Ethers, allyl vinyl oxidation

Conjugate addition of vinyllithium or a vinyl Grignard reagent to enones and subsequent oxidation afford the 1.4-diketone 16[25]. 4-Oxopentanals are synthesized from allylic alcohols by [3,3]sigmatropic rearrangement of their vinyl ethers and subsequent oxidation of the terminal double bond. Dihydrojasmone (18) was synthesized from allyl 2-octenyl ether (17) based on Claisen rearrangement and oxidation[25] (page 26). 

Among phosphonate esters (170) used in olefin synthesis were those with R = S-CeHi-Br-/ , S02 C6H4-Br-A CO-NHR, and S CHa CEi-XHa. The allyl vinyl thio-ethers (171) obtained using the last of these gave a-allyl-aldehydes on pyrolysis in the presence of red mercuric oxide. 

Bromination of the enol ether product with two equivalents of bromine followed by dehydrobromination afforded the Z-bromoenol ether (Eq. 79) which could be converted to the zinc reagent and cross-coupled with aryl halides [242]. Dehydrobromination in the presence of thiophenol followed by bromination/dehydrobromination affords an enol thioether [243]. Oxidation to the sulfone, followed by exposure to triethylamine in ether, resulted in dehydrobromination to the unstable alkynyl sulfone which could be trapped with dienes in situ. Alternatively, dehydrobromination of the sulfide in the presence of allylic alcohols results in the formation of allyl vinyl ethers which undergo Claisen rearrangements [244]. Further oxidation followed by sulfoxide elimination results in highly unsaturated trifluoromethyl ketonic products (Eq. 80). 

Although C—H insertion reactions rarely occur in intermolecular reactions with diazoacetates, these are common side reactions with diazomalonates3132 (equation 10) and diazo ketones (with a-allyl vinyl ethers).33 Several mechanistic pathways are available to generate the products of an apparent direct C—H insertion reaction and these include dipolar intermediates, ir-allyl complexes and ring opening of cyclopropanes.1 Oxidative problems due to the presence of oxygen are common with copper catalysts, but these are rarely encountered with rhodium catalysts except in systems where the carbenoid is ineffectively captured.34... 

Intramolecular hydroboration of allyl vinyl ethers.1 The hydroboration and subsequent oxidation of allyl vinyl ethers 1 with ThxBH2 (2 equiv.) leads to 1,3-diols with almost exclusive syn selectivity. High syn selectivity obtains regardless of the bulk of R1, but is lowered when R is phenyl. Apparently, electronic effects of R1 are important for stereoselectivity. 

Oxopentanals may be synthesized from allylic alcohols by 3,3 sigmatropic rearrangement of their vinyl ethers, and subs uent oxidation of the terminal double bond. Cinnamyl alcohol (31) was converted to the allyl vinyl ether (32), which was subjected to Qaisen reairangement to give 3-phenyl-4-pentenal (33) in 50% yield. Oxidation of the terminal double bond of (33) gave 3-phenyl-4-oxopentanal... 

The Claisen-Johnson rearrangement is also a reaction of choice for the construction of quaternary centers in the synthesis of triquinanes and related ter-penes. Accordingly, transposition of aUyUc alcohol 125 is a cornerstone reaction in the synthesis of tricyclic compound 124, a key intermediate for the access to several triquinanes described by Iwata [28] (Scheme 6.17). Acid 126 was obtained in 57% yield after transposition and saponification. Claisen allyl vinyl ether rearrangement was also applied to allylic alcohol 125. Oxidation of the aldehyde inter-... 

Reactions that proceed under neutral conditions are highly desirable. An important event in TT-allylpalladium chemistry is the introduction of highly reactive allylic carbonates (Sect. V.2.1.3), Their reactions can be carried out under mild neutral conditions. " Also, reactions of allylic carbamates, " allyl aryl ethers, and vinyl epoxides proceed without addition of bases. As shown by the mechanism in Scheme 6, the oxidative addition of allyl methyl carbonates is followed by decarboxylation as an irreversible process to afford TT-allylpalladium methoxide, and the generated methoxide picks up a proton from pronucleophiles (NuH), such as active methylene compounds. This in situ formation of the alkoxide is the reason why the reaction of aUyl carbonates can be carried out without addition of bases from outside. Alkoxides are rather poor nucleophiles, and alkyl allyl ethers are not formed from them. In addition, formation of TT-allylpalladium complexes from allylic carbonates involving decarboxylation is irreversible. In contrast, the formation of TT-allylpalladium acetate from allyl acetate is reversible. 

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