Iodine-catalyzed chemoselective dehydrogenation and aromatization of tetrahydro-β-carbolines: A short synthesis of Kumujian-C, Eudistomin-U, Norharmane, Harmane Harmalan and Isoeudistomine-M

Chilean papaya (Vasconcellea pubescens A. DC.) residues as a source of bioactive compounds: Chemical composition, antioxidant capacity, and antiglycation effects

The Chilean papaya (Vasconcellea pubescens A.DC.) is a climacteric fruit that grows in the north and center of Chile. During its processing, residues formed mainly by mucilage and seeds are produced and mostly discarded, despite being a potential source of bioactive metabolites. This work aimed to apply untargeted metabolic analysis by HPLC-DAD-QToF to study the chemical composition of ethyl acetate and methanol extracts from Chilean papaya residues and evaluate their antioxidant and antiglycation capacities. Twenty-three metabolites were tentatively identified in papaya residues, including one carboxylic acid, one glycosylated hydroquinone, four flavan-3-ols, three proanthocyanidins, twelve glycosylated flavonols, one carbohydrate, and one alkaloid reported for the first time. The antioxidant capacity measured as the scavenging of DPPH• and ABTS•+ radicals was comparable with that of ascorbic acid. Chilean papaya extracts decreased fluorescent Advanced Glycation End (AGE) products and oxidative modifications in proteins induced by glucose.

Divergent total syntheses of ITHQ-type bis-β-carboline alkaloids by regio-selective formal aza-[4 + 2] cycloaddition and late-stage C-H functionalization

We herein report the first total syntheses of several bis-β-carboline alkaloids, picrasidines G, S, R, and T, and natural product-like derivatives in a divergent manner. Picrasidines G, S, and T feature an indolotetrahydroquinolizinium (ITHQ) skeleton, while picrasidine R possesses a 1,4-diketone linker between two β-carboline fragments. The synthesis of ITHQ-type bis-β-carboline alkaloids could be directly achieved by a late-stage regio-selective aza-[4 + 2] cycloaddition of vinyl β-carboline alkaloids, suggesting that this remarkable aza-[4 + 2] cycloaddition might be involved in the biosynthetic pathway. Computational studies revealed that such aza-[4 + 2] cycloaddition is a stepwise process and explained the unique regioselectivity (ΔΔG = 3.77 kcal mol-1). Moreover, the successful application of iridium-catalyzed C-H borylation on β-carboline substrates enabled the site-selective C-8 functionalization for efficient synthesis and structural diversification of this family of natural products. Finally, concise synthesis of picrasidine R by the thiazolium-catalyzed Stetter reaction was also accomplished.

N-Iodosuccinimide-Mediated Synthesis of Benzo-Fused Bisimidazoles Enabled by a One-Pot Tandem Reaction of Fluorinated Propargyl Amidines

A N-iodosuccinimide (NIS)-mediated divergent and efficient tandem reaction between fluorinated propargyl amidines and aromatic o-diamines without any metal catalyst and additive under mild reaction conditions was developed for the synthesis of benzo-fused bisimidazoles in moderate to excellent yields. Preliminary mechanistic studies suggested that this reaction proceeded by an intermediate of secondary amine derived from 5-iodomethyl imidazole, and NIS played another role of oxidation reagent to promote the formation of a benzimidazole motif.

Comprehensive Metabolomic Comparison of Five Cereal Vinegars Using Non-Targeted and Chemical Isotope Labeling LC-MS Analysis

Vinegar is used as an acidic condiment and preservative worldwide. In Asia, various black vinegars are made from different combinations of grains, such as Sichuan bran vinegar (SBV), Shanxi aged vinegar (SAV), Zhenjiang aromatic vinegar (ZAV), and Fujian Monascus vinegar (FMV) in China and Ehime black vinegar in Japan (JBV). Understanding the chemical compositions of different vinegars can provide information about nutritional values and the quality of the taste. This study investigated the vinegar metabolome using a combination of GC-MS, conventional LC-MS, and chemical isotope labeling LC-MS. Different types of vinegar contained different metabolites and concentrations. Amino acids and organic acids were found to be the main components. Tetrahydroharman-3-carboxylic acid and harmalan were identified first in vinegar. Various diketopiperazines and linear dipeptides contributing to different taste effects were also detected first in vinegar. Dipeptides, 3-phenyllactic acid, and tyrosine were found to be potential metabolic markers for differentiating vinegars. The differently expressed pathway between Chinese and Japanese vinegar was tryptophan metabolism, while the main difference within Chinese vinegars was aminoacyl-tRNA biosynthesis metabolism. These results not only give insights into the metabolites in famous types of cereal vinegar but also provide valuable knowledge for making vinegar with desirable health characteristics.

Progress toward a biomimetic synthesis of pegaharmaline A

Efforts towards a biomimetic synthesis of the alkaloid pegaharmaline A began with attempted validation of the putative biosynthesis described in the isolation report. The reaction between vasicinone-derived pyrroloquinazoline 1 and tryptamines 2 and 9 proceeded under aqueous conditions at ambient temperature, forming the 1,6,10-triazaspiro[4.5]dec-7-anes 7 and 8. Alternative pyrroloquinazoline precursors were subsequently investigated; the reaction between dehydrodimethylisovasicinone (10) and tryptamine (9) led to the ring-opened product 13 that could not be converted into pegaharmaline A scaffold under Bischler-Napieralski conditions. The Pictet-Spengler reaction between a model isovasicinone (22) and tryptamine (9) was successful, but the resulting tetrahydro-β-carboline could not be converted into the natural product. These studies suggest an alternative biosynthetic pathway is potentially operating, while structural revision of the natural product cannot be ruled out at this time. As vasicinones and tryptamines are widely distributed throughout Nature, the novel scaffolds reported herein may be undiscovered natural products.

β-Carboline-based molecular hybrids as anticancer agents: a brief sketch

Cancer is a huge burden on the healthcare system and is foremost cause of mortality across the globe. Among various therapeutic strategies, chemotherapy plays an enormous role in overcoming the challenges of treating cancer, especially in late stage detection. However, limitations such as extreme side/adverse effects and drug resistance associated with available drugs have impelled the development of novel chemotherapeutic agents. In this regard, we have reviewed the development of β-carboline-based chemotherapeutic agents reported in last five years. The review mainly emphasizes on the molecular hybrids of β-carbolines with various pharmacophores, their synthetic strategies, and in vitro anticancer evaluation. In addition, the mechanisms of action, in silico studies, structural influence on the potency and selectivity among diverse cancer cell lines have been critically presented. The review updates readers on the diverse molecular hybrids prepared and the governing structural features of high potential molecules that can help in the future development of novel cytotoxic agents.

Recent Advances in the Synthesis of β-Carboline Alkaloids

β-Carboline alkaloids are a remarkable family of natural and synthetic indole-containing heterocyclic compounds and they are widely distributed in nature. Recently, these alkaloids have been in the focus of interest, thanks to their diverse biological activities. Their pharmacological activity makes them desirable as sedative, anxiolytic, hypnotic, anticonvulsant, antitumor, antiviral, antiparasitic or antimicrobial drug candidates. The growing potential inherent in them encourages many researchers to address the challenges of the synthesis of natural products containing complex β-carboline frameworks. In this review, we describe the recent developments in the synthesis of β-carboline alkaloids and closely related derivatives through selected examples from the last 5 years. The focus is on the key steps with improved procedures and synthetic approaches. Furthermore the pharmacological potential of the alkaloids is also highlighted.

Access to 1-indolyltetrahydro-β-carbolines via metal-free cross-dehydrogenative coupling: the total synthesis of eudistomin U, isoeudistomin U and 19-bromoisoeudistomin U

A highly selective and captivating metal-free cross-dehydrogenative coupling for the cross-coupling of two reactive nucleophiles such as tetrahydro-β-carboline and indoles is developed. A series of 1-indolyltetrahydro-β-carboline derivatives were synthesized in excellent to moderate yields. Temperature, time and concentration control resulted in mono indolylation selectively. Moreover, the total synthesis of eudistomin U and isoeudistomin U and the first total synthesis of 19-bromoisoeudistomin U were accomplished.

Revisiting applications of molecular iodine in organic synthesis

Molecular iodine contributes significantly to organic transformations in synthetic organic chemistry. It works effectively due to its mild Lewis acidic character, ability as an oxidizing agent, good moisture stability, and easy availability.

Selectivity-tunable oxidation of tetrahydro-β-carboline over an OMS-2 composite catalyst: preparation and catalytic performance

Controlling the reaction selectivity of organic transformations without losing high conversion is always a challenge in catalytic processes. In this work, a H3PO4·12WO3/OMS-2 nanocomposite catalyst ([PW]-OMS-2) was prepared through the oxidation of a Mn(ii) salt with sodium phosphotungstate by KMnO4. Comprehensive characterization indicates that different Mn2+ precursors significantly affected the crystalline phase and morphology of the as-synthesized catalysts and only MnSO4·H2O as the precursor could lead to a cryptomelane phase. Moreover, [PW]-OMS-2 demonstrated excellent catalytic activity toward aerobic oxidative dehydrogenation of tetrahydro-β-carbolines due to mixed crystalline phases, enhanced surface areas, rich surface oxygen vacancies and labile lattice oxygen species. In particular, β-carbolines and 3,4-dihydro-β-carbolines could be obtained from tetrahydro-β-carbolines with very high selectivity (up to 99%) over [PW]-OMS-2 via tuning the reaction solvent and temperature. Under the present catalytic system, scalable synthesis of a β-carboline was achieved and the composite catalyst showed good stability and recyclability. This work not only clarified the structure-activity relationship of the catalyst, but also provided a practical pathway to achieve flexible, controllable synthesis of functional N-heterocycles.