Lewis acid catalysis epoxidation

Nugent, W. A. (1992) Chiral Lewis acid catalysis Enantioselective addition of azide to meso-epoxides., J. Am. Chem. Soc, 114 2768-2769. 

The limitation of the pinacol is the need for symmetry. This section and the next suggest ways of avoiding this problem. Unsymmetrical epoxides are easily made from alkenes and open with Lewis acid catalysis to give the more substituted of the two possible cations.9 Even such a weak Lewis acid as LiBr opens the epoxide 51 to give the tertiary cation 52 which rearranges by ring contraction to the aldehyde 53. The authors prefer to have the bromocompound 54 as an intermediate.10... 

A single enantiomer of the epoxide below rearranges with Lewis acid catalysis to give a single enantiomer of product. Suggest a mechanism and comment on the stereochemistry. 

Epoxides too will react with indoles using Lewis acid catalysis with (l/ ,2 S -l,2-dihydronaphthalene oxide, high enantioselectivity (83% ee) shows the effectiveness of InBrj (Scheme 53) <2002JOC5386>. 

Epoxides can react smoothly with allylsilane reagents, for example, 1,3-bis-trimethylsilyl-l-propene under Lewis acid catalysis, to generate a highly functionalized alkenols (Scheme 41) <1998TL529>. 

The homologation of ketones by the addition of diazoalkanes complements the Tiffeneau-Demjanov rearrangement. Epoxide formation is a side reaction which can be minimized if polar aprotic solvents are avoided (Scheme 7). Rearrangement i.e. homologation) is maximized in ether solvents or by Lewis acid catalysis. The reaction is most effective in the ring expansion of cyclic ketones. 

Lithium derivatives of allyl silanes react in the y-position with alkyl halides, epoxides, and carbonyl compounds. The lithium derivative 110 of allyl silane 109 gives only the y-adduct 111 with ketones.31 Vinyl silanes such as 111 are usually converted into carbonyl compounds via epoxides which rearrange with Lewis acid catalysis and loss of silicon to give protected versions of ketones or aldehydes 112. 

Dithians 20 illustrate this approach and have been remarkably popular considering the problems in their use. They can be made from aldehydes with propane-1,3-dithiol and Lewis acid catalysis. They are deprotonated with BuLi and react with alkyl halides, epoxides, and carbonyl compounds (E+) to give 22 and hence 19 after hydrolysis. The hydrolysis is by no means easy there are many methods and this alone should warn us that none is very good.6... 

Acidic activation of the epoxide can be achieved either by Brpnsted-acidic catalysis via addition of a proton to the epoxide oxygen or by Lewis-acidic catalysis via coordination of the epoxide oxygen to a multivalent cation. In basic catalysis one of the epoxide carbons is attacked by a nucleophile. Only acid-catalyzed ringopening leads to an intermediate carbocation (3) which can easily result in the mi-... 

Nucleophilic substitutions proceeding via Sn2 pathways can be activated by pressure, as has been demonstrated in many examples. In particular, the ring opening of epoxides can be initiated by pressure, and also by Lewis acid catalysis. Consequently, combining these two activation modes might lead to an even more effective way to functionalize epoxides, and indeed, this strategy has been successfully applied. 

Epoxidation. With Mn-salen as a catalyst, enol derivatives give 2-hydroxy acetals. Unsaturated acids form lactones during oxidation in the presence of an iron porphyrin. Glycosylation. The action of a PhlO-MejSiOTf combination on thioglycosides consists of oxidation and Lewis acid catalysis, thus allowing the synthesis of disaccharides. 

When the attacking nucleophile is carbon-centered, intramolecular epoxide ring-opening reactions provide an entry intocarbocyclic systems. For example, epoxy allylsilane 70 cyclizes in an overwhelmingly 5-exo fashion under Lewis acid catalysis to form a putative silyl-stabilized carbocation intermediate (71) which then undergoes... 

Epoxides can also be converted to 1,3-dioxolanes by treatment with acetone in the presence of catalytic amounts of bismuth(lll) salts, with yields ranging from 87-99%. For example, the epoxy allyl ether 98 provided the dioxolane 99 in 97% yield using bismuth triflate as the catalyst <01SC3411>. When simple epoxides are treated with Aw(triphenylphosphine)-iminium cobalt tetracarbonyl (PPN-Co(CO)4) under Lewis acid catalysis, a carbonyl insertion reaction provides p-lactones regioselectively in good to high yields. The carbonylation occurs selectively at the unsubstituted position, and the reaction is... 

Schrock s asymmetric metathesis catalyst, Sharpless epoxidation of allylic alcohols, Noyori s isomerization of allylic amines hydrocyanation and carbene transfer Lewis acid catalysis. 

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