Epoxide-to-aldehyde rearrangement

This epoxide to aldehyde rearrangement was postulated to be the first step in the silver-mediated reaction of alkylzirconocene chlorides with epoxides, in which the aldehyde is subsequently alkylated by the alkylzirconocene species (cf. Scheme 8.44) [56], In a control experiment, it was shown that zirconocene dichloride (1 equivalent or less) and silver (catalytic amounts) do indeed induce the rearrangement of an epoxide to an aldehyde very quickly. 

Interestingly, 1,2-disubstituted epoxides led to a,a-disubstituted propargyl alcohols, indicating that 1,2 shifts and therefore epoxide-to-aldehyde rearrangements occurred during the reactions (Scheme 10.12). Mechanistic studies by NMR validated this rearrangement and also supported the transmetallation-abstraction mechanism as already proposed (Scheme 10.13).8... 

Bhatia KA, Eash KJ, Leonard NM, Oswald MC, Mohan RS (2001) A facile and efficient method for the rearrangement of aryl-substituted epoxides to aldehydes and ketones using bismuth triflate. Tetrahedron Lett 42 8129-8132... 

Epoxide-carbaayl rearrangement. Lithium bromide effects facile rearrangement of epoxides to aldehydes and/or ketones in benzene solution. The salt is insoluble in benzene but addition of 1 mole of HMPT or tri- -bulylphosphine oxide per mole of lithium bromide affords a soluble complex which effects the epoxide rearrangement. Evidence suggests a mechanism involving the salt of the bromohydrin as an intermediate. 

Rearrangement of epoxides Phis aluminum reagent (1 equiv.) effects rearrangement of trisubstituted epoxides to aldehydes with very high selectivity. Lewis acids are... 

Rearrangement of epoxides. This relatively inexpensive metal carbonyl can serve as a homogeneous catalyst for rearrangement of epoxides to aldehydes and for isomerization of p,y- to , S-unsaturated aldehydes. The olefin corresponding to the epoxide is a by-product. ... 

Rearrangements. The epoxide to aldehyde/ketone rearrangement is a classic reaction mediated by BF3-Et20. Recent... 

Catalytic quantities of bismuth(III) triflate initiate the rearrangement of epoxides to aldehydes which then react with (Z)-5-hydroxyalkenylsilanes to give 2,6-disubstituted 3,6-dihydro-277-pyrans (Scheme 58). ... 

Maruoka K, Murase N, Bureau R, Ooi T, Yamamoto H. Lewis acid-promoted selective rearrangement of tri-substituted epoxides to aldehydes or ketones. Tetrahedron 1994 50 3663-3672. 

Epoxides can also be rearranged to aldehydes or ketones on treatment with certain metallic catalysts.A good way to prepare p-diketones consists of heating a,P-epoxy ketones at 80-140°C in toluene with small amounts of (Ph3P)4Pd and 1,2-bis(diphenylphosphino)ethane. ... 

Phenylthio-l-trimethylsilylalkanes are easily prepared by the alkylation of (phenylthioXtrimethylsilyl)mcthane as shown in Scheme 10 [40], The treatment of (phenylthio)(trimethylsilyl)methane with butyllithium/tetramethylethylene-diamine (TMEDA) in hexane followed by the addition of alkyl halides or epoxides produces alkylation products which can be oxidized electrochemically to yield the acetals. Since acetals are readily hydrolyzed to aldehydes, (phenylthioXtrimethylsilyl)methane provides a synthon of the formyl anion. This is an alternative to the oxidative transformation of a-thiosilanes to aldehydes via Sila-Pummerer rearrangement under application of MCPBA as oxidant [40, 41]. 

Fig. 10.3. Mechanism of the acid-catalyzed proton shift in the rearrangement of some alkene epoxides (10.10) to aldehydes (10.11)...

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]. 

Reactions of salicylic aldehydes with the chloromethyl tolyl sulfone anion afford 2-hydroxydihydrobenzofurans in moderate to good yields (Scheme 98) <1998T6811>. Formation of an epoxide intermediate that rearranges to the corresponding homologated alcohol is believed to be involved. Decarboxylation of the reaction product is prevented by lactol formation <1998T6811>. 

Certain Lewis acids are known to induce an epoxide-aldehyde rearrangement <01TL8129>, and this chemistry has recently been combined in tandem with metal-mediated allylations. For example, epoxides react with tetraallyltin in the presence of bismuth(III) triflate to give homoallylic alcohols 116. The reaction involves an initial 1,2-shift to form an aldehyde 115, which is then attacked by the allyl tin species <03TL6501>. A similar but operationally more straightforward protocol is available by combining allyl bromide with indium metal, followed by the addition of epoxide <03TL2911>. 

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