Radical oxidative cyclization with ammonium nitrate

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). 

The reaction of /3-dicarbonyl compounds and simple ketones with Mn(OAc)3 or ceric ammonium nitrate is a general and effective method for C-H activation that has been extensively reviewed [1-6]. The oxidative free-radical cyclizations of la... 

Polyethers are readily accessible by tandem radical cyclizations. For example, bis-allylether 480a,b reacts with a trimethyl tin radical and then undergoes a sequential radical cyclization to provide 481a,b in 86 and 85% yield, respectively (equation (13)) (92JA3115). A ceric ammonium nitrate oxidation of 481 was carried out in methanol and converted the stannyl moiety into the corresponding dimethylacetal. 

The orf/ o-bromo-/7ara-methoxyphenyl group has been used for efficient generation of radicals in the /5-position to an oxygen atom [112], This unique reactivity is of very high synthetic interest, particularly when tertiary radicals are generated (>80% efficiency for the 1,6-hydrogen transfer). An example of a radical cyclization is depicted in Scheme 29. Deprotection of the /lara-methoxyphenyl ether is possible under standard oxidative conditions with ceric ammonium nitrate. 

Reaction with Allylic and Benzylic Electrophiles. The acyl radicals can be trapped with halogen- and silicon-based electrophiles. a -Allylation of a, -unsaturated ketones is done while using Mn(OAc)3 dihydrate and allyl bromide in refluxing benzene (eq 29). Better yields are usually observed for cyclopen-tenones compared to cyclohexenones. a -Benzylation is also possible using benzyl bromide as the electrophilic partner (eq 30). Both methods tolerate a range of substitution, including 8-alkoxy-a, -unsaturated ketones. It is possible to perform a sequential allylation/cyclization with an excess of allyltrimethylsilane (eq 31). Mn(OAc)3 offers good conversions, but the use of ceric ammonium nitrate (CAN) as co-oxidant improves yields. 

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