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An inverse-electron-demand Diels-Alder (IEDDA) reaction could complement the conventional normal-electron-demand Diels-Alder reaction in the synthesis of six-membered carbocycles. However, catalytic asymmetric all-carbon-based IEDDA reactions are underdeveloped. Herein, we disclosed a copper-catalyzed asymmetric IEDDA reaction using electron-deficient 3-carboalkoxyl-2-pyrones and electronically unbiased indenes as reactants. This method enables the rapid and enantioselective construction of a wide range of hexahydrofluorenyl bridged-lactone scaffolds. Using this method, asymmetric total syntheses of cephanolides A and B were accomplished.

Mechanochemistry is an emerging field with many potential applications in sustainable chemistry. But despite the growing interest in the field, its underlying mechanistic foundations are not fully understood yet. This work presents the application of computational tools, such as DFT calculations in continuum and microkinetic modeling, to the analysis of mechanically activated procedures. Two reactions reported in previous experimental publications were studied: (i) a series of Diels-Alder reactions and (ii) the synthesis of sulfonylguanidines. Calculations succeed in reproducing experimentally reported reaction times. The procedures were mostly standard, coupled with some sensitive choices in terms of starting concentrations and dielectric constant. This means that these particular reactions accelerated by ball milling followed the same mechanism as the equivalent reactions in solution. The implications of this result on the general picture of mechanochemical processes are discussed.

The heterogeneous metal-organic framework Bi-BTC successfully catalyzed the synthesis of para-xylene from bio-based 2,5-dimethylfuran and acrylic acid in a promising yield (92 %), under relatively mild conditions (160 °C, 10 bar), and with a low reaction-energy barrier (47.3 kJ mol-1 ). The proposed reaction strategy also demonstrates a remarkable versatility for furan derivatives such as furan and 2-methylfuran.

Identification of a common Diels-Alder pattern in three classes of bioactive natural products led us to study the synthesis and cycloaddition of a new class of cyclic dienes readily available from β,γ-unsaturated lactams. A practical and readily scalable route to the parent p-methoxybenzyl-protected 6- and 7-membered β,γ-unsaturated lactams was developed. These were readily transformed into the corresponding O-silylated dienes, which were reacted with dimethyl and diethyl fumarate to yield stereoselectively highly functionalized bicyclic adducts. These exhibited unexpected and versatile transformations upon acid hydrolysis depending on the nature of the dienophile substituents and the acid catalyst. All reactions have been performed on multigram quantities. These transformations provide a convenient, economical, and easily scalable pathway for the rapid construction of functionally and stereochemically dense privileged scaffolds for the construction of libraries of natural products-inspired molecules of pharmacological relevance.

Boron‐substituted 1,3‐dienes participate in ene reactions to afford new functionalized synthetic intermediates. After evaluating several enophiles as partners, the resulting products have been engaged in multistep sequences involving first a Diels Alder/allyboration process. A variety of skeletally diverse and complex polycyclic heterocycles were thus synthesized, such as tetrahydro‐ 1H ‐isoindole‐1,3( 2H )‐diones, eight‐membered lactones or tricyclic spiro compounds.

Nanographenes (NGs) with tunable electronic and magnetic properties have attracted enormous attention in the realm of carbon-based nanoelectronics. In particular, NGs with biradical character at the ground state are promising building units for molecular spintronics. However, most of the biradicaloids are susceptible to oxidation under ambient conditions and photolytic degradation, which hamper their further applications. Herein, we demonstrated the feasibility of tuning the magnetic properties of zigzag-edged NGs in order to enhance their stability via the controlled Diels-Alder reactions of peri-tetracene (4-PA). The unstable 4-PA (y0 =0.72; half-life, t1/2 =3 h) was transformed into the unprecedented benzo-peri-tetracenes (BPTs) by a one-side Diels-Alder reaction, which featured a biradical character at the ground state (y0 =0.60) and exhibited remarkable stability under ambient conditions for several months. In addition, the fully zigzag-edged circumanthracenes (CAs) were achieved by two-fold or stepwise Diels-Alder reactions of 4-PA, in which the magnetic properties could be controlled by employing the corresponding dienophiles. Our work reported herein opens avenues for the synthesis of novel zigzag-edged NGs with tailor-made magnetic properties.

Novel main-chain liquid crystalline Diels-Alder dynamic networks (LCDANs) were prepared that exhibit unprecedented ease for actuator programming and reprocessing compared to existing liquid crystalline network (LCN) systems. Following cooling from 125 °C, LCDANs are deformed with aligned mesogens self-locked at room temperature by slowly formed Diels-Alder (DA) bonds, which allows for the formation of solid 3D actuators capable of reversible shape change, and strip walker and wheel-capable light-driven locomotion upon either thermally or optically induced order-disorder phase transition. Any actuator can readily be erased at 125 °C and reprogrammed into a new one under ambient conditions. Moreover, LCDANs can be processed directly from melt (for example, fiber drawing) and from solution (for example, casting tubular actuators), which cannot be achieved with LCNs using exchangeable covalent bonds. The combined attributes of LCDANs offer significant progress toward developing easily programmable/processable LCN actuators.

Taking advantage of the reversible Diels-Alder (DA) reaction, a simple strategy to obtain recyclable epoxy resins is presented. For this purpose, blends of furan-functionalized and nonfunctionalized epoxy resin are prepared. After the addition of diamine and bismaleimide, blends are heated at 150 degrees C for 5 min, where the permanent amine/epoxy reaction has taken place and upon cooling to room temperature the reversible DA reaction has happened, giving rise to a dual permanent/nonpermanent network. Both reactions are confirmed by Fourier-transform infrared (FTIR) and C-13-crosspolarization, magic-angle spinning (CP/MAS) nuclear magnetic resonance (NMR). Modulated differential scanning calorimetry (MDSC) shows that the epoxy/amine and bismaleimide/amine curing reaction take place, after the DA reaction, simultaneously with the retroDA reaction and before the bismaleimide homopolymerization. Therefore, under the appropriate curing conditions, the Michael's addition and the bismaleimide homopolymerization do not avoid the formation of a hybrid network, as stated in other reports. The reversibility of the DA reaction in three consecutive cycles is confirmed by DSC. Finally, the dual-cured sample is reprocessed three times without significant loss of mechanical properties. The authors thank the Basque Government (IT1313-19), the University of the Basque Country (GIU19/077), and Mineco (MAT2017-84116-R), for the funding received to develop this work. Technical and human support provided by Macro-Behaviour-Mesostructure and Nanotechnology and NMR SGIker services of UPV/EHU are also gratefully acknowledged. The authors also thank Anne Bravo for the design of the table of contents. Document

The preparation of high value‐added chemicals from renewable resources is a crucial approach towards a sustainable economy. One prominent alternative to the production of petroleum‐based chemicals from fossil resources is through the sequential Diels‐Alder/aromatization reactions of biomass‐derived furan platforms. This Concept is focused on the recent boom in bio‐based furan DA strategies for aromatization of bio‐based platform chemicals, particularly that of furfurals, ranging from indirect use and activation strategies to recent examples of direct DA reaction of these electron‐withdrawing biomass‐derived furans., Ta‐DA! The Diels‐Alder (DA) reaction of electron‐rich furans with electron‐poor alkenes has been a benchmark for the preparation of complex scaffolds such as 7‐oxanorbornenes. Recently this technology has been used to prepare aromatics from bio‐based furanic platforms. The recently described direct DA reaction of bio‐based furfurals expands the scope of bioaromatic preparation to afford key biomass derivatives.

Calculations were partially performed at the TUBITAK ULAKBIM, High Performance and Grid Computing Center (TRUBA resources) as well as the computational resources at CCBG (www.ccbg.chem.boun.edu.tr) funded by the Bogazici University Research Fund (BAP Project No: 5156 and BAP-SUP Project No: 8245). B.D thanks Gebze Technical University Research Fund (BAP Project No: 2018-A105-32) for the financial support. The authors thank Assist. Prof. Yunus Emre Turkmen (Bilkent University, Ankara, Turkey) for fruitful discussions

A dual crosslinked self‐healing polyurethane was prepared with robust mechanical properties through the dynamic reversible pyridine‐Fe³⁺ coordination bonds and Diels–Alder (DA) covalent bonds dual crosslinking strategy. Moreover, the mechanical properties and self‐healing ability of polyurethane can be tuned readily by different ratio of the coordination bonds and DA bonds. Under external load, the coordination bonds serve as sacrificial bonds are broken to dissipate energy, the DA bonds can keep the shape of sample. With the coordination bonds participation, the damaged samples can be healed under moderate heating treatment or with the aid of FeCl₃ solution. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2228–2234

Carbon‐carbon bond formation is a fundamental transformation in both synthetic chemistry and biosynthesis. Enzymes catalyze such reactions with exquisite selectivity which often cannot be achieved using non‐biological methods but may suffer from an intolerance of high temperature and the presence of organic solvents limiting their applications. Here we report the thermodynamic and kinetic stability of the β‐barrel natural Diels‐Alderase AbyU, which catalyzes formation of the spirotetronate core of the antimicrobial natural product abyssomicin C, with creation of 3 new asymmetric centers. This enzyme is shown to catalyze [4 + 2] cycloadditions at elevated temperature (up to 65 oC), and in the presence of organic solvents (MeOH, CH3CN and DMSO) and the chemical denaturant guanidinium hydrochloride, revealing that AbyU has potential widespread value as a biocatalyst.

We gratefully acknowledge the National Research Foundation of Korea (NRF-2017K2A9A1A06042191) and the Belarusian Republican Foundation for Basic Research (X18KOP-003).

The unprecedented phospha-aza-Diels-Alder reaction between an activated electron-poor imine and 2H-phospholes yields 1-phospha-2-azanorbornenes in a highly chemoselective and moderately diastereoselective reaction. The intermediate 2H-phospholes, which act as dienes, are formed in situ from the corresponding 1H-phospholes. Theoretical calculations confirm that the phospha-aza-Diels-Alder reaction is of normal electron demand. The reactive P-N bond in 1-phospha-2-azanorbornenes can be cleaved by nucleophiles leading to the formation of 2,3-dihydrophospholes.

The first synthetic access to stable and acyclic 1,3-butadienes with two electron-withdrawing carbonyl groups and their potential to deliver new molecular scaffolds through intriguing endo-selective Diels-Alder cycloadditions are presented. The bicyclic scaffolds produced through the cycloaddition chemistry of electron-deficient dienes afforded potent Hedgehog signaling pathway inhibitors.