Ketones fragmentation reactions

Certain ketoximes can be converted to nitriles by the action of proton or Lewis acids. Among these are oximes of a-diketones (illustrated above), a-keto acids, a-dialkylamino ketones, a-hydroxy ketones, p-keto ethers, and similar compounds. These are fragmentation reactions, analogous to 17-25 and 17-26. For example, ot-dialkylamino ketoximes also give amines and aldehydes or ketones besides nitriles. 

Unfortunately, upon treating hydroxy ketone 56 with MeLi for extended reaction times, some sort of fragmentation reaction appeared to occur (Scheme 8.15). While the desired addition product 63 was not seen, the fragmentation that took place may be attributed to a retro-aldol reaction through intermediates such as 60 and 61. The tentatively assigned structure of final product 62 is shown, although it was not fully... 

The fragmentation reaction was applied most successfully in the total synthesis of sesquiterpenes 39). For example, the mesylate (110) underwent fragmentation reaction to provide the ketone (111), which could be converted to (112). The compound (112) was eventually transformed to the isolable mixtures of p-himachalene (113), trans-a-himachalene (114) as well as traHs-y-himachalene (115)40). 

Both the nitrosoenamine II and the nitrosamino acetate III demonstrated some unusual chemistry which led us to the study of nitrosamine fragmentation reactions which will be discussed below. Treatment of the nitrosoenamine II with dilute sulfuric acid led to the formation of benzyl phenyl ketone as anticipated, but the major product from this reaction was benzoin, as is illustrated in equation 2 (3). While this transformation and the proper-... 

Selenium dioxide is able to a-oxygenate ketones via their enol tautomers. As is demonstrated in Figure 12.10 by the reaction of selenium dioxide with cyclohexanone, the actual electrophilic substitution product C is unstable. The latter contains selenium in the oxidation state +2 that takes the opportunity to transform into selenium in the oxidation state 0, i.e., elemental selenium, by way of the fragmentation reaction indicated. Thereby, the a-C O single bond of the primary product C is transformed into the a-C=0 double bond of the final product B (which, however, is largely present as the tautomeric enol A). 

The second route of getting free radical center near is mediated by low-molecular free radicals or compounds which are either present in the polymer or are gradually formed there by fragmentation reactions. While tertiary peroxy radicals propagate only by abstraction of hydrogen atom from surrounding C —H bonds, secondary peroxy radicals may easily cleave to hydroxy radicals and ketones as follows ... 

A historically important use of the ozonolysis reaction was in the area of structure determination. In the days before the advent of spectroscopic techniques (Chapters 13-15), the structure of an unknown organic compound was determined by submitting it to a host of reactions. Often, a complex molecule was broken into several fragments to simplify the structural problem. After the individual fragments were identified, the original molecule could be mentally reconstructed from them. Alkenes were often cleaved to aldehydes and ketones by reaction with ozone. 

Olefin bonds and ketones are formed by the fragmentation reactions discussed above (Scheme VIII/12). To get alkynes with ketones by similar processes, an alkenol instead of the alkanol must be present in the starting material. 

The problem with reactions like this is that both the starting material and product are ketones, so they work cleanly only if the starting material is more reactive than the product. Cyclohexanone is more reactive as an electrophile than either cyclopentanone or cycloheptanone, so it ring expands cleanly to cycloheptanone. But expansion of cyclopentanone to cyclohexanone is messy and gives a mixture of products. We shall come back to diazo compounds in more detail in Chapter 40 diazonium salts will reappear in Chapter 38 where their decomposition will provide the driving force for fragmentation reactions. 

Photochemical reactions—Mannich ketobases, mainly, are submitted to photochemical reactions leading to ring formation (see Figs. 121 and 128). Irradiation in methanol of several aminoaryl ketones, however, gives the imines 371 (Fig. 148) and dcaminoal-kylated compounds are also formed as byproducts, due to fragmentation reactions. " ... 

Williams has used a spirodiepoxide derived from epoxidation of an allene to be a versatile electrophile for the formation of highly functionalized ketone-containing fragments. Reaction of a spiroepoxide formed with a... 

Esters have been prepared in 63-73% yields from several simple cycloalkyl and aryl alkyl ketones by reaction at room temperature with per-benzoic acid. The larger radical of the ketone appears as the alcohol fragment of the ester. Cyclic ketones are oxidized by potassium persulfate and sulfuric acid to esters from which o>-hydroxy aliphatic esters are obtained upon hydrolysis and reesterification. Peracetic acid in acetic anhydride converts salicylaldehyde to o-hydroxyphenyl formate (88%). ... 

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