Arynes Diels-Alder

Cycloaddition Reactions of Arynes (Diels-Alder Reaction)... 

Compared to cyclic dienes, acyclic dienes are less commonly used in aryne Diels-Alder reactions, although one example has been reported by Sarandeses et al. in the synthesis of helicenes (Scheme 12.48) [89]. Functionalized [4]/[5]heli-cenes 166 have been synthesized in five steps starting from 164, using a synthetic approach based on a Diels-Alder reaction between the l,3-bis(trimethylsilyloxy)-1,3-dienes 165 and benzyne [90]. 

Scheme 9 Synthesis of extended PAH 31 using an aryne Diels-Alder reaction [30]...

The benzyne created from 2-(trimethylsilyl)phenyl triflate was used in a highly diastereoselective aryne Diels-Alder reaction with a diene bearing Oppolzer s sultam. This approach to cis-functionalized 1,4-dihydronaphthalenes was reportedly the first aryne Diels-Alder reaction to provide enantioenriched cyclo-adducts. An unusual route to /3-aminoketones involves the treatment of 2-(trimethylsilyl)phenyl triflate with TBAF and an asymmetric vinyldihydropyridone. The resultant aryne Diels-Alder cycloadduct undergoes aromatization/eUmination to create the )V-acyl-/3-aminoketone (eq 13). This method was featured in a multistep synthesis of an unnatural a-amino acid. 

The heterocyclic aryne (trithiadiazepyne) 12.18, generated by treatment of the monobromo derivative of 12.16 with a strong base, can be trapped as a Diels-Alder adduct with furan (Scheme 12.3). ... 

A Diels-Alder reaction of arynes with 1,2,4-triazines 102 allows the preparation of isoquinolines substituted with electron-withdrawing groups in the nitrogen-containing ring. The isoquinoline-1-carboxylic esters bearing additional substituents are of particular interest because they are not readily available by the usual routes [100,101] (Scheme 2.42). 

New Diels-Alder reactions of 3-vinylindoles with an aryne selective access to functionalized [a]anellated carbazoles [145]... 

Species such as 5 and 6 are called benzynes (sometimes dehydrobenzenes), or more generally, arynes, and the mechanism is known as the benzyne mechanism. Benzynes are very reactive. Neither benzyne nor any other aryne has yet been isolated under ordinary conditions, but benzyne has been isolated in an argon matrix at 8 where its IR spectrum could be observed. In addition, benzynes can be trapped for example, they undergo the Diels-Alder reaction (see 15-58). It should be noted that the extra pair of electrons does not affect the aromaticity. The... 

The reaction of benzyne with furan was the first example of a Diels-Alder reaction of benzyne to be studied b. No authenticated examples of arynes are known which fail to give cyclo-adducts with furan i38>. The tetrahalogenobenzynes all form the expected adducts 103, X = F 6), Cl 57>i39)j gr 59) or i 59)). d0 other highly fluorinated arynes i40,i4i>. The isomeric adducts (104) and (105) have been detected by 19F n.m.r. spectroscopy when the dilithio-compound (12) was allowed to decompose in the presence of furan 28 103). 

The reaction of benzyne with cyclohexadiene has been known for some time 4>, but although a number of steroidal cis-dienes are readily available no reactions with arynes had been reported prior to our beginning such investigations 145>. This was somewhat surprising in view of the number of reports concerning the modification of steroids by means of reactions with carbenes 146 i49) and the known Diels-Alder reactions of steroidal dienes and trienes iso.isi). 

Benzyne itself has been isolated in solid argon at 8 K, and much evidence for the existence of arynes has come from trapping experiments and spectroscopy. Thus generation of benzyne (101) in the presence of furan (102) leads to the formation of the Diels-Alder (p. 197) adduct (103), which undergoes ready acid-catalysed... 

Similar alkene formations via dealkoxycarbonylation and denitration have been reported for the synthesis of novel heterocycles. Heterocyclic nitro compounds such as 4-nitroisoxazole undergo the Diels-Alder reaction subsequent dealkoxycarbonylation and denitration give the products, which are regarded as the Diels-Alder adducts of five-membered heterocyclic arynes (Eq. 7.142).121... 

Diels-Alder reaction of 3-vinylindole 131 with aryne in the presence of air gives, besides primary Diels-Alder product 132, the methyl 12-methyl-12H-[3]-benzoxepino[l,2-l7]indole-5-carboxylate 135. This can be explained by the formation of 1,2-dioxetane 133, its cyclo reversion and final intramolecular cyclization of dienol 134 or its tautomers (Scheme 26 (1996JCS(P1)1767)). 

The very reactive arynes have also been used as dienophiles in Diels-Alder cycloadditions with vinyl heterocycles and are included here, after the acetylenic esters, because of the similarity of their reactive functions. This reaction is of considerable importance because of its application in the synthesis of polycyclic compounds. 

THF at room temperature to produce substituted phenylallenes (231). Various arynes and terminal and internal alkynes can be used. The reaction of alkyne without propar-gylic hydrogen gave an acetylenic C-H addition product (a phenylalkyne) and a dehydro-Diels-Alder product (a phenanthrene).266... 

The elusive dibenzannulated dimethyldihydro-pyrene 69 was prepared by the group of Mitchell (Scheme 16) [48]. A Diels-Alder cycloaddition between an aryne and furan was employed to introduce the key benzene rings to the metacyclo-phanediene framework 68. The colorless 68 switches to green 69 on UV irradiation, and 69 reverts thermally or photochemically. More recently, the same group reported the synthesis of the three-way molecular switch 70 [49]. 

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