Radicals cyclization of unsaturated

Radical Cyclizations Section C of Table 11.3 shows some reactions involving cyclization of unsaturated radicals. This type of reaction is an important application of free radical chemistry in synthesis, and is discussed more thoroughly in Section 10.3.3 of Part B. Rates of cyclization reactions have also proven useful in mechanistic studies, where they can serve as reference points for comparison with other reaction rates. 

No product derived from the transannular hydrogen abstraction is observed in the addition of bromotrichloromethane, because bromine atom abstraction is sufficiently rapid to prevent effective competition by the hydrogen-abstraction process. Another example of transannular cyclization of unsaturated radicals is found in the reaction of 1,4-cyclooctadiene with acetaldehyde in the presence of benzoyl peroxide, which gives a cyclized ketone by a process involving an intramolecular addi-... 

As in the case of linear peroxidation products, the initiation step of the formation of isoprostanes is the abstraction of a hydrogen atom from unsaturated acids by a radical of initiator. Initiation is followed by the addition of oxygen to allylic radicals and the cyclization of peroxyl radicals into bicyclic endoperoxide radicals, which form hydroperoxides reacting with hydrogen donors. 

Oxidative radical cyclization of fi-keto esters. Radical cyclizations of unsaturated 0-keto esters initiated by Mn(III) acetate (1) can be terminated by oxidative 0-hydride elimination by Cu(OAc)2 (equation I). This radical reaction can... 

Scheme 24 shows the atom transfer radical cyclizations of unsaturated /3-keto esters 94 using MgfCKLh and chiral ligand 96. It was found that toluene as a solvent generally gave higher enantioselectivities than CH2CI2... 

The PET-oxidative cyclization of unsaturated O-alkyl-O-trimethylsilyl ketene acetals 23 and 27 yields cyclic esters 24, 25, and 28, accompanied by the formation of considerable amounts of non-cyclic esters 26 and 29, respectively [89], The cyclization mode is found to be in accordance with free radical cyclizations of the appropriate esters 26 and 29, performed by heating with organic peroxides [90]. Since organic electrochemistry can be used to oxidize... 

Mattay et al. examined the regioselective and stereoselective cyclization of unsaturated silyl enol ethers by photoinduced electron transfer using DCA and DCN as sensitizers. Thereby the regiochemistry (6-endo versus 5-exo) of the cyclization could be controlled because in the absence of a nucleophile, like an alcohol, the cyclization of the siloxy radical cation is dominant, whereas the presence of a nucleophile favors the reaction pathway via the corresponding a-keto radical. The resulting stereoselective cis ring juncture is due to a favored reactive chair like conformer with the substituents pseudoaxial arranged (Scheme 27) [36,37]. 

Reviews including aspects of P-lactam chemistry are ketene-imine cycloaddition reactions <98CHE1222>, radical cyclization processes <98MI169>, combinatorial synthesis <98AJC875>, electrophilic cyclization of unsaturated amides <98T13681> and theoretical studies on the synthesis of P-lactams <98MI245>. 

The use of Lewis acids in controlling the stereoselective outcome of radical cyclization reactions has been explored, in particular the effect of aluminium-based Lewis acids using low temperature Et3B/Bu3SnH-initiated procedures.171,172 For example, cyclization of propargyl ether (78) or allyl ether (79) in the presence of Lewis acid (80) can completely reverse the normal selectivity (Scheme 34).171 The effect of aluminium Lewis acids on the diastereoselectivity of 6-exo cyclization of unsaturated chiral menthol esters has been studied.172 Cyclization at low temperature in the presence of the Lewis acid MAD modified the de of the reaction from 31 to 98%. 

Cyclization of unsaturated enol silyl ethers.1 Oxidation of certain 8,8- and 8,p-unsaturated enol silyl ethers with either CAN or Cu(OTf)2 can result in radical cyclization. 

Triethylborane can also initiate radical cyclization of unsaturated alkynes to vinylstannanes. 

A preferential 2,5-trans relationship is also observed in the cyclization of unsaturated epoxythio-carbonylimidazolides to 2,5-disubstituted tetrahydrofurans. By treatment with tributyltin hy-dride/2,2 -azobisisobutyronitrile, an allyloxy radical, generated by epoxide cleavage, attacks the double bond, and the corresponding tetrahydrofurans are obtained in low to moderate yield. The diastereoselection is low, with the trans-isomer predominant in the reaction mixture46. 

Radical Cyclization of Unsaturated ALPhenylthioamides General Procedure139 ... 

The. radical cyclization of unsaturated JV-chloro amides is achieved by reactions promoted by benzoyl peroxide67, UV irradiation68,69, or chromium(II) chloride70. 

Preliminary studies on the cyclization of imino radicals generated by irradiation of suitable derivatives of unsaturated oximes 7 and 8 have been reported107. 

Intramolecular cyclization of unsaturated acyl chlorides. Tributyltin hydride reduces acyl chlorides to aldehydes by a free-radical chain reaction involving an acyl radical (I, 1193). Cekovic10 now finds that treatment of acyl chlorides with a double bond in the S- or 6-position with tributyltin hydride (azobisisobutyronitrile initiation) gives cyclohexanone derivatives. Thus 5-hexenoyl chloride (1) is converted into cyclohexanone (2), and citronelloyl chloride (3) is converted into menthone (4). 

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