Triple gold-catalyzed cyclization

The proposed mechanism for the formation of the eight-membered ring compound starts with an initial activation of the triple bond, followed by a gold-catalyzed cyclization at the C3 position of the substituted alkynylindole III-l to form the seven-membered ring iminium cation III-4 (Scheme 4.8). 1,2-Migration and proton loss then lead to III-6, from which the eight-membered ring compound m-3 is formed by protodemetalation. 

Analogous cleavage of a C=C triple bond has been found in the gold-catalyzed cyclization of (Z)-enynol 157 under an oxygen atmosphere (Scheme 7.54) [73]. Butenolide 159 was obtained by way of the cyclic alkenyl ether 158. 

Li has reported the water-promoted, gold(I)-catalyzed cascade addition/cyclization of terminal alkynes with o-alkynylbenzaldehyde derivatives to form 1-alkynyl-lH-isochromenes [41]. For example, reaction of l-(2-phenylethynyl)benzaldehyde with phenylacetylene catalyzed by a 1 4 mixture of (PMe3)AuCl and Hunig s base in a water/toluene mixture at 70 °C for 1 day led to isolation of isochromene 29 in 81% yield (Fq. (12.11)). The transformation is presumably initiated by gold-catalyzed addition of acetylide to the C=0 bond of the aldehyde moiety followed by addition of the resulting alkoxide across the pendant C=C triple bond. 

Gold-catalyzed 5-exo-type cyclization is also useful for oxazole synthesis. It was reported that AuCls is efficient in the electrophilic activation of carbon-carbon triple bonds of A-propargylamides 144 to afford 2,5-disubstituted oxazoles 150 under mild conditions (Scheme 19.36) [59]. The reaction proceeds through stereospecific oxyau-ration to generate vinylgold(III) intermediates 149 [60], which would be converted to the oxazoles 150 by protodeauration and subsequent isomerization. Vinylgold... 

A gold catalysis-initiated cascade process toward the synthesis of electron-rich arene-fused hexahydroquinolizinones 131 was developed by Zhang and Liu in 2012 (Scheme 12.57) [62]. In this cascade reaction, a gold-catalyzed amide cyclization to a tethered C-C triple bond initiates a subsequent Frie del-Crafts type cyclization, followed by a Ferrier rearrangement. Synthetic utility of this... 

Moreover, in a subsequent paper, a reaction based on a gold- or platinum-catalyzed tandem process that involves an intramolecular hydroalkoxylation of a triple bond followed by a Prins-type cyclization has been reported for the synthesis of [3.3.1]bicyclic compounds starting from easily available alkynol derivatives [152] (Scheme 89). 

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