Aldehydes 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... 

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>. 

The stereochemical outcome of the rearrangements of At,iV-dibenzylidene-l,2-cyclopropanedi-amines to 2,3-dihydro-2,3-diphenyl-l,4-diazepines parallels the rearrangement of 1,2-divinylcy-clopropanes. Substrates derived from cis-1,2-cyclopropanediamines with E,E geometry at the double bonds and aromatic aldehydes rearrange at room temperature to cis-diazepines 18. The trans-substrates require temperatures > 90 °C because of primary epimerization to the ra-sub-strates via a diradical intermediate766,1053. 

Oxonium intermediates resulting from the pinacol rearrangement of a-sulfonate acetal derivatives can be intercepted with nucleophilic reagents, affording protected ketones or aldehydes. Rearrangement of acetals in the presence of excess bis(2-methylpropyl)aluminum hydride affords the acetal products31. 

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