Rearrangement radical-induced

More than two dozen radical clocks are now known. They span a range of lifetimes from 10 to 10"7 s.12 The investigator must be aware of the possibility that the clock rearrangement is due to a side reaction or that the radical induced an efficient chain mechanism (Chapter 8). Also, radicals are not the only entities that can rearrange in this fashion. Carbanions, for example, have been shown to rearrange under certain conditions. 

The formation of 3,3-difluoro-3-arylpropanoates in good yields from the radical-induced rearrangement of 3-bromo-3,3-difluoroalanine Schiff bases has been explained by postulating a radical /pxo-substitution at the aromatic ring as shown in Scheme 14. 

In DNA free-radical chemistry allylperoxyl radicals play a major role in the free-radical-induced oxidation of Thy. Thus far, this kind of rearrangement has not yet been observed (but also not especially looked for) in this system. 

The modular nature of holoenzyme catalysis allows coenzyme B12 to be used in a variety of biological reactions with different apoenzymes. These reactions can involve homolysis of the Co—C bond to give an alkyl radical resulting in radical induced rearrangements, redox reactivity via reduction to Co (II) and Co (I), and alkylations. The nature of the coenzyme determines the type of the reaction, while the nature of the apoenzyme determines the selectivity of the reaction in terms of the substrate and the regiospecificity. 

V. Synthesis of 2-Deoxy Sugars by Radical-Induced Rearrangement. 130... 

A completely different reaction takes place if Si2me6 is reacted with meONa in presence of hexamethylphosphoric acid triamide498 (see p. 21). Trisilane thiols readly undergo a radical-induced skeletal rearrangement ... 

The term sigmatropic rearrangement is used here only in a formal sense, i.e., for a process described by the formulae above, without implication of reaction mechanism. Therefore, thermal, metal-catalyzed and even radical-induced rearrangements, which fulfill this requirement, are included in this section. However, with respect to nomenclature, sigmatropic rearrangements are described in the way introduced by Woodward and Hoffmann, i.e., by stating the number of atoms from the cleavage site to the centers where the new n-bond will be formed. 

Figure 7 H202-mediated, nonreducing terminal hexosamine cleavage. Terminal amino sugar loss is the major product (but some peeling of the n-1, n-2 sugar is seen), possibly via the rearrangement of a nitroso oxidation product to liberate NO and generate a free radical that rearranges to induce the partial loss of the next sugar, and so forth.

Reduction of bromide 320 under phase-transfer catalysis (PTC) conditions provided the 5-methylisoxazolidines 321 and 322 in 86% overall yield. Under the same conditions, the diastereomer 323 remained unchanged. The 5-methyl derivative 324 was obtained along with a substantial amount of pyrrolidine 325 by treatment with sodium cyanoborohydride in boiling DMF (Equations 58 and 59) <1997T739>. Isoxazolidines 320 and 323 underwent a radical-induced rearrangement upon treatment with tributyltin hydride (see Section 4.03.10). 

Thionocarbonates may proceed a radical-induced rearrangement to form thiosugars. Thus, treatment of 3,4-thionocarbonate 164 with azo-bis-isobutyronitrile (AIBN) and Bu3SnH gives the 3- and 4-thio sugar derivatives 165 and 166 with retention of configuration (O Scheme 74) [118],... 

Studies on the stability of 1,2-dihydro-2,4,6-triphenyl-1,3,5-triazine (369) included its synthesis from the aromatic 1,3,5-triazine 368. However, in the presence of aluminum chloride rearrangement occurs to give a 16% yield of 2,4,5-triphenylimidazole (370). A suggested mechanism proposed that the unstable dihydrotriazine undergoes a radical-induced ring opening and contraction to 370. ... 

Barton, D. H. R., Motherwell, R. S. H., Motherwell, W. B. Radical-induced ring opening of epoxides. A convenient alternative to the Wharton rearrangement. J. Chem. Soc., Perkin Tmns. 11981, 2363-2367. 

The synthesis of 9a-fluoro-18-methylprednisolone (282), a representative of still another new class of corticosteroids, has been described.The sequence, shown in Scheme 11, used the known radical-induced 1,4-rearrangement of... 

The ring expansion of substituted piperidines by means of free-radical-induced rearrangements has been used to prepare fully saturated azepines <91T4847>. Ethyl l-benzyl-3-oxo-4-phenyl-selenomethylpiperidine-4-carboxylate (259) is converted into the azepinone (260) by tri-n-butyl tin hydride in benzene under reflux in the presence of AIBN (Equation (21)). The reaction works also with ethyl 3-phenylselenomethyl-4-oxo-piperidine-3-carboxylate, preferably with the benzyl group replaced by a trityl group. 

For the last example in this group, once again the oxa-ene rearrangement seems to come into play. However, this reaction is really a radical-induced process involving the acyl group adding onto the double bond (Scheme 13) [48]. 

The radical-induced epoxide ring-opening of a,/3-epoxy-0-thiocarbonyl-imidazolides (23) [equation (3)] has been reported to be a convenient alternative to the Wharton rearrangement (action of hydrazine on epoxides of a,/3-unsaturated ketones) for production of allylic alcohols. /3,y-Disubstituted allylic alcohols with Z-conhguration are the major products formed on addition of alkyl-lithiums to the vinyl epoxide (24) [equation (4)]. ... 

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