A gold -catalyzed cyclization

Schreiber and Luo exploited domino sequences in both the coupling and pairing phases [25]. A gold-catalyzed cyclization of alkyne 157 was employed to obtain molecules that could subsequently be used in the pairing reaction. Activation of the alkyne by a gold(I) complex yielded the cydized cationic species 160,... 

A series of other structurally different cationic intermediates, generated following a gold-catalyzed cyclization step, are also susceptible to be trapped by an oxygen, nitrogen, or even carbon-centered nucleophile (Scheme 16.19) [23]. 

A valuable method for the synthesis of 2-aryl-3-fluoropyrroles 247 is based on a gold-catalyzed cyclization and dehydrofluorination of gan-difluorohomopropargylamines 246. Difluorinated homopropargylamines 246 can be prepared by the addition of gem-difluoropropargyllithium reagents to arylated A-tosyUmines 244 [84]. 

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. 

SCHEME 2.102 Synthesis of oxazepanones through a gold-catalyzed cyclization reaction [148]. 

In a spectacular work, Morita and Krause24 studied the cyclization of allenylthio-carbinols to dihydrothiophenes by copper, silver, and gold catalysts. The reaction was the first example of a gold-catalyzed C-S bond formation and was highly stereo-... 

A-vinylindoles took place. Moreover, the gold-catalyzed cyclization of 2-alkynylanilines can be combined with a gold-catalyzed Michael addition to enones, which afforded 3-alkylated indoles with good yield. Gold-catalyzed hydroamination/hydroarylation cascades of polyenyne-substituted anilines leading to condensed aromatics have been described recently. ... 

Acetylenic ketones are viable substrates as well for gold-catalyzed cyclizations to furans. Whereas alk-3-yn-l-ones readily cyclize to substituted furans in the presence of gold(I) or gold(III) chloride (possibly via isomerization to a-allenyl ketones see Section 5.2), the corresponding transformation of alk-4-yn-l-ones takes place in the presence of the cationic gold catalyst generated in situ from PhsPAuCl and AgOTf in toluene (Scheme 4-83). The cyclization is accelerated by... 

The cycloisomerization of a-allenyl ketones to the corresponding substituted furans was the first example of a gold-catalyzed addition of an oxygen nucleophile to an allene (Scheme 4-86). Traditionally, silver or palladium catalysts were employed for cyclizations of this type advantages of gold catalysis incluiie shorter reaction times, milder conditions, an or lower catalyst loadings. Variable amounts of... 

Instead of allenic carboxylic acids, the corresponding esters can also be used as substrates for gold-catalyzed cyclization reactions. Thus, heating t-butyl allenoates with gold(III) chloride in dichloromethane afforded butenolides in high yield (Scheme 4-104). In a similar way, Backvall et obtained 5-lactones by... 

A related gold-catalyzed cyclization of allyl acetates beariim a malonate as internal nucleophile leads to y-vinyl butyrolactones (Scheme 4-114). AuBrs and in situ formed Ph3PAuSbp6 are the best catalysts for this transformation, which occurred with good yield and excellent diastereoselectivity in most cases. Intermolecular C-O bond... 

In the total synthesis of (+)-lycopladine A by Toste et al. [30], a gold-catalyzed 5-endo cyclization of an iodoalkyne with a silyl enol ether has been used (Scheme 16.25). This transformation efficiently produces a -unsaturated bicyclic ketone that possesses the required quaternary asymmetric center at the position a to the carbonyl group. The vinyl iodide functionaiity generated during the cyclization was subsequently used in a palladium-catalyzed cross-coupling reaction in order to construct the pyridine ring of (-F)-lycopladine A. 

Intramolecular additions of alcohols or amines to olefins are recognized as useful methods to provide heterocyclic compounds. Kozmin and Zhang have achieved a gold-catalyzed synthesis of heterobicyclic alkenes by double cyclization of alcohols or sulfonamides which have a 1,5-enyne moiety [175]. As a typical example, unsaturated alcohol (95) is smoothly converted to 6-oxabicyclo[3.2.1]octane (96) in 90% yield using a 5 mol% of AuCls catalyst (Scheme 18.34). Cyclic ethers are also available from homopropargylic ethers with pendant alcohols through gold-catalyzed... 

Gold-catalyzed cyclization of 1,5-enynes allows the synthesis of a wide variety of synthetically useful products (Scheme 1.22) [24, 160]. The mechanism of 1,5-enynes resembles the one of 1,6-enynes, cyclizing through an endocycUc pathway. 

---