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The Diels–Alder adduct of p-benzoquinone and anthracene…
The Diels-Alder reaction (also known as the Diene Synthesis) is the reaction of a 1,3-butadiene with an alkene to form a cyclohexene. One of the first cycloadditions performed by Diels and Alder (Nobel Prize 1950) was the reaction of cyclopentadiene with p-benzoquinone (Diels, O.; Alder, K. 1928, , 98). Quinone cycloadditions have been frequently used in natural product syntheses, including the cyclopentadiene-benzoquinone adduct iteself in the synthesis of epi-Epoxydon.
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When 1,4-dimethoxyanthracene was used instead of anthracene (in acetic acid in the presence of -chloranil), a mixture of adducts and was obtained, which could be further oxidized by CAN to give the corresponding heptiptycene tri(-quinone) . Using a similar strategy, we synthesized a series of heptiptycene quinones by the reaction of pentiptycene quinones or triptycene quinones with anthracene or substituted anthracenes (Figure , compounds and –).
The Diels-Alder Reaction - Home | UW-Madison …
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In 1956, Wittig and Ludwig pioneered a convenient method for the synthesis of triptycene () via the Diels–Alder addition reaction between benzyne and anthracene. Obviously, this strategy provides a direct and convenient method for the synthesis of triptycenes with different functional groups.
In 1971, Kadosaka and co-workers reported the synthesis of a series of symmetrical trichloro-substituted triptycenes. As shown in Scheme ,5-dichloroanthracene () reacted with 3-chlorobenzyne, generated in situ from 6-chloroanthranilic acid, to provide 1,8,16-trichlorotriptycene () in 9% yield. A similar Diels–Alder reaction of 1,8-dichloroanthracene () with 3-chlorobenzyne in dimethoxymethane also gave 1,8,16-trichlorotriptycene () as the adduct, along with the generation of the adduct, 1,8,13-trichlorotriptycene ().
Isomerization and functionalization of 2:1 Diels–Alder adduct…
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Triptycene, with three phenyl rings being connected via two bridgehead carbons, has an abundance of reactive sites which are amenable to functionalization. Its unique structure makes triptycene derivatives useful for a wide range of potentially useful applications. There are two main synthetic strategies for the preparation of substituted triptycenes: (1) via the direct Diels–Alder addition reaction between the corresponding substituted anthracene and a benzyne, and (2) via the selective derivatization of unsubstituted triptycene () (Scheme ).
Diels–Alder adducts from the reaction of anthracene with dienophiles have been used in a variety of applications, including the synthesis of discrete molecular architectures such as molecular gears (Stevens & Richards, 1997). Although the crystal structures of a number of such derivatives have been disclosed, somewhat surprisingly the structure of the adduct (1) of -benzoquinone and anthracene has not been previously reported. Tautomer (2) does appear in the Cambridge Structural Database (Version 5.25; Allen 2002); however, its full structure has not been deposited (Hashimoto , 1999).
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Synthesis and Diels-Alder reactions of 9-(4 …
By employing the direct Diels–Alder reaction strategy, a wide range of substituted triptycene derivatives have been synthesized in reasonable yields, and in some cases, in relatively high yields. However, there are three notable deficiencies that restrict this synthetic strategy: (1) the Diels–Alder reactions between arynes and substituted anthracenes are complicated by the formation of the respective positional isomers, which results in decreased reaction yields and the necessity for (and associated difficulties involved with) separation of the desired products, (2) many substituted anthracenes were obtained in low yields over several reaction steps, which is not economical or practical, and (3) the triple bond in the benzyne or aryne is not a typical π bond and is highly reactive, hence the benzyne or aryne has to be produced in situ in order for it to be able to participate in the Diels–Alder reaction. The reactivity of the benzyne or aryne precursor plays the dominant role in the final yield of the Diels–Alder reaction. Moreover, there are benzynes or arynes with certain substituents that are difficult to generate in situ in high yields. Thus, additional convenient routes to obtain substituted triptycene derivatives in high yields are necessary.
Synthesis and Diels-Alder reactions of 9- ..
According to a similar method, we have also synthesized tweezer-shaped noniptycene triquinone and heptiptycene triquinone , with three equivalent U-shaped cavities, and noniptycene triquinone possessing two equivalent heptiptycene triquinone U-shaped cavities, in reasonable yields, by the addition reaction of pentiptycene triquinone with two equivalents of anthracene in acetic acid in the presence of -chloranil (Figure ).
anthracene with p-benzoquinone…
There are two main synthetic strategies for the synthesis of triptycene derivatives with useful functional groups. The direct Diels–Alder addition strategy involves a one-pot reaction demonstrating good suitability and scalability for many substituted triptycenes. However, the fact that the substrates (anthracenes or benzynes with specific substituents) are often difficult to prepare, and the formation of isomeric mixtures of products, both represent disadvantages to the wider application of the Diels–Alder strategy. To a certain degree, the selective derivatization of unsubstituted triptycene merely supplements these insufficiencies. Several important functional groups, such as acetyl, nitro and halo, can be introduced easily by properly chosen derivatization reactions. However, the strategies for selective derivatizations of the triptycene skeleton are still relatively demanding. Moreover, selective derivatization at the α-position of triptycene, due to its low reactivity, remains a major challenge. Remarkable headway has been made toward the synthesis of substituted pentiptycenes, in particular, central-ring functionalized pentiptycenes. On the other hand, with the difficulties associated with stereoselectivity and regioselectivity, the aromatic electrophilic substitution of pentiptycene has not been reported to date. The situation for the other higher order iptycenes is even worse with only few reports being available. Thus, considerable challenges still exist in the synthesis of substituted iptycenes. In relation to their gradually increasing applications as molecular machines, and in supramolecular chemistry, materials science, and many other research areas, the synthesis of substituted iptycenes, which are the cornerstone of iptycene chemistry, is also filled with both challenges and opportunities.
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