Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Oxygen-centered radicals cyclizations

Zlotorzynska, M., Zhai, H., M. Sammis, G. M. (2008). Chemoselective Oxygen-Centered Radical Cyclizations onto Silyl Enol Ethers. Organic Letters 10(21), 5083. [Pg.181]

Eq. 4.54 shows the reaction of n-heptanol (151) with Pb(OAc)4 under high-pressured carbon monoxide with an autoclave to generate the corresponding 8-lactone (152). This reaction proceeds through the formation of an oxygen-centered radical by the reaction of alcohol (151) with Pb(OAc)4,1,5-H shift, reaction with carbon monoxide to form an acyl radical, oxidation of the acyl radical with Pb(OAc)4, and finally, polar cyclization to provide 8-lactone [142-146]. This reaction can be used for primary and secondary alcohols, while (3-cleavage reaction of the formed alkoxyl radicals derived from tertiary alcohols occurs. [Pg.149]

The first step in the mechanism is the homolysis of the 0-N bond to form an oxygen-centered radical and a nitrogen-centered free radical. Next, the highly reactive alkoxyl radical abstracts a hydrogen atom from the 5-position (5-position) via a quasi chair-like six-atom transition state to generate a new carbon-centered radical that is captured by the initially formed NO free radical. If a competing radical source such as iodine is present, the reaction leads to an iodohydrin, which can cyclize to form a tetrahydrofuran derivative. Occasionally, tetrahydropyran derivatives are obtained in low yields. [Pg.42]

Selectivity in cyclizations of oxygen-centered radicals with the formation of furans 02S1469. [Pg.193]

This cleavage will therefore be discussed first. Several substrates have proven useful for the generation of the pivotal oxiranylcarbinyl radicals. Sharpless epoxides are valuable starting materials for radical precursors [18]. After formation of the oxygen-centered radical, reactions typical for radicals are observed. With a suitably positioned double bond, cyclization occurs as shown in Scheme 10 [19]. It is interesting that the cw-disubstituted carbon-centered radical can react further to yield a bicyclo[2.2.1] system. [Pg.710]

In stark contrast to azoalkanes, azoxy compounds rarely form radicals on heating or irradiation. Furthermore, they are unreactive to alkyl radical attack unless the reaction is intramolecular. For example, P-carbon centered radicals cyclize to azoxy nitrogen or oxygen and produce short-lived aminyl nitroxides that reopen or hydrazyl radicals that undergo fragmentation. The azoxy group is a powerful stabilizer of an adjacent radical center but the chemistry of a-azoxy radicals (hydrazonyloxides) and their dimers is not fully understood. [Pg.17]

Several useful synthetic methodologies are based on the generation of the oxygen-centered radicals from carboxylic acids and the (diacetoxyiodo)benzene-iodine system [613-617]. In particular, a direct conversion of 2-substituted benzoic acids 566 into lactones 567 via oxidative cyclization induced by [bis(acyloxy)iodo]arene/iodine has been reported (Scheme 3.224) [613,614]. [Pg.238]

The applications of such electronic effects extend to radical chemistry. For example, reactions of electrophilic oxygen-centered radicals display excellent chemoselectivity for cyclization onto the electron-rich silyl enol ether when competing with terminal alkene cyclization, 1,5-hydrogen abstraction, and p-fragmentation pathways (Figure 6.115). ... [Pg.160]

Lactone rings have also been constructed using carbonylation of appropriately functionalized compounds by the insertion of carbon monoxide. Ryu, Sonoda et al. have reported the synthesis of 8-lactones from saturated alcohols and carbon monoxide via remote carbonylation [110,111] (Scheme 64). Treatment of saturated alcohol 292 with lead tetraacetate led to oxygen-centered radical 293, which underwent a 1,5-hydrogen transfer reaction to produce carbon-centered radical 294. Trapping of this radical with carbon monoxide and oxidation followed by cyclization gave lactone 297. [Pg.129]

For recent reviews, see (a) Hartung, J., Gottwald, T, and Spehar, K., Selectivity in the chemistry of oxygen-centered radicals the formation of carbon-oxygen bonds. Synthesis, 1469, 2002 (b) Hartung, J., Stereoselective construction of the tetrahydrofuran nucleus by alkoxyl radical cyclizations, Eur. J. Org. Chem., 619, 2001. [Pg.1354]

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). [Pg.73]


See other pages where Oxygen-centered radicals cyclizations is mentioned: [Pg.10]    [Pg.814]    [Pg.147]    [Pg.248]    [Pg.192]    [Pg.18]    [Pg.709]    [Pg.710]    [Pg.711]    [Pg.535]    [Pg.1348]    [Pg.382]    [Pg.386]    [Pg.392]    [Pg.783]    [Pg.350]    [Pg.212]    [Pg.213]    [Pg.217]    [Pg.221]    [Pg.212]    [Pg.213]    [Pg.217]    [Pg.221]    [Pg.779]    [Pg.811]    [Pg.811]    [Pg.24]    [Pg.27]    [Pg.76]    [Pg.235]    [Pg.409]    [Pg.21]    [Pg.80]    [Pg.105]   
See also in sourсe #XX -- [ Pg.811 , Pg.812 , Pg.813 ]

See also in sourсe #XX -- [ Pg.4 , Pg.811 , Pg.812 , Pg.813 ]

See also in sourсe #XX -- [ Pg.4 , Pg.811 , Pg.814 ]




SEARCH



Centered Radicals

Cyclization oxygenative

Oxygen-centered radical

Radical centers

Radical cyclization

Radicals oxygen-centered radical

© 2024 chempedia.info