Fragmentation reactions Beckmann

The focus of the next four chapters (Chapters 14-17) is mainly on the theoretical/computational aspects. Chapter 14 by T. S. Sorensen and E. C. F. Yang examines the involvement of p-hydrido cation intermediates in the context of the industrially important heptane to toluene dehydrocyclization process. Chapter 15 by P. M. Esteves et al. is devoted to theoretical studies of carbonium ions. Chapter 16 by G. L. Borosky and K. K. Laali presents a computational study on aza-PAH carbocations as models for the oxidized metabolites of Aza-PAHs. Chapter 17 by S. C. Ammal and H. Yamataka examines the borderline Beckmann rearrangement-fragmentation mechanism and explores the influence of carbocation stability on the reaction mechanism. 

R. E. Gawley, The Beckmann reactions Rearrangements, elimination-additions, fragmentations, and rearrangement-cyclizations , in Organic Reactions, Vol. 35 (Ed. A. S. Kende), Wdey-Intersdence, London, 1988, pp. 1-415. 

R. E. Gawley, The Beckmann Reactions Rearrangements, Elimination-Additions, Fragmentations, and Rearrangement-Cyclizations, Org. React. 1988, 35, 1-420. 

The Beckmann Reactions Rearrangements, Elimination-Additions, Fragmentations, and Rearrangement-Cydizations... 

Natural products are derived from plant, marine, and microbial sources and often possess biological activity. They are a source of new drug products each year and continue to play a significant role in the drug discovery and development process. Herein, we will discuss examples of recent natural product synthesis that utilize the previously discussed Beckmann reactions rearrangement, fragmentation, and photo-Beckmann. 

When the Beckmann reaction was performed with mesylated oxime intermediate 113, lactam 112, from the Beckmann rearrangement, was obtained as the sole product tScheme 5.261. thus shutting down the Beckmann fragmentation pathway.This reaction is similar to what White and coworkers observed during their morphine synthesis. Beckmann reaction of an intermediate brosylated oxime in acetic acid provided the desired lactam, whereas the reaction of the corresponding oxime under acidic conditions did not yield any lactam... 

Gawley RE (1988) The Beckmann reactions rearrangement, elimination-additions, fragmentations and rearrangement-cyclizations. Org React 35 14-24... 

Gawley RE (1988) The Beckmann reactions rearrangements, elimination-additions, fragmentations, and rearrangement-cyclizations. In Organic reactions, vol 35. Hoboken, NJ Donaruma LG, Heldt WZ (1960) The Beckmann rearrangement. Org React 11 1-156... 

Aldoximes are among the most suitable starting materials for the synthesis of nitriles. A plethora of reagents [222-227] has been introduced for this Beckmann-type fragmentation (see Beckmann reaction, under Section 9.3.2.2). Among these are acetic anhydride [223], metal salts [224] or other metal catalysts [225], selenium dioxide [226] and Burgess reagent [227]. In the presence of hahde derivatives [228] or metal salts [229] aldoximes can also be oxidized to nitrile oxides. 

CycHc ketones with functional groups at the a-position of the carbonyl moiety are easily cleaved into the corresponding dicarbonyl compounds. The most commonly used substrates are a-hydroxy ketones [283], a-diketones [284], a-halo [285], and a-ni-tro ketones [286]. 1,3-Diketones are oxidized to dicarboxyUc acids, and in this case loss of one carbon atom has been observed [287]. Ketone derivatives are also cleaved. For example, ketoximes afford unsaturated nitrile derivatives by Beckmann fragmentation (see Beckmann reaction, under Section 9.3.2.2). 

Benzofurazan, 7-chloro-4-nitro-, 6, 394 as fluorigenic agents, 6, 410, 426 Benzofurazan, 4-chloro-7-sulfo-ammonium salt properties, 6, 426 Benzofurazan, 4-nitro-synthesis, 6, 408 Benzofurazans, 6, 393-426 Beckmann fragmentation, 6, 412 biological activity, 6, 425 bond angles, 6, 396 bond lengths, 6, 396 diazo coupling, 6, 409 dipole moments, 6, 400 electrochemical reduction, 5, 73 electrophilic reactions, 6, 409-410 ESR spectroscopy, 6, 400... 

The reaction that normally occurs on treatment of a ketoxime with a Lewis or proton acid is the Beckmann rearrangement (18-17) fragmentations are considered side reactions, often called abnormal or second-order Beckmann rearrangements. Obviously, the substrates mentioned are much more susceptible to fragmentation than are ordinary ketoximes, since in each case an unshared pair is available to assist in removal of the group cleaving from the carbon. However, fragmentation is a side reaction even with ordinary ketoximes and, in cases where a particularly stable carbocation can be cleaved, may be the main reaction. ... 

The natural antipode of corynantheine (35, 155, 20/ ) has elegantly been prepared by Autrey and Scullard (168), starting from yohimbone (305), synthesized and resolved previously by Swan (169). Yohimbone (305) was converted to 18-formylyohimbone (306) and then through 307 to oxime 308. On reaction with thionyl chloride, 308 underwent a Beckmann fragmentation to the trans-substituted indolo[2,3-a]quinolizine 310, which after desulfurization and esterification resulted in the levorotatory methyl corynantheate (304). This product... 

The reaction of compounds 465 with an excess of aryl isocyanate in acetonitrile at reflux leads to the formation of imidazooxadiazolones 466, via a Beckmann fragmentation that affords a presumed cyclic nitrone intermediate 187 (Equation 107) <1999SC3889>. 

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 ... 

Several pathways may be possible for the Beckmann fragmentation reaction (equations 68 and 69). Stepwise processes may occur (equation 68), but stereospecific concerted fragmentations are also common (equation 69). Stepwise processes may follow different routes, but in most cases the fragmentation may have the same intermediate as the Beckmann rearrangement the nitrilium ion. 

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