Catalytic base-induced isomerization

Building on Inoue s pioneering work and using Andersson s versatile diamine, Kozmin and coworkers have described a catalytic base-induced isomerization of silacyclopentene oxide 139 [Eq. (10.41)] as a potential route to acyclic polyol domains, important components of a number of natural products. Epoxida-tion of 140, epoxide ring opening, and oxidation of the C—Si bonds provides tetra-ols in good overall yield and selectivity ... 

It should be noted that addition of DBU (or HMPT) has often - particularly for the catalytic procedures - proven beneficial in terms of enantioselectivity. This effect has been attributed to the breaking-up of active but less enantioselective base aggregates [50, 56, 62-64]. Interestingly, when solid-phase-bound stoichiometric (achiral) bases were used instead of LDA, no addition of DBU was necessary [58, 59]. (An example of the stoichiometric use of a chiral solid-phase bound base is given elsewhere [52].) Both experimental and theoretical investigations [66-68] indicate that the base-induced isomerization of epoxides proceeds as a syn elimination, with the lithium ion of the base acting as a Lewis acid (Scheme 13.31). 

It has been shown that Lewis acid catalyzed isomerization of thionolactones provides access to thiolactones. For example, exposure of the substrate 22 to catalytic amounts of BF3 OEt2 led to efficient conversion to the thiolactone 23. Such transformations were also found to give minor amounts of lactone or dithiolactone side products <06TL6067>. Substituted tetrahydrothiophene derivatives have also been obtained from 1,4-dithiane-2,5-diol and 2-nitroethyl acetate derivatives by a base induced sequence featuring a Michael addition and a Henry reaction <06TL8087>. 

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