Oxidative cleavage 1-diketones

Enaminoketones undergo a clean oxidative cleavage to a-diketones, presumably through a dioxetane intermediate.180... 

The cyclobutanols (458) underwent oxidative cleavage with chromic acid to provide diketones (459) (Table 19)157). 

The ozonolysis of cyclobutene derivatives in the preparation of 1,4-diketones was also applied to the total synthesis of eyclopentanoid antibiotics 161 162k The oxidative cleavage of (470) by ozone and reductive work-up yielded the diketone (471) in 73 % yield. Diketone (471) underwent intramolecular aldol cyclization to give the key intermediate (472), which was used to synthesize ( )-xanthocidin161,162), (+)-epi-xanthocidin 162), ( )-p-isoxanthocidin161,162) as well as ( )-desdihydroxy-4,5-didehydroxanthocidin162). 

Although CY-diketones are only rarely isolated from aqueous permanganate oxidations there is conclusive proof that they are intermediates in the reaction (16). Of-Diketones are resistant to further oxidation under anhydrous conditions and can thus be isolated in good yields from the phase transfer assisted reactions. Howeverf under aqueous conditions oxidative cleavage takes place rapidly, thus preventing the accumulation of these products. 

Oxidative cleavage of alkynes by a variety of reagents has been reviewed [35, 60, 70, 71]. In most cases the CC bond is broken, but in some cases a-diketones are formed instead of, or in addition to, carboxylic acids. Examples of both types of reaction are given in Tables 3.3 and 3.6. 

As with alkene cleavage the main reagent for alkyne oxidations is RuO. Oxidative cleavage of alkynes by a variety of reagents has been reviewed [4, 6, 12, 14, 15], The first oxidation of alkynes was noted by Pappo and Becker in 1956 they showed that l,2-fc/x(l-acetoxycyclohexyl)ethyne (2) (Fig. 1.5) gave the diketone. Minimal experimental details were given [195],... 

Exposure of 144 to catalytic quantities of ruthenium tetroxide, generated in situ from ruthenium trichloride and sodium periodate, produces the symmetrical lactones 145 <2000JA9558>. It is proposed that the products form as a result of the ruthenium-catalyzed oxidative cleavage of the a-diketones to produce intermediate glycols (Equation 49). 

Terminal alkynes are prone to undergo facile oxidative cleavage to yield carboxylic acids with loss of the terminal carbon atom. In fact, most of the oxidizing agents that can be used in the selective oxidation of internal alkenes to 1,2-diketones [Ru04,711 PhIO with Ru catalysts,712 KMn04,713 T1(N03)3,716 0s04718] convert terminal alkynes to carboxylic acids. 

The reagent is effective for oxidative cleavage of a-glycols to aldehydes, of a-hydroxy ketones to an aldehyde and a carboxylic acid, and of a-diketones and a-keto iicids to acids.3... 

There is of course no need to use reconnection if you prefer another strategy but you are advised to try disconnection first. Disconnection of the 1,3-diCO relationship in the spiro-diketone 54 reveals a 1,6-diCO compound that could no doubt be made by oxidative cleavage of 58. But various authors10 preferred to ignore the 1,6-diCO relationship and simply disconnect to the enolate of cyclopentanone 56 and a bromoester 57. 

If the reaction mixture becomes warm or too basic, the diketone undergoes oxidative cleavage. The products are the carboxylate salts of carboxylic acids, which can be converted to the free acids by adding dilute acid. 

Base-catalysed conjugate addition of nitropropane to methyl vinyl ketone occurred smoothly to give the nitroketone. Formation of the salt with sodium methoxide was followed by oxidative cleavage of the C-N linkage with ozone. The product was a 1,4-diketone which was isolated without further aldol reaction by this route. 

Oxidative cleavage of the double bond in 168 (Scheme 33, Section 14.10.6.3) by ozonolysis was unsuccessful, while its dihydroxylation and treatment of resulting diol with lead(iv) acetate gave diketone 169 <1999T7471>. Ozonolysis of isopropyl 1,3,4,5,6,7-hexahydro-l-methylisobenzofuran-l-carboxylate 131 (Scheme 23, Section 14.10.5.6.3) proceeded smoothly and led to the corresponding oxonine carboxylate 132 <20020L3059>. 

Oxidations of aikynes (1, 947). A reinvestigation of this reaction indicates that oxidation to diketones is best effected in dry methylene chloride with excess powdered potassium permanganate and a phase-transfer catalyst (Adogen 464). Typical yields are 40-80%. If some water is also present, further oxidative cleavage of the enol of the diketone results in two carboxylic acids. An example is shown in equation (I). Oxidation of terminal aikynes to a carboxylic acid with one less carbon atom may also proceed through a dicarbonyl intermediate. 

Oxidative cleavage of a-diketones. Corey and Pearce used this reagent to effect oxidative cleavage of the hindered diketone 1 in their synthesis of the sesquiterpene picrotoxinin (3). 

Oxidative cleavage of ketones. Potassium superoxide in combination with Aliquat 336 oxidizes ketones in benzene solution at 20°. Ketones lacking an cf-hydrogen and 1,3-diketones do not react. 

Acyhin condensation (3, 311-3I2). The method of converting bis(trimethylsiloxy)-cyclobutene derivatives into the corresponding 1,2-diketocyclobutanes by oxidative cleavage with bromine in an aprotic medium has since been shown to be a useful method of obtaining nonenolizable a-diketones ... 

Terminal alkynes undergo the similar oxidative cleavage to afford carboxylic acids (Eq. 3.46), vhile internal alkynes are converted to diketones (Eq. 3.47) [79]. 

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