Gold catalysts cyclization

IH of 2-alkynylanihnes has been also studied in the presence of molybdenum [284], palladium [276, 277, 281], and gold catalysts [276]. It provides indoles in low to good yields as a result of 5-Endo-Dig cyclizations (Eq. 4.73) [277]. 

The use of gold catalysts is becoming more important in preparative organic chemistry and several examples have been provided where 4-allene-substituted /3-lactams 234 are cyclized in a short time and in high yield in the presence of gold(m) chloride to the carbapenem 235 (R3, R4 = H or R3, R4yf H) Equation (26) <2005AGE1840>. 

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-... 

The electrophilic cyclization of vinylallenes delivers cyclopentadienes with interesting substitution patterns.35 The anellated cyclopentadienes shown in Scheme 12.18 can be prepared in this way. Silver and gold catalysts have been compared using this substrate. 

Other cyclizations of allenols have been realized by means of gold-catalysts [186, 187], gold and platinum catalysts [188], and with lanthanum amide catalysts [189]. Intermolecular additions of alcohols to allenes were also catalyzed by cationic gold(I) complexes with carbene [190] or phosphane spectator ligands... 

The first gold-catalyzed addition reactions of carbon nucleophiles to allenes were only first disclosed in 2006, and the number of examples is still small. Toste and co-workers showed that allenic silyl enol ethers undergo a 5-endo- ng cyclization to hexahydroindenone derivatives in the presence of a cationic gold catalyst (Scheme 4-10). In these transformations, water or methanol is used as an external proton source for protodeauration of an intermediate vinylgold species. In an analogous manner, cyclopentenes were obtained in good yields from allenic P-ketoesters. In the presence of a palladium catalyst and an allylic halide, these substrates afford functionalized 2,3-dihydroflirans. 

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... 

In a related study, Uemura and co-workers obtained planar-chiral isochromene chromium complexes by gold-catalyzed enantioselective cyclization of prochiral (l,3-dihydroxymethyl-2-alkynyl)benzene chromium complexes. " The highest selectivities in this 6-e <3to-dig-hydroalkoxylation were observed in the presence of a chiral cationic gold catalyst prepared in situ from (/ )-xylyl-BINAP(AuC1)2 and AgSbFs. With (/ )-SEGPHOS(AuCl)2 and AgBp4, the enantiomeric product was obtained, albeit with low enantioselectivity. 

Intriguing variable cyclization modes were also observed in the reaction of allenic epoxyalcohols with cationic gold catalysts (Scheme 4-99). Whereas substrates bearing a 6w-(phenylsulfonyl)methyl or sulfonamide group in the tether afforded mainly or exclusively the 7-exo-trig cyclization product, a very imusual 9-ent/o-trig cycloisomerization was observed with the corresponding malonate derivative. 

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... 

Iwasima and colleagues engineered the Pt-catalyzed [3h-2] cycloaddition of A-(<9-alkynylphenyl)imines with activated alkenes into a synthesis of the mitosene skeleton (Scheme 2, equation 1) [6]. The gold catalyst, AuBrj, is equally effective. Nakamura s group described a novel Pt-catalyzed dehydroalkoxylation/cyclization cascade process... 

A further example for a metal-initiated reaction sequence providing an indole-inspired NP-based complex scaffold was reported by Van der Eycken and coworkers [7]. In this case, a Ugi reaction provided the desired substrate for the sequence of cyclization reactions initiated by the activation of the acetylene with a gold catalyst. The Ugi four-component reaction employed the indole-carboxaldehyde 12, the nitrile 13, amine 14, and carboxyhc acid 15 and led to the formation of the intermediate 16. In the presence of a gold(I) catalyst, instead of the expected indoloazepinone that could be formed via an endo-dig cyclization, a tetracyclic spiroindoline 17 was formed diastereoselectively (Scheme 13.3). 

Vinyl substituted tetrahydronaphthalenes, analogs to 14, can currently be obtained in high yield by employing a gold catalyst for the cyclization of 4-allenyl arenes. Although the scope of the reaction is limited to electron-rich arenes, acetals are tolerated, and compound 20 leads predominantly to /7ora-cyclized product 21 in high yield. 

---