Oxygen, singlet enol ethers

In the photo-oxygenation of enol ethers, where the ene reaction and [2 + 2]-cycloaddition compete, polar solvents favour cycloaddition whereas nonpolar solvents favour ene product formation [681, 683-685]. For example, 2,3-dihydro-4-methyl-4/f-pyran reacts with singlet oxygen to yield both a 1,2-dioxetane and an allylic hydro-... 

Two papers have appeared on the singlet photo-oxygenation of enol ethers. In the first of these, the (-t-)-methoxymethylenefenchanes (36) were found to give... 

Jefford, C.W., Kohmoto, S., Boukouvalas, J., and Burger, U., Reaction of singlet oxygen with enol ethers in the presence of acetaldehyde. Formation of 1,2,4-trioxanes,/. Am. Chem. Soc., 105,6498,1983. 

Acetylenic ethers 7 can be hydrozirconated, and subsequent iododezirconation leads to (fc)-iodo enol ethers 8 (Scheme 4.4) [18], These species undergo efficient Sonogashira couplings to give (E)-enynes, which are ultimately converted to stereodefined dienol ethers. These dienes have proven useful in studies of diastereoselective cycloaddition reactions with singlet oxygen, where R in 8 is a nonracemic auxiliary (e. g., menthyl) (Procedure 3, p. 140). 

SCHEME 12. Synthesis of hydroperoxides from silyl enol ethers TABLE 3. Preparation of alkyl hydroperoxides via singlet oxygen oxygenation... 

From singlet oxygen reaction with silyl enol ethers. 779... 

From singlet oxygen reaction with silyl enol ethers When a carbon tetrachloride solution of 1-methoxy-l-trimethylsiloxy-l-alkene in the presence of tetraphenylporphyrin and bubbling oxygen is irradiated with a 400-W Na lamp, a-trimethylsilyl peroxyesters were obtained in good yield (equation 11) . ... 

The cw effect , observed also in trisubstituted enol ethers , may as well be rationalized by a similar mechanism (Scheme 12). In this case, the electron-donating group that stabilizes the partially charged double bond carbon of the perepoxide in the syn transition state is the alkoxy moiety. Therefore, the favourable interactions of the singlet oxygen with the phenyl or alkoxy substituents direct the orientation of the perepoxide intermediate. 

Ab initio molecular orbital calculations, coupled with activation energies and entropies from experimental data, have been employed to determine the nature of the intermediates in the reaction of singlet oxygen with alkenes, enol ethers, and enamines.214 Allylic alkenes probably react via a perepoxide-like conformation, whereas the more likely pathway for enamines involves a zwitterionic cycloaddition mechanism. The reactions of enol ethers are more complex, since the relative stabilities of the possible intermediates (biradical, perepoxide, and zwitterionic) here depend sensitively on the substituents and solvent polarity. 

Photooxidation of enols.2 Although singlet oxygen adds to enol ethers, photooxidation of enols of ketones proceeds slowly, if at all, under usual conditions. However, in the presence of tetra-n-butylammonium fluoride (1 equiv.), /S-diketones, a-diketones, and / -keto esters undergo photooxidation3 at reasonable rates. In all cases, hydroperoxides are formed initially, which can undergo further reactions. 

However, in recent years, it has become apparent that several photoinduced oxidation reactions do not involve singlet oxygen as the reactive intermediate, and, consequently, the reaction products cannot be accounted for the mechanisms shortly reported above. Moreover, since the first report on the photochemical stereospecific synthesis of the most fascinating peroxide derivatives, i.e., 1,2-dioxetanes [32], it clearly appeared, with a few notable exceptions [33,38], that only electron-rich olefins, such as enamines, enol ethers, and thio-substituted... 

While the reaction of singlet oxygen with silyl enol ethers was governed by competing prototropic and silatropic ene processes (see Section 2.3.2.1.3.ii), the interaction with dienol ethers displays a different mode of reactivity. Singlet oxygen generated from triphenyl phosphite ozonide at low temperature... 

The high stereoselectivity of singlet oxygen attack can be explained by complete shielding of one of the two diastereotopic sides due to the substituents located a and a to the reactive enol ether double bond. 

Coverage in this chapter is restricted to the use of alkenes or alkynes as enophiles (equation 1 X = Y = C) and to the use of ene components in which a hydrogen is transferred. Coverage in Sections 1.2 and 1.3 is restricted to ene components in which all three heavy atoms are carbon (equation 1 Z = C). Thermal intramolecular ene reactions of enols (equation 1 Z = O) with unactivated alkenes are presented in Section 1.4. Metallo-ene reactions are covered in the following chapter. Use of carbonyl compounds as enophiles, which can be considered as a subset of the Prins reaction, is covered in depth in Volume 2, Chtqiter 2.1. Addition of enophiles to vinylsilanes and allylsilanes is covered in Volume 2, Chapter 2.2, while addition of enophiles to enol ethers is covered in Volume 2, Chapters 2.3-2.S. Addition of imines and iminium compounds to alkenes is presented in Volume 2, Part 4. Use of alkenes, aldehydes and acetals as initiators for polyene cyclizations is covered in Volume 3, Chapter 1.9. Coverage of singlet oxygen, azo, nitroso, S=N, S=0, Se=N or Se=0 enophiles are excluded since these reactions do not result in the formation of a carbon-carbon bond. 

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