Allyl benzyl ether, rearrangement

A high level of enantioselectivity in an acyclic system has been reported in the rearrangement of tricarbonylchromium(O) complexes of allyl benzyl ethers using chiral lithium amide base 73 (equation 38) . Upon treatment with 1.1 equivalents of lithium amide 73 and 1 equivalent of LiCl at —78 to —50°C, ether 74 afforded the rearrangement product R)-75 in 80% yield with 96% ee. The effect of substituents on the chemical yields and enantioselectivity of the [2,3]-Wittig rearrangement was also studied (see Table 3). 

As noted in Section 3.11.1, some of the earliest mechanistic work on the 2,3-Wittig rearrangement was carried out with allyl benzyl ethers. Selective benzyl deprotonation is typically effected with n-butyl-lithium in THF or THF-TMEDA at low temperature. Rautenstrauch s studies, summarized in Table 8, clearly showed that lower temperature favors 2,3- over 1,2-rearrangement, and established the potential... 

Table 8 2,3-Wittig Rearrangement of Allyl Benzyl Ethers...

A study of chiral induction in the asymmetric [2,3]-Wittig rearrangement of allyl benzyl ether in the presence of a bis(oxazoline) chiral ligand [(5,5)-Box-r-Bu] has been reported (Scheme 45). °... 

The rearrangement of an ether 1 when treated with a strong base, e.g. an organo-lithium compound RLi, to give an alcohol 3 via the intermediate a-metallated ether 2, is called the Wittig rearrangement. The product obtained is a secondary or tertiary alcohol. R R can be alkyl, aryl and vinyl. Especially suitable substrates are ethers where the intermediate carbanion can be stabilized by one of the substituents R R e.g. benzyl or allyl ethers. 

These ideas will be discussed in the following subsections, where most of the attention will be devoted to the mechanistic smdies with aromatic esters, which have been the subject of an overwhelming majority of the research efforts. Nevertheless, the same reaction mechanism has been shown to be valid for the PFR of anilides, thioesters, sulfonates, and so forth. Furthermore, it is also applicable to the photo-Claisen rearrangement [i.e. the migration of alkyl (or allyl, benzyl, aryl,)] groups of aromatic ethers to the ortho and para positions of the aromatic ring [21,22]. 

The Wittig rearrangement is the base-catalyzed 1.2-shift of an alkyl, allyl. benzyl, or phenyl group of an ether from oxygen to carbon. A patent claim describes the conversion of aryl arylmethyl ethers such as 21 into diarylmethanols such as 22.14... 

Use of benzyl alcohol resulted in formation of the benzyl ether corresponding to allyl ethers 52, but attempted Claisen rearrangement resulted in an 82 % yield of the product of a 1,3-benzyl shift (see Section 5.1.3.).20 To demonstrate the utility of the methodology outlined in Table 14, x-oxoester 53a was converted into the corresponding x-amino acid by hydrolysis and reductive animation.20... 

A 1,2-hydride shift has been invoked399 to account for the formation of p-methoxyphenylbutyraldehyde derivatives (337) during the treatment of />methoxy-benzyl-protected allylic alcohols (336) with zeolites. A similar C-glycosidation procedure involving Lewis acid-catalysed anomeric oxygen to carbon rearrangement of tetrahydropyranyl ether derivatives has been reported400 (see Scheme 82). It has been... 

Benzyl methyl ether or allyl methyl ethers can be selectively metalated at the benzylic/allylic position by treatment with BuLi or sBuLi in THF at -40 °C to -80 C, and the resulting organolithium compounds react with primary and secondary alkyl halides, epoxides, aldehydes, or other electrophiles to yield the expected products [187, 252, 253]. With allyl ethers mixtures of a- and y-alkylated products can result [254], but transmetalation of the lithiated allyl ethers with indium yields y-metalated enol ethers, which are attacked by electrophiles at the a position (Scheme 5.29). Ethers with ft hydrogen usually undergo rapid elimination when treated with strong bases, and cannot be readily C-alkylated (last reaction, Scheme 5.29). Metalation of benzyl ethers at room temperature can also lead to metalation of the arene [255] (Section 5.3.11) or to Wittig rearrangement [256]. Epoxides have been lithiated and silylated by treatment with sBuLi at -90 °C in the presence of a diamine and a silyl chloride [257]. 

When chiral substrates were employed in the cascade reaction, good transfer of the chiral information was observed (Scheme 7.43).119 Excellent diastereoselectivity was obtained with 152g, containing an allylic methyl group and even 152h, which contained a benzyl ether substituent that would lie outside the chairlike transition state (TS-156), controlled the facial selectivity of the [3,3]-sigmatropic rearrangement. 

In some cases, the steric constraints of the zeolite pores can enable or impede secondary reactions. A beautiful example is found in the rearrangement of benzyl allyl ethers over H-Beta zeolites (46). In the normal course of this reaction, the benzyl allyl ether is transformed to a 4-arylbutanal, probably via a five-membered ring intermediate ... 

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