Sulfur dioxide mediated syntheses of polyene containing natural products

The goal of this thesis was to open a new route to the synthesis of conjugated trienes using a new SO2-based C-C bond forming reactions developed in Lausanne. In addition, the potential of SO2 for applications in synthetic organic chemistry has been exploited. Firstly, we discovered that under suitable conditions sulfur dioxide is able to undergo ene reaction with different building blocks such as enoxysilanes and allylsilanes to form the corresponding sulfinates. The latter can be used as key intermediates for the preparation of β-ketones, β-keto-sulfonamides and β-keto-sulfonic esters. This approach allows preparing β-keto-sulfonamides in one-pot operations. Up to now, the same type of compounds would require multi-steps syntheses. Considering the importance of sulfonamides and sulfonic esters in medicinal chemistry we prepared representative examples of such compounds. Secondly, our studies demonstrate, for the first time, that sulfones containing (E)-alkene units can be obtained through the reaction of (E,E)-1-silyloxy-2-methylpenta-1,3-diene with substituted enoxysilanes and an excess of sulfur dioxide, followed by subsequent quenching with electrophiles. Importantly, this four-component reaction yields (E)-alkenes only in the presence of a protic acid promoter and only with 1-silyloxypentadienes. In all cases mixtures of α,β-syn and α,β-anti ketones are obtained. The α,β-diastereoselectivity of the oxyallylation (face selectivity of the enoxysilane addition) does not surpass 6:1 (3R,4S vs.3S,4S). Interestingly only β,ε-like substituted ε-sulfonylketones are formed, in contrast with the (Z)-isomers which present unlike β,ε-relative configuration. Thirdly, sulfur dioxide-based chemistry was applied for the asymmetric synthesis of the conjugated (E,E,E)-triene moiety of apoptolidinone, a highly potent apoptosis inducing agent with a daunting molecular architecture. Two main synthetic routes towards the synthesis of the trienic fragment of apoptolidinone were developed, one leading to the racemic fragment and a second route leading to the enantiomerically enriched C(1)-C(9) fragment. The latter was transformed further into the fragment already synthesized by Nicolaou and co-workers, demonstrating that this second route is significantly shorter than the route previously reported in the literature. The same methodology yields also facile access to the trienic part C(5)-C(14) fragment of restricticin. This underlines further the general applicability and the high flexibilities of our method toward the synthesis of trienes.