Arynes, synthesis

In this review we have shown that the tandem-aryne synthesis (Sec. 3.1) provides a useful general entry to m-terphenyls suitably substituted for the construction of cuppedophanes and cappedophanes, as well as other macrocyclic phanes. 

SCHEME 3.18 Palladium-catalyzed carbocyclization of aryl iodides, bicyclic alkenes, and arynes. Synthesis of annulated 9,10-dihydro-phenanthrenes. 

An unusual case of addition of a carbanion to an unconjugated carbon-carbon double bond is shown in Scheme 47a. The subsequent transfer of the amide group is also noteworthy (80CC1042). The intramolecular addition of a carbanion to an aryne is a more widely established process. Such reactions have been applied to the synthesis of indoles (Scheme 47b) (75CC745> and oxindoles (Scheme 47c) (63JOC1,80JA3646). 

Benzo[b]furan-4(5H)-one, 6,7-dihydro-synthesis, 4, 710 Benzofuranones H NMR, 4, 579 Benzo[b]furanones from 3-hydroxyflavanone, 3, 729 Benzo[b]furan-2(3H)-ones — see 2-Coumaranones Benzo[b]furan-3(2H)-ones IR spectra, 4, 590 reactions, 4, 650 Benzo[ c]furan-1 (3 W) -ones carbanions, 4, 579 H NMR, 4, 579 reactions with arynes, 1, 415 structure, 4, 552... 

Androcymbine, O-benzyl-, synthesis 282 Anilinium ion, as nucleophilic partner 49 f. Anthracene, trapping of arynes 187f. Antimony-de-diazoniation 275 Arenediazonium ions... 

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... 

Cyclization by addition of a side-chain carbanion to an aryne bond has been proposed as the method of choice for synthesis of the versatile 1-substituted benzocyclobutene system.7 This general procedure now has been modified to permit convenient large-scale preparations utilizing a commercially available base, a minimum amount of liquid ammonia, and distillation for isolation of the product. 

Alkanamines have acid strengths corresponding to Ka values of about 10 33, which means that their conjugate bases are powerfully basic reagents. Therefore they are very effective in causing elimination reactions by the E2 mechanism (Section 8-8) and aromatic substitution by the aryne mechanism (Section 14-6C). The following example illustrates this property in a useful synthesis of a benzenamine from bromobenzene ... 

Arynic substitution is a versatile technique of functional group transformation in aromatic systems and has found varied applications in preparation of simple compounds and in multi-step synthesis.3 4,3 The present section comprises examples illustrative of its synthetic scope. Attention is also drawn to some allied strategies which, when used in conjunction with the nucleophilic coupling of arynes, have opened convenient routes to complex natural products. 

If the moeity adding to the aryne carries a suitably placed electrophilic center, cyclization with the initially formed anion can occur as observed in the synthesis of dibenzothiophene (60).92 Of wider synthetic import in this context is enolate addition to arynes. Here, after the initial ring closure many types of products can be formed (Scheme ll).35 However, with some substrates, a particular reaction course can predominate 93 94 for example, reaction of cyclodecanone with bromobenzene provides an efficient method for benzo-annulation and ring expansion. Similarly, high yields of benzocyclobutenols can be obtained from the monoketals of ot-diketones.9s... 

Carbanions derived from side chain tertiary amides have also been cyclized to provide isoquinolones and isoindoles (equation 36).125 126 While benzyne intermediacy in the formation of the former is likely, the latter seems to arise through a SrnI reaction pathway. Synthesis of indole from the meta bromo compound (87), on the other hand, clearly involves an aryne cyclization. 27 A more versatile route to indoles is based on intramolecular addition of aminyl anions to arynes (equation 38).128 A somewhat similar dihydroindole preparation constitutes the first step in a synthesis of lycoranes (equation 39).129 The synthesis of (88) also falls in the same category of reactions, but it is noteworthy because only a few examples of ring closure of heteroarynes are mentioned in literature.27 28... 

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