Hydrogen abstraction-cyclization

Scheme 3.62. Domino radical hydrogen abstraction-cyclization procedure in the synthesis towards Aspidosperma alkaloids.

Methyl-o-benzyloxyphenylglyoxylate (32) cyclizes on irradiation to give an isomeric mixture of 2-phenyl-3-carboxymethyl-3-hydroxyl-2,3-dihydro-benzofurans (33), apparently by an internal hydrogen abstraction-cyclization mechanism.103 This abstraction must take place through a seven-membered... 

As described in Sec. 2.2, the o-xylylenols formed by hydrogen abstraction cyclize thermally to benzocyclobutenols, which at high temperatures reopen to the same o-xylylenols, which can undergo rapid [4+2]-cycloaddition... 

Formations of Cyclic and Spiro Acetals by a Tandem Intramolecular Hydrogen Abstraction-Cyclization Process (Schemes 30 through 36)... 

Cross-linked PVP can also be obtained by cross-linking the preformed polymer chemically (with persulfates, hydrazine, or peroxides) or with actinic radiation (63). This approach requires a source of free radicals capable of hydrogen abstraction from one or another of the labile hydrogens attached alpha to the pyrrohdone carbonyl or lactam nitrogen. The subsequently formed PVP radical can combine with another such radical to form a cross-link or undergo side reactions such as scission or cyclization (64,65), thus ... 

If selenide additions are carried out in the presence of tri- -butylstannane, the radical generated by addition can be reduced by hydrogen abstraction. The chain is then continued by selenide abstraction by the stannyl radical. This leads to nonselenated addition and cyclization products. 

By using various trapping reagents, it has been deduced that the transannular fragmentation is rapidly reversible. The cyclization of the fragmented radical C is less favorable, and it is trapped at rates which exceed that for recyclization under most circumstances. " Radicals derived from ethers and acetals by hydrogen abstraction are subject to fragmentation, with formation of a ketone or ester, respectively. 

The N,0- and N,S-heterocyclic fused ring products 47 were also synthesized under radical chain conditions (Reaction 53). Ketene acetals 46 readily underwent stereocontrolled aryl radical cyclizations on treatment with (TMSlsSiH under standard conditions to afford the central six-membered rings.The tertiary N,0- and N,S-radicals formed on aryl radical reaction at the ketene-N,X(X = O, S)-acetal double bond appear to have reasonable stability. The stereoselectivity in hydrogen abstractions by these intermediate radicals from (TMSlsSiH was investigated and found to provide higher selectivities than BusSnH. 

Figure 7.26. Photo-induced hydrogen abstraction from the y-carbon leads to biradical 72, which can (a) revert to the starting ketone, (b) cyclize, or (c) cleave the 2,3-CC bond. The structure for y-H abstraction for the starting ketone is also shown and the ideal parameters defined and listed.

Entries 18 to 19 pertain to cyclizations of electrophilic radicals generated by oxidations. Entry 18 is the prototype for cyclization of a number of more highly substituted systems. The reaction outcome is consistent with oxidation of the less-substituted enolic position followed by a 6-endo cyclization. The cyclized radical is then oxidized and deprotonated. In Entry 19, the vinyl radical formed by cyclization is reduced by hydrogen abstraction from the solvent ethanol. 

Evidence for the formation of alkyl and aryl radicals in some cases following loss of SO2 (Scheme 1) has been obtained. Thus, a small amount of M-pentane was formed in the decomposition of M-pentanesulphonyl azide in mineral oil ). Thermolysis of diphenyl sulphone-2-sulphonyl azide (8) in dodecane at 150 °C gave diphenyl sulphone 9 (27%) and diphenyl sulphone-2-sulphonamide 10 (9%) which arise by hydrogen abstraction by the aryl radical and sulphonyl nitrene, respectively. When this thermolysis was carried out in Freon E-4 at 150 °C, the products were diphenylene sulphone 77 (1.3%) (Pshorr-type cyclization product of the aryl radical) and 10 (1.5%) together with tars 16h Ferro-... 

Norrish Type II photocyclizations have been employed in many instances in the synthesis of nitrogen heterocycles. Type II cyclizations are the result of an intramolecular hydrogen abstraction by an excited carbonyl group followed by cyclization of the resulting biradical. Hydrogen abstraction from the y-carbon atom is normally preferred. The introduction of a nitrogen atom... 

The presence of heteroatoms in the N-alkyl chain can significantly affect the cyclization process. Benzylic hydrogen abstraction is preferred, for... 

Coumestrol has been prepared via a photocyclization of the glyoxylate ester shown below through consecutive 5-hydrogen abstraction and cyclization processes, and is followed by... 

Excitation of o-nitrodiphenylamine (111) and 3-nitro-2-phenylaminopyridine (112)64 causes the common intramolecular hydrogen abstraction, as the initial step, but subsequent steps involve the elimination of HNO2 and cyclization to give carbazole and a-carboline (113 and 114) in equation 55. 

Scheme 60). Griesbeck et al. assume that in a non-polar solvent such as benzene the intramolecular electron transfer from the methionic sulfur group is much faster than the abstraction of hydrogen from the hydroxyl group of the unprotected amino acid. C-Hydrogen abstraction leads to 313, whereas previous lactonization of the zwitterionic biradical 311 yields 314. Since the cis-hydroxy acid is not detected it is conceivable that it cyclizes immediately to the lactone 314. Photolysis of the corresponding methyl ester under the same conditions attains improved yields (84% combined) of two diastereomeric tricyclic products in a ratio of 48 52. 

Other researchers have reported that the cyclization step is believed to be rate determining in the cycloaromatization (Bergman) reaction of aliphatic enediynes." It has been found that the rate-limiting step is hydrogen abstraction by benzannelation. This effect should be attributable to the faster rate of retro-Bergman cyclization from the aromatic ring-condensed 1,4-didehydrobenzene diradicals and/or the slower rate of hydrogen abstraction by them. 

Thermolysis of benzoenyneallene (113) in cyclohexadiene at 75 °C produced the cycloaromatized adduct (116) in 22% yield. A biradical is believed to form through a cascade sequence involving an initial Myers cyclization. Trapping of the aryl radical centre in (114) with the tetrarylallenic moiety intramolecularly affords (115), having two triaryl radical centres. Hydrogen abstraction from cyclohexa-1,4-diene by (115)... 

The cyclization of 8, s-unsaturated acyl radicals has been the research subject of several groups [27]. The propagation steps for the prototype reaction are illustrated in Scheme 7.4. The 5-exo 6-endo product ratio varies with the change of the silane concentration due to the competition of hydrogen abstraction from the silane with the ring expansion path. 

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