Indole prenylation in alkaloid synthesis

Reversed Allylation of 3-Alkenyl Indoles Using Allyl Boronic Acids: Access to Tryptamine Based Alkaloids

All-carbon quaternary and tertiary stereocenters connected at the C2-position of functionalizable C3-alkylated indole nucleus are commonly occurring frameworks found in many indole alkaloids of medicinal importance. Their direct access is scarcely reported, a long-standing problem, and developing a unique yet simple method can pave the pathway to an entirely different retrosynthetic route for the total synthesis of these alkaloids. Herein, this problem is addressed by developing an unprecedented branch-selective allylation strategy employing a broad range of structurally and electronically different 3-alkenyl-indoles and allylboronic acids. The use of readily accessible starting precursors renders this method attractive for the total synthesis of indole alkaloids. The present strategy shows excellent functional group tolerance, atom economic, redox-neutral, scalable and easy to operate. The DFT calculation on the possible transition states indicated that allylboronic acid acts as a nucleophile rather than a more reactive boroxine. Mechanistically, this strategy is designed by exploiting the loss as well as regaining the indole's aromaticity. The utility of this method is demonstrated in the total syntheses of seven prenylated tryptamine alkaloids and the first total syntheses of dimeric indole alkaloids, fluevirine E and mucronatin B.

Construction of C2-indolyl-quaternary centers by branch-selective allylation: enabling concise total synthesis of the (±)-mersicarpine alkaloid

Herein we report a branch-selective allylation strategy for accessing C2-indolyl-all-carbon quaternary centers using allylboronic acids. This approach boasts broad functional group tolerance, scalability, and relies on easily accessible allyl alcohol precursors. Importantly, the C3-position of the indole remains free, offering a handle for further synthetic refinement. Mechanistic pathways, corroborated by density functional theory (DFT), suggest the involvement of an indolenine intermediate and a Zimmerman-Traxler-like transition state during allylboration. Demonstrating its efficacy, the method was applied to the total synthesis of the (±)-mersicarpine alkaloid and enabled formal synthesis of additional alkaloids, such as (±)-scholarisine G, (±)-melodinine E, and (±)-leuconoxine.

Prenylation of dimeric cyclo-L-Trp-L-Trp by the promiscuous cyclo-L-Trp-L-Ala prenyltransferase EchPT1

Prenyltransferases (PTs) from the dimethylallyl tryptophan synthase (DMATS) superfamily are known as efficient biocatalysts and mainly catalyze regioselective Friedel-Crafts alkylation of tryptophan and tryptophan-containing cyclodipeptides (CDPs). They can also use other unnatural aromatic compounds as substrates and play therefore a pivotal role in increasing structural diversity and biological activities of a broad range of natural and unnatural products. In recent years, several prenylated dimeric CDPs have been identified with wide range of bioactivities. In this study, we demonstrate the production of prenylated dimeric CDPs by chemoenzymatic synthesis with a known promiscuous enzyme EchPT1, which uses cyclo-L-Trp-L-Ala as natural substrate for reverse C2-prenylation. High product yields were achieved with EchPT1 for C3-N1' and C3-C3' linked dimers of cyclo-L-Trp-L-Trp. Isolation and structural elucidation confirmed the product structures to be reversely C19/C19'-mono- and diprenylated cyclo-L-Trp-L-Trp dimers. Our study provides an additional example for increasing structural diversity by prenylation of complex substrates with known biosynthetic enzymes. KEY POINTS: • Chemoenzymatic synthesis of prenylated cyclo-L-Trp-L-Trp dimers • Same prenylation pattern and position for cyclodipeptides and their dimers. • Indole prenyltransferases such as EchPT1 can be widely used as biocatalysts.

Enantioselective SN 2 Alkylation of Homoenolates by N-Heterocyclic Carbene Catalysis

The functionalization of the β-carbon of enals with electrophiles is a signature umpolung reactivity of N-heterocyclic carbene (NHC) derived homoenolates. However, only a limited number of electrophiles are shown to be compatible, with most of them being π-electrophiles. In this study, the successful enantioselective β-alkylation of homoenolates is reported using Csp3 electrophiles through an SN 2 strategy. The protocol shows a broad scope regarding alkyl electrophiles, delivering good yields, and excellent enantioselectivities (up to 99% ee). It enables the installation of drug-like structural motifs in either enals or alkylating agents, demonstrating its potential as a valuable tool for late-stage modification. Furthermore, a concise synthetic route is presented to chiral pyrroloindoline-type skeletons. Preliminary mechanistic studies support a direct SN 2 mechanism.

Photoredox-catalyzed diastereoselective dearomative prenylation and reverse-prenylation of electron-deficient indole derivatives

Prenylated and reverse-prenylated indolines are privileged scaffolds in numerous naturally occurring indole alkaloids with a broad spectrum of important biological properties. Development of straightforward and stereoselective methods to enable the synthesis of structurally diverse prenylated and reverse-prenylated indoline derivatives is highly desirable and challenging. In this context, the most direct approaches to achieve this goal generally rely on transition-metal-catalyzed dearomative allylic alkylation of electron-rich indoles. However, the electron-deficient indoles are much less explored, probably due to their diminished nucleophilicity. Herein, a photoredox-catalyzed tandem Giese radical addition/Ireland-Claisen rearrangement is disclosed. Diastereoselective dearomative prenylation and reverse-prenylation of electron-deficient indoles proceed smoothly under mild conditions. An array of tertiary α-silylamines as radical precursors is readily incorporated in 2,3-disubstituted indolines with high functional compatibility and excellent diastereoselectivity (>20:1 d.r.). The corresponding transformations of the secondary α-silylamines provide the biologically important lactam-fused indolines in one-pot synthesis. Subsequently, a plausible photoredox pathway is proposed based on control experiments. The preliminary bioactivity study reveals a potential anticancer property of these structurally appealing indolines.

Catalytic potential of a fungal indole prenyltransferase toward β-carbolines, harmine and harman, and their prenylation effects on antibacterial activity

The prenylation of compounds has attracted much attention, since it often adds bioactivity to non-prenylated compounds. We employed an enzyme assay with CdpNPT, an indole prenyltransferase from Aspergillus fumigatus with two naturally occurring β-carbolines, harmine (3) and harman (4) as prenyl acceptors, in the presence of dimethylallyl diphosphate (DMAPP) as the prenyl donor. The enzyme accepted these two prenyl acceptor substrates to produce 6-(3',3'-dimethylallyl)harmine (5) from 3 and 9-(3',3'-dimethylallyl)harman (6) and 6-(3',3'-dimethylallyl)harman (7) from 4. The X-ray crystal structure analysis of the CdpNPT (38-440) truncated mutant complexed with 4, and docking simulation studies of DMAPP to the crystal structure of the CdpNPT (38-440) mutant, suggested that CdpNPT could employ the two-step prenylation mechanism to produce 7, while the enzyme produced 6 with either one- or two-step prenylation mechanisms. Furthermore, the antibacterial assays revealed that the 3',3'-dimethylallylation of 3 and 4, as well as harmol (1), at C-6 enhanced the activities against Staphylococcus aureus and Bacillus subtilis.

Isoprene: A Promising Coupling Partner in C–H Functionalizations

Five-carbon dimethylallyl units, such as prenyl and reverse-prenyl, are widely distributed in natural indole alkaloids and terpenoids. In conventional methodologies, these valuable motifs are often derived from substrates bearing leaving groups, but these processes are accompanied by the generation of stoichiometric amounts of by-products. From an economical and environmental point of view, the basic industrial feedstock isoprene is an ideal alternative precursor. However, given that electronically unbiased isoprene might undergo six possible addition modes in the coupling reactions, it is difficult to control the selectivity. This article summarizes the strategies we have developed to achieve regioselective C–H functionalizations of isoprene under transition-metal and acid catalysis.

1 Introduction

2 Catalytic Coupling of Indoles with Isoprene

3 Catalytic Coupling of Formaldehyde, Arenes and Isoprene

4 Catalytic Coupling of 4-Hydroxycoumarins with Isoprene

5 Catalytic Coupling of Cyclic 1,3-Diketones with Isoprene

6 Conclusion and Outlook

Aza-Annulation of 1,2,3,4-Tetrahydro-β-carboline Derived Enaminones and Nitroenamines: Synthesis of Functionalized Indolizino[8,7-b]indoles, Pyrido[1,2-a:3,4-b']diindoles, Indolo[2,3-a]quinolizidine-4-ones and Other Tetrahydro-β-carboline Fused Heterocycles

Aza-annulation of novel 1,2,3,4-tetrahydro-β-carboline derived enaminones and nitroenamines with various 1,2- and 1,3-bis electrophiles, such as oxalyl chloride, maleic anhydride, 1,4-benzoquinone, 3-bromopropionyl chloride, itaconic anhydride, and imines (from formaldehyde and primary amines), has been investigated. These methodologies provide simple one-step pathways for efficient construction of highly functionalized tetrahydro-β-carboline 1,2-fused, five- and six-membered heterocyclic frameworks, such as indolizino[8,7-b]indoles, pyrido[1,2-a:3,4-b']diindoles, indolo[2,3-a]quinolizidines, and pyrimido[1',6':1,2]pyrido[3,4-b]indoles, which are core structures of many naturally occurring indole alkaloids with diverse bioactivity.

Friedel-Crafts Alkylation of Indoles with Trichloroacetimidates

Substituted indole scaffolds are often utilized in medicinal chemistry as they regularly possess significant pharmacological activity. Therefore the development of simple, inexpensive and efficient methods for alkylating the indole heterocycle continues to be an active research area. Reported are reactions of trichloroacetimidate electrophiles and indoles to address the challenges of accessing alkyl decorated indole structures. These alkylations perform best when either the indole or the imidate is functionalized with electron withdrawing groups to avoid polyalkylation.

Total synthesis of natural productsviairidium catalysis

An overview of the highlights in total synthesis of natural products using iridium as a catalyst is given.

Synthesis, Spectroscopic Characterization and Antimicrobial Potential of Certain New Isatin-Indole Molecular Hybrids

Molecular hybridization has a wide application in medicinal chemistry to obtain new biologically active compounds. New isatin-indole molecular hybrids 5a-n have been synthesized and characterized by various spectroscopic tools. The in vitro antimicrobial potential of the prepared compounds 5a-n was assessed using diameter of the inhibition zone (DIZ) and minimum inhibitory concentration (MIC) assays against a panel of Gram-negative bacteria, Gram-positive bacteria and fungi. Most of the synthesized compounds 5a-n showed weak activities against Gram-negative bacteria while compounds 5b and 5c exhibited good activities against Gram-positive bacteria. On the other hand, compound 5j emerged as the most active compound towards Candida albicans (C. albicans), with an MIC value of 3.9 µg/mL, and compound 5g as the most active congener towards Asperagillus niger (A. niger), with an MIC value of 15.6 µg/mL. Moreover, compound 5h manifested the best anti-P. notatum effect, with an MIC value of 7.8 µg/mL, making it equipotent with compound 5g.

Dual nickel and Lewis acid catalysis for cross-electrophile coupling: the allylation of aryl halides with allylic alcohols

Controlling the selectivity in cross-electrophile coupling reactions is a significant challenge, particularly when one electrophile is much more reactive. We report a general and practical strategy to address this problem in the reaction between reactive and unreactive electrophiles by a combination of nickel and Lewis acid catalysis. This strategy is used for the coupling of aryl halides with allylic alcohols to form linear allylarenes selectively. The reaction tolerates a wide range of functional groups (e.g. silanes, boronates, anilines, esters, alcohols, and various heterocycles) and works with various allylic alcohols. Complementary to most current routes for the C3 allylation of an unprotected indole, this method provides access to C2 and C4-C7 allylated indoles. Preliminary mechanistic experiments reveal that the reaction might start with an aryl nickel intermediate, which then reacts with Lewis acid activated allylic alcohols in the presence of Mn.

Bioinspired Indole Prenylation Reactions in Water

Isoprene units derived from dimethylallyl diphosphate (DMAPP) are an important motif in many natural products including terpenoids, carotenoids, steroids, and natural rubber. Understanding the chemical characteristics of DMAPP is an important topic in natural products chemistry, organic chemistry, and biochemistry. We have developed a direct bioinspired indole prenylation reaction using DMAPP or its equivalents as the electrophile in homogeneous aqueous acidic media in the absence of enzyme to provide prenylated indole products. After establishing the bioinspired indole prenylation reaction, this was then used to achieve the synthesis of a series of natural products, namely, N-prenylcyclo-l-tryptophyl-l-proline, tryprostatins, rhinocladins, and terezine D.

Alkyne Hydroheteroarylation: Enantioselective Coupling of Indoles and Alkynes via Rh-Hydride Catalysis

We report an enantioselective coupling between alkynes and indoles. A Rh-hydride catalyst isomerizes alkynes to generate a metal-allyl species that can be trapped with both aromatic and heteroaromatic nucleophiles.

Development of the Regiodivergent Asymmetric Prenylation of 3-Substituted Oxindoles

This paper describes our efforts to design a Pd-catalyzed asymmetric prenylation of 3-substituted oxindoles that affords access to both the linear and reverse-prenylated products. Both 3-alkyl- and 3-aryloxindoles performed well under our optimized reaction conditions. The regiodivergent alkylation of monoterpene-derived electrophiles using this methodology was also investigated. The utility of this methodology in natural product synthesis was demonstrated through the efficient total syntheses of four Flustra alkaloids, which also allowed the absolute stereochemistry of the prenylated oxindole products to be assigned. Surprisingly, the same enantiomer of ligand produced linear and branched regioisomers of opposite chirality.

Direct Assembly of Prenylated Heteroarenes through a Cascade Minisci Reaction/Dehydration Sequence

The prenyl group is an important component in bioactive compounds. Herein, we report the assembly of prenylated heteroarenes through a cascade Minisci reaction and acid-promoted dehydration sequence. The use of potassium (3-hydroxy-3-methylbut-1-yl)trifluoroborate as a new coupling reagent allows the direct introduction of prenyl and 3-hydroxy-3-methylbutyl groups to a wide variety of electron-deficient heteroarenes. Synthetic application is also demonstrated.

Diastereodivergent Reverse Prenylation of Indole and Tryptophan Derivatives: Total Synthesis of Amauromine, Novoamauromine, and epi-Amauromine

A regio- and stereoselective reverse prenylation of indole and tryptophan derivatives is presented. All four possible stereoisomers are accessible through this iridium-catalyzed reaction. The stereoselectivity is controlled by a chiral phosphoramidite ligand in combination with an achiral borane additive and can be switched by changing the nature of the borane. One enantiomer of the ligand is thus sufficient to prepare all possible isomers. The synthetic potential of this method was demonstrated by a short total synthesis of amauromine and its two natural diastereomers.

Exploiting the σ-phylic properties of cationic gold(i) catalysts in the ring opening reactions of aziridines with indoles

Gold(i) species catalyze the ring opening reactions of aziridines with indoles as nucleophiles for the synthesis of tryptamines and pyrroloindolines.

SmI2-mediated dimerization of indolylbutenones and synthesis of the myxobacterial natural product indiacen B

The synthesis and reactivity of indole derivatives substituted in the benzene section was studied. Starting materials 4- and 6-iodoindole were conveniently prepared via the Batcho–Leimgruber route and purified by sublimation. Novel vicinally indolyl-substituted cyclopentanols with unexpected cis-configuration were formed by SmI₂-mediated reductive dimerization of a 4-(indol-6-yl)butenone, obtained by Heck reaction. The two indolyl units appear to chelate Sm(II)/(III) leading to a gauche-type arrangement at the newly formed bond between the two β-carbons. Through a sequence of Sonogashira cross coupling and Meyer–Schuster rearrangement 6-prenoylindole was synthesized and reductively dimerized to a cyclopentane in a [3 + 2] cycloaddition by treatment with SmI₂ in THF. From 4-iodoindole, the natural product indiacen B from the myxobacterium Sandaracinus amylolyticus was synthesized for the first time, confirming its antimicrobial activity. The E-configuration of the chloroalkene moiety of indiacen B was confirmed by X-ray analysis.

Highly stereoselective [4+2] and [3+2] spiroannulations of 2-(2-oxoindolin-3-ylidene)acetic esters catalyzed by bifunctional thioureas

A new Michael-Michael cascade reaction between 2-(2-oxoindolin-3-ylidene)acetic esters 1 and nitroenoates 2, catalyzed by bifunctional thioureas, is investigated. The combination of the two Michael reactions results in a novel and facile [4+2] or [3+2] spiroannulation process, which is characterized by the following features: 1) two carbon-carbon bonds and four stereocenters, including a quaternary spiro carbon, are formed under mild conditions; 2) an unprecedented and stereochemically defined substitution pattern on the spirocarbocyclic unit is obtained; 3) the double-bond configuration of the donor-acceptor nitroenoate 2 determines the absolute configuration of the spiro center, whereas the remaining stereocenters are formed under control of the catalyst. The effect on the final stereochemical outcome of structural variations of each starting material, catalyst, and experimental conditions is analyzed in detail. In particular, the use of specifically designed chiral nitroenoates enables diverse polyfunctional spirocyclohexane derivatives containing six consecutive stereogenic centers to be constructed. To our knowledge, this is the first asymmetric organocatalytic strategy enabling both five- and six-membered β-nitro spirocarbocyclic oxindoles.

Mechanistic studies on the indole prenyltransferases

Covering: up to 2014. Prenylated indole alkaloids comprise a large and structurally diverse family of natural products that often display potent biological activities. In recent years a large family of prenyltransferases that install prenyl groups onto the indole core have been discovered. While the vast majority of these enzymes are evolutionarily related and share a common protein fold, they are remarkably versatile in their ability to catalyze reverse and normal prenylations at all positions on the indole ring. This highlight article will focus on recent studies of the mechanisms utilized by indole prenyltransferases. While all of the prenylation reactions may follow a direct electrophilic aromatic substitution mechanism, studies of structure and reactivity suggest that in some cases prenylation may first occur at the nucleophilic C-3 position, and subsequent rearrangements then generate the final product.

Simple indole alkaloids and those with a nonrearranged monoterpenoid unit

This review summarizes the isolation, structure determination, total syntheses and biological activities of simple indole alkaloids and those with a nonrearranged monoterpenoid unit, with literature coverage from 2012 to 2013.

Site-directed mutagenesis switching a dimethylallyl tryptophan synthase to a specific tyrosine C3-prenylating enzyme

The tryptophan prenyltransferases FgaPT2 and 7-DMATS (7-dimethylallyl tryptophan synthase) from Aspergillus fumigatus catalyze C(4)- and C(7)-prenylation of the indole ring, respectively. 7-DMATS was found to accept l-tyrosine as substrate as well and converted it to an O-prenylated derivative. An acceptance of l-tyrosine by FgaPT2 was also observed in this study. Interestingly, isolation and structure elucidation revealed the identification of a C(3)-prenylated l-tyrosine as enzyme product. Molecular modeling and site-directed mutagenesis led to creation of a mutant FgaPT2_K174F, which showed much higher specificity toward l-tyrosine than l-tryptophan. Its catalytic efficiency toward l-tyrosine was found to be 4.9-fold in comparison with that of non-mutated FgaPT2, whereas the activity toward l-tryptophan was less than 0.4% of that of the wild-type. To the best of our knowledge, this is the first report on an enzymatic C-prenylation of l-tyrosine as free amino acid and altering the substrate preference of a prenyltransferase by mutagenesis.

Ir-catalyzed reverse prenylation of 3-substituted indoles: total synthesis of (+)-aszonalenin and (-)-brevicompanine B

The selective reverse prenylation of 3-substituted-1H-indoles at C3 is described. The iridium-catalyzed reaction proceeds with high branched to linear selectivity (>20:1) for a variety of indoles. In addition, a diastereoselective reverse prenylation of tryptophan methyl ester is disclosed, and its synthetic utility is demonstrated in the synthesis of (+)-aszonalenin and (-)-brevicompanine B.

Regioselective prenylation of bromocarbazoles by palladium(0)-catalysed cross coupling – synthesis of O-methylsiamenol, O-methylmicromeline and carquinostatin A

We describe the regioselective prenylation of 3-bromo-carbazole by Pd(0)-catalysed cross coupling with a prenylstannane or a prenylboronate. The procedure is applied to the synthesis of bioactive carbazole alkaloids.

Catalytic mechanism of stereospecific formation of cis-configured prenylated pyrroloindoline diketopiperazines by indole prenyltransferases

Indole prenyltransferases AnaPT, CdpC3PT, and CdpNPT are known to catalyze the formation of prenylated pyrroloindoline diketopiperazines from tryptophan-containing cyclic dipeptides in one-step reactions. In this study, we investigated the different stereoselectivities of these enzymes toward all the stereoisomers of cyclo-Trp-Ala and cyclo-Trp-Pro. The stereoselectivities of AnaPT and CdpC3PT mainly depend on the configuration of the tryptophanyl moiety in the substrates, and they usually introduce the prenyl moiety from the opposite sides. CdpNPT showed lower stereoselectivity, and the structure of the second amino acid moiety in the substrates is important for the stereospecificity in its enzyme catalysis. Moreover, we determined the crystal structure of AnaPT in complex with thiolodiphosphate and compared it with the known structures of CdpNPT. Our results clearly revealed the presence of an indole binding mode that has so far not been characterized.

Rearrangements in the mechanisms of the indole alkaloid prenyltransferases

The indole prenyltransferases are a family of metal-independent enzymes that catalyze the transfer of a prenyl group from dimethylallyl diphosphate (DMAPP) onto the indole ring of a tryptophan residue. These enzymes are remarkable in their ability to direct the prenyl group in either a “normal” or “reverse” fashion to positions with markedly different nucleophilicity. The enzyme 4-dimethylallyltryptophan synthase (4-DMATS) prenylates the non-nucleophilic C-4 position of the indole ring in free tryptophan. Evidence is presented in support of a mechanism that involves initial ion pair formation followed by a reverse prenylation at the nucleophilic C-3 position. A Cope rearrangement then generates the C-4 normal prenylated intermediate and deprotonation rearomatizes the indole ring. The enzyme tryprostatin B synthase (FtmPT1) catalyzes the normal C-2 prenylation of the indole ring in brevianamide F (cyclo-L-Trp-L-Pro). It shares high structural homology with 4-DMATS, and evidence is presented in favor of an initial C-3 prenylation (either normal or reverse) followed by carbocation rearrangements to give product. The concept of a common intermediate that partitions to different products via rearrangements can help to explain how these evolutionarily related enzymes can prenylate different positions on the indole ring.

Potential rearrangements in the reaction catalyzed by the indole prenyltransferase FtmPT1

The indole prenyltransferase FtmPT1 catalyzes the C-2 normal prenylation of brevianamide F (cyclo-L-Trp-L-Pro) to give tryprostatin B. A previous structural analysis and studies with alternate substrates suggest that the reaction might not proceed through a direct C-2 attack, but could involve a C-3 prenylation followed by a rearrangement. In this work we investigated the reactivity of FtmPT1 with tryptophan, 5-hydroxybrevianamide, and 2-methylbrevianamide, and isolated products that had been reverse prenylated at C-3 and normal prenylated at N-1, C-3, or C-4. The formation of these products can be rationalized through mechanisms involving either an initial C-3 normal or C-3 reverse prenylation. In addition, we demonstrate that a C-3 reverse prenylated indole can undergo a nonenzymatic aza-Cope rearrangement at 37 °C to give an N-1 normal prenylated product. Together, these studies broaden the known product scope of this interesting catalyst and suggest that alternative mechanisms might be operating.