Gold-catalyzed cyclization

Alcohol addition was also studied by Hashmi et al. in intramolecular processes [28]. Through gold catalyzed cyclizations of (Z)-3-ethynylallyl alcohols 157, these authors were able to obtain furans 159. Reaction occurred via intermediate 158, which tautomerized the heteroaromatic furan, which is thermodynamically more stable (Scheme 8.15). 

Another example concerning furan synthesis by gold-catalyzed cyclization was also reported. Reaction of (Z)-enynols using AuClj furnish fully substituted furans in high yields under mild reaction conditions <050L5409>. 

Biannic, B. Gold-Catalyzed Cyclization of Mono-Allylic Diols and Ethers. Ph.D. Thesis, University of Florida Gainesville, FL, 2011. 

SCHEME 4 Synthesis of (+)-241D (13) using gold-catalyzed cyclization. 

Representative procedure for the gold-catalyzed cyclization of aryl propargyl ethers. 2H-Chromen ... 

Representative procedure for the gold-catalyzed cyclization of N-propargylindole-2-carboxamides. 4,9-Dimethyl-2-tosyl-2H-pyrido[3,4-b]indol-1 (9H)-one ... 

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. ... 

The exo-selective gold-catalyzed cyclization of alkenyl carbamates, amides, or ammonium salts affords the corresponding pyrrolidine or piperidine derivatives. Here, cationic gold(I) catalysts bearing sterically hindered, electron-rich phosphine ligands such as P(r-Bu)2(o-biphenyl) were much more reactive than PhsPAuOTf... 

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... 

Representative procedure for the gold-catalyzed cyclization of 4-hydroxyalky-2-yn-l-ones to 3(2H)-furanones. 2,2-Dimethyl-5-(2-phenylethyl)-3(2H)-furanone ... 

Representative procedure for the enantioselective gold-catalyzed cyclization of (I,3-dihydroxymethyl-2-alkynyl)benzene chromium complexes. (R -Tricarbonyl-(3-methyl-5-hydroxymethyl-lH-isochromene)chromiunP ... 

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... 

Representative procedure for the gold-catalyzed cyclization of allenic malonate esters. Methyl 3-(cyclohex-2-enyl)-3,6-dihydro-6,6-dimethyl-2-oxo-2H-pyran-3-carboxylat ... 

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... 

Even allylic alcohols participate in gold-catalyzed cyclizations of this type. Aponick and co-workers obtained substituted tetrahydrofui s and tetrapyrans by dehydrative cyclization of allylic diols in the presence of PhsPAuCl and AgOTf (Scheme 4-115). High cis-diastereoselectivities were observed when the reaction was carried out at low temperature (-10...-78 C). Bandini and co-workers ... 

General procedure for the synthesis of morpholines by gold-catalyzed cyclization of... 

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,... 

Liu, Y, Song, F., Song, Z., Liu, M. and Yan, B. 2005. Gold-catalyzed cyclization of (Z)-2-En-4-yn-l-ols Highly efficient synthesis of fuUy substituted dihydrofurans and furans. Org. 

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]. 

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