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1. Visible-Light Photocatalyzed peri -(3 + 2) Cycloadditions of Quinolines

   https://doi.org/10.1021/jacs.2c05687  

   Bellotti, Peter; Rogge, Torben; Paulus, Fritz; Laskar, Ranjini; Rendel, Nils; Ma, Jiajia; Houk, K. N.; Glorius, Frank (August 2022, Journal of the American Chemical Society)
2. Formal [4 + 2] Cycloadditions of Anhydrides and α,β-Unsaturated N -Tosyl Ketimines

   https://doi.org/10.1021/acs.orglett.8b04091  

   Burlow, Noah P.; Howard, Sara Y.; Saunders, Carla M.; Fettinger, James C.; Tantillo, Dean J.; Shaw, Jared T. (January 2019, Organic Letters)
3. Ambimodal Transition States in Diels–Alder Cycloadditions of Tropolone and Tropolonate with N ‐Methylmaleimide**

   https://doi.org/10.1002/anie.202109608  

   Zhang, Hong; Thøgersen, Mathias Kirk; Jamieson, Cooper S.; Xue, Xiao‐Song; Jørgensen, Karl Anker; Houk, Kendall N. (November 2021, Angewandte Chemie International Edition)
4. Getting excited about cycloadditions

   https://doi.org/10.1126/science.abg7834  

   Schmidt, Valerie A. (March 2021, Science)

   Synthetic chemists use photochemistry to achieve challenging or unusual chemical transformations, but not all compounds are photoactive. Photosensitization is a process by which a molecule that is incapable of efficiently absorbing a particular wavelength of light directly is promoted to its triplet excited state (T 1 ) by an intermolecular triplet energy transfer from a photosensitizer, which is a compound that ideally has a large extinction coefficient, rapid rate of intersystem crossing, and a long-lived T 1 . A particular advantage of photosensitization is that distinctive reactivity profiles not accessible through ground states become facile from corresponding excited states ( 1 ). The use of photosensitizers in chemical synthesis has paralleled the rise in popularity and use of various photoredox catalysts ( 2 ). On page 1338 of this issue, Ma et al. ( 3 ) report a photosensitized dearomative [4 + 2] cycloaddition that converts simple, unsaturated building blocks into products of increased molecular complexity using visible light. 

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5. Site-selective [2 + 2 + n ] cycloadditions for rapid, scalable access to alkynylated polycyclic aromatic hydrocarbons

   https://doi.org/10.1039/C9SC06102A  

   Kiel, Gavin R.; Bergman, Harrison M.; Tilley, T. Don (March 2020, Chemical Science)

   null (Ed.)

   Polycyclic aromatic hydrocarbons (PAHs) are attractive synthetic building blocks for more complex conjugated nanocarbons, but their use for this purpose requires appreciable quantities of a PAH with reactive functional groups. Despite tremendous recent advances, most synthetic methods cannot satisfy these demands. Here we present a general and scalable [2 + 2 + n ] ( n = 1 or 2) cycloaddition strategy to access PAHs that are decorated with synthetically versatile alkynyl groups and its application to seven structurally diverse PAH ring systems (thirteen new alkynylated PAHs in total). The critical discovery is the site-selectivity of an Ir-catalyzed [2 + 2 + 2] cycloaddition, which preferentially cyclizes tethered diyne units with preservation of other (peripheral) alkynyl groups . The potential for generalization of the site-selectivity to other [2 + 2 + n ] reactions is demonstrated by identification of a Cp 2 Zr-mediated [2 + 2 + 1]/metallacycle transfer sequence for synthesis of an alkynylated, selenophene-annulated PAH. The new PAHs are excellent synthons for macrocyclic conjugated nanocarbons. As a proof of concept, four were subjected to alkyne metathesis catalysis to afford large, PAH-containing arylene ethylene macrocycles, which possess a range of cavity sizes reaching well into the nanometer regime. Notably, these high-yielding macrocyclizations establish that synthetically convenient pentynyl groups can be effective for metathesis since the 4-octyne byproduct is sequestered by 5 Å MS. Most importantly, this work is a demonstration of how site-selective reactions can be harnessed to rapidly build up structural complexity in a practical, scalable fashion. 

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