Barton rearrangement

The above examples demonstrate that diastereoselectively labeled substrates can be used to probe stereochemical features in gas-phase reactions of flexible molecules. In addition, this approach is not confined to metal-ion chemistry and not even to charged species, cf. the Barton rearrangement of neutral alkoxy radicals mentioned above. Instead, the decisive factor for the observation of a significant SE is a rate-determining activation barrier in an appropriate energy regime. 

Porter reported the first decarboxylative azidation of thiohydroxamates (Barton PTOC esters). Moderate to good yields of the desired azides were obtained together with the S-pyridyl derivatives resulting from the direct trapping of the radical by the Barton ester. Interestingly, these reactions proved to be diastereoselective as demonstrated by the first example depicted in Scheme 8.12. The formation of the 5-pyridyl derivatives (Barton rearrangement) limits severely the scope of this process. The use of a new type of thio-hydroxamate esters (MPDOC esters) allows to overcome this difficulty (Scheme 8.26). These esters are more stable than the classical Barton esters and less prone to rearrange... 

The stereochemistry of formation and rearrangement of vicinal dihaUdes has been elucidated, chiefly by Barton s group. Trans diaxial addition occurs but the product may then equilibrate with the more stable... 

Baeyer-Villager oxidation, 10, 151, 433 Bamford-Stevens reaction, 402 Barton reaction, 253 Beckmann rearrangement, 140 Benzilic acid rearrangement, 418, 435 3 -Benzoyloxy-7-methylenecholest-5-ene, 60 Benzylmagnesium chloride, 64 3-Benzyloxycholesta-3,5-diene, 342... 

Barton and co-workers" performed flash vacuum pyrolysis (FVP) on trimethyl-silylvinylmethylchlorosilane (30), resulting in the production of trimethylchlorosi-lane (30%), trimethylvinylsilane (11.5%), and most interestingly, ethynylmethyl-silane (34, 11.9%). A proposed mechanism for the synthesis of 34 (Scheme 10) begins with the lo.ss of trimethylchlorosilane to form silylene 31, which can rearrange either to silaallene 32 or to silirene 33, both of which can lead to the isolated ethynylsilane. 

Barton has reported a wide variety of elegant studies in which various silenes or silylenes have been created, usually thermally, and their subsequent rearrangements investigated in terms of the observed products of trapping (51,53,65,145). It has been clearly established that interconversion between silenes and silylenes, especially where H atoms or Me3Si groups migrate, are facile processes. In some cases, radicals can be the precursors to silenes (65). 

An alternative method in the synthesis of alkaloids, photochemical rearrangement of endocyclic nitrones into bicyclic lactams has drawn special attention. Analyses of photochemical rearrangement and application of modified conditions of the Barton reaction testify to the comparability of results obtained in these approaches (Scheme 2.89) (460). 

The l-oxa-2,4,5-cycloheptatrienes 602 and 603 were postulated to be intermediates in the rearrangement of certain (ethynylfuryl)oxiranes to furo[3,4-b]furans [251]. The replacement of the methylene groups of 1,2-cycloheptadiene (465) by SiMe2 groups and the introduction of substituents at the allene moiety allowed the preparation of isolable seven-membered ring allenes. Thus, Barton et ah [177] obtained 604 and Ando et al. [178] 605. A few reactions of these systems have also been studied [177, 252]. Both groups [178, 253] synthesized the [4.4]betweenallene 606 and determined its structure by X-ray diffraction. 

An interesting neophyl-type radical rearrangement process has been established for the synthesis of azabicycles, which are not readily accessible by other means. Barton McCombie deoxygenation of xanthate 70 under slow addition of (TMS)3SiH and AIBN in refluxing toluene furnished the 2-azabenzonorbor-nane derivative in good yield (Reaction 7.72) [82]. 

Baeyer-Villiger reaction, 9, 3 43, 3 Bamford-Stevens reaction, 23, 3 Barbier Reaction, 58, 2 Bart reaction, 2, 10 Barton fragmentation reaction, 48, 2 Bechamp reaction, 2, 10 Beckmann rearrangement, 11, 1 35, 1 Benzils, reduction of, 4, 5 Benzoin condensation, 4, 5 Benzoquinones ... 

The transformation of endocyclic nitrone 56 (made from N,0-bis-protected hydroxylamine 55) to lactam 20 can be carried out by photochemical activation or by a two-step modification of Barton s protocol, that is, by trapping the nitrone oxygen followed by an alkali-promoted, semi-pinacol-like rearrangement (03JOC8065). 

Barton Beckwith Goosen/. Chem. Soc. 1965, 181 Petterson Wambsgans/. Am. Chem. Soc. 1964,86, 1648 Neale Marcus Schepers J. Am. Chem. Soc. 1966,88, 3051. For a review of N-halo amide rearrangements, see Neale Synthesis 1971, 1-15. 

The formation of the oxime (2) probably results from the presence of adventitious traces of hydroxylic material which catalyze the isomerization of the nitroso monomer, the primary product of the Barton reaction, to an oxime. More direct evidence for the formation of nitroso compounds in the photochemical rearrangement of nitrites is provided by the isolation of nitroso dimers. ... 

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