Copper-Catalyzed Preparation of γ-Alkylidenebutenolides and Isocoumarins under Mild Palladium-Free Conditions

Asymmetric Total Synthesis of Naturally Occurring (R)-2'-Methoxydihydroartemidin, (R)-(E)-3'-Hydroxyartemidin, and Its Structural Congeners: Method Optimization and Mechanistic Analysis

Asymmetric total synthesis of naturally occurring 3-substituted isocoumarins, 2'-methoxydihydroartemidin, (E)-3'-hydroxyartemidin, and two structural congeners is reported in this article. Enantiopure 3-alkylisocoumarin core present in those target molecules was accessed through an atom-economical and regiodefined 6-endo-dig cyclization from properly substituted 2-halobenzoic acids and chiral terminal alkynes through ligand-free Cu(I) catalysis. A detailed mechanistic investigation was carried out through control experiments, and density functional theory (DFT) analysis reveals that solvent directed a non-Sonogashira pathway through direct insertion of alkyne through a regiodefined way is operating. The synthesized 3-alkylisocoumarins are synthetically manipulated into various small molecular entities.

Synthesis and biological evaluation of natural Lachnophyllum methyl ester, Lachnophyllum lactone and their synthetic analogs

(2Z)-Lachnophyllum methyl ester and (4Z)-Lachnophyllum lactone were recently identified as major components in essential oils and extracts of Conyza bonariensis from Togo. Extended biological evaluation of these acetylenic compounds was however hampered by the reduced amounts isolated. A synthetic route was designed providing access to larger quantities of these two natural products as well as to original non-natural analogs with the prospect of exploring for the first time the structure-activity relationships in this series. Using LC/MS analysis, synthetic samples allowed confirming the presence of the two previously isolated natural products in plant extracts obtained by the accelerated solvent extraction technique. The nematocidal activity of the synthesized compounds confirmed the potency of the natural products, which remain the most active among all analogs tested. The synthesized compounds were also assessed against Leishmania infantum axenic amastigotes and the Mycobacterium tuberculosis H37Rv pathogenic strain. (2Z)-Lachnophyllum methyl ester, (4Z)-Lachnophyllum lactone and lactone analogs exhibited the strongest antileishmanial potency. As expected, a longer alkyl chain was necessary to observe significant antimycobacterial activity. The lactone analog bearing a C10 lipophilic appendage displayed the highest antimycobacterial potency. The notable activities of lactones, naturally occurring or analogs, either nematicidal, antileishmanial or antimycobacterial, were compared to their cytotoxicity for mammalian cells and revealed moderate selectivity index values. In this regard, the innocuous (2Z)-Lachnophyllum methyl ester and its analogs open up more promising perspectives for the discovery of bioactive agents to protect both agricultural crops and human health.

A Platform for the Synthesis of Oxidation Products of Bilirubin

Bilirubin is the principal product of heme catabolism. High concentrations of the pigment are neurotoxic, yet slightly elevated levels are beneficial. Being a potent antioxidant, oxidative transformations of bilirubin occur in vivo and lead to various oxidized fragments. The mechanisms of their formation, intrinsic biological activities, and potential roles in human pathophysiology are poorly understood. Degradation methods have been used to obtain samples of bilirubin oxidation products for research. Here, we report a complementary, fully synthetic method of preparation. Our strategy leverages repeating substitution patterns in the parent tetracyclic pigment. Functionalized ready-to-couple γ-lactone, γ-lactam, and pyrrole monocyclic building blocks were designed and efficiently synthesized. Subsequent modular combinations, supported by metal-catalyzed borylation and cross-coupling chemistries, translated into the concise assembly of the structurally diverse bilirubin oxidation products (BOXes, propentdyopents, and biopyrrins). The discovery of a new photoisomer of biopyrrin A named lumipyrrin is reported. Synthetic bilirubin oxidation products made available in sufficient purity and quantity will support future in vitro and in vivo investigations.

Copper-catalyzed deacetonative Sonogashira coupling

A convenient Pd- and phosphine-free protocol for assembling internal alkynes from tertiary propargyl alcohols and (het)aryl halides has been developed. The proposed tandem approach includes the base-promoted retro-Favorskii fragmentation followed by Cu-catalyzed C(sp)-C(sp²) cross-coupling. The use of inexpensive reagents (e.g. a catalyst, additives, a base, and a solvent) and good functional group tolerance make the procedure practical and cost-effective. The synthetic utility of the method was demonstrated by a smooth alkynylation of vinyl iodides derived from natural steroidal hormones.

Synthetic studies towards naturally occurring γ-(Z)/(E)-alkylidenebutenolides through bimetallic cascade cyclization and an adventitious photoisomerization method

A general and flexible visible light-induced photoisomerization method of γ-(Z)-alkylidenebutenolides to their corresponding E-components was reported in this article. Initially, a series of naturally occurring enantiopure γ-(Z)-alkylidenebutenolides was synthesized by employing a "Pd-Cu" bimetallic cascade cyclization protocol. In the later part, the synthesized γ-(Z)-alkylidenebutenolides were photoisomerized in the presence of a triplet photosensitizer to γ-(E)-alkylidenebutenolides in reasonably acceptable yields. Total synthesis of goniobutenolides, hygrophorones, ramariolide D, melodorinols/acetyl-melodorinols, versicolactones, and phomopsolidones was achieved by employing the developed methods.

Recent reports on the synthesis of γ-butenolide, γ-alkylidenebutenolide frameworks, and related natural products

γ-Butenolides are fundamental frameworks found in many naturally occurring compounds, and they exhibit tremendous biological activities. γ-Butenolides also have proven their potential as useful synthetic intermediates in the total synthesis of natural compounds. Over the years, many γ-butenolide natural products have been isolated, having exocyclic γ-δ unsaturation in their structure. These natural products are collectively referred to as γ-alkylidenebutenolides. Considering the different biological profiles and wide-ranging structural diversity of the optically active γ-butenolide, the development of synthetic strategies for assembling such challenging scaffolds has attracted significant attention from synthetic chemists in recent times. In this report, a brief discussion will be provided to address isolation, biogenesis, and current state-of-the-art synthetic protocols for such molecules. This report aims to focus on synthetic strategies for γ-butenolides from 2010-2020 with a particular emphasis on γ-alkylidenebutenolides and related molecules. Metal-mediated catalytic transformation and organocatalysis are the two main reaction types that have been widely used to access such molecules. Mechanistic considerations, enantioselective synthesis, and practical applications of the reported procedures are also taken into consideration.

An Approach for the Generation of γ-Propenylidene-γ-butenolides and Application to the Total Synthesis of Rubrolides

Design and synthesis of a new class of γ-butenolides, viz. β-aryl-γ-propenylidene-γ-butenolides, have been reported from β-aryl-Z-enoate propargylic alcohols in the presence of acid. Isolation of β-aryl-γ-propenylidene-γ-butenolides and their O¹⁸-isomer confirmed the intermediacy of the allenyl-lactonium ion as well as the cyclic-hemiacetal during the proposed mechanism. By utilizing the β-aryl-γ-methylenecyclohexenylidene-γ-butenolides as starting materials, a highly stereoselective and efficient approach has been developed for the syntheses of frameworks of rubrolide natural products. This strategy was further extended for the total synthesis of rubrolide E.

Palladium complexes with an annellated mesoionic carbene (MIC) ligand: catalytic sequential Sonogashira coupling/cyclization reaction for one-pot synthesis of benzofuran, indole, isocoumarin and isoquinolone derivatives

A Pd(ii) complex containing a mesoionic carbene and phosphine ligands is an efficient catalyst for tandem coupling/cyclization reaction.

Asymmetric total synthesis of cryptoconcatone I

Asymmetric total synthesis of the naturally occurring cryptoconcatone I, which possesses a γ-Z-butenolide framework, is described here for the first time. Asymmetric propargylation, E-selective cross metathesis, and regioselective reductive epoxide ring opening were employed to access the enantiopure terminal alkyne. Late stage bimetallic cascade cyclization of the alkyne with (Z)-3-bromoacrylic acid afforded the natural product in an efficient way.

Controllable construction of isoquinolinedione and isocoumarin scaffolds via RhIII-catalyzed C–H annulation of N-tosylbenzamides with diazo compounds

An efficient protocol for the synthesis of isoquinolinediones by Rh^III-catalyzed C–H activation/annulation/decarboxylation of N-tosylbenzamides with diazo compounds is reported.

When cobalt-mediated [2 + 2 + 2] cycloaddition reaction dares go astray: synthesis of unprecedented cobalt(iii)-complexes

The cobalt-mediated cycloaddition with γ-alkylidenebutenolide diverges dramatically from what is expected, leading to unprecedented polycyclic scaffold cobalt(iii)-complexes. The mechanism of this reaction has been studied by DFT calculations.

7-Step total synthesis of (+)-EBC-329: Photoisomerisation reveals new seco-casbane family member

The first seco-casbane, EBC-329, isolated from the Australian rainforest, was synthesised from (+)-2-carene in seven steps. This endeavour not only established the absolute stereochemical assignment as (8R,9S)-EBC-329, but also identified, via photoisomerisation, a new seco-casbane family member.

Asymmetric total synthesis of naturally occurring spirocyclic tetranorsesquiterpenoid lanceolactone A

Asymmetric total synthesis of naturally occurring γ-butenolide containing [4.4]spiro-tetrahydrofuran lanceolactone A has been reported in this present work. Bimetallic (“Pd–Cu”) cascade cyclization was the crucial reaction employed for the construction of the γ-butenolide framework of the natural product.

Ag2O nanoparticle-catalyzed substrate-controlled regioselectivities: direct access to 3-ylidenephthalides and isocoumarins

Herein, we disclose the first example of an efficient, silver oxide nanoparticle-catalyzed, direct regioselective synthesis of 3-ylidenephthalides 11–16 and isocoumarins 17–20via sonogashira type coupling followed by substrate-controlled 5-exo-dig or 6-endo-dig cyclization reaction, respectively. This one pot coupling involves reaction of substituted 2-halobenzoic acid with meta/para-substituted and ortho-substituted terminal alkynes, which proceeded in a regioselective manner resulting in the formation of 3-ylidenephthalides or isocoumarins, respectively, in excellent yields (up to 95%) with complete Z-selectivity. This protocol features relatively broad substrate scope, mild conditions, operational simplicity, and is favourable with aromatic/alicyclic terminal alkynes. The competition experiments and gram-scale synthesis further highlight the importance and versatility of the methodology. The proposed mechanistic pathways illustrate that the regioselectivity is substantially being controlled by the substituent(s) present on the acetylenic phenyl ring.

We report the first example of an efficient, Ag₂O nanoparticle-catalyzed, direct regioselective synthesis of 3-ylidenephthalides and isocoumarins via Sonogashira type coupling followed by substrate-controlled 5-exo-dig or 6-endo-dig cyclization reaction, respectively.

A general two-step one-pot synthesis process of ynones from α-keto acids and 1-iodoalkynes

A two-step one-pot synthesis process of ynones was developed by cycloaddition of α-keto acids and 1-iodoalkynes followed by a ring-opening reaction.

Divergent synthesis of 3,4-dihydrodibenzo[b,d]furan-1(2H)-ones and isocoumarins via additive-controlled chemoselective C–C or C–N bond cleavage

Rh(iii)-Catalyzed C–C/C–O bond formation between cyclic 2-diazo-1,3-diketones and salicylamides with additive-controlled chemoselectivity.

Gold-silver catalyzed straightforward one pot synthesis of pyrano[3,4-b]pyrrol-7(1H)-ones

Pyrano[3,4-b]pyrrol-7(1H)-one is a bicyclic structure that is rarely described in the literature but is found in numerous polycyclic natural products as lamellarins. This work presents a one-pot synthesis of pyrano[3,4-b]pyrrol-7(1H)-one substituted in the 2- and 5-position. The reaction proceeds via a one-pot two step 5-endo-dig and 6-endo-dig cyclization catalyzed by a cationic gold complex with high regioselectivity.

Ru-Catalysed synthesis of fused heterocycle-pyridinones and -pyrones

Heterocycle-pyridinones and heterocycle-pyranones have been prepared by Ru-catalysed oxidative coupling of N-unprotected primary heterocycle-amides and heterocycle-carboxylic acids with internal alkynes.

Achmatowicz Reaction and its Application in the Syntheses of Bioactive Molecules

Substituted pyranones and tetrahydropyrans are structural subunits of many bioactive natural products. Considerable efforts are devoted toward the chemical synthesis of these natural products due to their therapeutic potential as well as low natural abundance. These embedded pyranones and tetrahydropyran structural motifs have been the subject of synthetic interest over the years. While there are methods available for the syntheses of these subunits, there are issues related to regio and stereochemical outcomes, as well as versatility and compatibility of reaction conditions and functional group tolerance. The Achmatowicz reaction, an oxidative ring enlargement of furyl alcohol, was developed in the 1970s. The reaction provides a unique entry to a variety of pyranone derivatives from functionalized furanyl alcohols. These pyranones provide convenient access to substituted tetrahydropyran derivatives. This review outlines general approaches to the synthesis of tetrahydropyrans, covering general mechanistic aspects of the Achmatowicz reaction or rearrangement with an overview of the reagents utilized for the Achmatowicz reaction. The review then focuses on the synthesis of functionalized tetrahydropyrans and pyranones and their applications in the synthesis of natural products and medicinal agents.

Expeditious Synthesis of Isoquinolones and Isocoumarins with a Vinyl Borane as an Acetylene Equivalent

An innovative and simple expeditious synthesis of 3,4-unsubstituted isoquinolones and isocoumarins starting from safe and easy to handle two-carbon acetylene equivalent was developed. The synthetic potential of this new method was further demonstrated in the facile total synthesis of two naturally occurring alkaloids: corydaldine and doryanine.

Ruthenium-catalyzed decarbonylative addition reaction of anhydrides with alkynes: a facile synthesis of isocoumarins and α-pyrones

The first example of a ruthenium catalyzed decarbonylative addition reaction of anhydrides with disubstituted alkynes is reported.

First studies directed towards the diastereoselective synthesis of the BCD tricyclic core of brownin F

The BCD tricyclic core of brownin F was prepared in eight synthetic operations for the first time. Our synthesis features a diastereo-, chemo- and regioselective intramolecular [3 + 2] cycloaddition between a cyclic carbonyl ylide and a γ-alkylidenebutenolide.