Radical cyclizations carbon-centered radicals

The success of intramolecular conjugate additions of carbon-centered radicals in multifunctional contexts is noteworthy. Compound 57 (see Scheme 10), prepared by an interesting sequence starting from meto-toluic acid (54) (see 54 > 55 > 56 > 57), can be converted to the highly functionalized perhydroindane 58 through an intramolecular conjugate addition of a hindered secondary radical.21-22 This radical cyclization actually furnishes a 6 1 mixture of perhydroindane diastereoisomers, epimeric at C-7, in favor of 58 (96 % total yield). It should be noted that a substantially less strained cis-fused bicyclo[4.3.0] substructure is formed in this cyclization. 

Carbon-centered radicals generated by Barton s thiohydroxamate method can also participate in ring-forming reactions (see Scheme 26).52b,s3 For example, irradiation of 129 results in the formation of compound 130 (82% yield). The outcome of this transformation is reminiscent of Stork s elegant radical cyclization/trapping processes (see Schemes 7 and 8), in that/botn alkene carbon atoms have become functionalized. / I... 

The transition structure trans-42 to fra s-43 TiCp2Cl was also calculated and a barrier of 11.7 kcal mol 1 was obtained for the cyclization. This value is similar to the activation energy for the formation of cis-43 TiCp2Cl, although the distance between the carbon-centered radical and oxygen atom is shorter (2.04 A). 

Rate constants for reactions of Bu3SnH with some a-substituted carbon-centered radicals have been determined. These values were obtained by initially calibrating a substituted radical clock on an absolute kinetic scale and then using the clock in competition kinetic studies with Bu3SnH. Radical clocks 24 and 25 were calibrated by kinetic ESR spectroscopy,88 whereas rate constants for clocks 26-31 were measured directly by LFP.19,89 90 For one case, reaction of Bu3SnH with radical 29, a rate constant was measured directly by LFP using the cyclization of 29 as the probe reaction.19... 

Similar to the intramolecular addition of neutral carbon-centered radicals to alkenes, the formation of radical cations starting from alkenes with subsequent cyclization offers a convenient method for constructing carbocyclic ring systems. In contrast to the regioselective 1,5-ring closure (5-cxo-trig cyclization) of the... 

Pandey and co-workers developed two photosystems useful for initiating one-electron reductive chemistry and applied them to activate a, 3-unsaturated ketones. The resulting carbon-centered radicals cyclize stereoselectively with proximate olefins. Their concept involved a secondary and dark electron transfer from... 

By using either one of these photosystems, one-electron (3-activation of a,(3-unsaturated carbonyl compounds produced carbon-centered radical precursors which cyclize efficiently and stereoselectively to tethered activated olefins or carbonyl groups. The 1,2-anti-stereochemistry observed contrasts with the general trend of syn-stereochemistry expected in 5-hexenyl radical cyclizations. Application of this methodology was successfully demonstrated by the stereoselective synthesis of optically pure C-furanoside, starting from L-tartaric acid (Scheme 38) [57,58]. 

Cyclization of nitro-stabilized radicals provides another method for the generation of cyclic nitronates (221). Oxidation of the aci-foim of nitroalkanes with ceric ammonium nitrate generates the ot-carbon centered radical, which in the presence of an alkene, leads to the homologation of the a-radical. In the case of a tethered alkene of appropriate length, radical addition leads to a cyclic nitronate (Scheme 2.20). 

Selenides are more readily cleaved by tin radicals than are thioethers. Two examples of the tin radical mediated cleavage of selenides are listed in Table 3.48 (Entries 8 and 9) more examples have been reported [768,773,821-823], The carbon-centered radicals initially formed by homolytic C-Se bond cleavage can either be directly reduced to the alkane by treatment with a tin hydride, or may add to multiple bonds before reduction. Entry 10 in Table 3.48 is an example of the formation of a polycyclic indoline through radical cyclization. Radical-mediated cleavage proceeds under mild,... 

CYCLIZATIONS OF CARBON-CENTERED RADICALS TO CARBON-CARBON MULTIPLE BONDS 789... 

Alkoxyl radicals can be generated by a variety of methods including peroxide reduction, nitrite ester photolysis, hypohalite thermolysis, and fragmentation of epoxyalkyl radicals (for additional examples of alkoxyl radical generation, see Section 4.2.S.2). Hypohalites are excellent halogen atom donors to carbon-centered radicals, and a recent example of this type of cyclization from the work of Kraus is illustrated in Scheme 43.182 Oxidation of the hemiketal (57) presumably forms an intermediate hypoiodite, which spontaneously cyclizes to (58) by an atom transfer mechanism. Unfortunately, the direct application of the Barton method for the generation of alkoxyl radicals fails because the intermediate pyridine-thione carbonates are sensitive to hydrolytic reactions. However, in a very important recent development, Beckwith and Hay have shown that alkoxyl radicals are formed from N-alkoxypyridinethiones.183 Al-... 

If, oil the other hand, R- is unsaturated and can undergo cyclization rapidly, it will do so. This competition between reduction of the first formed radical R and its cyclization to a new cyclic radical R - is the same as discussed for the formation of free radicals from alkyl halides and tributyltin radicals. The only difference is in the way in which the carbon-centered radical is produced. 

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