubiquitous Plant Enzyme Involved in Secondary Metabolism

Abstract L-Alanine: aldehyde aminotransferase occurs ubiquitously in higher plants. The enzyme catalyzes the reaction: L-alanine + monoaldehyde -> monoamine + pyruvate; it is responsible for the formation of aliphatic plant amines and involved in the biosynthesis of hemlock alkaloids as shown by Roberts. A continuous coupled photometric test was developed to determine the low activities of the transaminase. The enzyme from the "amine-free" plant Spinacia oleracea was purified 77-fold and separated from other aminotransferases. A comparison of the Spinacia enzyme with that isolated from spadix-appendices of the amine-producing Arum maculatum during anthesis revealed very similar characteristics in pH-dependence, ATm-values for alanine and aliphatic aldehydes, and inhibition by 2-oxoacids. In contrast to the Spinacia enzyme the Arum aminotransferase is rapidly inactivated in the absence of pyridoxal-5'-phosphate. The enzymes of S. oleracea, A. maculatum and Mercurialis perennis are localized in mitochondria, but not in chloroplasts or peroxisomes. The results are discussed in relation to the function of alanine: aldehyde aminotransferase in secondary metabolism. It is suggested that some enzymes may be expressed in plants at low levels, even in the absence of any metabolic function.

Biosynthesis and metabolism of the hemlock alkaloids

Biosynthesis and metabolism of the hemlock alkaloids

The alkaloids of hemlock ( Conium maculatum L.)—III : The presence of bound forms in the plant

Evidence is produced to indicate that the major alkaloids of hemlock, coniine and γ-coniceine, occur as bound forms as well as the normal free salts. These bound forms vary from ethanol-soluble substances which readily break down into coniine or γ-coniceine, to larger, more resistant, water-soluble complexes which on vigorous treatment break down to alkaloid-like compounds. The quantities vary during fruit development and it is suggested that the alkaloids are built up into a series of compounds of increasing complexity which may play an important part in the metabolism of the fruit.

The alkaloids of hemlock ( Conium maculatum L.)—IV : Isotopic studies of the bound forms of alkaloids in the plant

When radioactive γ-coniceine is fed to stalks subtending actively growing umbels almost all the radioactivity passes into the umbels. Here it is rapidly converted into the bound forms previously described p1 and into coniine. Conversely, when radioactive coniine is fed, radioactive bound forms and γ-coniceine are produced. The specific activities of all these compounds fluctuated fairly rapidly indicating that they are involved in important metabolic processes. The specific activities of the free alkaloids did not fall markedly even after a period of 19 days and this indicates a re-cycling system. The similarity of this situation to that obtaining with the pyridine nucleotides is discussed and it is suggested that the alkaloids may represent moieties of nucleotide-like compounds concerned with oxidation-reduction processes in this plant.