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1. 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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2. Selectivity between an Alder–ene reaction and a [2 + 2] cycloaddition in the intramolecular reactions of allene-tethered arynes

   https://doi.org/10.1039/d1qo00459j  

   Le, Anh; Lee, Daesung (June 2021, Organic Chemistry Frontiers)

   Substituent-dependent reactivity and selectivity in the intramolecular reactions of arynes tethered with an allene are described. With a 1,3-disubstituted allene moiety, an Alder–ene reaction of an allenic C–H bond is preferred over a [2 + 2] cycloaddition, whereas a [2 + 2] cycloaddition of the terminal π-bond of the allene is preferred with a 1,1-disubstituted allene. With a 1,1,3-trisubstituted allene-tethered aryne, an Alder–ene reaction with an allylic C–H bond is preferred over a [2 + 2] cycloaddition. 

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3. Photoinduced [4 + 2]-cycloaddition reactions of vinyldiazo compounds for the construction of heterocyclic and bicyclic rings

   https://doi.org/10.1039/D4SC03558E  

   Bao, Ming; Bohórquez, Arnold_R Romero; Arman, Hadi; Doyle, Michael P (January 2024, Chemical Science)

   Highly selective formal [4 + 2]-cycloaddition of vinyldiazoacetates with azoalkenes from a-halohydrazones, as well as with cyclopentadiene and furan, occurs with light irradiation at room temperature, producing highly functionalized heterocyclic and bicyclic compounds in good yields and excellent diastereoseletivity. Under blue light these vinyldiazoacetate reagents selectively form unstable cyclopropenes that undergo intermolecular cycloaddition reactions at a faster rate than their competitive ene dimerization. [4 + 2]-cycloaddition of vinyldiazoacetates with in situ formed azoalkenes produces bicyclo[4.1.0]tetrahydropyridazine derivatives and, together with their cycloaddition using cyclopentadiene and furan that form tricyclic compounds, they occur with high chemoselectivity and diastereocontrol, good functional group tolerance, and excellent scalability. Subsequent transformations portray the synthetic versatility of these structures. 

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4. Intramolecular cycloaddition/rearrangement cascade from gold( iii )-catalysed reactions of propargyl aryldiazoesters with cinnamyl imines

   https://doi.org/10.1039/C8CC07885H  

   Qiu, Huang; Arman, Hadi; Hu, Wenhao; Doyle, Michael P. (November 2018, Chemical Communications)

   Conjugated cycloheptene-1,4-dione-enamines are cycloaddition products from a surprising rearrangement in a Au( iii )-catalysed reaction between propargyl aryldiazoacetates and cinnamyl imines. This complex transformation occurs through an initial rapid Au( iii )-catalysed [4+3]-cycloaddition to form dihydroazepinyl aryldiazoacetates followed by a subsequent uncatalyzed domino transformation that occurs by sequential [3+2]-cycloaddition-/nitrogen extrusion and acyloxy migration/retro-Michael addition/tautomerization. 

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5. Formal oxo- and aza-[3 + 2] reactions of α-enaminones and quinones: a double divergent process and the roles of chiral phosphoric acid and molecular sieves

   https://doi.org/10.1039/d0sc02078h  

   Luo, Weiwei; Sun, Zhicheng; Fernando, E. H.; Nesterov, Vladimir N.; Cundari, Thomas R.; Wang, Hong (September 2020, Chemical Science)

   null (Ed.)

   A double divergent process has been developed for the reaction of α-enaminones with quinones through facile manipulation of catalyst and additive, leading to structurally completely different products. The two divergent processes, which involve formal aza- and oxo-[3 + 2] cycloaddition reactions, are mediated by chiral phosphoric acid and molecular sieves, respectively. While inclusion of phosphoric acid in the reaction switched the reaction pathway to favor the efficient formation of a wide range of N -substituted indoles, addition of 4 Å molecular sieves to the reaction switched the reaction pathway again, leading to enantioselective synthesis of 2,3-dihydrobenzofurans in excellent yields and enantioselectivities under mild conditions. Studies in this work suggest that the chiral phosphoric acid acts to lower the transition state energy and promote the formation of amide intermediate for the formal aza-[3 + 2] cycloaddition and the molecular sieves serve to facilitate proton transfer for oxo-[3 + 2] cycloaddition. The reactivity of α-enaminones is also disclosed in this work. 

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