New access to long-chain polyketides and spiroketals by umpolung of electron rich dienes with sulfur dioxide
Natural polyketides show important biological activity. The inherent stereochemical complexity present in this class of molecules has captured the imagination of organic chemists. Their stereocontrolled, asymmetric total synthesis is a permanent and stimulating area of research. Previous efforts in our group demonstrated that the Umpolung of electron rich 1,3-dienes with SO2 could represent an efficient synthetic strategy to polyketides. Under appropriate conditions sulfur dioxide reacts with 1,3-dienes in the hetero-Diels-Alder fashion. In the case of 1-alkoxy-1,3-dienes, the 6-alkoxy-3,6-dihydro-1,2-oxatiin-2-oxides (sultines) so-obtained can be ionized in the presence of a Lewis or protic acid. The resulting zwitterionic intermediates can be trapped with enoxysilanes (oxyallylation) to generate the corresponding substituted alkenylsulfinates. Desilylation of the latter under acidic conditions leads to allylsulfinic acid intermediates, which, after retro-ene desulfitation, produce 4-alkoxyhepta-6-en-2-one core, a valuable fragment in polyketide natural product synthesis. This new SO2 cascade reaction, hetero-Diels-Alder addition/oxyallylation/retro-ene desulfitation (Vogel's cascade) has been extended to 1-alkoxy-3-acyloxy-1,3-pentadienes. Their use in this cascade reaction allowed the generation of 1,3-polyols array containing keto and enol ester functions, suitable for further chain elongation. In addition, this work highlights the extension of this methodology to other types of p-nucleophiles. The use of carefully elaborated allylsilanes allowed the preparation in a single operation (one pot double oxyallylation reaction) of long chain polyketides of 13 carbons units. Finally, a new route to the spiroketals A/B and C/D and of Spongistatins is presented in order to demonstrate the utility and the versatility of this approach.
Section de chimie et génie chimique
Faculté des sciences de base
Institut des sciences et ingénierie chimiques
Jury: Janine Cossy, Manuel Gomez Guillén, Stefan Pitsch, Raymond Roulet
Public defense: 2005-12-16
Record created on 2005-10-12, modified on 2016-08-08