Cycloisomerization gold oxide

Building on the earlier work of Hashmi [15], Liu has reported the gold-catalyzed cycloisomerization and oxidative cyclization of (Z)-enynols [16,17]. As an example of the former transformation, treatment of the tetraphenyl enynol 7 with a catalytic amount of AuCh in dichloromethane at room temperature led to isolation of... 

The Diels-Alder reaction outlined above is a typical example of the utilization of axially chiral allenes, accessible through 1,6-addition or other methods, to generate selectively new stereogenic centers. This transfer of chirality is also possible via in-termolecular Diels-Alder reactions of vinylallenes [57], aldol reactions of allenyl eno-lates [19f] and Ireland-Claisen rearrangements of silyl allenylketene acetals [58]. Furthermore, it has been utilized recently in the diastereoselective oxidation of titanium allenyl enolates (formed by deprotonation of /3-allenecarboxylates of type 65 and transmetalation with titanocene dichloride) with dimethyl dioxirane (DMDO) [25, 59] and in subsequent acid- or gold-catalyzed cycloisomerization reactions of a-hydroxyallenes into 2,5-dihydrofurans (cf. Chapter 15) [25, 59, 60],... 

Aminoalkenes, oxidative cyclization, 10, 710-711 Aminoalkoxides, on zinc compounds, 2, 371 a-Aminoalkylallenes, cycloisomerizations, 10, 720 a-Aminoalkylcuprates, preparation, 9, 519-520 -Aminoalkylidynes, diiron carbonyl complexes with cyclopentadienyl ligands, 6, 248 Aminoalkynes, hydroamination, 10, 717 a-Aminoallenes, activation by gold, 9, 574 Amino r]5-amides, in Ru and Os half-sandwich rf3-arenes,... 

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... 

With the electTOTi-poor allenic esters, palladium(0) is able to catalyze the reaction without gold. The reactiOTi then is initiated at the other end, after oxidative addition of the aryl halide to the electrophilic palladium(II) species cycloisomerizes the allenic ester and then forms the product by reductive elimination. With o-alkynylbenzoates, the intermediate vinylgold species contains an enol ether substructure and is able to directly intercept the activated allyl donors, even in the absence of palladium. In both cases, by careful trace analysis (ICP), the presence of the other metal was excluded [78]. 

The cycloisomerization of l, -enynes promoted by gold complex catalysts, are known to go through only the third pathway. This high selectivity is due to two main reasons (1) the fragment [AuL]" has only one vacant site, thus it can not coordinate simultaneously the alkyne and the alkene moieties, (2) oxidative addition processes are not facile for gold complexes [24, 157, 158]. In general, gold(I) complexes surpass the reactivity shown by platinum(ll) and other electrophilic metals for the reaction of enynes. ... 

Gold-catalyzed cycloisomerizations of alkynyl ketones/aldehydes provide convenient access to 2-benzopyrylium salts, which are important intermediates for natural product syntheses. For example, the benzaldehyde derivative 151 was converted directly to the 2-benzopyrylium salt 152 in the presence of a catalytic amount of Au(OAc)3 in (CH2C1)2/TFA (Scheme 19.37) [62], IBX-mediated oxidation of 152 in the presence of n-Bi NI (as a phase-transfer catalyst) cleanly afforded the azaphilone 153 in 84% yield after reductive workup, which is a precursor of ( )-S-15183a (154). Similarly, AgNOs-promoted cyclization of the alkyne 155 furnished direct construction of the tricyclic 8/7-pyrano[2,3,4-rfe]chromen-8-one core of cassiarin A (158) (Scheme 19.38) [63]. 

SCHEME 7.26 Platinum and gold cycloisomerization of 1,5-enynes with subsequent (a) migration of an acetyl group or (b) and (c) oxidation of a hydroxy group. 

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