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4 pages, 194 KiB

Open AccessEditorial

Modern Strategies for Heterocycle Synthesis

by Gianfranco Favi

Molecules 2020, 25(11), 2476; https://doi.org/10.3390/molecules25112476 - 27 May 2020

Cited by 7 | Viewed by 2911

Abstract

Heterocycles constitute the largest and most diverse family of organic compounds that have received extensive interest owing to their popularity in many natural products, pharmaceuticals, and materials [...] Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

Research

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15 pages, 2744 KiB

Open AccessArticle

TMSBr-Promoted Cascade Cyclization of ortho-Propynol Phenyl Azides for the Synthesis of 4-Bromo Quinolines and Its Applications

by Fengyan Jin, Tao Yang, Xian-Rong Song, Jiang Bai, Ruchun Yang, Haixin Ding and Qiang Xiao

Molecules 2019, 24(21), 3999; https://doi.org/10.3390/molecules24213999 - 5 Nov 2019

Cited by 7 | Viewed by 3812

Abstract

Difficult-to-access 4-bromo quinolines are constructed directly from easily prepared ortho-propynol phenyl azides using TMSBr as acid-promoter. The cascade transformation performs smoothly to generate desired products in moderate to excellent yields with good functional groups compatibility. Notably, TMSBr not only acted as an [...] Read more.

Difficult-to-access 4-bromo quinolines are constructed directly from easily prepared ortho-propynol phenyl azides using TMSBr as acid-promoter. The cascade transformation performs smoothly to generate desired products in moderate to excellent yields with good functional groups compatibility. Notably, TMSBr not only acted as an acid-promoter to initiate the reaction, and also as a nucleophile. In addition, 4-bromo quinolines as key intermediates could further undergo the coupling reactions or nucleophilic reactions to provide a variety of functionalized compounds with molecular diversity at C4 position of quinolines. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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15 pages, 1687 KiB

Open AccessArticle

Catalyst-Free Synthesis of Polysubstituted 5-Acylamino-1,3-Thiazoles via Hantzsch Cyclization of α-Chloroglycinates

by Mara Tomassetti, Gabriele Lupidi, Pamela Piermattei, Federico V. Rossi, Samuele Lillini, Gianluca Bianchini, Andrea Aramini, Marco A. Ciufolini and Enrico Marcantoni

Molecules 2019, 24(21), 3846; https://doi.org/10.3390/molecules24213846 - 25 Oct 2019

Cited by 6 | Viewed by 4493

Abstract

A catalyst-free heterocyclization reaction of α-chloroglycinates with thiobenzamides or thioureas leading to 2,4-disubstituted-5-acylamino-1,3-thiazoles has been developed. The methodology provides straightforward access to valuable building blocks for pharmaceutically relevant compounds. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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10 pages, 6555 KiB

Open AccessCommunication

Nucleophilic Arylation of Halopurines Facilitated by Brønsted Acid in Fluoroalcohol

by Naoko Takenaga, Toshitaka Shoji, Takayuki Menjo, Akiko Hirai, Shohei Ueda, Kotaro Kikushima, Tomonori Hanasaki and Toshifumi Dohi

Molecules 2019, 24(21), 3812; https://doi.org/10.3390/molecules24213812 - 23 Oct 2019

Cited by 4 | Viewed by 3598

Abstract

Various aryl-substituted purine derivatives were synthesized through the direct arylation of halopurines with aromatic compounds, facilitated by the combination of triflic acid and fluoroalcohol. This metal-free method is complementary to conventional coupling reactions using metal catalysts and reagents for the syntheses of aryl-substituted [...] Read more.

Various aryl-substituted purine derivatives were synthesized through the direct arylation of halopurines with aromatic compounds, facilitated by the combination of triflic acid and fluoroalcohol. This metal-free method is complementary to conventional coupling reactions using metal catalysts and reagents for the syntheses of aryl-substituted purine analogues. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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13 pages, 3191 KiB

Open AccessArticle

Sequential MCR via Staudinger/Aza-Wittig versus Cycloaddition Reaction to Access Diversely Functionalized 1-Amino-1H-Imidazole-2(3H)-Thiones

by Cecilia Ciccolini, Giacomo Mari, Gianfranco Favi, Fabio Mantellini, Lucia De Crescentini and Stefania Santeusanio

Molecules 2019, 24(20), 3785; https://doi.org/10.3390/molecules24203785 - 21 Oct 2019

Cited by 8 | Viewed by 3743

Abstract

A multicomponent reaction (MCR) strategy, alternative to the known cycloaddition reaction, towards variously substituted 1-amino-1H-imidazole-2(3H)-thione derivatives has been successfully developed. The novel approach involves α-halohydrazones whose azidation process followed by tandem Staudinger/aza-Wittig reaction with CS₂ in a sequential [...] Read more.

A multicomponent reaction (MCR) strategy, alternative to the known cycloaddition reaction, towards variously substituted 1-amino-1H-imidazole-2(3H)-thione derivatives has been successfully developed. The novel approach involves α-halohydrazones whose azidation process followed by tandem Staudinger/aza-Wittig reaction with CS₂ in a sequential MCR regioselectively leads to the target compounds avoiding the formation of the regioisomer iminothiazoline heterocycle. The approach can be applied to a range of differently substituted α-halohydrazones bearing also electron-withdrawing groups confirming the wide scope and the substituent tolerance of the process for the synthesis of the target compounds. Interestingly, the concurrent presence of reactive functionalities in the scaffolds so obtained ensures post-modifications in view of N-bridgeheaded heterobicyclic structures. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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17 pages, 1520 KiB

Open AccessFeature PaperArticle

One-Pot Metal-Free Synthesis of 3-CF₃-1,3-Oxazinopyridines by Reaction of Pyridines with CF₃CO-Acetylenes

by Vasiliy M. Muzalevskiy, Zoia A. Sizova, Kseniya V. Belyaeva, Boris A. Trofimov and Valentine G. Nenajdenko

Molecules 2019, 24(19), 3594; https://doi.org/10.3390/molecules24193594 - 6 Oct 2019

Cited by 15 | Viewed by 3760

Abstract

The reaction of pyridines with trifluoroacetylated acetylenes was investigated. It was found that the reaction of various pyridines with two molecules of CF₃CO-acetylenes proceeds under mild metal-free conditions. As a result, efficient stereoselective synthesis of 3-arylethynyl-3-trifluoromethyl-1,3-oxazinopyridines was elaborated. Target heterocycles can [...] Read more.

The reaction of pyridines with trifluoroacetylated acetylenes was investigated. It was found that the reaction of various pyridines with two molecules of CF₃CO-acetylenes proceeds under mild metal-free conditions. As a result, efficient stereoselective synthesis of 3-arylethynyl-3-trifluoromethyl-1,3-oxazinopyridines was elaborated. Target heterocycles can be prepared in up to quantitative yields. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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11 pages, 2400 KiB

Open AccessCommunication

A Brønsted Acid-Catalyzed Multicomponent Reaction for the Synthesis of Highly Functionalized γ-Lactam Derivatives

by Xabier del Corte, Edorta Martinez de Marigorta, Francisco Palacios and Javier Vicario

Molecules 2019, 24(16), 2951; https://doi.org/10.3390/molecules24162951 - 14 Aug 2019

Cited by 24 | Viewed by 3972

Abstract

Brønsted acids catalyze a multicomponent reaction of benzaldehyde with amines and diethyl acetylenedicarboxylate to afford highly functionalized γ-lactam derivatives. The reaction consists of a Mannich reaction of an enamine to an imine, both generated in situ, promoted by a phosphoric acid catalyst and [...] Read more.

Brønsted acids catalyze a multicomponent reaction of benzaldehyde with amines and diethyl acetylenedicarboxylate to afford highly functionalized γ-lactam derivatives. The reaction consists of a Mannich reaction of an enamine to an imine, both generated in situ, promoted by a phosphoric acid catalyst and a subsequent intramolecular cyclization. The hydrolysis of the cyclic enamine substrate can provide enol derivatives and, moreover, a second attack of the amine on the carboxylate can afford amide derivatives. An optimization of the reaction conditions is presented in order to obtain selectively cyclic enamines that can afford the enol species after selective hydrolysis. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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19 pages, 9639 KiB

Open AccessArticle

Design, Synthesis, and Biological Evaluation of Novel N-Acylhydrazone Bond Linked Heterobivalent β-Carbolines as Potential Anticancer Agents

by Xiaofei Chen, Liang Guo, Qin Ma, Wei Chen, Wenxi Fan and Jie Zhang

Molecules 2019, 24(16), 2950; https://doi.org/10.3390/molecules24162950 - 14 Aug 2019

Cited by 14 | Viewed by 3821

Abstract

Utilizing a pharmacophore hybridization approach, we have designed and synthesized a novel series of 28 new heterobivalent β-carbolines. The in vitro cytotoxic potential of each compound was evaluated against the five cancer cell lines (LLC, BGC-823, CT-26, Bel-7402, and MCF-7) of different origin—murine [...] Read more.

Utilizing a pharmacophore hybridization approach, we have designed and synthesized a novel series of 28 new heterobivalent β-carbolines. The in vitro cytotoxic potential of each compound was evaluated against the five cancer cell lines (LLC, BGC-823, CT-26, Bel-7402, and MCF-7) of different origin—murine and human, with the aim of determining the potency and selectivity of the compounds. Compound 8z showed antitumor activities with half-maximal inhibitory concentration (IC₅₀) values of 9.9 ± 0.9, 8.6 ± 1.4, 6.2 ± 2.5, 9.9 ± 0.5, and 5.7 ± 1.2 µM against the tested five cancer cell lines. Moreover, the effect of compound 8z on the angiogenesis process was investigated using a chicken chorioallantoic membrane (CAM) in vivo model. At a concentration of 5 μM, compound 8z showed a positive effect on angiogenesis. The results of this study contribute to the further elucidation of the biological regulatory role of heterobivalent β-carbolines and provide helpful information on the development of vascular targeting antitumor drugs. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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17 pages, 2142 KiB

Open AccessArticle

Alcohol Participates in the Synthesis of Functionalized Coumarin-Fused Pyrazolo[3,4-b]Pyridine from a One-Pot Three-Component Reaction

by Wei Lin, Cangwei Zhuang, Xiuxiu Hu, Juanjuan Zhang and Juxian Wang

Molecules 2019, 24(15), 2835; https://doi.org/10.3390/molecules24152835 - 4 Aug 2019

Cited by 6 | Viewed by 4674

Abstract

A concise and efficient approach to synthesizing coumarin-fused pyrazolo[3,4-b]pyridine via silica sulfuric acid (SSA) catalyzed three-component domino reaction under microwave irradiation has been demonstrated. Participation of various alcohols in construction of coumarin derivatives has been described for the first time. Short [...] Read more.

A concise and efficient approach to synthesizing coumarin-fused pyrazolo[3,4-b]pyridine via silica sulfuric acid (SSA) catalyzed three-component domino reaction under microwave irradiation has been demonstrated. Participation of various alcohols in construction of coumarin derivatives has been described for the first time. Short reaction time, high yields, one-pot procedure, usage of eco-friendly catalyst, and solvent are the key features of this method. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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26 pages, 8729 KiB

Open AccessArticle

Synthesis and Properties of 6-Aryl-4-azidocinnolines and 6-Aryl-4-(1,2,3-1H-triazol-1-yl)cinnolines

by Natalia A. Danilkina, Nina S. Bukhtiiarova, Anastasia I. Govdi, Anna A. Vasileva, Andrey M. Rumyantsev, Artemii A. Volkov, Nikita I. Sharaev, Alexey V. Povolotskiy, Irina A. Boyarskaya, Ilya V. Kornyakov, Polina V. Tokareva and Irina A. Balova

Molecules 2019, 24(13), 2386; https://doi.org/10.3390/molecules24132386 - 27 Jun 2019

Cited by 9 | Viewed by 5205

Abstract

An efficient approach towards the synthesis of 6-aryl-4-azidocinnolines was developed with the aim of exploring the photophysical properties of 6-aryl-4-azidocinnolines and their click reaction products with alkynes, 6-aryl-4-(1,2,3-1H-triazol-1-yl)cinnolines. The synthetic route is based on the Richter-type cyclization of 2-ethynyl-4-aryltriazenes with the [...] Read more.

An efficient approach towards the synthesis of 6-aryl-4-azidocinnolines was developed with the aim of exploring the photophysical properties of 6-aryl-4-azidocinnolines and their click reaction products with alkynes, 6-aryl-4-(1,2,3-1H-triazol-1-yl)cinnolines. The synthetic route is based on the Richter-type cyclization of 2-ethynyl-4-aryltriazenes with the formation of 4-bromo-6-arylcinnolines and nucleophilic substitution of a bromine atom with an azide functional group. The developed synthetic approach is tolerant to variations of functional groups on the aryl moiety. The resulting azidocinnolines were found to be reactive in both CuAAC with terminal alkynes and SPAAC with diazacyclononyne, yielding 4-triazolylcinnolines. It was found that 4-azido-6-arylcinnolines possess weak fluorescent properties, while conversion of the azido function into a triazole ring led to complete fluorescence quenching. The lack of fluorescence in triazoles could be explained by the non-planar structure of triazolylcinnolines and a possible photoinduced electron transfer (PET) mechanism. Among the series of 4-triazolylcinnoline derivatives a compound bearing hydroxyalkyl substituent at triazole ring was found to be cytotoxic to HeLa cells. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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13 pages, 3789 KiB

Open AccessArticle

Synthesis of (1,2,3-triazol-4-yl)methyl Phosphinates and (1,2,3-Triazol-4-yl)methyl Phosphates by Copper-Catalyzed Azide-Alkyne Cycloaddition

by Anna Tripolszky, Krisztina Németh, Pál Tamás Szabó and Erika Bálint

Molecules 2019, 24(11), 2085; https://doi.org/10.3390/molecules24112085 - 31 May 2019

Cited by 9 | Viewed by 4315

Abstract

An efficient and practical method was developed for the synthesis of new (1,2,3‑triazol‑4‑yl)methyl phosphinates and (1,2,3-triazol-4-yl)methyl phosphates by the copper(I)‑catalyzed azide-alkyne cycloaddition (CuAAC) of organic azides and prop-2-ynyl phosphinate or prop-2-ynyl phosphate. The synthesis of (1‑benzyl-1H-1,2,3-triazol-4-yl)methyl diphenylphosphinate was optimized with respect [...] Read more.

An efficient and practical method was developed for the synthesis of new (1,2,3‑triazol‑4‑yl)methyl phosphinates and (1,2,3-triazol-4-yl)methyl phosphates by the copper(I)‑catalyzed azide-alkyne cycloaddition (CuAAC) of organic azides and prop-2-ynyl phosphinate or prop-2-ynyl phosphate. The synthesis of (1‑benzyl-1H-1,2,3-triazol-4-yl)methyl diphenylphosphinate was optimized with respect to the reaction parameters, such as the temperature, reaction time, and catalyst loading. The approach was applied to a range of organic azides, which confirmed the wide scope and the substituent tolerance of the process. The method elaborated represents a novel approach for the synthesis of the target compounds. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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16 pages, 4852 KiB

Open AccessArticle

Synthesis of Terpyridines: Simple Reactions—What Could Possibly Go Wrong?

by Dalila Rocco, Catherine E. Housecroft and Edwin C. Constable

Molecules 2019, 24(9), 1799; https://doi.org/10.3390/molecules24091799 - 9 May 2019

Cited by 17 | Viewed by 6912

Abstract

The preparation of 2⁴-functionalized 1²,2²:2⁶,3²-terpyridines (4′-functionalized 3,2:6′,3″-terpyridines) by the reaction of three 4-alkoxybenzaldehydes with 3-acetylpyridine and ammonia was investigated; under identical reaction conditions, two (R =ⁿC₄H₉, C [...] Read more.

The preparation of 2⁴-functionalized 1²,2²:2⁶,3²-terpyridines (4′-functionalized 3,2:6′,3″-terpyridines) by the reaction of three 4-alkoxybenzaldehydes with 3-acetylpyridine and ammonia was investigated; under identical reaction conditions, two (R =ⁿC₄H₉, C₂H₅) gave the expected products whereas a third (R = ⁿC₃H₇) gave only a cyclohexanol derivative derived from the condensation of three molecules of 3-acetylpyridine with two of 4-(n-propoxy)benzaldehyde. A comprehensive survey of “unexpected” products from reactions of ArCOCH₃ derivatives with aromatic aldehydes is presented. Three different types of alternative product are identified. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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14 pages, 1973 KiB

Open AccessArticle

Novel 6a,12b-Dihydro-6H,7H-chromeno[3,4-c] chromen-6-ones: Synthesis, Structure and Antifungal Activity

by Jin-ping Bao, Cui-lian Xu, Guo-yu Yang, Cai-xia Wang, Xin Zheng and Xin-xin Yuan

Molecules 2019, 24(9), 1745; https://doi.org/10.3390/molecules24091745 - 5 May 2019

Cited by 15 | Viewed by 3791

Abstract

A new series of coumarin derivatives, 7-hydroxy-7-(trifluoromethyl)-6a,12b-dihydro-6H,7H-chromeno[3,4-c]chromen-6-ones 3a–p, were synthesized via Michael addition, transesterification and nucleophilic addition from the reaction of 3-trifluoroacetyl coumarins and phenols in the presence of an organic base. The products were characterized [...] Read more.

A new series of coumarin derivatives, 7-hydroxy-7-(trifluoromethyl)-6a,12b-dihydro-6H,7H-chromeno[3,4-c]chromen-6-ones 3a–p, were synthesized via Michael addition, transesterification and nucleophilic addition from the reaction of 3-trifluoroacetyl coumarins and phenols in the presence of an organic base. The products were characterized by infrared spectroscopy (IR), hydrogen nuclear magnetic resonance spectroscopy (¹H-NMR), carbon nuclear magnetic resonance spectroscopy (¹³C-NMR) and high-resolution mass spectrometer (HRMS). Single crystal X-ray analysis of compounds 3a and 3n clearly confirmed their assigned chemical structures and their twisted conformations. Compound 3a crystallized in the orthorhombic system, Pbca, in which a = 8.6244(2) Å, b = 17.4245(4) Å, c = 22.5188(6) Å, α = 90°, β = 90°, γ = 90°, v = 3384.02(14) Å³, and z = 8. In addition, the mycelial growth rate method was used to examine the in vitro antifungal activities of the title compounds 3a–p against Fusarium graminearum and Fusarium monitiforme at 500 µg/mL. The results showed that compound 3l exhibited significant anti-Fusarium monitiforme activity with inhibitory index of 84.6%. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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9 pages, 743 KiB

Open AccessArticle

Traceless Solid-Phase Synthesis of Ketones via Acid-Labile Enol Ethers: Application in the Synthesis of Natural Products and Derivatives

by Eva Schütznerová, Anna Krchňáková and Viktor Krchňák

Molecules 2019, 24(7), 1406; https://doi.org/10.3390/molecules24071406 - 10 Apr 2019

Cited by 5 | Viewed by 3337

Abstract

In solid-phase organic synthesis, Wang resin is traditionally used for the immobilization of acids, alcohols, phenols, and amines. We report the use of Wang resin for the traceless synthesis of ketones via acid-labile enol ethers. We demonstrate the practicality of this synthetic strategy [...] Read more.

In solid-phase organic synthesis, Wang resin is traditionally used for the immobilization of acids, alcohols, phenols, and amines. We report the use of Wang resin for the traceless synthesis of ketones via acid-labile enol ethers. We demonstrate the practicality of this synthetic strategy on the solid-phase synthesis of pyrrolidine-2,4-diones, which represent the core structure of several natural products, including tetramic acid. Base-triggered condensation of pyrrolidine-2,4-diones yielded 4-hydroxy-1,1′,2′,5-tetrahydro-2H,5′H-[3,3′-bipyrrole]-2,5′-diones. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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30 pages, 2098 KiB

Open AccessArticle

Green and Facile Assembly of Diverse Fused N-Heterocycles Using Gold-Catalyzed Cascade Reactions in Water

by Xiuwen Jia, Pinyi Li, Xiaoyan Liu, Jiafu Lin, Yiwen Chu, Jinhai Yu, Jiang Wang, Hong Liu and Fei Zhao

Molecules 2019, 24(5), 988; https://doi.org/10.3390/molecules24050988 - 11 Mar 2019

Cited by 23 | Viewed by 4726

Abstract

The present study describes an AuPPh₃Cl/AgSbF₆-catalyzed cascade reaction between amine nucleophiles and alkynoic acids in water. This process proceeds in high step economy with water as the sole coproduct, and leads to the generation of two rings, together with [...] Read more.

The present study describes an AuPPh₃Cl/AgSbF₆-catalyzed cascade reaction between amine nucleophiles and alkynoic acids in water. This process proceeds in high step economy with water as the sole coproduct, and leads to the generation of two rings, together with the formation of three new bonds in a single operation. This green cascade process exhibits valuable features such as low catalyst loading, good to excellent yields, high efficiency in bond formation, excellent selectivity, great tolerance of functional groups, and extraordinarily broad substrate scope. In addition, this is the first example of the generation of an indole/thiophene/pyrrole/pyridine/naphthalene/benzene-fused N-heterocycle library through gold catalysis in water from readily available materials. Notably, the discovery of antibacterial molecules from this library demonstrates its high quality and potential for the identification of active pharmaceutical ingredients. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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Review

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52 pages, 13620 KiB

Open AccessReview

Last Decade of Unconventional Methodologies for the Synthesis of Substituted Benzofurans

by Lucia Chiummiento, Rosarita D’Orsi, Maria Funicello and Paolo Lupattelli

Molecules 2020, 25(10), 2327; https://doi.org/10.3390/molecules25102327 - 16 May 2020

Cited by 42 | Viewed by 5564

Abstract

This review describes the progress of the last decade on the synthesis of substituted benzofurans, which are useful scaffolds for the synthesis of numerous natural products and pharmaceuticals. In particular, new intramolecular and intermolecular C–C and/or C–O bond-forming processes, with transition-metal catalysis or [...] Read more.

This review describes the progress of the last decade on the synthesis of substituted benzofurans, which are useful scaffolds for the synthesis of numerous natural products and pharmaceuticals. In particular, new intramolecular and intermolecular C–C and/or C–O bond-forming processes, with transition-metal catalysis or metal-free are summarized. (1) Introduction. (2) Ring generation via intramolecular cyclization. (2.1) C7a–O bond formation: (route a). (2.2) O–C2 bond formation: (route b). (2.3) C2–C3 bond formation: (route c). (2.4) C3–C3a bond formation: (route d). (3) Ring generation via intermolecular cyclization. (3.1) C7a-O and C3–C3a bond formation (route a + d). (3.2) O–C2 and C2–C3 bond formation: (route b + c). (3.3) O–C2 and C3–C3a bond formation: (route b + d). (4) Benzannulation. (5) Conclusion. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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22 pages, 5407 KiB

Open AccessReview

An Overview of Saturated Cyclic Ethers: Biological Profiles and Synthetic Strategies

by Qili Lu, Dipesh S. Harmalkar, Yongseok Choi and Kyeong Lee

Molecules 2019, 24(20), 3778; https://doi.org/10.3390/molecules24203778 - 21 Oct 2019

Cited by 54 | Viewed by 7486

Abstract

Saturated oxygen heterocycles are widely found in a broad array of natural products and other biologically active molecules. In medicinal chemistry, small and medium rings are also important synthetic intermediates since they can undergo ring-opening and -expansion reactions. These applications have driven numerous [...] Read more.

Saturated oxygen heterocycles are widely found in a broad array of natural products and other biologically active molecules. In medicinal chemistry, small and medium rings are also important synthetic intermediates since they can undergo ring-opening and -expansion reactions. These applications have driven numerous studies on the synthesis of oxygen-containing heterocycles and considerable effort has been devoted toward the development of methods for the construction of saturated oxygen heterocycles. This paper provides an overview of the biological roles and synthetic strategies of saturated cyclic ethers, covering some of the most studied and newly discovered related natural products in recent years. This paper also reports several promising and newly developed synthetic methods, emphasizing 3–7 membered rings. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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16 pages, 4758 KiB

Open AccessReview

Recent Advances in the Synthesis of 2H-Pyrans

by David Tejedor, Samuel Delgado-Hernández, Raquel Diana-Rivero, Abián Díaz-Díaz and Fernando García-Tellado

Molecules 2019, 24(16), 2904; https://doi.org/10.3390/molecules24162904 - 9 Aug 2019

Cited by 18 | Viewed by 5700

Abstract

In this review, we discuss the nature of the different physicochemical factors affecting the valence isomerism between 2H-pyrans (2HPs) and 1-oxatrienes, and we describe the most versatile synthetic methods reported in recent literature to access to 2HPs, with the only exception [...] Read more.

In this review, we discuss the nature of the different physicochemical factors affecting the valence isomerism between 2H-pyrans (2HPs) and 1-oxatrienes, and we describe the most versatile synthetic methods reported in recent literature to access to 2HPs, with the only exception of 2HPs fused to aromatic rings (i.e., 2H-chromenes), which are not included in this review. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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33 pages, 6195 KiB

Open AccessReview

Recent Advances on Metal-Free, Visible-Light- Induced Catalysis for Assembling Nitrogen- and Oxygen-Based Heterocyclic Scaffolds

by Robert Pawlowski, Filip Stanek and Maciej Stodulski

Molecules 2019, 24(8), 1533; https://doi.org/10.3390/molecules24081533 - 18 Apr 2019

Cited by 39 | Viewed by 6870

Abstract

Heterocycles are important class of structures, which occupy a major space in the domain of natural and bioactive compounds. For this reason, development of new synthetic strategies for their controllable synthesis became of special interests. The development of novel photoredox systems with wide-range [...] Read more.

Heterocycles are important class of structures, which occupy a major space in the domain of natural and bioactive compounds. For this reason, development of new synthetic strategies for their controllable synthesis became of special interests. The development of novel photoredox systems with wide-range application in organic synthesis is particularly interesting. Organic dyes have been widely applied as photoredox catalysts in organic synthesis. Their low costs compared to the typical photocatalysts based on transition metals make them an excellent alternative. This review describes proceedings since 2015 in the area of application of metal-free, visible-light-mediated catalysis for assembling various heterocyclic scaffolds containing five- and six-membered rings bearing nitrogen and oxygen heteroatoms. Full article

(This article belongs to the Special Issue Modern Strategies for Heterocycle Synthesis)

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