Systems gold-catalyzed

Research in this area was developed further by Li et at. using 1,3-dienes 64 for the gold-catalyzed annulation of phenols and naphtols [58, 59]. These generated various dihydrobenzofuran derivatives. The best yields were achieved when the catalytic system included enough AuCl3 and silver salt to remove halogen atoms and deliver cationic gold. 

A novel gold catalyzed example of three-component addition was recently reported by Shi et al. (Equation 8.44) [106]. Terminal aryl alkynes, alcohols and 2-(arylmethy-lene) cyclopropylcarbinols provided an intermolecular tandem hydroalkoxylation/ Prins-type reaction to form 3-oxabicyclo[3.1.0]hexanes from simple materials and under mild conditions, catalyzed by the system AuClPPh3/AgOTf. The proposed mechanism for this reaction is shown in Scheme 8.19. 

Another version of the Conia-ene reaction is the focus of this research. Here the reaction of y-alkynic (3-ketoesters was tested.48 The screening resulted in a copper / silver cocatalysis system. It remains unclear why the authors did not at least run some reactions with gold catalysts, too, especially since they even cite the work on the gold-catalyzed Conia-ene reaction in the introduction (Section 12.1). 

A gold-catalyzed tandem reaction of 1,7-diynes using p-nitrobenzyl alcohol as a nucleophile was developed to construct the 6-5-bicycHc ring systems in an enantio- and diastereoselective manner. The modularity of this approach enabled the efficient preparation of a variety of natural product analogs (14AGE1837). 

The cycloisomerization of a- or (3-hydroxyallenes can also be carried out in water, for example with tetrachloroauric acid as catalyst. This system was used for the first example of a tandem lipase/gold-catalyzed transformation. The one-pot kinetic resolution/cycloisomerization of racemic allenic acetates with Burkholderia cepacia lipase (PS Amano SD Amano Enzyme USA Co., Ltd., Elgin, IL) and HAuCU afforded 2,5-dihydrofurans as well as unreacted starting material with 28-50% isolated yield and 86-98% ee (Scheme 4-96). The mutual tolerance of the Lewis-acidic gold catalyst with the Ixwis-basic lipase is maintained as long as low amounts of the former are used. 

The sequential intramolecular hydroarylation of alkynes is applied to the synthesis of structurally complex extended x-systems. The gold-catalyzed sequential intramolecular hydroarylation of triynes followed by aromatization with DDQ proceeded to give triaryl-substituted diacenaphtho[l,2-y l, 2 -l] fluoranthenes, which can be used for organic light-emitting devices (Scheme 21.49) [55]. 

Gold-catalyzed addition of N-H and O-H to a triple bond has proven to be a rich source of protic acid-based multicatalytic systems. The setup for hydrogen transfer from Hantzsch esters has already been discussed other recent examples of Au/protic acid systems include combinations with well-known acid-catalyzed reactions such as the Fischer indole synthesis [114], Povarov reaction [115], and Diels-Alder reaction[116] (Scheme 26.26). 

Another possibility would be the use of allylic esters, which after a gold-catalyzed cycloisomerization with the carbonyl oxygen atom as the nucleophile deliver activated allylic intermediates which at the same time contain a vinylgold substructure. After transfer of an allyl cation to palladium(O), an oxidative addition to palladium, the vinylgold intermediate could transfer the organic moiety to palladium(II). A final reductive elimination would close the catalytic cycle. At the same time, no halide that potentially could deactivate the cationic gold(I) catalyst would be present. Indeed, Blum et al. [30] presented such systems. But... 

The gold-catalyzed reaction of alkynes with aromatic units has been extensively studied [105-107]. This reaction allows the synthesis of polycyclic aromatic and heteroaromatic systems via Friedel-Crafts-type processes. Although, the C-H activation of aryl compounds by gold(lll) has been known for more than 70 years, it is accepted that the Friedel-Crafts-type reaction proceeds via [Au(alkyne)] complexes and subsequent electrophilic aromatic substitution with the arenes or heteroarene compounds. 

The next step toward the completion of the preparation of Lundurine A would involve the synthesis of alkynylindole XVII and the subsequent cyclization via a gold-catalyzed reaction. The intermediate reactions have already been identified and optimized on a model system. The key steps in the synthesis will be the conversion of alkynylindole XVII into XVIII and a subsequent intramolecular cyclopropanation to obtain Lunderine A [16-26]. 

Using a similar dual gold catalysis of diynes model, Hashmi, Hansmann, and coworkers explored a dual gold-catalyzed cyclization of 3,4-diethynylthiophenes to generate pentaleno[c]thiophenes through gold-vinylidenes and C-H bond activation. In these cases, 5-e (io-dig cyclization pathway was observed. This protocol can be used to construct thiophene/furan or thiophene/pyrrole 101 system skeleton from 3,4-diethynylthiophene derivatives 100 (Scheme 12.46) [49]. 

Using a combination of AuClg/AgOTf as the catalytic system, Jean and Weghe realized a gold-catalyzed intramolecular hydroarylation of unactivated olefins, which provided a facile access to dihydrobenzopyrans, tetralins, and tetrahydro-quinolines 122 (Scheme 12.53) [57]. A variety of homoallyl aryl ethers 121 with... 

Gagosz extensively investigated on the gold-catalyzed cyclizations of ether-alkyne systems 145 and 147 bearing both terminal and internal alkynes in 2010. A wide range of structurally important spiro or fused dihydrofurans 146 and dihy-dropyrans 148 were dexterously constructed via a 1,5-hydride shift/cyclization sequence using alkyne as hydride acceptors (Scheme 12.64) [68]. This hydroalkylation process, which could be applied to the terminal as well as ester-substituted alkynes, allows the efficient conversion of secondary or tertiary C(sp )—H bonds into new C-C bonds under practical conditions. The stereoselectivity of the cycloisomerization process toward the formation of a new five- or six-membered cycle appears to be dependent on steric factors and alkyne substitution. 

Despite the statement above concerning the acid lability of cyclic formals, Gold and Sghibartz have shown that the acid catalyzed hydrolysis of these compounds is markedly depressed by some metal ions . Although the smaller cyclic formals did not exhibit a substantial rate reduction even in the presence of small cations like lithium, in certain larger systems the rate reduction was more than an order of magnitude. 

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