Total Synthesis of (−)-Agelastatin A

Applications of Clauson-Kaas Reaction in Organic Synthesis

Pyrrole-embedded organic molecules received a considerable importance due to their numerous biological and material applications. Hence, several synthetic strategies have been devised for the construction of diverse pyrrole analogues over the years. Among these, the Clauson-Kaas reaction is one of the most widely used protocols for the synthesis of various N-substituted pyrroles. This review briefly describes the Clauson-Kaas reaction along with modifications and a detailed account on its applications in the various sectors of organic synthesis.

Clauson-Kaas pyrrole synthesis using diverse catalysts: a transition from conventional to greener approach

Pyrrole is an important aromatic heterocyclic scaffold found in many natural products and predominantly used in pharmaceuticals. Continuous efforts are being made to design and synthesize various pyrrole derivatives using different synthetic procedures. Among them, the Clauson-Kaas reaction is a very old and well-known method for synthesizing a large number of N-substituted pyrroles. In recent years, due to global warming and environmental concern, research laboratories and pharmaceutical industries around the world are searching for more environmentally friendly reaction conditions for synthesizing compounds. As a result, this review describes the use of various eco-friendly greener protocols to synthesize N-substituted pyrroles. This synthesis involves the reaction of various aliphatic/aromatic primary amines, and sulfonyl primary amines with 2,5-dimethoxytetrahydrofuran in the presence of numerous acid catalysts and transition metal catalysts. The goal of this review is to summarize the synthesis of various N-substituted pyrrole derivatives using a modified Clauson-Kaas reaction under diverse conventional and greener reaction conditions.

Electrophilic Aminating Agents in Total Synthesis

Classical amination methods involve the reaction of a nitrogen nucleophile with an electrophilic carbon center; however, in recent years, umpoled strategies have gained traction where the nitrogen source acts as an electrophile. A wide range of electrophilic aminating agents are now available, and these underpin a range of powerful C-N bond-forming processes. In this Review, we highlight the strategic use of electrophilic aminating agents in total synthesis.

Pyrrole carboxamide introduction in the total synthesis of pyrrole-imidazole alkaloids

In this review various strategies for the incorporation of the signature pyrrole carboxamide moiety in the total syntheses of pyrrole-imidazole alkaloids (PIA) are discussed. These so-called oroidin alkaloids have a broad range of biological activities and display interesting skeletal diversity and complexity. These alkaloids are sponge-derived secondary metabolites and thus far more than 200 members of the PIA family have been isolated over the past few decades. Methods range from classical amide bond forming processes to non-traditional bond formation including the de novo synthesis of the pyrrole itself.

First synthesis of merged hybrids phosphorylated azirino[2,1-b]benzo[e][1,3]oxazine derivatives as anticancer agents

This work describes a straightforward diastereoselective synthetic access to azirino[2,1-b]benzo[e][1,3]oxazines containing phosphorus substituents such as phosphonate or phosphine oxide, by means of nucleophilic addition of functionalized phenols to the C-N double bond of 2H-azirine derivatives. In addition, the cytotoxic effect on cell lines derived from human lung adenocarcinoma (A549) and human embryonic kidney (HEK293) was also screened. Some azirino[2,1-b]benzo[e][1,3]oxazines 4 and 6 exhibited very good activity against the A549 cell line in vitro. Furthermore, selectivity towards cancer cell (A549) over (HEK293), and non-malignant cells (MCR-5) has been detected.

Visceral Adipose Tissue Drives Cardiac Aging Through Modulation of Fibroblast Senescence by Osteopontin Production

BACKGROUND

Aging induces cardiac structural and functional changes linked to the increased deposition of extracellular matrix proteins, including OPN (osteopontin), conducing to progressive interstitial fibrosis. Although OPN is involved in various pathological conditions, its role in myocardial aging remains unknown.

METHODS

OPN deficient mice (OPN-/-) with their wild-type (WT) littermates were evaluated at 2 and 14 months of age in terms of cardiac structure, function, histology and key molecular markers. OPN expression was determined by reverse-transcription polymerase chain reaction, immunoblot and immunofluorescence. Luminex assays were performed to screen plasma samples for various cytokines/adipokines in addition to OPN. Similar explorations were conducted in aged WT mice after surgical removal of visceral adipose tissue (VAT) or treatment with a small-molecule OPN inhibitor agelastatin A. Primary WT fibroblasts were incubated with plasma from aged WT and OPN-/- mice, and evaluated for senescence (senescence-associated β-galactosidase and p16), as well as fibroblast activation markers (Acta2 and Fn1).

RESULTS

Plasma OPN levels increased in WT mice during aging, with VAT showing the strongest OPN induction contrasting with myocardium that did not express OPN. VAT removal in aged WT mice restored cardiac function and decreased myocardial fibrosis in addition to a substantial reduction of circulating OPN and transforming growth factor β levels. OPN deficiency provided a comparable protection against age-related cardiac fibrosis and dysfunction. Intriguingly, a strong induction of senescence in cardiac fibroblasts was observed in both VAT removal and OPN-/- mice. The addition of plasma from aged OPN-/- mice to cultures of primary cardiac fibroblasts induced senescence and reduced their activation (compared to aged WT plasma). Finally, Agelastatin A treatment of aged WT mice fully reversed age-related myocardial fibrosis and dysfunction.

CONCLUSIONS

During aging, VAT represents the main source of OPN and alters heart structure and function via its profibrotic secretome. As a proof-of-concept, interventions targeting OPN, such as VAT removal and OPN deficiency, rescued the heart and induced a selective modulation of fibroblast senescence. Our work uncovers OPN's role in the context of myocardial aging and proposes OPN as a potential new therapeutic target for a healthy cardiac aging.

Enantioselective PCCP Brønsted acid-catalyzed aza-Piancatelli rearrangement

An enantioselective aza-Piancatelli rearrangement has been developed using a chiral Brønsted acid based on pentacarboxycyclopentadiene (PCCP). This reaction provides rapid access to valuable chiral 4-amino-2-cyclopentenone building blocks from readily available starting material and is operationally simple.

Ruthenium-catalyzed ring-opening reaction of a 3-aza-2-oxabicyclo[2.2.1]hept-5-ene with amines — an unexpected mode of ring-opening

The ruthenium-catalyzed ring-opening reaction of a 3-aza-2-oxabicyclo[2.2.1]hept-5-ene with amines was investigated. In the presence of an amine and a ruthenium catalyst, the N–O bond is cleaved, forming a ring-opened allylic alcohol product. Through a possible ruthenium-catalyzed redox isomerization, the allylic alcohol is transformed into a saturated carbonyl product. This unexpected mode of ruthenium-catalyzed ring-opening reaction of a 3-aza-2-oxabicyclo[2.2.1]hept-5-ene leads to the formation of a cyclopentanone derivative in moderate yields.

Total synthesis of (−)-agelastatin A: an SH2′ radical azidation strategy

A new synthetic approach to (−)-agelastatin A has been established through the strategic implementation of brominative olefin transposition of a silyl enol ether and subsequent S_H2′ radical azidation of the resultant allylic bromide.

Asymmetric Radical Bicyclization of Allyl Azidoformates via Cobalt(II)-Based Metalloradical Catalysis

Cobalt(II)-based metalloradical catalysis has been successfully applied to radical bicyclization of allyl azidoformates to construct aziridine/oxazolidinone-fused bicyclic structures. The Co(II) complex of D2-symmetric chiral amidoporphyrin 3,5-DitBu-QingPhyrin has been identified as an effective metalloradical catalyst for the intramolecular radical aziridination of this type of carbonyl azides, allowing for high-yielding formation of synthetically useful chiral [3.1.0]-bicyclic aziridines with high diastereo- and enantioselectivity.

Compounds, compositions, and methods of agelastatin alkaloids: patent evaluation of WO2015042239 (A1)

Agelastatins are a family of tetracyclic alkaloids isolated from marine sponges. The patent examined in this publication covers the chemical synthesis of agelastatins A to F and eight analogues and their therapeutic use against hematologic malignancies. The claim on the chemical synthesis features a novel biomimetic cyclization of a tricyclic precursor, which streamlines scalable access to agelastatins and their analogues. This new synthetic approach can potentially expedite the research on these compounds for therapeutic use against cancers by making them more easily available. The claim on the therapeutic use against hematologic malignancies is based on the in vitro cytotoxicity against a limited number of cell lines and can be further strengthened by in vivo therapeutic evaluations focusing on specific hematologic malignancies. The comparative evaluation of the cytotoxicities of the natural alkaloids described in the application greatly enhances the understanding of their structure-activity relationships (SARs) relevant to the development of novel medicinal leads. Overall, the patent application is strong and has the potential to advance the rapidly expanding agelastatin research.

Synthesis of α-C-Galactosylceramide via Diastereoselective Aziridination: The New Immunostimulant 4'-epi-C-Glycoside of KRN7000

A new immunostimulant, the 4'-epimer of α-C-GalCer, was synthesized from a C2-symmetric dienediol and α-C-allyl galactoside. The intramolecular aziridination and the following reductive ring opening provided the core of the aliphatic amino alcohol with excellent regio- and stereocontrol. The new immunostimulants 3d and 3e gave a better polarized Th1-type cytokine response in murine NKT cells than the benchmarked α-C-GalCer.

Syntheses of Cyclic Guanidine-Containing Natural Products

Naturally occurring guanidine derivatives frequently display medicinally useful properties. Among them, the higher order pyrrole-imidazole alkaloids, the dragmacidins, the crambescidins/batzelladines, and the saxitoxins/tetradotoxins have stimulated the development of many new synthetic methods over the past decades. We provide here an overview of the syntheses of these cyclic guanidine-containing natural products.

Amant AH, Hein JE, Read de Alaniz J. Cascade rearrangement of furylcarbinols with hydroxylamines: practical access to densely functionalized cyclopentane derivatives

The aza-Piancatelli rearrangement with hydroxylamines to 4-aminocyclopentenones is described. Subsequent transformations highlight the versatility of the cyclopentene scaffold and the value of the hydroxylamine in this transformation.

Asymmetric synthesis of cis-aminocyclopentenols, building blocks for medicinal chemistry

A highly efficient one-pot multistep process involving an asymmetric Pd(II)-catalyzed Overman rearrangement and a Ru(II)-catalyzed ring-closing metathesis reaction has been developed for the preparation of (R)- or (S)-aminocyclopenta-2-enes. The rapid strategy employed and the relatively mild conditions of the one-pot process allowed the multigram synthesis of the carbocycles in high enantiomeric excess (92% ee). The synthetic utility of these compounds was demonstrated by the stereoselective incorporation of hydroxyl groups, generating cis-4- and cis-5-aminocyclopenta-2-en-1-ols, important building blocks for medicinal chemistry.

Dimeric pyrrole-imidazole alkaloids: synthetic approaches and biosynthetic hypotheses

The pyrrole-imidazole alkaloids are a group of structurally unique and biologically interesting marine sponge metabolites. Among them, the cyclic dimers have caught synthetic chemists' attention particularly. Numerous synthetic strategies have been developed and various biosynthetic hypotheses have been proposed for these fascinating natural products. We discuss herein the synthetic approaches and the biosynthetic insights obtained from these studies.

Endeavors to access molecular complexity: strategic use of free radicals in natural product synthesis

Free radicals, which in the past were considered unruly chemical species, have become manageable and indispensable for synthetic organic chemistry. The unique nature of free radicals has allowed practitioners in organic synthesis to design flexible approaches to produce various materials ranging from small molecules to polymers. The present Personal Account describes the author's endeavors to create molecular complexity by the strategic use of free radicals, with an emphasis on the synthesis of bioactive natural products.

Synthesis and anticancer activity of all known (-)-agelastatin alkaloids

The full details of our enantioselective total syntheses of (-)-agelastatins A-F (1-6), the evolution of a new methodology for synthesis of substituted azaheterocycles, and the first side-by-side evaluation of all known (-)-agelastatin alkaloids against nine human cancer cell lines are described. Our concise synthesis of these alkaloids exploits the intrinsic chemistry of plausible biosynthetic precursors and capitalizes on a late-stage synthesis of the C-ring. The critical copper-mediated cross-coupling reaction was expanded to include guanidine-based systems, offering a versatile preparation of substituted imidazoles. The direct comparison of the anticancer activity of all naturally occurring (-)-agelastatins in addition to eight advanced synthetic intermediates enabled a systematic analysis of the structure-activity relationship within the natural series. Significantly, (-)-agelastatin A (1) is highly potent against six blood cancer cell lines (20-190 nM) without affecting normal red blood cells (>333 μM). (-)-Agelastatin A (1) and (-)-agelastatin D (4), the two most potent members of this family, induce dose-dependent apoptosis and arrest cells in the G2/M-phase of the cell cycle; however, using confocal microscopy, we have determined that neither alkaloid affects tubulin dynamics within cells.

Formal synthesis of (-)-agelastatin A: an iron(II)-mediated cyclization strategy

An iron(II)-mediated aminohalogenation of a cyclopentenyl N-tosyloxycarbamate provided new access to the key intermediate for the synthesis of (-)-agelastatin A (AA, 1), a potent antiproliferative alkaloid. The present synthetic endeavour offered an insight into the mechanism underlying the iron(II)-mediated aminohalogenation of N-tosyloxycarbamate, in which the radical properties of the N-iron intermediates in the redox states were operative.

Bioinspired total synthesis of agelastatin A

A one-two punch: two potentially biosynthetically relevant cyclizations of a keramadine analogue give agelastatin A. A diastereoselective C-ring formation, which proceeds through a 5-exo-trig cyclization or a Nazarov cyclization of a red-colored N-acyliminium intermediate, generates the three contiguous stereocenters of the cyclopentane core. A silica gel assisted cyclization of a nagelamide J analogue gives agelastatin A.

Stereocontrolled synthesis of vicinal diamines by organocatalytic asymmetric Mannich reaction of N-protected aminoacetaldehydes: formal synthesis of (-)-agelastatin A

The 1,2-diamine (vicinal diamine) motif is present in a number of natural products with interesting biological activity and in many chiral molecular catalysts. The efficient and stereocontrolled synthesis of enantioenriched vicinal diamines is still a challenge to modern chemical methodology. We report here both syn- and anti-selective asymmetric direct Mannich reactions of N-protected aminoacetaldehydes with N-Boc-protected imines catalyzed by proline and the axially chiral amino sulfonamide (S)-3. This organocatalytic process represents the first example of a Mannich reaction using Z- or Boc-protected aminoacetaldehyde as a new entry of α-nitrogen functionalized aldehyde nucleophile in enamine catalysis. The obtained optically active vicinal diamines are useful chiral synthons as exemplified by the formal synthesis of (-)-agelastatin A.

Total syntheses of (-)-hanishin, (-)-longamide B, and (-)-longamide B methyl ester via a novel preparation of N-substituted pyrrole-2-carboxylates [corrected]

A novel preparation of N-substituted pyrrole-2-carboxylates has been developed based upon 1,3-dipolar cycloaddition and a conventional hydrogenolysis. By using this method as the key step, total syntheses of natural alkaloids (-)-hanishin, (-)-longamide [corrected] B, and (-)-longamide [corrected] B methyl ester were accomplished in the highest overall yields, respectively.

Biosynthesis, asymmetric synthesis, and pharmacology, including cellular targets, of the pyrrole-2-aminoimidazole marine alkaloids

The pyrrole-2-aminoimidazole (P-2-AI) alkaloids are a growing family of marine alkaloids, now numbering well over 150 members, with high topographical and biological information content. Their intriguing structural complexity, rich and compact stereochemical content, high N to C ratio (~1 : 2), and increasingly studied biological activities are attracting a growing number of researchers from numerous disciplines world-wide. This review surveys advances in this area with a focus on the structural diversity, biosynthetic hypotheses with increasing, but still rare, verifying experimental studies, asymmetric syntheses, and biological studies, including cellular target receptor isolation studies, of this stimulating and exciting alkaloid family.