Cyclizations gold chloride

Gold chloride forms an unstable complex with cyclo-octatetraene at low temperatures. This decomposes at — 20°C to l,2-dichlorocyclo-octa-3,5,7-triene, which gradually cyclizes to (562). In an attempt to form pentalene-metal complexes, cyclo-octatetraene was reduced to a mixture of trienes and bicyclo [4,2,0] octadiene, which was then treated with substituent ruthenium carbonyls, affording a variety of complexes, including (563). °... 

The substitution of propargyhc alcohols by amides gives propargyUc amides which can be isomerized into allenyl amides and cyclized into oxa-zoles in the presence of catalytic amounts of gold chloride (Scheme 49). The catalytic transformations can be performed by introduction of aU the starting products including the catalysts at the beginning of the reaction, but better yields are obtained when AuCls is introduced after completion of the rutheniiun-catalyzed reaction [106]. 

In a more recent publication Hashmi et al. reported that propargyl ketones such as 20 and allenyl ketones such as 21 can be cyclized to furans such as 22 in reactions catalyzed by gold(III) chloride (Scheme 5) [12]. This type of reaction had already been described as a... 

However, the ability of gold(III) chloride to provide protic catalysis under exceptionally mild conditions is further demonstated by two recent examples the hydroxyallene 35 bearing a silyl protecting group is efficiently cyclyzed to give the 2,5-dihydrofuran 36 without deprotection [20] other acidic catalysts which in principle sufficiently promote this type of cyclization - such as HC1 gas or Amberlyst 15 resin - are of course much less compatible with acid sensitive functionalities. Also for the formation of macrocycle 39 gold(III) chloride turned out to be the catalyst of choice [21],... 

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>. 

Functionalized 2-hydroxy-3-allenoates can be converted into 2,5-dihydrofurans by using 5-10 mol% of gold(lll) chloride as catalyst. This mild and efficient cyclization method can be applied to alkyl- and alkenyl-substituted allenes at room temperature, furnishing tri- and tetrasubstituted dihydrofurans in good to excellent yields and with complete axis-to-center chirality transfer (Equation 50) <20010L2537>. 

CycUzation. 3-Alkynones cyclize to afford furan derivatives in the presence of gold(III) chloride. Furans bearing an alkynyl chain undergo intramolecular cycloaddition further. Transformation into bicyclic phenols is noteworthy because the hydroxy group is at a pen-position. 

The catalyst is formed in situ either from a phosphinegold(I) chloride and a silver salt, or by reaction of PhaPAuMe with a Bronsted acid, e.g., HBF4 (Scheme 4-30). No cyclization takes place with protic acids in the absence of gold(I). 

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... 

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... 

Toste has reported a gold(I)-catalyzed protocol for the intramolecular addition of an alkyl azide to an alkyne with loss of dinitrogen (acetylenic Schmidt reaction) to form pyrrole derivatives [19]. For example, treatment of homopropargyl azide 16 with a catalytic 1 2 mixture of (dppm)Au2Cl2 (dppm = l,2-bis(diphenylphosphino)meth-ane) and AgSbFfi in methylene chloride at 35 °C led to isolation of2,5-dibutylpyrrole in 82% yield (Eq. (11.13)). The method was also effective for the cyclization of a-unsubstituted homopropargyl azides and tolerated both alkyl and aryl substitution of the alkyne. 

Alkynylindole III-29 cyclized in the presence of AuCla at room temperature to give indoloazocine III-30 as the major product in a 55 % isolated yield (Table 4.1, entry 1). Indoloazocine III-30 contains the tetracyclic core present in lundurine A. Traces of vinyl chloride III-31, which is the product of Markonikov gold-catalyzed hydrochlorination in this reaction [103-105], were also observed using AUCI3. 

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