Cyclization of 1,6-enynes

Scheme 12 Mechanism of the titanium alkoxide catalyzed cyclization of enynes...

Similar reactivity is observed in the cyclization of enynes in the presence of the yttrium-based catalyst 70 and a silane reductant [53,54]. The 1,6- and 1,7-enynes 90 and 91 provide -E-alkylidene-cyclopentancs 92 and -cyclohexanes 93 in very good yield (Eq. 15, Scheme 20) [55]. These transformations likely proceed by syn hydrometallation of the 7r-basic alkyne, followed by insertion of the alkene and a-bond metathesis. The reaction of 1,6-enynes tolerated... 

Rhodium and rhodium-cobalt based catalysts using silanes as the stoichiometric reductant were initially reported in 1992 to reductively couple enyne substrate (Eq. 30) [90,91]. Further investigation showed this reaction to be an effective method for the cyclization of enyne substrates 129 to... 

The reductive cyclization of enynes has been used to prepare exo-methylene-cycloalkanes. Two systems have proven successful in this transformation, namely PMHS/Pd2(dba)3 CHCl3245 (Eq. 102) and Et3SiH/Pd(dppe)Cl2/HC02H (Eq. 103).246... 

The palladium-catalyzed hydrostannylative cyclization of enynes is dealt with first, since mechanistically it is closely related to hydrometallation. Lautens262 reported the formation of homoallyl stannanes through the reaction of 1,6-enynes with tributyltin hydride in the presence of a catalytic amount of Pd(OAc)2.263 The active catalytic species is... 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

Table 9 Palladium-catalyzed silastannative cyclization of enyne derived from diethyl malonate...

Cyclization of enynes with isocyanides.1 In the presence of Ni(COD)2 (1 equiv.) complexed with PBu3, 1,6-enynes and an isocyanide cyclize to l-imino-2-cyclopentenes, which can be hydrolyzed to cyclopentenones. 

Cyclization of 1,6- and -enynes.1 This combination produces an unstable Ni-Cr complex that can catalyze cyclization of enynes. The usefulness is markedly improved by coordination to a cross-linked polystyrene. The polymeric Ni-Cr complex (1) has several advantages over Pd(II) catalysts used previously for this cyclization, particularly the ability to effect cyclization of enynes to both five- and six-membered rings. 

Scheme 20.2S Synthesis and cyclization of enyne-allenylphosphine oxides.

Fig. 2.23. Cyclization of enynes with electrophilic and nucleophilic carbene complexes.

A new cyclization strategy via PET-generated electrophilic selenium species starting from diphenyldiselenide was introduced by Pandey et al. They succeeded in the cyclization of enyne compounds in good yields in the range 50-60% as well as in synthesizing a,a -trans dialkyl cyclic ethers via diastereoselective oxyselenation of l,n-diolefins (Scheme 31) [42,43],... 

Table 9.14 Rh(l)-Catalyzed arylative and alkenylative cyclization of enynes.

Furans having a double bond at position 2 can be obtained by a phosphine-initiated cyclization of enyne ketones in the presence of an aldehyde. The reaction may involve the 1,6-addition of the phosphine toward the enyne, ring closure and Wittig reaction of the ylide with the aldehyde <99TL3753>. 

Rhode and Hoffmann have investigated sequential radical-mediated cyclizations of enynes as a method for the stereocontrolled synthesis of heteroannular acetals. The sequence generally followed a S-exo-trigfi-endo-dig-patlmay and gave the products in a stereocontrolled manner <2000T6479>. 

Trost and Tanoury found an interesting skeletal reorganization of enynes using a palladium catalyst.In this reaction, the second product is derived from a metathesis reaction (Equation (5)). It was speculated that the reaction would proceed by oxidative cyclization of enynes with the palladium complex followed by reductive elimination and then ring opening. To confirm this reaction mechanism, they obtained a compound having a cyclobutene ring, which was considered to be formed by the reductive elimination (Equation (6)). 

The cyclization of enynes containing propargylic carbonate fragment involves allene intermediate, which takes part in intramolecular acylpalladation by endo-frig-modc to give a six-membered ring (Scheme 23). " ... 

A study of kinetic isotope effects, theoretical calculations, and dynamic trajectories of the C(2)-C(6) (Schmittel)/ene cyclization of enyne-allenes has been reported.178 For (74), the isotope effect l=h/ l=d = 1-43 is consistent with a highly asynchronous... 

Kinetic isotope effects have been reported that suggest that the thermal cyclization of enyne-allenes proceeds through a stepwise diradical mechanism (Scheme 121). This is even true if steric bulk at the alkyne terminus is large, contrary to theoretical predictions.182... 

Pancrazi and co-workers have reported a 6-endo-trig cyclization of enyne 112 in a synthetic approach to the A-B ring of forskolin [95TL7247], The tandem addition process using tributyltin hydride provides the tricyclic compound 113 in 60% yield. 

Nickel-mediated intramolecular cyclizations of enynes, dienynes, bis-dienes and diynes leading to heterocycles 92SL539. 

Quite recently, the alkenylative cyclization of enynes with ethylene was achieved using Cp RuCl(cod) as a precatalyst at room temperature [92]. This mild and selective transformation was applied to a sulfonamide 95 to produce the corresponding pyrrolidine derivative 96 (Eq. 37). A ruthenacyclopentene intermediate was proposed for this novel cyclization. 

An intramolecular version of olefin cross-metathesis has been demonstrated in cyclization of a,u -alkadienes 293462 (Equation (85)), cyclization of enyne to provide 1,3-dienylboronic esters 297406 (Equation (86)), and in cyclization of boron-tethered enynes obtained from 1-alkynylboronates and allylic alcohols (298 463 (Equation (87)) or allyl boronates and propargyl alcohols.464... 

Cyanosilanes can be isocyanide sources since a tautomeric equilibrium exists between cyanosilanes and the corresponding isocyanides. The equilibrium largely favors the cyano tautomer. The use of such dilute isocyanide donors realizes efficient Ti(n)- and Ni(0)-catalyzed cyclizations of enynes to iminocyclopentenes via metallacyclopentene intermediates (Scheme 19).266,266 266b Treatment of zirconacyclopentanes and -pentenes with Me3SiCN provides zirconocene-imine complexes, which serve for carbon-carbon bond formation with various unsaturated bonds.267... 

Engels, B. Lennartz, C. Hanrath, M. Schmittel, M. Strittmatter, M. Regioselectivity of biradical cyclizations of enyne-allenes influence of substituents on the switch from the Myers-Saito to the novel C —C cyclization, Angew. Chem. Int. Ed. 1998, 37, 1960-1963. 

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