Hydroxyketones 0-diketones

In this chapter, we discuss the rate coefficients and the mechanisms of oxidation of ketones. The classes covered include alkanones, hydroxyketones, diketones, unsaturated ketones, ketenes, cyclic ketones, ketones derived from biogenic compounds, and halogen-substituted ketones. Photolysis is a major atmospheric process for many ketones, and will be discussed in chapter IX. The major bimolecular reactions removing ketones from the atmosphere are with OH. Although less important than the OH... 

They are formed by treatinga-diketones, a-hyd-roxyaldehydes, hydroxyketones, aminoalde-hydes or aminoketones with arylhydrazines. Sugars can be identified by their osazones which have characteristic melting-points, formation times or crystal appearance. 

Diketones can be prepared by oxidation of the corresponding monoketone (287) or a-hydroxyketone (288). 1,2-Diketones are used extensively as intermediates in the preparation of pharmaceuticals, flavors, and fragrances. Toxicity data for selected diketones are shown in Table 11. 

Most frequent are oxidations of alkenes that can be converted to a series of compounds such as epoxides, halohydnns and their esters, ozonides (1,2,4 tri-oxolanes), a-hydroxyketones, a-hydroxyketone fluorosulfonates, ot-diketones, and carboxylic acids and their denvatives... 

Fluonnated alkenes with one fluonne atom attached to the double bond are converted to a-hydroxyketones by potassium permanganate [30] (equation 22) a-Diketones are formed by permanganate hydroxylation of double bonds flanked by fluonne atoms [31] (equation 23)... 

After the first hydrolytic step, secondary alcohols seem to continue biodegradation through ketone, hydroxyketone, and diketone. Diketones then produce a fatty acid and a linear aldehyde which is further oxidized to fatty acid. Finally, these two fatty acids continue biodegradation by enzymatic 3 oxidation [410],... 

Both the diketone and the cleavage products were shown to arise from an a-hydroxyketone intermediate (benzoin) 9. 

Hydride and 1,2-alkyl shifts represent the most common rearrangement reactions of carbenes and carbenoids. They may be of minor importance compared to inter-molecular or other intramolecular processes, but may also become the preferred reaction modes. Some recent examples for the latter situation are collected in Table 23 (Entries 1-10, 15 1,2-hydride shifts Entries 11-15 1,2-alkyl shifts). Particularly noteworthy is the synthesis of thiepins and oxepins (Entry 11) utilizing such rearrangements, as well as the transformations a-diazo-p-hydroxyester - P-ketoester (Entries 6, 7) and a-diazo-p-hydroxyketone -> P-diketone (Entry 8) which all occur under very mild conditions and generally in high yield. 

Trisubstituted imidazoles have been synthesized from 1,2-diketones or a-hydroxyketones with ammonium acetate in very short reaction times with excellent yields in the presence of l,l,3,3-VAr V ,(V -tetramethylguanidinium trifluoroacetate as an ionic liquid <06SC65>. Iodine acted as an efficient catalyst in the synthesis of 1,2,4,5-tetraarylimidazoles 93 using benzoin 91,... 

Under these solvent-free conditions, the oxidation of primary alcohols (e. g. benzyl alcohol) and secondary alcohols (e.g. 1-phenyl-l-propanol) is rather sluggish and poor and is of little practical utility. Consequently, the process is applicable only to a-hydroxyketones as exemplified by various examples including a mixed benzylic/ali-phatic a-hydroxyketone, 2-hydroxypropiophenone that delivers the corresponding vicinal diketone [106,107]. 

The first intermediate product of ketone oxidation is a-ketohydroperoxide. All other molecular products are formed by decay and reactions of this hydroperoxide and its adduct with ketone. Among these products, aldehydes, diketones, a-hydroxyketones, acids, esters, and C02 were observed. The information about the products of the oxidation of ketones by dioxygen are available in monographs [4,7],... 

In the hydrogenation of diketones by Ru-binap-type catalysts, the degree of anti-selectivity is different between a-diketones and / -diketones [Eqs (13) and (14)]. A variety of /1-diketones are reduced by Ru-atropisomeric diphosphine catalysts to indicate admirable anti-selectivity, and the enantiopurity of the obtained anti-diol is almost 100% (Table 21.17) [105, 106, 110-112]. In this two-step consecutive hydrogenation of diketones, the overall stereochemical outcome is determined by both the efficiency of the chirality transfer by the catalyst (catalyst-control) and the structure of the initially formed hydroxyketones having a stereogenic center (substrate-control). The hydrogenation of monohydrogenated product ((R)-hydroxy ketone) with the antipode catalyst ((S)-binap catalyst) (mis-... 

Biotransformation of or-Bromo and a,a -Dibromo Alkanone into or-Hydroxyketone and ar-Diketone by Spirulina platensis... 

Isoxazoles and their dihydro and tetrahydro analogues serve as immensely flexible building blocks in synthesis through their ability to function as masked forms of /3-diketones, /3-hydroxyketones (and thus enones) and y-amino alcohols. All of these interrelationships are possible because of the relatively labile nature of the nitrogen-oxygen bond. 

This explains, for example, the tendency of some 1,2-diols to suffer oxidative carbon-carbon bond breakage under the action of PDC. Thus, although many 1,2-diols can be uneventfully oxidized to a-hydroxyketones with PDC,176 very often a cleavage of a carbon-carbon bond occurs, resulting in two carbonyl functionalities.177 Vicinal tertiary diols, sometimes, are smoothly oxidized to diketones by PDC.178... 

Oxidation of alkenes and alkynes—sometimes with oxidative breakage of the carbon-carbon multiple bond—affording 1,2-diols,46,47 a-hydroxyketones 46 diketones,46 aldehydes,311,46 ketones46 or carboxylic acids31,44,46,29... 

Oxidation of the a-hydroxyketone with concentrated nitric acid, or by catalytic amounts of copper(n) salts in acetic acid solution which are regenerated continuously by ammonium nitrate, yields the diketone (e.g. benzil and furil, Expt 6.143). 

Just recently, a cyclo-cyanosilylation has been described321. Ryu and his coworkers have taken dicyanodimethylsilane (J>43)310) as a reagent under the conditions of the common cyanosilylation and obtained cyclic silyl enol ether (544,545) when (S-hydroxyketones or j8-diketones were employed (Scheme 87). 

A /3-hydroxyketone (542) forms with 543 the corresponding l,3-dioxa-2-sila-4-cyanocyclohexane (544) and the unsaturated isomer 545. With methanol, de-cyano-silylation is obtained. The /3-diketone 546 forms 544, too in that case it is better cleaved by AgF/THF or only methanol. 

Desymmetrization via proline-catalyzed asymmetric intramolecular aldol reaction can, however, also be performed with acydic diketones of type 109 as has been reported by the Agami group [106], In the first step a prochiral acyclic diketone reacts in the presence of L-proline as catalyst (22-112 mol%) with formation of the aldol adduct 111 (Scheme 6.49). In this step reaction products with two stereogenic centers, 110, are formed. These chiral hydroxyketones 110 are subsequently converted, via dehydration, into the enones 111, by treatment with p-toluenesulfonic acid. 

This chapter deals with target molecules of two main types hydroxyketones 1 and 1,3- or P-diketones 4. Both have a 1,3-relationship between the two functionalised carbons. Both can be disconnected at one of the C-C bonds between the functional groups to reveal the enolate 2 of one carbonyl compound reacting with either an aldehyde 3 or acid derivative 5 such as an ester. 

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