Acyl radicals, generation

Acyl radicals can be generated and they cyclize in the usual manner. A polyene-cyclization reaction generated four rings, initiating the sequence by treatment of a phenylseleno ester with Bu3SnH/AIBN to form the acyl radical, which added to the first alkene unit. The newly formed carbon radical added to the next alkene, and so on. Acyl radicals generated firom Ts(R)NCOSePh derivatives cyclize to form lactams. ... 

Cyclization of both alkyl and acyl radicals generated by selenide abstraction have also been observed. 

Entry 11 involves generation and cyclization of an alkoxymethyl radical from a selenide. The cyclization mode is the anticipated 5-exo with a cis ring juncture. This is a case in which the electronic characteristics of the radical are not particularly favorable (ERG oxygen in the radical), but cyclization nevertheless proceeds readily. The reaction in Entry 12 was used to prepare a precursor of epibatidine. Entry 13 shows a 6-endo cyclization that is favored by steric factors. The 6-endo cyclization is also favored with a tetrahydropyranyloxy substituent in place of the ester, indicating that the electronic effect is not important. Entries 14 to 16 involve acyl radicals generated from selenides. The preferred 6-endo cyclization in Entry 15 is thought to be due to the preference for the less-substituted end of the double bond. Entry 17 is an example of a 5-exo-dig cyclization. 

Minisci-type substitution is one of the most useful reactions for the synthesis of alkyl- and acyl-substituted heteroaromatics. The acyl radicals are formed by the redox decomposition from aldehyde and /-butyl hydroperoxide or by silver-catalyzed decarboxylation of a a-keto acid with persulfate. Synthesis of acylpyrazines 70 as ant pheromones are achieved by this methodology using trialkyl-substituted pyrazines 69 with the acyl radicals generated from aldehydes or a-keto acids (Equation 10) <1996J(P1)2345>. The latter radicals are highly effective for the acylation. Homolytic alkylation of 6-chloro-2-cyanopyrazine 71 is performed by silver-catalyzed decarboxylation of alkanoic acids to provide 5-alkyl-substituted pyrazines 72 (Scheme 18) <1996CCC1109>. 

Cyclization of an acyl radical generated from a carbohydrate-derived hept-6-enoic acid selenoester (to a cyclohexanone) has been studied even though the full potential of this reaction is yet to be established [38],... 

Acyl radicals from RCOSeCfH. Acyl radicals generated from phenylseleno-esters can participate in inter- and intramolecular alkene addition. The latter tandem... 

Bridged derivatives of thiochroman-3-one 545 arise from the capture of the acyl radical generated by the Mn(m) oxidation of the thioether by the aryl ring rather than by the ally 1 group (Equation 197) <1999TA4427>. 

Benzothiazole is acylated selectively in the 2-position by acyl radicals generated from a variety of aldehydes under the influence of the redox system f-butylhydroperoxide-ferrous sulfate. The nucleophilicity of acyl radicals is confirmed by the higher reactivity of 6-nitrobenzothiazole. The reaction, which could detect acyl radicals in the oxidation of aldehydes by various oxidizing agents, could serve as a diagnostic test for the presence of such radicals (71JCS(C)1747). 

Acyl radical sources, other than aldehydes, are also available. The group transfer addition of an acyl radical has been reported by Zard and co-workers, where S-acyl xanthates serve as acyl radical sources [44]. Crich and co-workers reported that an acyl radical, generated from an aromatic acyl telluride by photolysis, adds to an allylic sulfide which contains an ethoxycarbonyl group to form the corre-.sponding y-unsaturated ketones [45]. The addition pathway involves Sh2 type reaction with extrusion of a /ert-butylthiyl radical. 

Sibi and Ji reported that acyl radicals, generated from acyl bromides, can participate in Lewis acid-mediated diastereoselective radical addition reactions (Scheme 4-26) [49]. Using triethylborane/02 as a radical initiator, the reaction was conducted at -78 °C. 

Ryu, Sonoda and coworkers have investigated the macrocyclization of acyl radicals generated by the carbonylation of alkyl radicals which employ (TMS)3SiH as chain carrier [46], Ring sizes from 10 and upwards could be successfully synthesized by [n-l-1] type annulation. Competing macrocyclizations of the initial alkyl radical do not appear to be a problem in this work when a combination of high dilution and 30 atm of CO pressure was used. 

In 1988, Boger and co-worker found that the intramolecular free-radical cyclization reactions of acyl radicals generated from selenol esters proceeded efficiently, suppressing competing reduction and decarbonylation (Eq. 47) [98]. 

The Pd-catalysed carbonylation of o-iodophenols with allenes is regioselective and generally yields 3-methylenechroman-4-ones <97JOC1566>, which are also formed via cyclisation of the acyl radicals generated by the decomposition of the allyloxybenzoyl diazonium salts (39) in the presence of Nal <97JOC5982>. 

Other examples of addition to the oxygen atom in the Cy5/Cy4 case have been observed with acyl radicals, generated by photolysis of o-phthalaldehyde, " " or of aryl phthalates " (X = O) and thiophthalates (X = S) (194). Isomerized products (196) (X = O) or coupling products (X = S) from 195 have been rationalized according to Scheme 85. As in the preceding... 

In this protocol, cyclohexanone peroxide and 2-methyl-l-oxa-2-azaspiro[2.5]octane were used, in the presence of Fe(II) salt, as free-radical sources. Afterward, in 1969 Caronna et al. reported the acylation of aromatic bases by acyl radicals (Scheme 7(c)) [2], The nucleophilic character of acyl radicals, generated in situ by Fe(II)-mediated hydroperoxide decomposition (Scheme 7(a) and (b)), was demonstrated to play the key role in the process. 

Caddick et al. have successfully applied the aerobic acylation of the electron-deficient vinyl phosphonates (668) by acyl radicals generated from aldehydes (669) via their auto-oxidation, to the synthesis of y-ketophos-phonates (670) (Scheme 168) ... 

FIGURE 48.5 Reaction coordinate (RC) derived from computational studies of the Norrish type I reaction. The RC shows a-cleavage from both Sj and Tj surfaces to yield the acyl-alkyl biradical/radical pair. Subsequent decar-bonylation of the acyl radical generates the alkyl-alkyl biradical/radical pair. 

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