Pictet-Spengler Condensation Research Articles

C-H functionalization of 2-alkyl tryptamines: direct assembly of azepino[4,5-b]indoles and total synthesis of ngouniensines.

The Pictet-Spengler type condensation of tryptamine derivatives and aldehydes or ketones is a classic reaction, and has been previously applied to assemble indole-annulated 5-, 6- and 8-membered heterocyclic rings. In this work, we further expand the synthetic scope of this reaction to the 7-membered azepino[4,5-b]indole skeleton through the direct C-H functionalization of 2-alkyl tryptamines, in which the non-activated methylene group participates in a 7-membered ring formation with aldehydes. By combining this unprecedented ring-forming process with a second C-H olefination at the same carbon, the concise total synthesis of natural products ngouniensines is achieved, demonstrating the synthetic potential of the developed chemistry in simplifying retrosynthetic disconnections.

Phylogenetic analysis and functional characterization of norcoclaurine synthase involved in benzylisoquinoline alkaloids biosynthesis in Stephania tetrandra.

Benzylisoquinoline alkaloids (BIAs) are a class of secondary metabolites that possess diverse pharmaceutical properties and are exclusively accumulated in specific plant genera. The Pictet-Spengler condensation, catalyzed by norcoclaurine synthase (NCS), represents a key enzymatic reaction in the biosynthetic pathway of BIAs. While NCS genes have been identified in several plant families such as Papaveraceae, Berberidaceae, and Ranunculaceae, no NCS genes have been reported in Menispermaceae, which is another genus known to accumulate BIAs. Here, NCSs were isolated and functionally characterized from the Menispermaceae family plant Stephania tetrandra. In vitro enzyme assay identified two functional StNCSs which could catalyze the formation of (S)-norcoclaurine. These functionally characterized genes were then integrated into engineered yeast to enable the production of norcoclaurine. Phylogenetic analysis of the NCS enzymes revealed that the StNCSs predominantly clustered into two clades. The functional StNCSs clustered with known NCSs, highlighting the presence of a specific NCS catalytic domain. This study not only provides additional genetic components for the synthetic biology-based production of BIAs in yeast but also contributes to the understanding of the phylogenetic relationships and structure-function relationship of NCS genes involved in the origin and production of BIAs.

Low Energy Collision-Induced Dissociation of Azepine Pictet-Spengler Adducts of Nω-Methylserotonin.

Cimitrypazepines are a class of natural products produced by Pictet-Spengler condensation of Nω-methylserotonin and aldehydes in a manner that produces a seven-membered azepine ring. In this study, the fragmentation behavior of this class of molecules under low-energy CID was investigated in detail. Proposed mechanisms of fragmentation were supported by deuterium labeling and DFT calculations. Loss of methylamine and methylenimine were dominant fragmentation pathways of analogs containing an aliphatic side chain. Loss of methylenimine was found to proceed via unusual methyl cation transfer. Fragmentation of analogs containing an aromatic side chain was strongly influenced by the nature of the substituents and proceeded via a novel retro-Pictet-Spengler pathway and involvement of ion-neutral complexes. In some cases, a gas-phase interconversion between the azepine and β-carboline ring was observed during fragmentation. Detailed analysis of fragmentation behavior provided in this study will serve as a valuable guide for the discovery of new analogs from natural sources.

Elucidation of the 1-phenethylisoquinoline pathway from an endemic conifer Cephalotaxus hainanensis

Cephalotaxines harbor great medical potential, but their natural source, the endemic conifer Cephalotaxus is highly endangered, creating a conflict between biotechnological valorization and preservation of biodiversity. Here, we construct the whole biosynthetic pathway to the 1-phenethylisoquinoline scaffold, as first committed compound for phenylethylisoquinoline alkaloids (PIAs), combining metabolic modeling, and transcriptomemining ofCephalotaxushainanensis to infer the biosynthesis for PIA precursor. We identify a novel protein, ChPSS, driving the Pictet-Spengler condensation and show that this enzyme represents the branching point where PIA biosynthesis diverges from the concurrent benzylisoquinoline-alkaloids pathway. We also pinpoint ChDBR as crucial step to form 4-hydroxydihydrocinnamaldehyde diverging from lignin biosynthesis. The elucidation of the early PIA pathway represents an important step toward microbe-based production of these pharmaceutically important alkaloids resolving the conflict between biotechnology and preservation of biodiversity.

3-(1,2,3-Triazol-4-yl)-β-Carbolines and 3-(1H-Tetrazol-5-yl)-β-Carbolines: Synthesis and Evaluation as Anticancer Agents.

Herein, the synthesis and anticancer activity evaluation of a series of novel β-carbolines is reported. The reactivity of nitrosoalkenes towards indole was explored for the synthesis of novel tryptophan analogs where the carboxylic acid was replaced by a triazole moiety. This tryptamine was used in the synthesis of 3-(1,2,3-triazol-4-yl)-β-carbolines via Pictet-Spengler condensation followed by an oxidative step. A library of compounds, including the novel 3-(1,2,3-triazol-4-yl)-β-carbolines as well as methyl β-carboline-3-carboxylate and 3-tetrazolyl-β-carboline derivatives, was evaluated for their antiproliferative activity against colorectal cancer cell lines. The 3-(1H-tetrazol-5-yl)-β-carbolines stood out as the most active compounds, with values of half-maximal inhibitory concentration (IC50) ranging from 3.3 µM to 9.6 µM against colorectal adenocarcinoma HCT116 and HT29 cell lines. The results also revealed a mechanism of action independent of the p53 pathway. Further studies with the 3-tetrazolyl-β-carboline derivative, which showed high selectivity for cancer cells, revealed IC50 values below 8 μM against pancreatic adenocarcinoma PANC-1, melanoma A375, hepatocarcinoma HEPG2, and breast adenocarcinoma MCF-7 cell lines. Collectively, this work discloses the 3-tetrazolyl-β-carboline derivative as a promising anticancer agent worthy of being further explored in future works.

Efficient Synthesis of Novel 1-Substituted β- Carboline Derivatives via Pictet-Spengler Cyclization of 5-Hydroxy-L-Tryptophan

A facile synthesis of novel 1-substituted β-carboline derivatives by using three efficient reaction steps was described. The synthetic route began with the construction of β-carboline frameworks involving the coupling of 5-hydroxy-L-tryptophan with different substituted phenylglyoxal via Pictet-Spengler condensation. Subsequent reduction of carbonyl functionality on carbon-7’ by using Wolff-Kishner reaction followed by N-alkylation afforded a practical access to a series of 1-substituted β-carboline derivatives in moderate yields. These novel derivatives were successfully synthesized without the use of expensive metal catalyst, prolonged reaction hours or critical reaction conditions. The molecular structures of all synthesized derivatives were confirmed by infrared (IR), gas chromatography–mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) spectroscopy.

The Simultaneous Formation of Acrylamide, β-carbolines, and Advanced Glycation End Products in a Chemical Model System: Effect of Multiple Precursor Amino Acids.

This study investigated the effect of multiple precursor amino acids on the simultaneous formation of acrylamide, β-carbolines (i. e., harmane and norharmane), and advanced glycation end products (AGEs) [i.e., Nε-(carboxymethyl)lysine and Nε-(carboxyethyl)lysine] via a chemical model system. This model system was established with single or multiple precursor amino acids, including lysine–glucose (Lys/Glu), asparagine–glucose (Asn/Glu), tryptophan–glucose (Trp/Glu), and a combination of these amino acids (Com/Glu). Kinetic parameters were calculated by multiresponse non-linear regression models. We found that the k values of the AGEs and of acrylamide decreased, while those of harmane increased in the Com/Glu model when heated to 170 and 200°C. Our results indicated that the precursor amino acid of acrylamide and AGEs compete for α-dicarbonyl compounds, leading to a decrease in the formation of AGEs and acrylamide. Moreover, compared with asparagine, the precursor amino acid of β-carbolines was more likely to react with acetaldehyde by Pictet–Spengler condensation, which increased the formation of harmane and decreased the formation of acrylamide via the acrolein pathway.

Convenient Synthesis of Alternatively Bridged Tryptophan Ketopiperazines and Their Activities against Trypanosomatid Parasites.

There is an urgent need for the development of new treatments against trypanosomatid parasites; the causative agents of some of the most debilitating diseases in the developing world. This work targets an interesting 6-5-6-6 fused carboline scaffold, accessing a range of substituted derivatives through stereospecific intramolecular Pictet-Spengler condensation. Modification of the cyclisation conditions allowed retention of the carbamate protecting group and gave insight into the reaction mechanism. Compounds' bioactivities were measured against T. brucei, T. cruzi, L. major and HeLa cells. We have identified promising pan-trypanocidal lead compounds based on the core scaffold, and highlight key SAR trends which will be useful for the future development of these compounds as potent trypanocidal agents.

Bioinspired Divergent Oxidative Cyclization from Strictosidine and Vincoside Derivatives: Second-Generation Total Synthesis of (-)-Cymoside and Access to an Original Hexacyclic-Fused Furo[3,2-b]indoline.

The second-generation synthesis of (-)-cymoside as well as the formation of a new hexacyclic-fused furo[3,2-b]indoline framework is reported. After a Pictet-Spengler condensation between secologanin tetraacetate and tryptamine, the course of the cyclization of the 7-hydroxyindolenine intermediate, generated by oxidation with an oxaziridine, depended on the stereochemistry of the 3-position. The 3-(S)-strictosidine stereochemistry delivered efficiently the scaffold of cymoside via intramolecular coupling with the C16-C17 enol ether, while the 3-(R)-vincoside stereochemistry directed towards the reaction with the C18-C19 terminal alkene and the formation of the unexpected caged compound.

Asymmetric Biocatalytic Synthesis of 1-Aryltetrahydro-β-carbolines Enabled by "Substrate Walking".

Stereoselective catalysts for the Pictet–Spengler reaction of tryptamines and aldehydes may allow a simple and fast approach to chiral 1‐substituted tetrahydro‐β‐carbolines. Although biocatalysts have previously been employed for the Pictet–Spengler reaction, not a single one accepts benzaldehyde and its substituted derivatives. To address this challenge, a combination of substrate walking and transfer of beneficial mutations between different wild‐type backbones was used to develop a strictosidine synthase from Rauvolfia serpentina (RsSTR) into a suitable enzyme for the asymmetric Pictet–Spengler condensation of tryptamine and benzaldehyde derivatives. The double variant RsSTR V176L/V208A accepted various ortho‐, meta‐ and para‐substituted benzaldehydes and produced the corresponding chiral 1‐aryl‐tetrahydro‐β‐carbolines with up to 99 % enantiomeric excess.

Simultaneous generation of acrylamide, β-carboline heterocyclic amines and advanced glycation ends products in an aqueous Maillard reaction model system.

The simultaneous formation of acrylamide; β-carboline heterocyclic amines (HAs): harmane and norharmane; and advanced glycation end products (AGEs) (Nε-(carboxymethyl)lysine (CML) and Nε-(carboxyethyl)lysine (CEL)) was analyzed based on an aqueous model system. The model systems included lysine-glucose (Lys/Glu), asparagine-glucose (Asn/Glu), tryptophan-glucose (Trp/Glu), and a mixture of these amino acids (Mix/Glu). Only AGEs were generated when heated at 100°C, Asn and Trp competed with Lys for glucose and methylglyoxal (MGO), and glyoxal (GO) decreased AGE content. The k value of CML, CEL, and acrylamide decreased when heated at 130°C, whereas that of harmane increased in the Mix/Glu, owing to the competition between Lys and Asn for glucose, GO, and MGO. Harmane preferably formed via the Pictet-Spengler condensation between Trp and acetaldehyde, which further reduced acrylamide formation via the acrolein pathway.

Solution-Phase DNA-Compatible Pictet-Spengler Reaction Aided by Machine Learning Building Block Filtering.

SummaryThe application of machine learning toward DNA encoded library (DEL) technology is lacking despite obvious synergy between these two advancing technologies. Herein, a machine learning algorithm has been developed that predicts the conversion rate for the DNA-compatible reaction of a building block with a model DNA-conjugate. We exemplify the value of this technique with a challenging reaction, the Pictet-Spengler, where acidic conditions are normally required to achieve the desired cyclization between tryptophan and aldehydes to provide tryptolines. This is the first demonstration of using a machine learning algorithm to cull potential building blocks prior to their purchase and testing for DNA-encoded library synthesis. Importantly, this allows for a challenging reaction, with an otherwise very low building block pass rate in the test reaction, to still be used in DEL synthesis. Furthermore, because our protocol is solution phase it is directly applicable to standard plate-based DEL synthesis.

Characterization of Reaction Products and Mechanisms between Serotonin and Methylglyoxal in Model Reactions and Mice.

Serotonin is an important endogenous regulatory neurotransmitter and has also been found in fruits, vegetables, and nuts. Methylglyoxal (MGO) is a reactive dicarbonyl metabolite and also a food toxin that modifies protein and DNA to cause the development of many chronic diseases. The objective of this study is to understand the reaction mechanisms between serotonin and MGO and determine whether serotonin could trap MGO in vivo. Five products were detected in phosphate buffer (pH 7.4) at 37 °C. Four products (compounds 2 and 4-6) were purified from the reaction mixture, and their structures were characterized by the analysis of their high-resolution mass and one- and two-dimensional nuclear magnetic resonance spectra. One product (compound 3), as a result of its instability, could not be properly purified and was tentatively characterized on the basis of its high-resolution mass spectrum and corresponding mass fragments. On the basis of the structures of these five products, two reaction pathways were proposed. Compounds 2, 3, 5, and 6 were produced through the Pictet-Spengler condensation pathway between the primary amine of serotonin and the ketone of MGO, and compound 3 was identified as the intermediate product to form products 2, 5, and 6, whereas compound 4 was formed through nucleophilic substitution by the benzene ring of serotonin, which is a new reaction pathway between biogenic amines and reactive carbonyl species. More importantly, the detection of adducts 2 and 4-6 in mice supports our hypothesis that the reaction between serotonin and MGO also happens in vivo through the same pathways as those in model reactions, suggesting that dietary or endogenous serotonin has the capacity to trap MGO in vivo.

Inverted Binding of Non-natural Substrates in Strictosidine Synthase Leads to a Switch of Stereochemical Outcome in Enzyme-Catalyzed Pictet-Spengler Reactions.

The Pictet–Spengler reaction is a valuable route to 1,2,3,4-tetrahydro-β-carboline (THBC) and isoquinoline scaffolds found in many important pharmaceuticals. Strictosidine synthase (STR) catalyzes the Pictet–Spengler condensation of tryptamine and the aldehyde secologanin to give (S)-strictosidine as a key intermediate in indole alkaloid biosynthesis. STRs also accept short-chain aliphatic aldehydes to give enantioenriched alkaloid products with up to 99% ee STRs are thus valuable asymmetric organocatalysts for applications in organic synthesis. The STR catalysis of reactions of small aldehydes gives an unexpected switch in stereopreference, leading to formation of the (R)-products. Here we report a rationale for the formation of the (R)-configured products by the STR enzyme from Ophiorrhiza pumila (OpSTR) using a combination of X-ray crystallography, mutational, and molecular dynamics (MD) studies. We discovered that short-chain aldehydes bind in an inverted fashion compared to secologanin leading to the inverted stereopreference for the observed (R)-product in those cases. The study demonstrates that the same catalyst can have two different productive binding modes for one substrate but give different absolute configuration of the products by binding the aldehyde substrate differently. These results will guide future engineering of STRs and related enzymes for biocatalytic applications.

Pictet-Spengler condensations using 4-(2-aminoethyl)coumarins.

Androgen-deprivation therapy (ADT) is only a palliative measure, and prostate cancer invariably recurs in a lethal, castration-resistant form (CRPC). Prostate cancer resists ADT by metabolizing weak, adrenal androgens to growth-promoting 5α-dihydrotestosterone (DHT), the preferred ligand for the androgen receptor (AR). Developing small-molecule inhibitors for the final steps in androgen metabolic pathways that utilize 17-oxidoreductases required probes that possess fluorescent groups at C-3 and intact, naturally occurring functionality at C-17. Application of the Pictet-Spengler condensation to substituted 4-(2-aminoethyl)coumarins and 5α-androstane-3-ones furnished spirocyclic, fluorescent androgens at the desired C-3 position. Condensations required the presence of activating C-7 amino or N,N-dialkylamino groups in the 4-(2-aminoethyl)coumarin component of these condensation reactions. Successful Pictet-Spengler condensation, for example, of DHT with 9-(2-aminoethyl)-2,3,6,7-tetrahydro-1H,5H,11H-pyrano[2,3-f]pyrido[3,2,1-ij]quinolin-11-one led to a spirocyclic androgen, (3R,5S,10S,13S,17S)-17-hydroxy-10,13-dimethyl-1,2,2',3',4,5,6,7,8,8',9,9',10,11,12,12',13,13',14,15,16,17-docosahydro-7'H,11'H-spiro-[cyclopenta[a]phenanthrene-3,4'-pyrido[3,2,1-ij]pyrido[4',3':4,5]pyrano[2,3-f]quinolin]-5'(1'H)-one. Computational modeling supported the surrogacy of the C-3 fluorescent DHT analog as a tool to study 17-oxidoreductases for intracrine, androgen metabolism.

A modular approach to the bisbenzylisoquinoline alkaloids tetrandrine and isotetrandrine.

An efficient racemic total synthesis of the bisbenzylisoquinoline alkaloids tetrandrine and isotetrandrine in four different routes is reported herein. Key steps of the synthesis include N-acyl Pictet-Spengler condensations to access the tetrahydroisoquinoline moieties, as well as copper-catalyzed Ullmann couplings for diaryl ether formation. Starting from commercially available building blocks tetrandrine and isotetrandrine are accessed in 12 steps. Depending on the sequence of the four central condensation steps, equimolar mixtures of both diastereomers or predominantly tetrandrine or its diastereomer isotetrandrine are obtained. Through computational analysis we were able to rationalize the differences in the observed diastereomeric specificities.

Scalable Formal Synthesis of (-)-Quinocarcin.

A scalable and unified strategy is described for the synthesis of (-)-quinocarcin, an important tetrahydroisoquinoline antitumor alkaloid. The strategy allows the practical formal synthesis of (-)-quinocarcin in 13 steps and 4.8% overall yield using N-phthaloyl-l-alanine as a chiral pool. It features the gram-scale and stereoselective synthesis of the tetrahydroisoquinoline moiety (AB ring) via Pd-catalyzed C(sp3)-H arylation and Pictet-Spengler condensation and a Cu(I)-catalyzed exo-selective [C + NC + CC] coupling reaction to generate the chiral pyrrolidine motif (D ring).

Not Just from Ethanol. Tetrahydroisoquinolinic (TIQ) Derivatives: from Neurotoxicity to Neuroprotection.

The 1,2,3,4-tetrahydroisoquinolines (TIQs) are compounds frequently described as alkaloids that can be found in the human body fluids and/or tissues including the brain. In most circumstances, TIQs may be originated as a consequence of reactions, known as Pictet-Spengler condensations, between biogenic amines and electrophilic carbonyl compounds, including ethanol's main metabolite, acetaldehyde. Several TIQs may also be synthesized enzymatically whilst others may be formed in the body as by-products of other compounds including TIQs themselves. The biological actions of TIQs appear critically dependent on their metabolism, and nowadays, among TIQs, 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol), N-methyl-1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (N-methyl-(R)-salsolinol), 1-[(3,4-dihydroxyphenyl)methyl]-1,2,3,4-tetrahydroisoquinoline-6,7-diol (norlaudanosoline or tetrahydropapaveroline or THP) and 1-benzyl-1,2,3,4-tetrahydroisoquinoline (1BnTIQ) are considered as those endowed with the most potent neurotoxic actions. However, it remains to be established whether a continuous exposure to TIQs or to their metabolites might carry toxicological consequences in the short- or long-term period. Remarkably, recent findings suggest that some TIQs such as (1-[(4-hydroxyphenyl)methyl]-1,2,3,4-tetrahydroisoquinoline-6,7-diol) (higenamine) and 1-methyl-1,2,3,4-tetrahydroisoquinoline (1-MeTIQ) as well as N-methyl-tetrahydroisoquinoline (N-methyl-TIQ) exert unique neuroprotective and neurorestorative actions. The present review article provides an overview on these aspects of TIQs and summarizes those that presently appear the most significant highlights on this puzzling topic.

A New Approach to the Synthesis of Benzo[c][1,7]naphthyridin-4(3H)-ones

A simple and efficient approach has been developed for the synthesis of previously inaccessible 6-arylbenzo[c][1,7]naphthyridin-4(3H)-ones, which is based on the interaction of 3-amino-6-methyl-4-arylpyridin-2(1H)-ones and aromatic aldehydes in a strong acidic media (trifluoroacetic, phosphorous or polyphosphorous acids) under heating. The initial imine derivatives undergo Pictet–Spengler condensation to form intermediate 6-aryl-5,6-dihydrobenzo[c]-1,7-naphthyridin-4(3H)-ones, which are oxidized by atmospheric oxygen to give 3-amino-6-methyl-4-arylpyridin-2(1H)-ones as the final products.

Genes encoding norcoclaurine synthase occur as tandem fusions in the Papaveraceae.

Norcoclaurine synthase (NCS) catalyzes the enantioselective Pictet-Spengler condensation of dopamine and 4-hydroxyphenylacetaldehyde as the first step in benzylisoquinoline alkaloid (BIA) biosynthesis. NCS orthologs in available transcriptome databases were screened for variants that might improve the low yield of BIAs in engineered microorganisms. Databases for 21 BIA-producing species from four plant families yielded 33 assembled contigs with homology to characterized NCS genes. Predicted translation products generated from nine contigs consisted of two to five sequential repeats, each containing most of the sequence found in single-domain enzymes. Assembled contigs containing tandem domain repeats were detected only in members of the Papaveraceae family, including opium poppy (Papaver somniferum). Fourteen cDNAs were generated from 10 species, five of which encoded NCS orthologs with repeated domains. Functional analysis of corresponding recombinant proteins yielded six active NCS enzymes, including four containing either two, three or four repeated catalytic domains. Truncation of the first 25 N-terminal amino acids from the remaining polypeptides revealed two additional enzymes. Multiple catalytic domains correlated with a proportional increase in catalytic efficiency. Expression of NCS genes in Saccharomyces cereviseae also produced active enzymes. The metabolic conversion capacity of engineered yeast positively correlated with the number of repeated domains.