Epoxides rearrangement with acid

Epoxides rearrange with Lewis acids in a pinacol fashion... 

Interrelationships between the kaurene and atiserene series have also been established by a Wagner-Meerwein rearrangement. For example treatment of isokaurene epoxide (62) with acid gave atisiran-15-one (63), possibly through the sequence shown in Scheme 24. ... 

The synthesis of vinyl epoxides from dienes is mentioned in Chapter 19. The monoepoxide is formed first as the diene is more nucleophilic than the alkene in the monoepoxide. The main difficulty is that the monoepoxide rearranges with acid catalysis from the by-product, the carboxylic acid of the peroxyacid used in the epoxidation. The solution is simple the mixture must be buffered to keep the acidity low. 

Fucoxanthin (40) in a series of reactions involving lithium aluminium hydride reduction of the keto and acetate function to fucoxanthinol, allylic dehydration and subsequent epoxide rearrangement in acidic chloroform of fucoxanthinol to the epimeric fucochromes and further reaction with lithium aluminium hydride under forcing conditions, finally provided zeaxanthin (26) (22, 26). 

Campholenic Aldehyde Manufacture. Campholenic aldehyde is readily obtained by the Lewis-acid-catalyzed rearrangement of a-pinene oxide. It has become an important intermediate for the synthesis of a wide range of sandalwood fragrance compounds. Epoxidation of (+)- Ct-pinene (8) also gives the (+)-o -a-pinene epoxide [1686-14-2] (80) and rearrangement with zinc bromide is highly stereospecific and gives (-)-campholenic aldehyde... 

Again, difluorohalomethyl,50,58 aminodifluoromethyl59 or difluoro(sulfonyloxy)methyl derivatives (e.g., 8)58,60-63 Can be aminolyzed in a similar manner to yield amidines59,63 or amides.50 58,60-62 In the latter process the intermediacy of acid fluorides (see Section 6.1.2.) has been proposed. By analogy, reaction of monoalkylated perfluorinated epoxides with amines leads to a-functionalizcd amides.64 65 In the presence of fluoride ions, perfluorinated epoxides can rearrange to acid fluorides which can be further elaborated with amines to provide the respective amides.63... 

The three mono-epoxides of humulene 1 are naturally occurring, and it is believed that they are in vivo precursors of other bicyclic and tricyclic sesquiterpenes. In vitro experiments have demonstrated that the 1,2- and 4,5-epoxides undergo facile acid-catalysed rearrangement, and it has been shown recently that treatment of a chloroform solution of the 8,9-epoxide with tin(IV) chloride at -60°C for 15 minutes gives a variety of hydrocarbons and one major product (25%), the alcohol 2. 

The propensity for epoxides to undergo various rearrangements under Lewis acidic conditions is also well-known. Typically, Lewis acid "catalysts", such as BF3-OEt2, must be used in quantities approaching stoichiometric amounts due to their instability under the reaction conditions. However, Yamanoto has recently demonstrated that tris(pentafluoro phenyl)boron, an air-stable and powerful Lewis acid, was not only active in catalytic quantities, but also effective in mediating the alkyl-shift rearrangement of epoxide 70 with... 

The intermediate cation in a pinacol rearrangement can equally well be formed from an epoxide, and treating epoxides with acid, including Lewis acids such as MgBr2, promotes the same type of reaction. 

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