Ruthenium-Catalyzed Cyclocarbonylation of Allenyl Alcohols and Amines: Selective Synthesis of Lactones and Lactams

Synthesis of Natural Products by C-H Functionalization of Heterocycless

Total synthesis is considered by many as the finest combination of art and science. During the last decades, several concepts were proposed for achieving the perfect vision of total synthesis, such as atom economy, step economy, or redox economy. In this context, C-H functionalization represents the most powerful platform that has emerged in the last years, empowering rapid synthesis of complex natural products and enabling diversification of bioactive scaffolds based on natural product architectures. In this review, we present an overview of the recent strategies towards the total synthesis of heterocyclic natural products enabled by C-H functionalization. Heterocycles represent the most common motifs in drug discovery and marketed drugs. The implementation of C-H functionalization of heterocycles enables novel tactics in the construction of core architectures, but also changes the logic design of retrosynthetic strategies and permits access to natural product scaffolds with novel and enhanced biological activities.

Palladium-Catalyzed Cascade Carbonylation to α,β-Unsaturated Piperidones via Selective Cleavage of Carbon-Carbon Triple Bonds

A direct and selective synthesis of α,β-unsaturated piperidones by a new palladium-catalyzed cascade carbonylation is described. In the presented protocol, easily available propargylic alcohols react with aliphatic amines to provide a broad variety of interesting heterocycles. Key to the success of this transformation is a remarkable catalytic cleavage of the present carbon-carbon triple bond by using a specific catalyst with 2-diphenylphosphinopyridine as ligand and appropriate reaction conditions. Mechanistic studies and control experiments revealed branched unsaturated acid 11 as crucial intermediate.

Unified Total Syntheses of Rhamnofolane, Tigliane, and Daphnane Diterpenoids

Rhamnofolane, tigliane, and daphnane diterpenoids are structurally complex natural products with multiple oxygen functionalities, making them synthetically challenging. While these diterpenoids share a 5/7/6-trans-fused ring system (ABC-ring), the three-carbon substitutions at the C13- and C14-positions on the C-ring and appending oxygen functional groups differ among them, accounting for the disparate biological activities of these natural products. Here, we developed a new, unified strategy for expeditious total syntheses of five representative members of these three families, crotophorbolone (1), langduin A (2), prostratin (3), resiniferatoxin (4), and tinyatoxin (5). Retrosynthetically, 1-5 were simplified into their common ABC-ring 6 by detaching the three-carbon units and the oxygen-appended groups. Intermediate 6 with six stereocenters was assembled from four achiral fragments in 12 steps by integrating three powerful transformations, as follows: (i) asymmetric Diels-Alder reaction to induce formation of the C-ring; (ii) π-allyl Stille coupling reaction to set the trisubstituted E-olefin of the B-ring; and (iii) Eu(fod)₃-promoted 7-endo cyclization of the B-ring via the generation of a bridgehead radical. Then 6 was diversified into 1-5 by selective installation of the different functional groups. Attachment of the C14-β-isopropenyl and isopropyl groups led to 1 and 2, respectively, while oxidative acetoxylation and C13,14-β-dimethylcyclopropane formation gave rise to 3. Finally, formation of an α-oriented caged orthoester by C13-stereochemical inversion and esterification with two different homovanillic acids delivered 4 and 5 with a C13-β-isopropenyl group. This unified synthetic route to 1-5 required only 16-20 total steps, demonstrating the exceptional efficiency of the present strategy.

Strategies for the construction of γ-spirocyclic butenolides in natural product synthesis

Over the last four decades, a number of γ-spirocyclic butenolide containing natural products, drugs, and medicinally useful synthetic compounds have been reported. In this review, we discuss diverse chemical approaches to synthesize γ-spiro butenolides and their application towards natural product synthesis. The collective perception of various methods may allow superior approaches capable of delivering efficient synthetic approaches to obtain γ-spiro butenolide comprising natural products and their hybrid analogues for further drug discovery and development.

Ruthenium-Catalyzed Cycloadditions to Form Five-, Six-, and Seven-Membered Rings

Ruthenium-catalyzed cycloadditions to form five-, six-, and seven-membered rings are summarized, including applications in natural product total synthesis. Content is organized by ring size and reaction type. Coverage is limited to processes that involve formation of at least one C-C bond. Processes that are stoichiometric in ruthenium or exploit ruthenium as a Lewis acid (without intervention of organometallic intermediates), ring formations that occur through dehydrogenative condensation-reduction, σ-bond activation-initiated annulations that do not result in net reduction of bond multiplicity, and photochemically promoted ruthenium-catalyzed cycloadditions are not covered.

Deciphering the Chameleonic Chemistry of Allenols: Breaking the Taboo of a Onetime Esoteric Functionality

The allene functionality has participated in one of the most exciting voyages in organic chemistry, from chemical curiosities to a recurring building block in modern organic chemistry. In the last decades, a special kind of allene, namely, allenol, has emerged. Allenols, formed by an allene moiety and a hydroxyl functional group with diverse connectivity, have become common building blocks for the synthesis of a wide range of structures and frequent motif in naturally occurring systems. The synergistic effect of the allene and hydroxyl functional groups enables allenols to be considered as a unique and sole functionality exhibiting a special reactivity. This Review summarizes the most significant contributions to the chemistry of allenols that appeared during the past decade, with emphasis on their synthesis, reactivity, and occurrence in natural products.

Yeast-driven whey biorefining to produce value-added aroma, flavor, and antioxidant compounds: technologies, challenges, and alternatives

Whey is a liquid residue generated during the production of cheese and yogurt. It has a pH between 3.9 and 5.6, and a high chemical oxygen demand (COD), from 60 to 80 g/L. Whey contains lactose, proteins, and minerals. Globally, approximately 50% of the whey generated is untreated and is released directly into the environment, which represents an environmental risk. To overcome whey management problems, conventional thermo-physical valorization treatments have been explored, which are complex, costly and energy-intensive. As an alternative, whey fermentation processes employing bacteria, fungi and yeast are economical and promising methods. Among them, yeast fermentation creates value-added products such as antimicrobials, biofuels, aromas, flavors, and antioxidants with no need for previous conditioning of the whey, such as hydrolysis of the lactose, prior to whey biorefining. The biorefining concept applied to whey is discussed using chemical and biological transformation pathways, showing their pluses and minuses, such as technical drawbacks. The main challenges and solutions for the production of fusel alcohols, specifically for 2-phenylethanol, are also discussed in this review.

Nanocrystalline ZnO: A Competent and Reusable Catalyst for the Preparation of Pharmacology Relevant Heterocycles in the Aqueous Medium

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Nanoparticle catalyzed synthesis is a green and convenient method to achieve most of the chemical transformations in water or other green solvents. Nanoparticle ensures an easy isolation process of catalyst as well as products from the reaction mixture avoiding the hectic work up procedure. Zinc oxide is a biocompatible, environmentally benign and economically viable nanocatalyst with effectivity comparable to the other metal nanocatalyst employed in several reaction strategies. This review mainly focuses on the recent applications of zinc oxide in the synthesis of biologically important heterocyclic molecules under sustainable reaction conditions.

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Application of zinc oxide in organic synthesis: Considering the achievable advantages of this nanocatalyst, presently several research groups are paying attention in anchoring zincoxide or its modified structure in several types of organic conversions e.g. multicomponent reactions, ligand-free coupling reactions, cycloaddition reaction, etc. The advantages and limitations of this nanocatalyst are also demonstrated. The present study aims to highlight the recent multifaceted applications of ZnO towards the synthesis of diverse heterocyclic motifs. Being a promising biocompatible nanoparticle, this catalyst has an important contribution in the fields of synthetic chemistry and medicinal chemistry.

Butenolide Synthesis from Functionalized Cyclopropenones

A general method to synthesize substituted butenolides from hydroxymethylcyclopropenones is reported. Functionalized cyclopropenones undergo ring-opening reactions with catalytic amounts of phosphine, forming reactive ketene ylides. These intermediates can be trapped by pendant hydroxy groups to afford target butenolide scaffolds. The reaction proceeds efficiently in diverse solvents and with low catalyst loadings. Importantly, the cyclization is tolerant of a broad range of functional groups, yielding a variety of α- and γ-substituted butenolides.

An efficient Nozaki-Hiyama allenylation promoted by the acid derived MIL-101 MOF

A concise synthesis of the sulfonic acid-containing MIL-101 MOF catalyst was reported using commercially available materials. A series of characterization of as-synthesized MIL-101-SO3H including SEM, XRD, FTIR, BET and TGA was also demonstrated. Using MIL-101-SO3H as a catalyst, an efficient Nozaki-Hiyama allenylation reaction was achieved to generate various polyfunctionalized α-allenic alcohols in high yield and good selectivity. Taking advantage of the high acidity of the MIL-101-SO3H MOF structure, such transformations were also achieved under mild reaction conditions and short reaction times. Based on our observed evidence during this study, a mechanism was proposed involving a substrate activation/γ-nucleophilic addition reaction sequence. In addition, the MIL-101-SO3H catalyst can be recycled ten times during the Nozaki-Hiyama allenylation reaction without compromising the yield and selectivity.

Total Synthesis, Biological Evaluation, and Target Identification of Rare Abies Sesquiterpenoids

Abiespiroside A (1), beshanzuenone C (2), and beshanzuenone D (3) belong to the Abies sesquiterpenoid family. Beshanzuenones C (2) and D (3) are isolated from the critically endangered Chinese fir tree species Abies beshanzuensis and demonstrated weak inhibiting activity against protein tyrosine phosphatase 1B (PTP1B). We describe herein the first total syntheses of these Abies sesquiterpenoids relying on the sustainable and inexpensive chiral pool molecule (+)-carvone. The syntheses feature a palladium-catalyzed hydrocarbonylative lactonization to install the 6,6-fused bicyclic ring system and a Dreiding-Schmidt reaction to build the oxaspirolactone moiety of these target molecules. Our chemical total syntheses of these Abies sesquiterpenoids have enabled (i) the validation of beshanzuenone C's weak PTP1B inhibiting potency, (ii) identification of new synthetic analogs with promising and selective protein tyrosine phosphatase SHP2 inhibiting potency, and (iii) preparation of azide-tagged probe molecules for target identification via a chemoproteomic approach. The latter has resulted in the identification and evaluation of DNA polymerase epsilon subunit 3 (POLE3) as one of the novel cellular targets of these Abies sesquiterpenoids and their analogs. More importantly, via POLE3 inactivation by probe molecule 29 and knockdown experiment, we further demonstrated that targeting POLE3 with small molecules may be a novel strategy for chemosensitization to DNA damaging drugs such as etoposide in cancer.

Total Syntheses of Bisdehydroneostemoninine and Bisdehydrostemoninine by Catalytic Carbonylative Spirolactonization

The first total syntheses of the stemona alkaloids bisdehydroneostemoninine and bisdehydrostemoninine in racemic forms have been achieved. The synthetic strategy features a novel palladium-catalyzed carbonylative spirolactonization of a hydroxycyclopropanol to rapidly construct the oxaspirolactone moiety. It also features a Lewis acid promoted tandem Friedel-Crafts cyclization and lactonization to form the 5-7-5 tricyclic core of the target stemona alkaloids.

Asymmetric formal synthesis of (+)-cycloclavine

The asymmetric synthesis of Szántay's amine (+)-2, the pivotal precursor for direct access to (+)-cycloclavine (1), is described for the first time in eleven steps with 19.7% overall yield from the commercially available 4-bromoindole.

Dehydrogenative lactonization of diols with a platinum-loaded titanium oxide photocatalyst

The Pt loaded rutile TiO₂ photocatalyst promotes the dehydrogenative lactonization of diols. The reaction rate is improved by the addition of Al₂O₃.

A Synthesis of Substituted α-Allenols via Iron-Catalyzed Cross-Coupling of Propargyl Carboxylates with Grignard Reagents

α-Allenols are attractive and versatile compounds whose preparation can be a nontrivial task. In this Letter, we provide a method for the prompt synthesis of substituted α-allenols via a catalytic cross-coupling reaction which makes use of a nontoxic and cost-effective iron catalyst. The catalyst loading is typically as low as 1-5 mol %. The mild reaction conditions (-20 °C) and the short reaction time (15 min) allow for the presence of a variety of functional groups. Moreover, the reaction was shown to be scalable up to gram-scale and the propargyl substrates are readily accessible by a one-pot synthesis.

Total synthesis, biosynthesis and biological profiles of clavine alkaloids

This review highlights noteworthy synthetic and biological aspects of the clavine subfamily of ergot alkaloids. Recent biosynthetic insights have laid the groundwork for a better understanding of the diverse biological pathways leading to these indole derivatives. Ergot alkaloids were among the first fungal-derived natural products identified, inspiring pharmaceutical applications in CNS disorders, migraine, infective diseases, and cancer. Pergolide, for example, is a semi-synthetic clavine alkaloid that has been used to treat Parkinson's disease. Synthetic activities have been particularly valuable to facilitate access to rare members of the Clavine family and empower medicinal chemistry research. Improved molecular target identification tools and a better understanding of signaling pathways can now be deployed to further extend the biological and medical utility of Clavine alkaloids.

Enzyme- and Ruthenium-Catalyzed Enantioselective Transformation of α-Allenic Alcohols into 2,3-Dihydrofurans

An efficient one-pot method for the enzyme- and ruthenium-catalyzed enantioselective transformation of α-allenic alcohols into 2,3-dihydrofurans has been developed. The method involves an enzymatic kinetic resolution and a subsequent ruthenium-catalyzed cycloisomerization, which provides 2,3-dihydrofurans with excellent enantioselectivity (up to >99 % ee). A ruthenium carbene species was proposed as a key intermediate in the cycloisomerization.

Synthesis of diversely substituted 2-(furan-3-yl)acetates from allenols through cascade carbonylations

Novel synthesis of diversely substituted 2-(furan-3-yl)acetatesviapalladium-catalyzed one-pot multi-component reactions of allenols, aryl iodides, alcohols, and carbon monoxide has been developed. Moreover, the 2-(furan-3-yl)acetates obtained were found to be ready intermediates for the construction of the biologically significant naphtho[1,2-b]furan-5-ol scaffold.

Total synthesis of rugulovasine A

A concise total synthesis of rugulovasine A is achieved by using Uhle's ketone derivative as the key intermediate, which was synthesized by intramolecular cyclization via metal-halogen exchange. Two different routes to construct a spirocyclic butyrolactone subunit involving a Ru-catalyzed cyclocarbonylation and a special Ru-catalyzed double bond rearrangement were studied.

Algicidal lactones from the marine Roseobacter clade bacterium Ruegeria pomeroyi

Volatiles released by the marine Roseobacter clade bacterium Rugeria pomeroyi were collected by use of a closed-loop stripping headspace apparatus (CLSA) and analysed by GC–MS. Several lactones were found for which structural proposals were derived from their mass spectra and unambiguously verified by the synthesis of reference compounds. An enantioselective synthesis of two exemplary lactones was performed to establish the enantiomeric compositions of the natural products by enantioselective GC–MS analyses. The lactones were subjected to biotests to investigate their activity against several bacteria, fungi, and algae. A specific algicidal activity was observed that may be important in the interaction between the bacteria and their algal hosts in fading algal blooms.

(E)-2-boryl-1,3-butadiene derivatives of the 10-TMS-9-BBDs: highly selective reagents for the asymmetric synthesis of anti-1,2-disubstituted 3,4-pentadien-1-ols

The efficient stepwise construction of optically pure trans-4-substituted 2-boryl-1,3-butadienes 6 is described. Hydroboration of 1-alkynes with either enantiomeric form of 3 leads to the pure trans-1-alkenylboranes 4 which undergo addition of alpha-ethoxyvinyllithium followed by a BF(3)-mediated 1,2-B-->C vinylic group migration to provide 6. These organoboranes 6 serve as a new type of asymmetric allylborating agent providing an extremely selective protocol for the preparation of anti-1,2-disubstituted 3,4-pentadien-1-ols 8 as essentially single diastereomers in enantiomerically pure form. One example of a cis-2-boryl-1,3-butadiene (9) was prepared through a Grignard procedure. It was found to provide the corresponding syn-alcohol 11. The utility of 8 was demonstrated in their conversion to substituted beta-hydroxy acids 12 through ozonolysis and to substituted alpha,beta-unsaturated-delta-lactones 13 through Ru-catalyzed cyclocarbonylation.

General, regiodefined access to alpha-substituted butenolides through metal-halogen exchange of 3-bromo-2-silyloxyfurans. Efficient synthesis of an anti-inflammatory gorgonian lipid

A variety of alpha-substituted butenolides were efficiently prepared from 3-bromo-2-triisopropylsilyloxyfuran via lithium-bromine exchange and in situ quench with carbon or heteroatom electrophiles. The inherent flexibility of this methodology is illustrated by a short and efficient synthesis of an anti-inflammatory marine natural product.