Giexner, Renate Maria (2012)
Ph.D. thesis, University of Birmingham.
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The addition of allylmetals to aldehydes, particularly in an asymmetric fashion, is of prime importance in the synthetic chemist’s arsenal of C–C bond-forming methodologies, as the secondary homoallylic alcohol products are primed for further transformations. So far, reagent-controlled approaches with chiral metal-based Lewis acids have been the most effective means for controlling the enantioselectivity of this reaction. However, this approach cannot be used when acetals are employed as the latent electrophiles. Since Brønsted acids also mediate the Hosomi-Sakurai reaction of acetals, we hypothesised that a chiral Brønsted acid HY* has the potential to introduce stereoselectivity into this bond-forming process via the formation of a chiral contact ion pair in which the chiral conjugate base controls the nucleophilic addition to the achiral oxacarbenium ion with which it will be associated. For the investigation, three types of Brønsted acids with different acidity, all based on a binaphthyl core, have been synthesised. These catalysts were employed in the reaction of 1-alkoxyisochromans with allyltrimethylsilane. An optimisation study was carried out and the scope and the limitations of the method were probed. In summary, the first enantioselective asymmetric organocatalytic allylation of acetals is presented.
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