Oxidation of enol ether

A retroaldol fragmentation subsequent to the addition of p-TsOI I and a small amount of water to epoxide 206, obtained by oxidation of enol ether 205 with DMDO, resulted in the direct formation of dialdehyde hydrate 208, possessing the spirostructure necessary for the construction of the fused-rings core of ( )-ginkoli-de B. Apparently, hydrolysis of the epoxide produces the hemiacetal 207, which undergoes retroaldol fragmentation of the cydobutane to afford the dialdehyde, which forms the stable hydrate 208 (Scheme 8.52) [94]. 

The oxidation of enol ethers and their derivatives is a useful method for the synthesis of a-hydroxy-ketones or their derivatives, which are versatile building blocks for organic synthesis. Since enol ethers and esters are types of olefin, some asymmetric epoxidation and dihydroxylation reactions have been applied to their oxidation. 

In 1992, Thornton et al. reported that Mn(salen) (43) catalyzed the asymmetric oxidation of silyl enol ethers to give a mixture of a-siloxy and a-hydroxy ketones, albeit with moderate enantioselectivity (Scheme 28).135 Jacobsen et al. examined the oxidation of enol esters with Mn(salen) (27) and achieved good enantioselectivity.136 Adam et al. also reported that the oxidation of enol ethers with (27) proceeded with moderate to high enantioselectivity.137 Good substrates for these reactions are limited, however, to conjugated enol ethers and esters. Based on the analysis of the stereochemistry,137 enol ethers have been proposed to approach the oxo-Mn center along the N—Mn bond axis (trajectory c, vide supra). 

Another useful method for the asymmetric oxidation of enol derivatives is osmium-mediated dihydroxylation using cinchona alkaloid as the chiral auxiliary. The oxidation of enol ethers and enol silyl ethers proceeds with enantioselectivity as high as that of the corresponding dihydroxylation of olefins (vide infra) (Scheme 30).139 It is noteworthy that the oxidation of E- and Z-enol ethers gives the same product, and the E/Z ratio of the substrates does not strongly affect the... 

The oxidation of enol ethers at a reticulated vitreous carbon anode [2, 3] (Scheme 1) in a mixture of methanol/THF containing tetraethylammonium tosylate as the electrolyte and 2,6-lutidine as the base leads to substituted tetrahydrofu-ran and tetrahydropyran rings in good yields (51-96%). The major product obtained had a trans-stereochemistry. The cyclization failed to make seven-membered ring products. In order to determine the... 

Azide ions are oxidised at low positive potentials and generate azide radicals. Azide radicals will add to an alkene. Thus the anodic oxidation of enol ethers in... 

Oxidation of enol ethers 9-70 Reaction between aldehydes and aluminum ethoxide (Tishchenko) 9-72 Reaction of acetophenones with AgN03-I2 or other reagents... 

The addition of medioxy groups to an unsaturated carbon takes place by anodic oxidation of enol ethers in methanol containing sodium methoxide yields are generally satisfactory (equation 21). 

The anodic oxidation of enol ethers at a graphite anode in methanol containing 2,6-lutidine and sodium perchlorate results in the dimerization of the enol ethers to acetals of 1,4-dica nyl compounds (equation 22). The mechanism of dimerization is thought to involve a tail-tail coupling of the cation radicals generated by the one-electron oxidation of the enol ethers. 

The anodic oxidation of enol ethers in methanol yields a-methoxylated carbonyl conqKxinds, which are useful intermediates for the synthesis of carbonyl compounds utilizing the technique of oxidative cleavage of glycols (equation 4S). ... 

Another reaction in which epoxy ethers have been suggested io be intermediates is the oxidation of enol ethers with perbenzoie acid. topic need only be mentioned briefly here, since olefin ozidaticu with perbenzoio add has been taken up in gieateir detail in section IIM./I,... 

The required a-hydroxy ketones are accessible via a variety of synthetic methods, including the acyloin condensation (see Chapter 9), oxidation of ketone enolates, oxidation of enol ethers,and oxidation of a, 3-unsaturated ketones,as depicted below. 

The Mn(salen) complex 8 has also been appHed towards the synthesis of a-hydroxy carbonyl compounds from enol ethers and ketene acetals [87,88,89]. These substrates are a special class of trisubstituted olefins, previously demonstrated to be excellent substrates for AE. Indeed, the observed sense of stereoinduction in the oxidation of enol ether derivatives adheres to the skewed side-on approach model developed for trisubstituted olefins. In the presence of 7 mol % of 8, silyl enol ether 37 was oxidized under bleach conditions to afford the a-hy-droxy ketone in 87% ee (Scheme 14) [89]. 

Oxidation of enol ethers. The procedure of Sheng and Zajacek (2, 287) when applied to 5,6,7,8-tetrahydrochromane (1) leads to cleavage of the double bond to give a dicarbonyl compound, 6-ketononanolide (2), in 50% yield.1 The reaction... 

Oxidation of enol ethers. The reagent oxidizes linear or cyclic enol ethers directly to esters or lactones in high yield. The reaction evidentiy involves attack of the double bond. ... 

A convenient method for the oxidation of enol ethers to esters has been achieved using pyridinium chlorochromate [equation (5)]. Cyclic ethers give the corresponding lactone. 

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