Cyclic Epoxides Research Articles

Comparison of catalytic activity of graphitic carbon nitrides derived from different precursors for carbon dioxide conversion

The outrageous level of carbon dioxide (CO2) accumulation in the atmosphere due to persistent burning of fossil fuels is a serious problem to manhood, and there is upthrust growth in CO2 mitigation using various methodologies. In this regard, graphitic carbon nitride (GCN) based catalysts have been extensively used for CO2 conversion to cyclic carbonates. However, the catalytic activity and the product yields reported are different in different literature, which could be attributed mainly to the precursors used in the preparation of GCN. Hence, in this work, we have performed systematic investigations on GCN, derived from various commonly used precursors, including dicyandiamide, melamine, urea and thiourea, as a catalyst for CO2 conversion reaction involving its cycloaddition to epichlorohydrin to form cyclic carbonates. Suitable justifications were provided for superior activity of one over other. The results indicate that the amount of GCN obtained from different precursors follows the order as, dicyandiamide > melamine > thiourea > urea, using the same amount of initial precursor. Nevertheless, the urea-derived GCN possesses the highest surface area among all GCN and showed the best catalytic activity, giving 56% cyclic carbonate yield, which is almost 3 times higher of that obtained from GCN, derived from melamine. On the other hand, exfoliated GCN samples show a moderate decrease in catalytic activity, which could be attributed to a decrease in terminal amino groups, the active sites for catalyzing this reaction. This work provides some insightful findings for screening of best GCN precursor for catalytic CO2 cycloaddition to cyclic epoxides.

β-cyclodextrin Mediated Green Synthesis of Bioactive Heterocycles

Abstract: In this review, we report β-cyclodextrin catalyzed green transformations of biologically active heterocycles. β-Cyclodextrin is a seminatural product, water-soluble, highly efficient, and biodegradable catalyst. β-Cyclodextrin is a versatile catalyst and promotes a variety of multicomponent transformations, biomimetic reactions, C-C bond formation, and synthesis of some biologically active natural products. It has been applicable to attain some name reactions, deprotection of THP/MOM/Ac/Ts ethers, oxidative cleavage of epoxides, oxidative dehydrogenation of alcohol, regioselective cyclization of chalcone epoxides and 2’-aminochalcones. The catalyst is useful to carry out diastereoselective reactions, and it also plays a very important role in phase transfer catalysts.

Ring-Opening Copolymerization Using a Chromium Complex with a Readily Available Aminotriphenolate Ligand

A new chromium(III) complex (2) with a readily available aminotriphenolate ligand (LtBu,tBu) was developed as an efficient catalyst for alternating ring-opening copolymerization of cyclic epoxides and anhydrides to afford polyesters. The single-crystal structure of 2 exhibits an octahedral geometry despite the high steric profile of LtBu,tBu. After the reaction conditions were optimized, complex 2 in the presence of a cocatalyst produced perfectly alternating polyesters with high molecular weights (up to Mn = 44.2 kg/mol) and low dispersity at high conversion. In addition, various polyesters prepared using combinations of comonomers exhibited relatively high glass transition temperatures.

G-type Halohydrin Dehalogenases Catalyze Ring Opening Reactions of Cyclic Epoxides with Diverse Anionic Nucleophiles.

Halohydrin dehalogenases are promiscuous biocatalysts, which enable asymmetric ring opening reactions of epoxides with various anionic nucleophiles. However, despite the increasing interest in such asymmetric transformations, the substrate scope of G-type halohydrin dehalogenases toward cyclic epoxides has remained largely unexplored, even though this subfamily is the only one known to display activity with these sterically demanding substrates. Herein, we report on the exploration of the substrate scope of the two G-type halohydrin dehalogenases HheG and HheG2 and a newly identified, more thermostable member of the family, HheG3, with a variety of sterically demanding cyclic epoxides and anionic nucleophiles. This work shows that, in addition to azide and cyanide, these enzymes facilitate ring-opening reactions with cyanate, thiocyanate, formate, and nitrite, significantly expanding the known repertoire of accessible transformations.

Hydrothermal Modification of TS-1 Zeolites with Organic Amines and Salts to Construct Highly Selective Catalysts for Cyclopentene Epoxidation

Developing efficient heterogeneous catalysts for cyclic olefins epoxidation is highly attractive for meeting the growing need for various cyclic epoxides. Herein, hierarchical TS-1 zeolite with relatively abundant mesopores and less amount of surface hydroxyl groups was obtained by hydrothermal modification of an as-synthesized TS-1 zeolite with a mixed solution of ammonia, tetrapropylammonium bromide (TPABr) and KCl. The post-modified TS-1 zeolite exhibited much higher catalytic activity (52% conversion) and epoxide selectivity (98%) for the epoxidation of cyclopentene than the conventional TS-1 zeolites. The excellent catalytic activity of the hierarchical TS-1 could be mainly assigned to the enhancement of the mass transport ability and the accessibility of the active Ti species, while the improvement of epoxidation selectivity may be mainly related to the introduction of a certain amount of K+ that can effectively modulate the coordination environment of Ti species as well as the polarity of the zeolite. This work demonstrated that a highly active and selective catalyst for the H2O2-mediated cyclopentene epoxidation could be obtained by concurrently generating mesopore and extinguishing the unfavorable defective hydroxyl groups through the simple hydrothermal treatment of the conventional TS-1 zeolite with a mixed base/salt solution.

Formal Synthesis of Teadenols via Palladium-Catalyzed 6-endo Cyclization of an Epoxyphenol

AbstractFormal syntheses of teadenols A and B are achieved via a key Pd-catalyzed 6-endo cyclization of a phenol possessing a vinyl ­epoxide moiety. Although 5-exo and 6-endo cyclizations compete during cyclizations of epoxides with a nucleophilic moiety at the 5-position, 6-endo cyclization is realized by using a palladium catalyst.

A Novel Catalytic Route to Polymerizable Bicyclic Cyclic Carbonate Monomers from Carbon Dioxide.

A new catalytic route has been developed for the coupling of epoxides and CO2 affording polymerizable six‐membered bicyclic carbonates. Cyclic epoxides equipped with a β‐positioned OH group can be transformed into structurally diverse bicyclic cyclic carbonates in good yields and with high selectivity. Key to the chemo‐selectivity is the difference between the reactivity of syn‐ and anti‐configured epoxy alcohols, with the latter leading to six‐membered ring carbonate formation in the presence of a binary AlIII aminotriphenolate complex/DIPEA catalyst. X‐ray analyses show that the conversion of the syn‐configured substrate evolves via a standard double inversion pathway providing a five‐membered carbonate product, whereas the anti‐isomer allows for activation of the oxirane unit of the substrate opposite to the pendent alcohol. The potential use of these bicyclic products is shown in ring‐opening polymerization offering access to rigid polycarbonates with improved thermal resistance.

A Novel Catalytic Route to Polymerizable Bicyclic Cyclic Carbonate Monomers from Carbon Dioxide

AbstractA new catalytic route has been developed for the coupling of epoxides and CO2 affording polymerizable six‐membered bicyclic carbonates. Cyclic epoxides equipped with a β‐positioned OH group can be transformed into structurally diverse bicyclic cyclic carbonates in good yields and with high selectivity. Key to the chemo‐selectivity is the difference between the reactivity of syn‐ and anti‐configured epoxy alcohols, with the latter leading to six‐membered ring carbonate formation in the presence of a binary AlIII aminotriphenolate complex/DIPEA catalyst. X‐ray analyses show that the conversion of the syn‐configured substrate evolves via a standard double inversion pathway providing a five‐membered carbonate product, whereas the anti‐isomer allows for activation of the oxirane unit of the substrate opposite to the pendent alcohol. The potential use of these bicyclic products is shown in ring‐opening polymerization offering access to rigid polycarbonates with improved thermal resistance.

Construction of Benzo‐Fused Heterocycles by Epoxide–Heteronucleophile Cyclization: Applications in the Synthesis of Natural Products and Designed Molecules

AbstractConstruction of benzo‐fused heterocycles, specifically which have one or more sp3 stereogenic carbon atoms on the heterocyclic ring, by intramolecular ring‐opening cyclization of epoxides with O‐ and N‐nucleophiles is a powerful synthetic tool in organic synthesis. In this Review, we analyze the efficiency of such a heterocyclization approach in natural product synthesis and synthetic methodologies published since the year 2000. In addition to briefly discussing the synthetic methods of accessing the requisite epoxide substrates, important aspects of the subsequent cyclization reaction, such as reaction conditions, endo‐ versus exo‐regioselectivity, protecting or functional group compatibility, and enantio‐ and distereoselectivity are surveyed. Although the literature of organic chemistry in the last four decades has witnessed a large number of reviews/book chapters on the epoxide ring‐opening chemistry, the title topic has never been reviewed. Herein, we have systematized it as a dedicated review for the first time.

Site-selective reactions mediated by molecular containers.

In this review, we summarize various site-selective reactions mediated by molecular containers. The emphasis is on those reactions that give different product distributions on the potential reactive sites inside the containers than they do outside, free in solution. Specific cases include site-selective cycloaddition and addition of arenes, reduction of epoxides, α,β-unsaturated aldehydes, azides, halides and alkenes, oxidation of remote C–H bonds and alkenes, and substitution reactions involving ring-opening cyclization of epoxides, nucleophilic substitution of allylic chlorides, and hydrolysis reactions. The product selectivity is interpreted as the consequence of the space shape and environment inside the container. The containers include supramolecular structures self-assembled through metal/ligand interactions or hydrogen bonding and open-ended covalent structures such as cyclodextrins and cavitands. Challenges and prospects for the future are also provided.

Biomimetic Construction of a syn-2,7-Dimethyloxepane Ring via 7-Endo Cyclization.

syn-2,7-Dimethyloxepane is a unique structure observed in natural ladder-shaped polycyclic ethers (LSPs), such as Caribbean ciguatoxins, gymnocin-B, and brevisulcenal-F that exhibit potent biological activities. Thus, the successful construction of this seven-membered ring is desirable, but its ring strain and the 1,3-repulsion between its two methyl groups makes this process difficult. Herein, we prepared syn-2,7-dimethyloxepanes via 7-endo cyclizations of vinyl epoxides that break Baldwin's rules. Such a biomimetic approach to syn-2,7-dimethyloxepanes has not yet been reported; however, we achieved this challenging cyclization with the aid of a cis-olefin tether and an unsubstituted vinyl group. The NO-ring fragment of gymnocin-B was also prepared from one of these 7-endo cyclized products, demonstrating the potential application of this strategy in constructing bioactive LSPs.

Synthesis of the C1–C13 Fragment of Eribulin on a Kilogram Scale

An improved kilogram-scale synthesis of the C1–C13 fragment of eribulin is disclosed. Rh-mediated 6-endo-tet cyclization of a vinyl epoxide provides a crystalline tetrahydropyran intermediate, which was transformed to a lactol intermediate having the bicyclic motif of the C3–C12 fragment. This intermediate was further elaborated into the C1–C13 fragment of eribulin following the precedent sequence of reactions.

Cellulose sulfate: An efficient heterogeneous catalyst for the ring-opening of epoxides with alcohols and anilines

Cellulose sulfate was synthesized by esterification of α-cellulose with concentrated sulfuric acid at −10°C in ethanol. Cellulose is mainly sulfated on 3-, 6- and 3, 6-positions of the cellulose. It acts as a heterogeneous catalyst for the ring-opening of epoxides with alcohols or anilines and the Friedel-Crafts reaction between N-benzylindole and crotonaldehyde at room temperature. Methanolysis of cyclic epoxides, styrene oxide, terminal aliphatic epoxides, and glycidyl ethers were carried out using the catalyst (0.4–6.8 mg/mmol of epoxide) and afforded the corresponding products in 53–97% isolated yields after 10 min–24 h. Cellulose sulfate was successfully recycled and reused up to 3 catalytic cycles for the ring-opening of styrene oxide with methanol.

Asymmetric Total Synthesis of Phomarol

Asymmetric Total Synthesis of Phomarol

Annulative Methods in the Synthesis of Complex Meroterpene Natural Products.

From the venerable Robinson annulation to the irreplaceable Diels-Alder cycloaddition, annulation reactions have fueled the progression of the field of natural product synthesis throughout the past century. In broader terms, the ability to form a cyclic molecule directly from two or more simpler fragments has transformed virtually every aspect of the chemical sciences from the synthesis of organic materials to bioconjugation chemistry and drug discovery. In this Account, we describe the evolution of our meroterpene synthetic program over the past five years, enabled largely by the development of a tailored anionic annulation process for the synthesis of hydroxylated 1,3-cyclohexanediones from lithium enolates and the reactive β-lactone-containing feedstock chemical diketene.First, we provide details on short total syntheses of the prototypical polycyclic polyprenylated acylphloroglucinol (PPAP) natural products hyperforin and garsubellin A, which possess complex bicyclo[3.3.1]nonane architectures. Notably, these molecules have served as compelling synthetic targets for several decades and induce a number of biological effects of relevance to neuroscience and medicine. By merging our diketene annulation process with a hypervalent iodine-mediated oxidative ring expansion, bicyclo[3.3.1]nonane architectures can be easily prepared from simple 5,6-fused bicyclic diketones in only two chemical operations. Leveraging these two key chemical reactions in combination with various other stereoselective transformations allowed for these biologically active targets to be prepared in racemic form in only 10 steps.Next, we extend this strategy to the synthesis of complex fungal-derived meroterpenes generated biosynthetically from the coupling of 3,5-dimethylorsellinic acid (DMOA) and farnesyl pyrophosphate. A Ti(III)-mediated radical cyclization of a terminal epoxide was used to rapidly prepare a 6,6,5-fused tricyclic ketone which served as an input for our annulation/rearrangement process, ultimately enabling a total synthesis of protoaustinoid A, an important biosynthetic intermediate in DMOA-derived meroterpene synthesis, and its oxidation product berkeleyone A. Through a radical-based, abiotic rearrangement process, the bicyclo[3.3.1]nonane cores of these natural products could again be isomerized, resulting in the 6,5-fused ring systems of the andrastin family and ultimately delivering a total synthesis of andrastin D and preterrenoid. Notably, these isomerization transformations proved challenging when employing classic, acid-induced conditions for carbocation generation, thus highlighting the power of radical biomimicry in total synthesis. Finally, further oxidation and rearrangement allowed for access to terrenoid and the lactone-containing metabolite terretonin L.Overall, the merger of annulative diketene methodology with an oxidative rearrangement transformation has proven to be a broadly applicable strategy to synthesize bicyclo[3.3.1]nonane-containing natural products, a class of small molecules with over 1000 known members.

Semi-aromatic polyester synthesis via alternating ring-opening copolymerization using a chromium complex based on a pentapyridine ligand

The present study examined the use of a Cr(III) complex bearing a pentapyridine ligand as a catalyst in alternating ring-opening copolymerization (ROCOP) to prepare polyesters. Alternating ROCOP of anhydride (phthalic anhydride or 1,8-naphthalic anhydride) and cyclic epoxide (cyclohexene oxide) was performed using a Cr(III) pentapyridine complex (1) and a cocatalyst, DMAP (4-dimethylaminopyridine) or PPNCl ((bis(triphenylphosphine)iminium chloride). After optimizing the monomer ratio and reaction conditions, fully alternating polyesters with narrow polydispersity were synthesized (Mn up to 10.4 kg/mol, Ð below 1.3). These results demonstrate that Cr(III)-based catalysts bearing pentapyridine ligands can generate fully alternating polyesters.

Preparation and characterization of novel hybrid nanomaterial catalyst MCM-41@AzaCrown-SB-Cu and its application in synthesis of 1, 2, 3-triazole derivatives in click chemistry

Synthesis of efficient and reusable catalyst was carried out by immobilization of azacrown ether onto the covalently linked amine-functionalized mesoporous MCM-41 via a simple method. Application of this linked azacrown ether functionalized MCM-41 catalyst was also investigated as an effective, heterogeneous and host material in the regioselective ring opening and triazole cyclization of epoxides by phenylacetylene and sodiumazide in click reaction. The heterogeneous catalyst was characterized by TGA,XRD,TEM, 1HNMR, 13CNMR and FT-IR techniques. The results of theses analysis showed that, MCM-41@azacrown-Cu with a 22.99% mol loading of copper has given excellent catalytic activity and high efficiency in short time. It is noted that one of the most valuable factors in this work is the recyclability of the catalyst.

Regioselective Cross-Electrophile Coupling of Epoxides and (Hetero)aryl Iodides via Ni/Ti/Photoredox Catalysis.

A cross-electrophile coupling reaction of epoxides and (hetero)aryl iodides that operates via the merger of three catalytic cycles involving a Ni-, Ti-, and organic photoredox catalyst has been developed. Three distinct classes of epoxides, styrene oxides, cyclic epoxides, and terminal aliphatic epoxides, all undergo coupling in moderate to good yield and high regioselectivity with the use of three different nitrogen-based ligands for Ni under otherwise identical reaction conditions. The mild reaction conditions accommodate a broad scope of abundant and complex coupling partners. Mechanistic studies suggest that when styrene oxides are employed radical intermediates are involved via Ti-radical ring-opening of the epoxide. Conversely, for terminal aliphatic epoxides, involvement of an iodohydrin intermediate enables the formation of the unexpected linear product.

Anti-inflammatory polyether triterpenoids from the marine macroalga Gracilaria salicornia: Newly described natural leads attenuate pro-inflammatory 5-lipoxygenase and cyclooxygenase-2

5-Lipoxygenase (5-LOX)/cyclooxygenase-2 (COX-2) were found to be the two major inducible pro-inflammatory enzymes catalysing the rate-limiting stage in the development of pro-inflammatory prostaglandins/thromboxanes by COX-2 pathway and leukotrienes by 5-LOX pathway. Instantaneous quenching of COX-2/5-LOX-associated cascade is an important therapeutic objective in the attenuation of inflammatory pathologies. Two polyether triterpenoids, characterised as 15-(octahydro-7-(4,5-dihydro-3-methoxy-2,6-dimethyl-2H-pyran-6-yl)-10-methylpyrano[3,2-b]pyran-14-yl)-18-(19-methyl-23-methyleneoxepan-19-yl)pent-15-en-18-ol (1) and tetrahydro-6-(hexahydro-13-((tetrahydro-18-(23-hydroxy-23-methylheptan-19-yl)-15-methylfuran-15-yl)methyl)-10-methyl-2H-furo[3,2-b]pyran-7-yl)-2,2,6-trimethyl-2H-pyran-3-yl butyrate (2) were isolated from the organic extract of the marine macroalga, Gracilaria salicornia harvested from southeast-coastal zones of the Indian peninsular. Polyether analogue bearing trimethyl-2H-pyran-3-yl butyrate moiety (2) disclosed potent attenuation properties against 5-LOX (IC50 1.89 mM) and COX-2 (IC50 1.87 mM) enzymes. In-silico molecular docking methods designated the anti-inflammatory mechanisms of isolated compounds and their comparison of docking factors assigned that the polyether 2 displayed the smallest binding energy of −10.29 and −10.96 kcal mol−1 in COX-2 and 5-LOX active sites, respectively and designated competent hydrogen-bonding associations with the enzymatic catalytic regions. Greater electronic factors along with lesser steric bulk of the polyether 2 bearing furanyl-furo[3,2-b]pyran-2H-pyran moiety was found to have noteworthy functional roles to attenuate the inflammatory enzymes. Proposed bio-synthetic origin leading to the polyether analogues comprising the cyclization of squalene epoxides through the sequences of enzyme-catalysed cascade corroborated their structural attributions. These reports designated that polyether triterpenoid enclosing furanyl-furo[3,2-b]pyran-2H-pyran skeleton might be considered as a prospective anti-inflammatory therapeutic source to alleviate COX-2 and 5-LOX-mediated inflammatory pathologies.

Ring-opening copolymerization of cyclic epoxide and anhydride using a five-coordinate chromium complex with a sterically demanding amino triphenolate ligand

This work describes polyester synthesis via alternating ring-opening copolymerization of epoxides and anhydrides using a trigonal bipyramidal chromium complex containing a sterically demanding ligand.