Enyne cydization

Scheme 25 Mechanism of the reductive enyne cydization promoted by Pd and silane...

Scheme 31 Scope and selectivity of reductive enyne cydization by Rh4(CO)i...

Kozmin has reported the formation of polyqrclic ethers via the gold(I) or gold(III)-catalyzed tandem enyne cydization/alkoxylation of 1,5-enynes that contained a tethered hydroxy group [126]. As an example, treatment of homopropargyUc alcohol 86 with a catalytic 1 1 mixture of (PPhs) AuCl and AgC104 at 20 °C led to isolation of 6-oxabicyclo[3.2.1]octene 87 in 89% yield (Scheme 12.15). Conversion of 86 to 87 presumably occurs via attack of the pendant alkene on a goldhydroxy group at the disubstituted alkenyl carbon atom. Protodeauration then releases heterocycle 87 (Scheme 12.15). 

Palladium complexes are effective catalysts for the reductive cydization of enyne substrates [53,54], The first report of catalytic cydization of 1,6- and 1,7-enynes 115a,b to cyclopentane 116a and cyclohexane 116b derivatives appeared in 1987 (Eq. 19) [70]. The authors proposed that the Pd(II) species 117 forms by oxidative addition of acetic acid to Pd(0) (Scheme 25). Complex 117 hydrometallates the alkyne to give 118, which cyclizes to provide... 

For reviews encompassing the Pd-cata-lyzed cycloisomerization and reductive cydization of 1,6-enynes, see (a) B.M. Trost, Acc. Chem. Res. 1990, 23, 34 ... 

The enyne-allene 20b having two sterically demanding iert-butyl groups exhibited a slower rate of cydization (t /2 60 min at 76 °C) when compared with 20a (Eq. 20.2). It is worth noting that the benzylic radical center in 22 is an a,a-di-t-butyl-benzylic radical, which has been shown to be persistent in dilution solution at room temperature for several days [32],... 

The presence of a methyl substituent at the acetylenic terminus of the enyne-allenes 28e-n appears to reduce the rate of cydization, presumably for steric reasons. The higher thermal stability allows their isolation and purification at ambient temperature without special precautions. 

Very recently, Genet and Michelet developed a diastereoselective reaction that took place by hydroamination of an unactivated alkene followed by a cydization process under very mild conditions. This 1,6-enyne reaction was compatible with electron-poor aromatic amines, including amines bearing chloride atoms (Equation 8.73), which is very useful for further functionalization of the substituted aniline ring [164]. 

Scheme 25 NHC/Pd-catalyzed bismetallative cydization of enyne with Bu3SnSiMe3...

RXN52 Reductive Cydization with Diynes and Enynes 220... 

RXN64 Tandem Cydization-Capture Process of Enynes 260... 

Genet and Michelet have reported the gold(I)-catalyzed cydization/amination of 1,6-enynes with carboxamides and electron-deficient aromatic amines [27]. Reaction of 1,6-enyne 32 with o-cyanoaniline and a catalytic 1 1 mixture of (PPh3)AuCl and AgSbFg in dioxane at room temperature for 20 h led to isolation of methylene cyclopentane 33 in 93% yield as a single diastereomer (Eq. (11.19)). Gold(I)-catalyzed... 

Hong prepared Au complexes of extremely hindered ADC ligands (e.g., 32, Scheme 16.11) and examined their catalytic activities in the tandem indole addi-tion/cydization of enyne 29 [32]. ADC complex 32 favored the alkene product... 

The cross-coupling of alkynylzinc halides or fluorinated alkenylzinc halides with fluorinated alkenyl iodides allows the preparation of a range of fluorinated dienes or enynes [265]. Functionalized allylic boronic esters can be prepared by the cross-coupling of (dialkylboryl)methylzinc iodide 428 with functionalized alkenyl iodides. The intramolecular reaction provides cydized products, such as 429 (Scheme 7.109) [53c-ej. In some cases, reduction reactions [266] or halogen-zinc exchange reactions [40] are observed. 

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