One-Pot Reaction Sequence: N-Acylation/Pictet-Spengler Reaction/Intramolecular [4 + 2] Cycloaddition/Aromatization in the Synthesis of β-Carboline Alkaloid Analogues

One-pot synthesis of tetrahydro-β-carbolines, fused with an isoindole core, was proposed starting from maleic anhydride and azomethines easily available from tryptamines and 3-(hetaryl)acroleins. This sequence includes four key steps: an acylation of the aldimine with maleic anhydride, a Pictet-Spengler cyclization, an intramolecular Diels-Alder reaction, and a concluding [1,3]-H shift. As a result, six- or seven-nuclear alkaloid-like heterocyclic systems, containing a benzo[1,2]indolizino[8,7-b]indole fragment annulated with furan, thiophene, or pyrrole, are formed in a diastereoselective manner.

Studies on the total syntheses of β‑carboline alkaloids orthoscuticellines A and B

Orthoscuticellines A and B are newly isolated natural β-carboline alkaloids from the moss animal Orthoscuticella ventricosa. Herein, we report the first targeted total synthesis of orthoscuticelline B and an analogous synthetic method for the preparation of dihydro derivate of orthoscuticelline A. The new synthetic approach is based on commercially available and inexpensive reagents leading to a practical synthesis of the target molecules. The reaction sequence consisting of a T3P®-catalyzed amide formation followed by a Bischler-Napieralski cyclisation and a DDQ-assisted dehydrogenation step ensures a practical access to orthoscuticelline B in three steps with 58% overall yield.

Thiazole-5-carbaldehyde Synthesis by Cascade Annulation of Enaminones and KSCN with Dess-Martin Periodinane Reagent

The Dess-Martin periodinane (DMP) reagent-mediated reactions of tertiary enaminones with potassium thiocyanate for the synthesis of thiazole-5-carbaldehydes are developed. The product formation involves cascade hydroxyl thiocyanation of the C═C double bond, intramolecular hydroamination of the C≡N bond, and thiazole annulation by condensation on the ketone carbonyl site, representing novel reaction pathways in the reactions between enaminones and thiocyanate salt. DMP plays dual roles in mediating the free radical thiocyanation and inducing the unconventional selective thiazole-5-carbaldehyde formation by masking the in situ generated formyl group during the reaction process.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Design and synthesis of β-carboline and combretastatin derivatives as anti-neutrophilic inflammatory agents

A series of β-carboline derivatives was synthesized by the Pictet-Spengler reaction with or without the combretastatin skeleton. The structures of these derivatives were elucidated by spectroscopic techniques. All synthesized compounds were evaluated for their anti-inflammatory activity in human neutrophils. Among them, two compounds, NTU-228 and HK-72, showed significant inhibitory effects on N-formyl-Met-Leu-Phe (fMLF)-induced superoxide anion generation in human neutrophils with IC₅₀ values of 5.58 ± 0.56 and 2.81 ± 0.07 μM, respectively. Neither NTU-228 nor HK-72 caused cytotoxicity in human neutrophils. NTU-228 inhibited the phosphorylation of p38 mitogen-activated protein kinase (MAPK) and intracellular Ca²⁺ levels ([Ca²⁺]_i) in fMLF-activated human neutrophils. Additionally, HK-72 selectively inhibited the fMLF-induced phosphorylation of p38 and [Ca²⁺]_i in human neutrophils. Molecular docking analysis showed a favorable binding affinity of HK-72 toward p38 MAPK. The proposed synthetic strategy opens up new opportunities for the synthesis of novel potential candidates against neutrophilic inflammation.

Synthesis of aza-quaternary centers via Pictet-Spengler reactions of ketonitrones

Despite the array of advances that have been made in Pictet-Spengler chemistry, particularly as it relates to the synthesis of β-carboline derivatives of both natural and designed origin, the ability to use such reactions to generate aza-quaternary centers remains limited. Herein, we report a simple procedure that enables the synthesis of a variety of such products by harnessing the distinct reactivity profiles of ketonitrones as activated by commercially available acyl chlorides. Notably, the reaction process is mild, fast, and high-yielding (54-97%) for a diverse collection of substrates, including some typically challenging ones, such as indole cores with electron-deficient substituents. In addition, by deploying an acyl bromide in combination with a thiourea promoter, a catalytic, asymmetric version has been established, leading to good levels of enantioselectivity (up to 83% ee) for several ketonitrones. Finally, the resultant N-O bonds within the products can also be functionalized in several unique ways, affording valuable complementarity to existing Pictet-Spengler variants based on the use of imines.

Recent evolution on synthesis strategies and anti-leishmanial activity of β-carboline derivatives - An update

Leishmaniasis is the most widespread pathogenic disease in several countries. Currently, no effective vaccines are available, and the control of Leishmaniasis primarily relies on decade-old chemotherapy. The treatment for the Leishmaniasis is not up to the mark. Current therapy for Leishmaniasis is ancient and requires hospitalization for the administration. These medications are also highly toxic and resistant. β-carboline, a natural indole containing alkaloid, holds a vital position in the field of medicinal chemistry with a diversified pharmacological action. The current review focuses mainly on the anti-leishmanial effects of β-carboline analogs and their synthetic strategies, structural activity relationship studies (SAR). The past ten years alterations unveiled by β-carboline analogs present in phytoconstituents and various derivatives of synthesized analogs with the mechanism of action were briefly shortlisted and illustrated.

Tricyclic Imidazolidin-4-ones by Witkop Oxidation of Tetrahydro-β-carbolines

1-Substituted and 1,1-disubstituted tetrahydro-β-carbolines undergo sodium periodate oxidative ring expansion in the presence of formaldehyde and other aldehydes to form 5,6-dihydro-7H-1,4-methanobenzo[e][1,4]diazonine-2,7(3H)-diones in 30-81% yield. In most cases, the reaction to form this new 6/8/5-tricyclic ring system proceeds with high diastereoselectivity. These benzannulated medium-ring keto imidazolidin-4-ones expand the menu of tetrahydro-β-carboline oxidation products.

The Pictet-Spengler Reaction Updates Its Habits

The Pictet-Spengler reaction (P-S) is one of the most direct, efficient, and variable synthetic method for the construction of privileged pharmacophores such as tetrahydro-isoquinolines (THIQs), tetrahydro-β-carbolines (THBCs), and polyheterocyclic frameworks. In the lustro (five-year period) following its centenary birthday, the P-S reaction did not exit the stage but it came up again on limelight with new features. This review focuses on the interesting results achieved in this period (2011-2015), analyzing the versatility of this reaction. Classic P-S was reported in the total synthesis of complex alkaloids, in combination with chiral catalysts as well as for the generation of libraries of compounds in medicinal chemistry. The P-S has been used also in tandem reactions, with the sequences including ring closing metathesis, isomerization, Michael addition, and Gold- or Brønsted acid-catalyzed N-acyliminium cyclization. Moreover, the combination of P-S reaction with Ugi multicomponent reaction has been exploited for the construction of highly complex polycyclic architectures in few steps and high yields. The P-S reaction has also been successfully employed in solid-phase synthesis, affording products with different structures, including peptidomimetics, synthetic heterocycles, and natural compounds. Finally, the enzymatic version of P-S has been reported for biosynthesis, biotransformations, and bioconjugations.

Selective construction of alkaloid scaffolds by alcohol-based direct and mild aerobic oxidative Pictet–Spengler reactions

Tetrahydro-β-carboline and β-carboline alkaloid scaffolds can be selectively obtained by direct aerobic oxidative Pictet–Spengler reactions of tryptamines with alcohols using TBN/TEMPO as the catalysts and oxygen as the oxidant under mild conditions.

A Unified Strategy for the Synthesis of β-Carbolines, γ-Carbolines, and Other Fused Azaheteroaromatics under Mild, Metal-Free Conditions

An efficient, unified approach for the synthesis of β-carbolines, γ-carbolines, and other fused azaheteroaromatics has been realized under metal-free conditions, from propargylic amines and (hetero)aromatic aldehydes. This unified strategy provides β- and γ-carbolines as well as a range of fused azaheteroaromatics with a broad substrate scope and excellent functional group compatibility. The formal synthesis of oxopropalines D and G has been achieved on gram scale (3a), in a one-pot reaction from commercially available materials (previous shortest reported route to 3a was 5 steps). NMR studies of the conversion of imine intermediate 3aa to β-carboline 3a were conducted and revealed that the reaction proceeded through an allene intermediate.

Enzyme catalysed Pictet-Spengler formation of chiral 1,1'-disubstituted- and spiro-tetrahydroisoquinolines

The Pictet–Spengler reaction (PSR) involves the condensation and ring closure between a β-arylethylamine and a carbonyl compound. The combination of dopamine and ketones in a PSR leads to the formation of 1,1′-disubstituted tetrahydroisoquinolines (THIQs), structures that are challenging to synthesize and yet are present in a number of bioactive natural products and synthetic pharmaceuticals. Here we have discovered that norcoclaurine synthase from Thalictrum flavum (TfNCS) can catalyse the PSR between dopamine and unactivated ketones, thus facilitating the facile biocatalytic generation of 1,1′-disubstituted THIQs. Variants of TfNCS showing improved conversions have been identified and used to synthesize novel chiral 1,1′-disubstituted and spiro-THIQs. Enzyme catalysed PSRs with unactivated ketones are unprecedented, and, furthermore, there are no equivalent stereoselective chemical methods for these transformations. This discovery advances the utility of enzymes for the generation of diverse THIQs in vitro and in vivo.

The Pictet-Spengler condensation of β-arylethylamine and carbonyl compounds is an important step in the synthesis of bioactive alkaloids. Here, the authors report a Pictet-Spengler reaction between dopamine and unactivated ketones catalysed by norcoclaurine synthase and its engineered variants.

Fluorinated alcohol-mediated [4 + 3] cycloaddition reaction of indolyl alcohols with cyclopentadiene

This paper describes a catalyst- and additive-free, [4 + 3] cycloaddition reaction of 3-indolylmethanols with cyclopentadiene in HFIP, producing cyclohepta[b]indole derivatives in high yields and with wide substrate scope.

Intramolecular Friedel-Crafts Acylation Reaction Promoted by 1,1,1,3,3,3-Hexafluoro-2-propanol

Simple dissolution of an arylalkyl acid chloride in 1,1,1,3,3,3-hexafluoro-2-propanol promotes an intramolecular Friedel-Crafts acylation without additional catalysts or reagents. This reaction is operationally trivial in both execution and product isolation (only requiring concentration followed by purification) and accommodates a broad range of substrates. Preliminary studies that bear upon potential reaction mechanisms are reported.