Lithio-epoxide, rearrangement

For example, 1-donor-substituted cyclopropancmethanols may be efficiently produced by cyclopropanation of suitably substituted enol ethers, by reaction of 1-donor-substituted 1-lithio-cyclopropanes with carbonyl compounds, or by addition of carbon nucleophiles to 1-donor-substituted cyclopropanecarbaldehydes. Oxaspiropentanes, important precursors of cyclobutanones, may as easily be obtained by epoxidation of methylenecyclopropanes, or by reaction of carbonyl compounds with diphenylsulfonium cyclopropanide and l-bromo-1-lithiocyclopropanes, respectively. Moreover, as the stereochemistry of most rearrangements may be efficiently controlled, asymmetric syntheses begin to appear. 

Multicomponent Linchpin Reactions. 2-Lithio-2-TMS-1,3-dithiane has also been employed as a bi-directional nucleophile or linchpin in epoxide opening reactions where both openings occur in an intermolecular fashion. The dithiane anion first attacks the less hindered carbon of the epoxide then, in the presence of THF and 12-crown-4,1,4-Brook rearrangement occurs with transfer of the TMS group to reveal a new dithiane anion that can react with an additional equivalent of epoxide (eq 27). This reaction, first reported by Tietze et al. in 1994, suffered from a lack of control over the rearrangement step, thus limiting the use to the formation of symmetric 1,5-diols. ... 

Sn2 VersusSN2 Reactionof2-TMS-l,3-Dithiane With Vinyl Epoxides. In 2002 Smith et al. demonstrated that 2-lithio-2-TMS-1,3-dithiane reacts with vinyl epoxides in an Sn2 fashion exclusively, while other, larger silyl dithianes afford mixtures of Sn2 and Sn2 products. Particularly useful, the large 2-lithio-2-triisopropyl-1,3-dithiane provides solely the Sn2 product. Furthermore, it was found that trans epoxides furnish syn products and cis epoxides produced anti products, albeit in modest yields (eq 28). However, under the reaction conditions (THF, HMPA) the TMS group underwent a 1,4-Brook rearrangement to afford a 1 1 mixture of the anticipated homoaUyhc alcohol and the rearranged silyl ether. 

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