The insertion of lithiated epoxides
into boronic esters followed
by thermal syn-elimination provides a stereospecific
entry to alkenes. This process avoids transition metals and is amenable
to iteration to provide higher substitution patterns.
A synthesis of S,S-DICHED (dicyclohexylethane-1,2-diol), a C
2-symmetrical chiral director for Matteson homologations, is described. It relies on the insertion of lithiated S-2-cyclohexyloxirane into cyclohexylboronic acid pinacol ester and proceeds in three linear steps from readily available starting materials. No step requires chromatography or any specialized equipment.
Stereoselective preparation of highly substituted olefins is still a severe challenge that requires well defined elimination precursors. Organoboron chemistry is particularly suited for the preparation of molecules with adjacent stereocenters. As organo boron substrates with leaving groups in β‐position can undergo stereospecific syn‐ or anti‐elimination, this chemistry harbors great potential for the synthesis of complex olefins. In recent years three main strategies emerged, which differ in their approach to the β‐functionalized organoboron elimination precursor. (i) Stereoselective preparation of such elimination precursor can be achieved by addition of a boron‐stabilized anion (d1) to an aldehyde or ketone (a1) or diastereoselective 1,3‐rearrangement of suitable boron‐ate‐complexes. Stereospecific methods rely either on (ii) diastereospecific 1,2‐metalate rearrangement of boron‐ate‐complexes that involve opening of appropriate heterocycles or (iii) addition of chiral carbenoids (d1*) to chiral boronates (a1*) with a leaving group in α‐position.
The first total synthesis of resveratrone and iso‐resveratrone based on an epoxide olefination approach is described. The pivotal reaction proceeds by insertion of the lithiated epoxide into a boronic ester and subsequent syn‐elimination. Resveratrone has been described to have remarkable photophysical properties, including two‐photon absorption. Therefore, an azide derivative has been prepared to allow for use as a biological label.
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