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1. Diels−Alder Reactions of 1‐Alkoxy-1-amino-1,3-butadienes: Direct Synthesis of 6‐Substituted and 6,6-Disubstituted 2‐Cyclohexenones and 6‐Substituted 5,6-Dihydropyran-2-ones

   https://doi.org/10.1021/acs.orglett.1c01031  

   Elkin, P. K.; Durfee, N. D. (June 2021, Organic letters)

   Gaich, T. (Ed.)

   We report the cycloaddition reactions of 1-alkoxy-1- amino-1,3-butadienes. These doubly activated dienes are prepared on a multigram scale from crotonic acid chloride and its derivatives. The dienes undergo Diels−Alder (DA) and hetero-Diels−Alder (HDA) reactions under mild reaction conditions with a variety of electron- deficient dienophiles to afford cycloadducts in good yields with excellent regioselectivities. The hydrolysis of the DA cycloadducts provides 6-substituted and 6,6-disubstituted 2-cylohexenones, which are versatile building blocks for complex molecule synthesis. The corresponding HDA cycloadducts afford 6-substituted 5,6-dihydropyr- an-2-ones. 

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2. Enantioselective Diels-Alder-lactamization organocascades employing a furan-based diene

   https://doi.org/10.1039/c6ob02738e  

   Abbasov, Mikail E.; Hudson, Brandi M.; Kong, Weixu; Tantillo, Dean J.; Romo, Daniel (January 2017, Organic & Biomolecular Chemistry)

   null (Ed.)

   α,β-Unsaturated acylammonium salts are useful dienophiles enabling highly enantioselective and stereodivergent Diels-Alder-initiated organocascades with furan-based dienes. Complex polycyclic systems can thus be obtained from readily prepared dienes, commodity acid chlorides, and a chiral isothiourea organocatalyst under mild conditions. We describe the use of furan-based dienes bearing pendant sulfonamides leading to the generation of oxa-bridged, trans -fused tricyclic γ-lactams. This process constitutes the first highly enantio- and diastereoselective, organocatalytic Diels-Alder cycloadditions with these typically problematic dienes due to their reversibility. Computational studies suggest that the high diastereoselectivity with these furan dienes may be due to a reversible Diels-Alder cycloaddition for the endo adducts. In addition, the utility of this methodology is demonstrated through a concise approach to a core structure with similarity to the natural product isatisine A and a nonpeptidyl ghrelin-receptor inverse agonist. 

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3. Mechanochemical Reactivity of a 1,2,4‐Triazoline‐3,5‐dione‐Anthracene Diels‐Alder Adduct

   https://doi.org/10.1002/asia.202300850  

   Chang, Hao‐Chun; Liang, Min‐Chieh; Luc, Van‐Sieu; Davis, Chelsea; Chang, Chia‐Chih (November 2023, Chemistry – An Asian Journal)

   Abstract Force‐responsive molecules that produce fluorescent moieties under stress provide a means for stress‐sensing and material damage assessment. In this work, we report a mechanophore based on Diels‐Alder adductTAD‐Anof 4,4′‐(4,4′‐diphenylmethylene)‐bis‐(1,2,4‐triazoline‐3,5‐dione) and initiator‐substituted anthracene that can undergo retro‐Diels‐Alder (rDA) reaction by pulsed ultrasonication and compressive activation in bulk materials. The influence of having C−N versus C−C bonds at the sites of bond scission is elucidated by comparing the relative mechanical strength ofTAD‐Anto another Diels‐Alder adductMAL‐Anobtained from maleimide and anthracene. The susceptibility to undergo rDa reaction correlates well with bond energy, such that C−N bond containingTAD‐Andegrades faster C−C bond containingMAL‐Anbecause C−N bond is weaker than C−C bond. Specifically, the results from polymer degradation kinetics under pulsed ultrasonication shows that polymer containingTAD‐Anhas a rate constant of 1.59×10⁻⁵ min⁻¹, whileMAL‐An(C−C bond) has a rate constant of 1.40×10⁻⁵ min⁻¹. Incorporation ofTAD‐Anin a crosslinked polymer network demonstrates the feasibility to utilizeTAD‐Anas an alternative force‐responsive probe to visualize mechanical damage where fluorescence can be “turned‐on” due to force‐accelerated retro‐Diels‐Alder reaction. 

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4. A sequential cyclization/π-extension strategy for modular construction of nanographenes enabled by stannole cycloadditions

   https://doi.org/10.1039/d2sc00397j  

   Bergman, Harrison M.; Beattie, D. Dawson; Kiel, Gavin R.; Handford, Rex C.; Liu, Yi; Tilley, T. Don (May 2022, Chemical Science)

   The synthesis of polycyclic aromatic hydrocarbons (PAHs) and related nanographenes requires the selective and efficient fusion of multiple aromatic rings. For this purpose, the Diels–Alder cycloaddition has proven especially useful; however, this approach currently faces significant limitations, including the lack of versatile strategies to access annulated dienes, the instability of the most commonly used dienes, and difficulties with aromatization of the [4 + 2] adduct. In this report we address these limitations via the marriage of two powerful cycloaddition strategies. First, a formal Cp 2 Zr-mediated [2 + 2 + 1] cycloaddition is used to generate a stannole-annulated PAH. Secondly, the stannoles are employed as diene components in a [4 + 2] cycloaddition/aromatization cascade with an aryne, enabling π-extension to afford a larger PAH. This discovery of stannoles as highly reactive – yet stable for handling – diene equivalents, and the development of a modular strategy for their synthesis, should significantly extend the structural scope of PAHs accessible by a [4 + 2] cycloaddition approach. 

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5. Stereochemical Shape Morphing in Diels‐Alder Polymer Networks

   https://doi.org/10.1002/smll.202407858  

   Moon, Junho; Sang, Zhen; Rajagopalan, Kartik Kumar; Gardea, Frank; Sukhishvili, Svetlana (November 2024, Small)

   Abstract The intrinsic reversibility of dynamic covalent bonding, such as the furan‐maleimide Diels‐Alder (DA) cycloaddition reactions, enables reprocessable, self‐healing polymer materials that can be reconfigured via the mechanism of solid‐state plasticity. In this work, the temperature‐dependent exchange rates of stereochemically distinctendoandexoDA bonds are leveraged to achieve tunable, temperature‐ and stress‐activated shape morphing in Diels‐Alder polymer (DAP) networks. Through thermal annealing, ≈35% ofendoDA isomers are converted in neat DAP networks to the thermodynamically favoredexoform, achieving ≈97%exoafter complete annealing at 60 °C. This conversion results in a ≈1.7 fold increase in elastic modulus, from 1.7 to 3.0 MPa, and significantly alters the stress relaxation and shape recovery behavior. Spatially resolved annealing, is further showcased enabling the precise control of spatial distributions ofendoandexoDA bonds across planar geometries. The locally distinct concentrations ofendo/exoisomers, achieved by temperature‐induced conversion ofendoDA isomers to the thermodynamically stableexoDA isomers, gave rise to the spatial distributions of stress relaxation rates and elastic strain recovery mismatch to enable controlled stereochemical shape morphing. This approach provides a simplified, thermally driven method for shape morphing, with potential applications in soft robotics and flexible electronics. 

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