Carbocyclizations enyne cycloisomerization

Scheme 48. Carbocyclic product formation during Pt(II)-enyne cycloisomerization.

Michael additions of 7r-allyl species to alkynes were employed for the synthesis of elaborated carbocycles as in the ruthenium-catalyzed cycloisomerization of 1,6-enynes (Equation (188)).1... 

Enantioselective hydrogenation of 1,6-enynes using chirally modified cationic rhodium precatalysts enables enantioselective reductive cyclization to afford alky-lidene-substituted carbocycles and heterocycles [27 b, 41, 42]. Good to excellent yields and exceptional levels of asymmetric induction are observed across a structurally diverse set of substrates. For systems that embody 1,2-disubstituted alkenes, competitive /9-hydride elimination en route to products of cycloisomerization is observed. However, related enone-containing substrates cannot engage in /9-hydride elimination, and undergo reductive cyclization in good yield (Table 22.12). 

Platinum(ll)-catalyzed carbocyclization of a.ty-enynes has been intensively researched lately. For instance, 2-disubstituted l-en-6-ynes react with methanol in the presence of PtCl2 as catalyst to form carbocycles with exocyclic aUcenes. The analogous reactions of 3-allyl propargyl ether or 3-allyl propargyl tosylamine in water using the same catalysts will generate cyclopropyl aldehydes besides the product of cycloisomerization (Scheme 82). The coordination of the... 

Cyclopentadienyl compounds of the form CpCo(CO)2 are also known to convert l,n-enynes into the tranditional 1,3-diene cycloisomerization products (486, 487). Remarkably, the metathesis reaction is independent of tether length between both unsaturated functional groups, and leads to five-membered ring carbocycles exclusively (Scheme 60). 

Cycloisomerization of enynes. Formation of 5-membered heterocycles and carbocycles from open-chain enynes may lead to 1,3- or 1,4-dienes by changing the substrates slightly. The product profiles are different from reactions catalyzed by the (dba),Pd2 CHCl3-HOAc-(o-Tol)3P system. " ... 

Transition metal-catalyzed carbocyclizations of 1,6-enynes A [7-11] such as cycloisomerization [12-18], metathesis [19-30], skeletal reorganization [31-34], and ene reactions [35] are useful synthetic methods leading to five-membered rings (Scheme 4). Transition metal-catalyzed carbocyclizations of... 

Transition-metal-catalyzed intramolecular cycloisomerization is one of the most useful carbocyclization reactions, and specifically, the Rh-catalyzed cycloisomerization of 1,6-enynes provides a powerful tool in organic synthesis. Cyclization of l,6-en5me 452 catalyzed by Wilkinson s catalyst gave 1-exo-methylene-2-cyclohexene 453 via a 6-exo-trig mode in 83% yield (Scheme 2-50). Terminal substitution on the alkene moiety dramatically suppressed the cyclization, and substitution of the terminal alkyne moiety was detrimental to the reaction as well. 

There are many examples of preparing cyclopentane structures from enynes by gold-catalyzed carbocyclization reactions. Toste et al. have reported that Au(l)-phosphine complexes act as superior catalysts for isomerization of 1,5-enynes to bicyclo[3.1.0]hexenes [124]. For example, treatment of 1,5-enyne (86) with 1 mol% of PhsP-AuPFfi in dichloromethane at room temperature results in formation of cyclopropane-fused cyclopentene (87) in 99% yield (Scheme 18.30). 1,6-Enynes also undergo similar cycloisomerization to five-membered cyclic compounds under the influence of cationic gold(l) catalysts [125, 126], Hydroxylated enynes are versatile precursors for cyclopentenones by gold-catalyzed cycloisomerization... 

Cycloisomerization of dienes and enynes into cyclic molecules with Grubbs second-generation catalysts modified by reaction with DMF has been reported to give a species able to catalyse the cyclization reaction.Metathesis of a bicyclic diene in the presence of ethane has been reported to give di-fused[3.0.3]carbocycles via a ring-rearrangement metathesis transformation (Scheme 95). " ... 

---