Enol ethers, reaction with singlet oxygen

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). 

The trimethylsilyl group of enol silyl ethers can participate in ene reactions with singlet oxygen (60,61). 

The enol tautomers of 1-(2, 4, 6 -trialkylphenyl)-2-methyl-1,3-diketones form a range of alkene, epoxide, ether, and hydroperoxide products on reaction with singlet oxygen. The product distribution is substantially affected by the solvent, apparently owing to the disruption of intramolecular hydrogen bonding of the enols in polar solvents. 

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. 

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-... 

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 reaction of singlet oxygen with alkenes constitutes an important chapter in organic chemistry. The discovery that certain enol ethers from zwitterionic peroxides as the primary event has led to the development of new methods for preparing a variety of 1,2,4-trioxanes . 

Schmid and Hofheinz described an important application of this reaction to the total synthesis of the antimalarial drug artemisinin. The key step in the (-)-isopulegol-based synthesis is the ene-reaction of singlet oxygen with an enol ether (Scheme 12). This reaction leads to an allylic hydroperoxide 20 when the reaction is performed in the nonpolar solvent, CHjClj. The use of the more polar, protic methanol inverts the regioselectivity, and the a-hydroperoxy acetal 21 is formed (presumably by trapping of an intermediary 1,4-zwitterion), which is cycHzed to the target molecule 22 by acid catalysis. 

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. 

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) . ... 

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... 

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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