Epoxide rearrangements

Scheme 5. Payne rearrangement/epoxide opening reaction hydroxide nucleophile.

A careful analysis of this problem led to the identification of an exceedingly simple solution (see Scheme 10). The Masamune-Sharpless solution to the threo 2,3-diol problem actually takes advantage of the ready availability of the erythro 2,3-diol diastereoisomer. As we have seen in Scheme 9, erythro 2,3-diols such as 20 can be conveniently assembled from trans allylic alcohols via sequential SAE and Payne rearrangement/epoxide opening reac-... 

Payne rearrangement. The Payne rearrangement2 of a primary cts-2,3-epoxy alcohol to a secondary 1,2-epoxy alcohol usually requires a basic aqueous medium, but it can be effected with BuLi in THF, particularly when catalyzed by lithium salts. As a consequence, the rearrangement becomes a useful extension of the Sharpless epoxidation, with both epoxides available for nucleophilic substitutions. Thus the more reactive rearranged epoxide can be trapped in situ by various organometallic nucleophiles. Cuprates of the type RCu(CN)Li are particularly effective for this purpose, and provide syn-diols (3).3... 

The formation of the oxepin is reasonably explained by an electrocyclic ring opening of rearranged epoxide 299 in a thermal reaction. As mentioned above, two routes to 299 are possible. If the rearrangement is concerted, a 1,5-sigmatropic reaction with inversion of the reaction center (oxygen) in 299 is photochemically allowed. It is possible to separate a nonconcerted process... 

Two new tetraphenylporphyrin (TPP) catalysts have been reviewed.331 Cr(TPP)m triflate is highly regio- and stereo-selective in rearranging epoxides into aldehydes, while the iron perchlorate analogue affords the corresponding ketones. 

LiBr acts as a Lewis acid which rearranges epoxides. The reactions afforded aldehydes leading to ring contractions (Sch. 68) [117]. 

Contraction and Expansion of Steroid Rings The Westphalen and Backbone Rearrangements Epoxide Rearrangements Aromatization... 

Ibuka, T., Nakai, K., Akaji, M., Tamamura, H., Fujii, N., Yamamoto, Y. An aza-Payne rearrangement-epoxide ring opening reaction of 2-aziridinemethanols in a one-pot manner a regio- and stereoselective synthetic route to diastereomerically pure N-protected 1,2-amino alcohols. Tetrahedron 1996, 52, 11739-11752. 

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. 

The syntheses of the C1-C12 AB and C13-C28 CD segments, 528 and 535, are summarized in Scheme 75. The syntheses of the two segments are based on a one-pot unsymmetric bisalkylation of 2-TBS-l,3-dithiane (525) with Brook rearrangement. Epoxides 521, 522, 523, and 524 were synthesized as coupling partners. 

Skeletal rearrangement of alkanes, olefins, and functionalized compounds pinacolone rearrangement Wagner-Meerwein rearrangement epoxide rearrangement rearrangement of cyclic acetals... 

A stereoselective approach to tetrasubstituted (Z)-/3-hydroxy-o -TMS (trimethylsi-lyl)enol ethers occurs via a sequential addition/[ 1,2]-Brook rearrangement/epoxide-opening process of TMS-substituted oxiranyl anions with acylsilanes (Scheme 46). ... 

A stereoselective approach to tetrasubstituted (ii)-/ -hydroxy silyl enol ethers from aryl-substituted oxiranyl anions and acylsilanes is reported to occur via a sequential addition - [l,2]-Brook rearrangement - epoxide-opening (Scheme 87). ... 

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