Optically active ether

This reaction is the reverse of the initial ketyl radical formation by the benzophenone triplet and is therm Q4ynamically favorable. The experiments using optically active alcohols as source of hydrogen atoms show, however, that under normal conditions this reaction is unimportant. This is probably due to other, more efficient pathways for reaction of the ketyl radicals or perhaps to diffusion rates which separate the radicals before reverse transfer can occur. That this reaction can be important in some cases even without the presence of sulfur compounds was shown by studying the photoreduction of benzophenone in optically active ethers.<68) Although the reaction of benzophenone in methyl 2-octyl ether is only 0.17 times as fast as that in isopropanol, ethers can be used as sources of hydrogen atoms for photoreduction ... 

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Of these reactions, the [2,3]-Wittig rearrangement in particular has often been used as a means of transferring chirality. The product of this reaction has potential stereogenic centers at C-3 and C-4 (if R ), and if the starting ether is optically active because of a stereogenic center at C-1, the product may be optically active as well. Many examples are known in which an optically active ether was converted to a product that was optically active because of chirality at C-3, C-4, or both. If a... 

Aplysin and (—) debromoaplysin were prepared analogously via lithiation (t-BuLi) of the following optically active ether. 

The previous results discussed here were put in question by a stereochemical study for the optically active ether (+)-50-2-g (68). In this case a one-center... 

Nevertheless, the samples in this series show melting temperatures that are very dependent on the length of the optically active ether segment, in spite of their identical chirality and optical purity. 

Cleavage of allylic ethers can occur using acetic anhydride in the presence of iron(III) chloride (eq 45). The reaction takes place without isomerization of a double bond, but optically active ethers are cleaved with substantial racemization. ... 

The same species, DUophus fasciola, led to the isolation of an optically active ether of glycerol and geranylgeraniol. This is currently the only linear diterpene to be characterized in a Dictyotale (Amico et al., 1977), although this compound was later found in two other Dictyotales Taonia lacheana and Taonia atomaria ciliata (Tringali, Piattelli, and Spatafora, 1995). 

On a number of occasions, the Wittig rearrangement has proven instrumental for the stereoselective preparation of homoallylic alcohols. One of the earliest examples was demonstrated by Felkin and showased the ability of optically active ethers to participate in stereospecific rearrangements (Equation 21) [100], Treatment of optically active allyl ether 177 (> 90% ee) with n-BuLi leads to stereospecific formation of a benzylic alcohol 179 (>90% ee) as a side product after a series of manipulations. 

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